# Rear-fill speakers



## GlassWolf

*rear-fill speakers​*
While rear speakers might seem natural or necessary to some people, it is deemed undesirable and at best useless to those who are into good sound quality. There is nothing inherently evil about rear speakers, it's just that rear speakers often interfere too much with the ability of the front speakers to produce a believable sound stage and imaging characteristics. These effects can be minimized if care is taken to set up the rear speakers, but they are still there.

First, we discuss the "purist" view on rear speakers. Rear speakers grew out of necessity rather than the pursuit for better sound quality. A lot of cars, especially compacts and sub-compacts, have very small speaker openings in the front of the car due to space restrictions. It is rather rare to find a stock speaker location that can fit something as desirable as a 6.5 inch driver, while 4" and 4x6" speaker openings are quite popular. Small speakers are usually incapable of producing low bass (below 100Hz) at a satisfying level (say 90dB for casual listening.) Rear speaker mounting locations, especially the rear decks of sedans, offer a lot of area to mount a sizable driver, thus car manufactures rushed to mount large speakers in the rear to fill in the low bass region. This would have worked out if the stock stereos fed a low-pass filtered signal to the rear speakers so that they only produced the low bass frequencies, in which case those rear speakers would be called woofers or subwoofers. But no. Car manufactures didn't want to let all that volume go to waste so they fed the whole signal spectrum to the rear speakers. To make matters worse, the signal sent to the rear speakers is in stereo. The end result is that the center of the sound stage is somewhere to the far-right behind the driver and far-left behind the passenger. The instrument and vocal image floats all over the rear of the listener and shifts dramatically depending on the position of the listener's head. This is not how a live performance sounds.

For the reason discussed above, rear speakers of any configuration will interfere with the proper sound stage production and imaging of the front speakers. However, there are measures that can be taken to minimize the effect. The simplest thing to do is to turn down the rear speaker volume. Close your eyes, keep your head straight and adjust the front-rear fader control while you listen to a piece of music with strong central vocal content (check this with your home stereo or sit in the middle of the back seat with the rear volume turned all the way down.) First, turn the rear speaker volume all the way down, and then slowly turn it up until the vocal image starts to drift to the opposite side of the car. If you are sitting in the driver seat, listen for it to drift toward the passenger side and vice versa. This is the point where the rear speakers are still noticeable but it is not interfering too much with the proper operation of your front speakers.

There are two more things you can do but they require that you add components to your stereo system and the improvement is not as dramatic as simply turning down the rear speaker volume. Rear speakers should never be allowed to operate in full range unless you are going for the THX or Dolby Digital AC-3 theater surround sound setup. If you have a separate subwoofer, band-limit the signal going to the rear speakers to approximately between 200Hz and 3kHz. You don't need complex crossovers for this, just something like a 6dB per octave first-order high-pass filter at 300Hz and a 6dB per octave first-order low-pass filter at 2kHz. We're talking about one coil and one non-polarized electrolytic capacitor ($20 if you get the real snazzy stuff like polypropylene capacitors and low gauge air core inductors). Minimizing the high frequency content of the signal fed to the rear speakers is much more important than the low frequency content. In fact, if you do not have a dedicated subwoofer, you can do without the 300Hz high-pass filter and let the rear speakers produce the bass frequencies. But keep in mind that rear speakers should have a lower relative volume than the front so the effectiveness of the rear speaker to double as a subwoofer is severely limited.

To wring out the last bit of negative side effects, the rear speakers should be in mono. This can be done only if you have an external amplifier. In other words, this is impossible with a stock stereo system. The simplest way to do this is if the amplifier has a stereo/mono switch built-in, or to use an amp that is bridgeable. Then just put the two speakers in series and bridge them across the amplifier. If the amp is not bridgeable, you will have to find a crossover or some other signal processor that has a mono output. However, most of the crossovers and signal processors only have mono output for subwoofers (music below 200Hz) and thus are not suitable for this purpose. So the simplest way to do this is to get an inexpensive bridgeable amplifier.

If you do decide to get rear speakers, you would have to decide which type or brand of speakers to get. After reading what's written above, if you think all this rear speaker negative side effect crap is hog wash, just pick out your rear speakers. But if you are now a faithful believer in low-volume band-limited mono rear speakers, read the next paragraph

Your cheapest and simplest solution is not to have ANY rear speakers. The next cheapest solution is to keep your stock rear speakers. The reasoning behind this is that most stock speakers are quite OK in the 200Hz to 3kHz region. It is in the bass and high frequency region where they run into trouble. Besides, you are going to be running it low-volume, band-limited and in mono, so the difference between a pair of stock rear speakers and say a $250 pair of high quality mid-bass drivers is not going to be all that noticeable. But again, it is important that you keep it low-volume, band-limited and in mono. If you have a lot of money (send me some) and really want a fancy spare-no-expense type of system, then go ahead and find a good solid pair of mid-bass speakers. DO NOT, I repeat DO NOT, I'll say it again DO NOT buy co-axials or component speakers to use as rear fill since you are going to be wasting a lot of money on the useless tweeter and crossovers. You might be rich but you don't have to be stupid. A $200 set of components or co-axials might be just so-so but a pair of $200 mid-bass drivers is going to be killer. A lot of companies make really good mid-bass drivers. Look into Kicker, JL, Boston Acoustics, MB Quart, Audax, or Morel just to mention a few. There are also a few high dollar brands such as Dynaudio, USD Audio, Image Dynamics and Focal. This is not an inclusive list because there are a LOT of good mid-bass drivers out there.


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## ebrahim

Well I have a KIA SPECTRA SX sedan and I am doing two way components around but the fronts are getting 180 x 2 and the rears 80 x 2. The reason why I am doing that is because my headunit plays dvds and my mom likes watching dvds in my car when we go see my brother.


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## GlassWolf

I believe I noted somewhere in that article when I wrote it some time ago, that multi-channel sources like DVD audio, or entertaining passengers in the back seats were exceptions to the no-rear fill iew.

That noted, this article was written purely from a 2.0 audio source SQ viewpoint.


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## jasondplacetobe

i always thought rear fill added ambience so thats why iam gonna run them. so i'll follow your advice to the letter. could you also make your amp run mono by combining the signal from the rcas?


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## Notloudenuf

I remember reading about attaching the + from the R speaker and the - from the L speaker and vice versa to create a mono signal. If I remember correctly (and I'm sure I don't) this was some version of Dolby surround processing.

Does what I said make any sense or am I completely off base here?


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## TREETOP

Notloudenuf said:


> I remember reading about attaching the + from the R speaker and the - from the L speaker and vice versa to create a mono signal. If I remember correctly (and I'm sure I don't) this was some version of Dolby surround processing.
> 
> Does what I said make any sense or am I completely off base here?


You're probably thinking about "L-R", left minus right. It's a much more effective method for rear fill than regular mono.

Some good info on L-R here:
http://www.diymobileaudio.com/forum...m/22523-hey-werewolf-confusion-about-l-r.html

http://www.diymobileaudio.com/forum...71218-rear-fill-l-r-stereo-mono-question.html

And a 19-page thread on rear fill in general, with some more good L-R info here:
http://www.diymobileaudio.com/forum/product-selection-comparisons/9806-rear-fill-do-you-use.html


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## jasondplacetobe

thanks tree top. i'll check those links out. nice avatar by the way


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## GlassWolf

you're talking about bridging an amplifier,and the amp has to support bridged mode by inverting one channel to operate this way.

you can't rally combine RCA line signals with a splitter/combiner that way. you need to wire the head unit or XO outputs to the amp, then set the two rear amp channels to bridged mode and run the rear speakers in series @ 8 ohms (thus the amp "sees" 4 ohms and delivers the same power as it's giving the front two channels in stereo @ 4 ohms)
The rears will also need a bandpass filter between about 500Hz to 5KHz. If you can't do this with one filter network, you can do it with the amp's crossover and the head unit combined, using one XO as the HPF, and the other as your LPF, combining to create your bandpass crossover filter. I'd recommend a 2nd order, or 12dB slope for both HP and LP filters for the rears.


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## lycan

TREETOP said:


> You're probably thinking about "L-R", left minus right. It's a much more effective method for rear fill than regular mono.
> 
> Some good info on L-R here:
> http://www.diymobileaudio.com/forum...m/22523-hey-werewolf-confusion-about-l-r.html
> 
> http://www.diymobileaudio.com/forum...71218-rear-fill-l-r-stereo-mono-question.html
> 
> And a 19-page thread on rear fill in general, with some more good L-R info here:
> http://www.diymobileaudio.com/forum/product-selection-comparisons/9806-rear-fill-do-you-use.html


thanks for posting the links 

To argue _against_ rear-fill, most people always parrot a silly statement, usually something like "when you go see a band, the band is playing in front of you". Two fundamental things wrong with this concept:

1. In _any_ venue, there will always be sound coming from behind you _also_. These "ambient cues" give you a sense of space.

2. When you go see a band, you're probably not enjoying the music in a box the size of a phone booth.

Proper use of an attenuated, bandlimited, and _delayed_ (on the order of 20msec, in accordance with Haas) L-R difference signal for rear-fill will _enhance_ the sense of space inside the vehicle, without detracting from the front stage.

Yes ... there are many "flavors" of rear-fill. It would be a huge mistake to lump them all together.


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## Knobby Digital

lycan said:


> To argue _against_ rear-fill, most people always parrot a silly statement, usually something like "when you go see a band, the band is playing in front of you".


Speak for yourself. I'm looking for a convertible right now in which I can install 20 ft. baffles on the rear deck so I can listen to pipe organ music more accurately.


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## GlassWolf

I like L-R differential band-limited rear fill with delay, sure, but it's far more practical for most home audio than for car applications, since very few car audio signal processors appear to offer balanced inputs, and the ones that do tend to be extremely expensive (I'm looking at you, Zapco SymbioLink.)

It's a great idea, but how would you implement this in a typical car audio system with, say, a head unit, a 4 channel full range amplifier, a sub amp, and the typical 2-way component front stage and a pair of midbass drivers as rear fill, and of course the sub in the trunk. That's what most people would have to work with, short of spending a load of dough for signal processors that don't use optical connections and still offer (XLR-like) balanced interconnects, as after reading the threads above, it appears that most people are advocating this method of rear fill be done pre-amplification with unbalanced source and balanced signal processing to handle 1/3 octave equalization, time alignment, and crossover (cut/boost, Q, and HPF, LPF) requirements, taking all of that work away from the usual offerings in modern head unit offerings.

The reason I'd suggested the mono bridged amp + serial rears above was mainly due to the simplicity and cost effectiveness of doing this with the existing gear in an average aftermarket mobile audio system, as opposed to just running two rear stereo channels with some attenuation. (As noted I can't edit my original posts for some reason, so I'm unable to revise any of this stuff, and I forgot to mention the addition of manual delay adjustment for the rears.)

Thanks for the input.


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## GlassWolf

I've given some thought to running rear fill, myself. The issue I ran into was that my head unit does all of the signal processing, and it's a 4-way output, so I either run my 3-way front stage passive, using one pair of pre-outs for rear fill, or I run 3-way active front stage, and sub outputs, using all 4 of the pre-outs. The other option is spending a bunch of more money, endering all of the internal goodness of the Pioneer P99RS useless, and do external digital signal processing via something like an AudioControl product.


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## lycan

this is, after all, a DIY forum ...

Simple subtraction of two audio signals should not be discouraged. An opamp plus two resistors will work, if there's no balanced signal to be found. As far as the prohibitively difficult tasks of bandlimiting & attenuation, well .... 

I do agree that ~20msec of delay is not necessarily as easy as the above, but again ... compared to what many tackle, and describe on a DIY forum, it's not that difficult.

Adding, or enhancing, ambience makes much more sense to me in a _smaller_ environment, than a _larger_ one


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## GlassWolf

do you happen to have an image of a circuit diagram for the op amp and resistors I could borrow?
I don't have any circuit design software on this box, and I wanted it for use in updating the article above.

if ya do, thanks. appreciate it.


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## lycan

GlassWolf said:


> do you happen to have an image of a circuit diagram for the op amp and resistors I could borrow?
> I don't have any circuit design software on this box, and I wanted it for use in updating the article above.
> 
> if ya do, thanks. appreciate it.


not "handy" ... i'm pretty image-illiterate, sadly 

But maybe what's needed is a new tutorial, on how to create the appropriate bandlimited & delayed difference signal? Maybe these dandy new minidsp's (that bikinpunk turned us onto) can provide delay, in addition to bandlimiting? Remember, pristine signal paths are not required for ambience enhancement 

Check the web for simple opamp inverters, inverting opamp, stuff like that


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## GlassWolf

The Basic Op Amp Inverter










"An input voltage, Vin is applied to the input resistor, Rin. The amplifier, represented by the triangle, amplifies the input voltage it receives and inverts its polarity, producing an output voltage, Vout. This same output voltage is also applied to a feedback resistor, Rf, which is connected to the amplifier input along with Rin.

The amplifier itself has a very high voltage gain. As a result, the junction of the two resistors, which is also the amplifier input, must be virtually at ground potential. A non-zero input voltage will be amplified so much that the output voltage would try to exceed its electronic limits. At the same time, the amplifier requires almost zero input current to operate. Therefore, the input current (Vin/Rin) must be the same as the feedback current (Vout/Rf). This in turn means that the effective gain of the circuit with feedback in place is simply the resistance ratio, Rf/Rin. This is the beauty and value of the operational amplifier: we can obtain precision results if we use precision resistors."

Now I just need to find the values for the resistors, and maybe I'll give it a try. With unbalanced input and output, it shouldn't be very expensive to do. Most head units can handle time delay, and the head unit and amplifier combined can handle a bandpass filter, if one handles the HPF and the other handles the LPF accordingly, given that you don't have a single bandpass filter network to do it for you, or don't want to build a passive one to do the job at the speaker(s). Actually that diagram link is using a 15VDC power supply so the opamp and resistor values listed should just "drop in" for this.


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## GlassWolf

lycan said:


> But maybe what's needed is a new tutorial, on how to create the appropriate bandlimited & delayed difference signal? Maybe these dandy new minidsp's (that bikinpunk turned us onto) can provide delay, in addition to bandlimiting? Remember, pristine signal paths are not required for ambience enhancement


I'm not familiar with these miniDSPs you mentioned. I'm also a lousy programmer, so that's probably not within my scope of skills, but it does sound intriguing. If you want to work on that idea, or just writing something on how to set up the delay and bandpass filtering for people with existing unbalanced systems to follow, I'd be all for it. Let me know if you wanted to collaborate on it, or just do it without help.. whatever.


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## GlassWolf

Hmm I found the thread on the MiniDSP. That looks incredibly promising. All you'd appear to need is a ~20msec delay, a L-R differential signal inversion, and a bandpass filter for maybe 250Hz-3KHz or so. With the plug-ins and their software that should be all too hard.
This could even be an item that could be programmed and resold as a niche market "plug in and go" solution for people.

I like that.


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## MarkZ

lycan said:


> this is, after all, a DIY forum ...
> 
> Simple subtraction of two audio signals should not be discouraged. An opamp plus two resistors will work, if there's no balanced signal to be found. As far as the prohibitively difficult tasks of bandlimiting & attenuation, well ....


It could even be easier than that. Some folks bridge their amps to run their speakers. Some amps sum the inputs when bridging. All you have to do is reverse the polarity of one of the RCAs, which is as simple as soldering.

Also, there are more and more people running carPCs these days. L-R and 20ms delay is a snap.

Edit: To the carPC crowd... A simple (and free) VST plugin that computes L-R is the Voxengo MSED plugin. It also computes L+R in the same step. So you can have one plugin handle your rear fill and your center channel output with very little CPU usage.


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## Negolien

I like my rear SoundStream SST6.9's Bi-Amped baby :<) with the 4x6's and tweets in the sails all I needs some bump in the trunk which will be fixed tommorrow.


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## lycan

MarkZ said:


> It could even be easier than that. Some folks bridge their amps to run their speakers. Some amps sum the inputs when bridging. All you have to do is reverse the polarity of one of the RCAs, which is as simple as soldering.


careful ... some amps ground the shield pins of the input RCAs to local power ground (it's a bad practice, but it persists nonetheless). Reversing the polarity of RCA's will only work with amplifiers that accept differential or balanced inputs (even if they "look" like RCA jax), so it's not recommended as a universally-good thing to do.

A line-level transformer would come in handy, and it can be a cheaper one to boot ... since we're not expecting low bass, or extended treble, from the L-R ambience signal  Big bonus of transformers : you get isolation also, and no power supply needed.


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## MarkZ

lycan said:


> careful ... some amps ground the shield pins of the input RCAs to local power ground (it's a bad practice, but it persists nonetheless). Reversing the polarity of RCA's will only work with amplifiers that accept differential or balanced inputs (even if they "look" like RCA jax), so it's not recommended as a universally-good thing to do.
> 
> A line-level transformer would come in handy, and it can be a cheaper one to boot ... since we're not expecting low bass, or extended treble, from the L-R ambience signal  Big bonus of transformers : you get isolation also, and no power supply needed.


Yes, thank you. I don't know what I was thinking.


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## GlassWolf

lycan said:


> careful ... some amps ground the shield pins of the input RCAs to local power ground (it's a bad practice, but it persists nonetheless). Reversing the polarity of RCA's will only work with amplifiers that accept differential or balanced inputs (even if they "look" like RCA jax), so it's not recommended as a universally-good thing to do.
> 
> A line-level transformer would come in handy, and it can be a cheaper one to boot ... since we're not expecting low bass, or extended treble, from the L-R ambience signal  Big bonus of transformers : you get isolation also, and no power supply needed.


got a part nujmber for the line xformer?


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## lycan

i don't have a part number. The Jensen's are world-class, but expensive. And much more expensive than needed for a bandlimited, rear-fill application. The Jensen webpage, however, is a TREASURE of information:

WELCOME TO JENSEN TRANSFORMERS, INC.

YES ... it's worth a FULL DAY to read through all the white papers alone!

Many uses for line-level transformers :

1. Isolation. Does your power amp ground the RCA negative input to it's local power ground, thereby creating a nasty ground loop when connected to a head unit? Bad move, but a transformer solves it.

2. Voltage gain, or attenuation (not power gain!). Turns ratio tells you voltage gain.

3. Inversion. Apply an audio signal to the primary. Depending on how you connect, or "observe", the secondary ... you either have the positive version of the signal, or the negative version of the signal.

4. Subtraction. Apply left-positive to one terminal of the primary, and right-positive to the other terminal of the primary. What you "observe" on the secondary is now L-R (or, R-L) 

Yes ... line-level transformers are wonderful little things.


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## psycle_1

I'm thinking a transformer like this should, work, correct?

EDCOR - WSM10K/10K


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## lycan

psycle_1 said:


> I'm thinking a transformer like this should, work, correct?
> 
> EDCOR - WSM10K/10K


looks fine to me! And for ten bux, how can you go wrong?

Tell ya what ... just to protect everything ... wire it like this :

LEFT PLUS --> 1k resistor --> *primary positive*
RIGHT PLUS --> 1k resistor --> *primary negative*

(ignore the center taps for now)

then, connect the *secondary positive* to one RCA terminal (plug/tip) of an amplifier, and connect the *secondary negative* to the other RCA terminal (shield/ring) of the _same_ amplifier input. It won't matter if the amp accepts balanced inputs, or (instead) grounds the negative RCA terminal ... that's one of the beauties of transformer coupling. You should now have an amplified, L-R difference signal 

Of course, we still need to bandlimit & delay our lovely "ambience" signal ...

Feel free to post a proposed sketch, before you commit.


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## psycle_1

Bandlimit: either use an amp with an internal crossover that will run in bandpass, or use passives...

Delay: use a Boss Digital Delay pedal. Since it already runs on a standard 9 volt battery is should not be an issue to wire it into the car's electrical system. I remember Bill Burton (RIP) mentioned using this in an old Car Stereo Review magazine article.


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## GlassWolf

I found a good transformer line for sale on eBay!


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## sqshoestring

Might as well tell my point of view on rears. I run them in many cars for two reasons: expand the left channel to my left, and secondly to add more midbass.

As we can find in other threads, you can add midbass in the cone of confusion, as long as the fronts lead them in volume (dB) to cue your hearing that they are in front. So you need some MB in front, more is better. You need to make rears that have a good MB response or, dominant MB if you will. Do this with install, driver type, EQ, whatever works.

Next I need some midrange and into the vocals, but not really treble. This is where most directional cues are, this will pull the left back to me and outside my window instead of in front of me. 

I like some residual treble in the rears, so I can't hear it but passengers close to them can. Once all this is working right, it works well. Is it full SQ, maybe not, but close, and maybe I highly value response and channel separation. 

What I typically start with is a larger driver with more MB, yep a 6x9 coax works really well if you can fit them, and mounted way off axis or under heavy cloth/felt if you need to kill the highs down some. Often run them at same or lightly less power than fronts, as the fronts are closer they sound louder to start with depending on your car/install. I often have to fade/swap drivers/EQ/change install to get it right so they blend in. If they have a peak they stand out and it will not work well. Less bright drivers work better, like a plain coax that has more bass compared to others. I only run them full power for outside the car listening, where they help greatly over just the fronts.

I can run TA, though in this car they are near the same distance to me. If I process the signal otherwise they will not meet my two reasons for having them, even though the difference signal sounds like fun to play with.


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## casual

how about putting a pair of decent tweets in the rear and blocking low frequencies? Is this a bad ideal also?


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## negativegain

i have experimented with rear fill in my audi for a few months now, and I have concluded that i prefer to have it. i have settled on a bandpass on my rear 6.5's of 80hz to around 5khz.

i primarily listen to electronic music, and maybe it sounds better to me because in a warehouse/club, the sound is coming from all directions. having rear fill playing just gives a much better sense of space in my application.


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## sqshoestring

I like that too, if you get it perfect it sounds like headphones.

You can get strange things if you have multiple tweeters like combing, its not a good idea unless they have some processing in them. I find it best if I mute the treble so the front is quite dominant, then no problems. Into the vocal areas l want some to pull the stage back and get my left channel to the left. Midbass I want just for more midbass, because just like a sub multiple midbass raise the dB. I will run them right down to the subs. If you experiment with rears, be prepared to change them out and/or EQ them, etc to get it right. I rarely put them in and it works great, so don't make it permanent until you are satisfied is my advice.


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## casual

i wanted to do ib in the rear deck so i was figuring i could put 8's in the stock locations and put some tweeters and eq them just to get vocals. Any thoughts on this?


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## sqshoestring

casual said:


> i wanted to do ib in the rear deck so i was figuring i could put 8's in the stock locations and put some tweeters and eq them just to get vocals. Any thoughts on this?


Sure, EQ on only rears would help a lot because you still need to EQ the front so it sounds good on its own. You might want to consider a dome mid, or other very low playing larger tweeter or 3" full range/etc instead of a normal tweeter. Most tweeter can't reach an 8 that well plus you don't want much of the upper treble anyway. The 8 would be great down to the 80Hz range and get you some MB, again you want a woofer not a sub or it will not reach very high if you are trying to get midrange too. (That is a problem with 8s and tweeters, not to do with running rears)


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## eviling

meh, i sitll prefer them, especily if your running passive, theirs no reason really not to with 4 channel amps.


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## ebrahim

To end this dilemma on rear fill I will fill you in on something. The guy from Morel tech support in NY told me that I do not need rear fill. Scott told me if I wanted a SQ system I should not do rear fill. An IASCA Judge told me rear speakers are a waste but he told me it is my car.


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## narvarr

lycan said:


> looks fine to me! And for ten bux, how can you go wrong?
> 
> Tell ya what ... just to protect everything ... wire it like this :
> 
> LEFT PLUS --> 1k resistor --> *primary positive*
> RIGHT PLUS --> 1k resistor --> *primary negative*
> 
> (ignore the center taps for now)
> 
> then, connect the *secondary positive* to one RCA terminal (plug/tip) of an amplifier, and connect the *secondary negative* to the other RCA terminal (shield/ring) of the _same_ amplifier input. It won't matter if the amp accepts balanced inputs, or (instead) grounds the negative RCA terminal ... that's one of the beauties of transformer coupling. You should now have an amplified, L-R difference signal
> 
> Of course, we still need to bandlimit & delay our lovely "ambience" signal ...
> 
> Feel free to post a proposed sketch, before you commit.


I know this is an old thread, but has anyone tried this yet?


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## narvarr

Looking at this closer, if you had the room, couldn't you actually get a better sound stage with 5 drivers up front using this methodology? It would be more of a 7.1 but with a little twist to it:

*O------O------O------O------O*
L-R, L, L+R, R, R-L,





*O-----------------------------O*
L-R, R-L
20ms T/A, 20ms T/A

Just a thought...
Sorry, my diagram didn't come out right


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## Gary S

cajunner said:


> so what single box option exists that can do this?


 - I've been running rear surround speakers in my cars for over twenty years now, in various ways... early on, I used a hafler AKA simple L-R signal from a NON-Trimode amp... before Fosgate made trimode amps popular, with the old school NON trimode amps, you could wire L-R directly from the speaker outputs on the amp... but because almost all amps are now trimode, you can't do that anymore.

I was looking for a plug-n-play black box solution I could recommend to friends and family. I now use a Sony deck with Dolby Prologic ll. Prologic ll works with any stereo source.

Sony MEX-DV2200










I believe there is also another Sony model that will do it.


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## narvarr

Gary S said:


> - I've been running rear surround speakers in my cars for over twenty years now, in various ways... early on, I used a hafler AKA simple L-R signal from a NON-Trimode amp... before Fosgate made trimode amps popular, with the old school NON trimode amps, you could wire L-R directly from the speaker outputs on the amp... but because almost all amps are now trimode, you can't do that anymore.
> 
> I was looking for a plug-n-play black box solution I could recommend to friends and family. I now use a Sony deck with Dolby Prologic ll. Prologic ll works with any stereo source.
> 
> Sony MEX-DV2200
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I believe there is also another Sony model that will do it.


This is all a very interesting concept, but my question at this point is why is only L-R being discussed? Is it because the focus is on the drivers seat sound only? If R-L is the opposite of L-R and can be derived in the same manner as the other, why not do both? L-R for the rear left speaker and R-L for the rear right speaker? It looks like this is what the Hafler circuit does...so to speak.


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## Gary S

Usually we try to setup cars for optimum sound in the two front seats. For car stereo shows also.

The rear surround signal is mono, and usually called L-R (read "Left minus Right").

Hooking up one speaker R-L and the other L-R simply puts the rears out of phase with each other. This is recommended, it makes the rears harder to localize. I think Prologic does this already for you in their processor.


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## narvarr

Gary S said:


> Usually we try to setup cars for optimum sound in the two front seats. For car stereo shows also.
> 
> The rear surround signal is mono, and usually called L-R (read "Left minus Right").
> 
> Hooking up one speaker R-L and the other L-R simply puts the rears out of phase with each other. This is recommended, it makes the rears harder to localize. I think Prologic does this already for you in their processor.


So your saying one speaker L-R and the other R-L is recommended? Sorry for my slowness today. I haven't slept since I got off work at 6:00 this morning.


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## SoulFly

i like rear fill personally. i don't care what anyone says, i like the sound to surround me, not just in front of me. i don't care whats considered "proper" for SQ as if it was some robotic scientific formula cause i'm not a robot and my ears don't adhere to a math problem.


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## narvarr

Gary S said:


> Usually we try to setup cars for optimum sound in the two front seats. For car stereo shows also.
> 
> The rear surround signal is mono, and usually called L-R (read "Left minus Right").
> 
> Hooking up one speaker R-L and the other L-R simply puts the rears out of phase with each other. This is recommended, it makes the rears harder to localize. I think Prologic does this already for you in their processor.


I just stumbled upon something you posted a while back in this thread http://www.diymobileaudio.com/forum/302008-post79.html and it answered my question.


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## Notloudenuf

Here are some links I found helpful when researching my rear-fill install.

Surround stereo system
Build a High-end Surround Stereo System
Accurate Ambience Reproduction - Guides - Car Audio and Electronics


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## narvarr

Notloudenuf said:


> Here are some links I found helpful when researching my rear-fill install.
> 
> Surround stereo system
> Build a High-end Surround Stereo System
> Accurate Ambience Reproduction - Guides - Car Audio and Electronics


Verry good info there, thanks! How is your rear fill setup? 

Sent from my X10i using Tapatalk


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## Notloudenuf

narvarr said:


> Verry good info there, thanks! How is your rear fill setup?
> 
> Sent from my X10i using Tapatalk


I took it out. I was never able to get it to where I wanted it to be. My goal was to use rear fill to add width to the sound stage. 
I got it to sound more "wraparound" but never able to add width outside the mirrors, which is what I wanted.


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## narvarr

Notloudenuf said:


> I took it out. I was never able to get it to where I wanted it to be. My goal was to use rear fill to add width to the sound stage.
> I got it to sound more "wraparound" but never able to add width outside the mirrors, which is what I wanted.


Where were your rear speakers located, back door or rear deck?

Sent from my X10i using Tapatalk


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## Notloudenuf

narvarr said:


> Where were your rear speakers located, back door or rear deck?
> 
> Sent from my X10i using Tapatalk


They were in the back doors.


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## Complacent_One

This thread seems to be going back and forth between surround sound and rear fill.

These are two completely different animals.

Rear Fill, is utilized to create space, yes, but is used more along the lines to widen space for the front sound stage. If used properly it will widen the front stage recreation while not detracting for accurate and highly detailed imaging.

If you want accurate surround sound, then you need to start with material that was recorded with that intent....surround sound (pro-logic, dolby, etc...) utilizes complex algorithms to re-create space and effects to give the sense of live events. Yes, there are studio selections, but the rear effects when "studio" is selected from pro logic is the least invasive of the algorithms with the lowest rear channel output.

Since all of this is subjective to what the listener is looking for, I will include my opinion.
I prefer the True studio sound, which means that the recording engineer is in a sound booth that is acoustically treated and will absorb pretty much all of the side and rear reflections from his or her near field monitors. Most of the time, those monitors can be placed much further apart than we can in our cars, so the soundstage will be represented as much wider than we can achieve without TA and potentially well thought out rear fill. The delay in the rear should only be set to compensate for distance, and not for any additional effect(IMHO). Believe or not, simpler is usually better. I have the Same Sony SACD/DVD/CD deck mentioned earlier and it provides the ability to custom set the TA for the speakers and I measured (tape measure) then tweaked by ear...I am running the rears at approximately 12db down from the rear and I noticed my soundstage width jumped out from inside the a-pillars to what I perceive to just outside of the ears. The best thing is I feel that, with my eyes closed, I could chew some paper and put a spitwad right between the eyes of ole S. Winwood sitting smack dab in the middle of my hood.

Again...some facts, lots of opinion...


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## The real Subzero

post very good information


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## bigfastmike

I do like the info here. Do have to remember that all installs will benefit from different methods. I do think before trying the mono or phase tricks you need ability to do time correction and be sure it's optimal. I thought my time alignments were good until I retweaked them and wow what a difference. I had always kept rear volume low Ana crossedover around 120hz. Once I got ta just right I dropped crossover and was amazed. My midbass actually moved forward. I always thought that wasn't supposed to happen. So I dropped em down more to 80hz. Wow! Dropped my sub crossover down to 63hz. Even better. I keep thinking nobody's gonna believe those settings sound so good but they do. 
Now I can't imagine it would work in most vehicles. 
What I am saying is get your basic settings as close to perfect as possible. If you think they are then document them and try something different. It's amazing how simple adjustments available on even most head units can really well when you find the perfect balance in your ride.


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## RNBRAD

Anytime you take an extra set of speakers and play them in a car environment you throw in an extra set of parameters or variables you have to deal with that can create havoc on your front stage. When done correctly the rewards are pleasant. I couldn't help but think when Complacent_One mentioned stage width with rear fill. It does have a way of fooling the senses in making you think the space is bigger than it is. This is not easily achieved, at least not for me and honestly, I spent hours and hours trying different x/o points, speaker sizes, resistors, direction etc. to get the right feel. In no way do I want to notice I have music coming from the rear. That's missing the concept. It has an ambience, an air or presence about it that literally makes the front stage wider and fuller. When I turn my rears off it is night and day though but yet turning and facing the rear you can't hear or focus on them. But I only use 4''mids, play a very narrow range and reverse phased. Tweeters were just too hard to get right in the mix.


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## Negolien

Still a big fan of the Bi-Amplified SoundStreams running off an amp in my rear deck. I have a ported box with Bridged 12s running 2ohms into a mono 1000 Watt amp. My fronts consist of 4x6 plates and Tweets in the sails. Using multiple amps but I'am not having any issues even without running a Capacitor.I really like the Olds Alero set up as a base for a decent low budget system.Now If I would have only pulled my donut tire out before I installed my box lofl.


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## F150Man

How about this for a rear fill ?


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## rxh0272

GlassWolf said:


> *rear-fill speakers*​
> While rear speakers might seem natural or necessary to some people, it is deemed undesirable and at best useless to those who are into good sound quality. There is nothing inherently evil about rear speakers, it's just that rear speakers often interfere too much with the ability of the front speakers to produce a believable sound stage and imaging characteristics. These effects can be minimized if care is taken to set up the rear speakers, but they are still there.
> 
> First, we discuss the "purist" view on rear speakers. Rear speakers grew out of necessity rather than the pursuit for better sound quality. A lot of cars, especially compacts and sub-compacts, have very small speaker openings in the front of the car due to space restrictions. It is rather rare to find a stock speaker location that can fit something as desirable as a 6.5 inch driver, while 4" and 4x6" speaker openings are quite popular. Small speakers are usually incapable of producing low bass (below 100Hz) at a satisfying level (say 90dB for casual listening.) Rear speaker mounting locations, especially the rear decks of sedans, offer a lot of area to mount a sizable driver, thus car manufactures rushed to mount large speakers in the rear to fill in the low bass region. This would have worked out if the stock stereos fed a low-pass filtered signal to the rear speakers so that they only produced the low bass frequencies, in which case those rear speakers would be called woofers or subwoofers. But no. Car manufactures didn't want to let all that volume go to waste so they fed the whole signal spectrum to the rear speakers. To make matters worse, the signal sent to the rear speakers is in stereo. The end result is that the center of the sound stage is somewhere to the far-right behind the driver and far-left behind the passenger. The instrument and vocal image floats all over the rear of the listener and shifts dramatically depending on the position of the listener's head. This is not how a live performance sounds.
> 
> For the reason discussed above, rear speakers of any configuration will interfere with the proper sound stage production and imaging of the front speakers. However, there are measures that can be taken to minimize the effect. The simplest thing to do is to turn down the rear speaker volume. Close your eyes, keep your head straight and adjust the front-rear fader control while you listen to a piece of music with strong central vocal content (check this with your home stereo or sit in the middle of the back seat with the rear volume turned all the way down.) First, turn the rear speaker volume all the way down, and then slowly turn it up until the vocal image starts to drift to the opposite side of the car. If you are sitting in the driver seat, listen for it to drift toward the passenger side and vice versa. This is the point where the rear speakers are still noticeable but it is not interfering too much with the proper operation of your front speakers.
> 
> There are two more things you can do but they require that you add components to your stereo system and the improvement is not as dramatic as simply turning down the rear speaker volume. Rear speakers should never be allowed to operate in full range unless you are going for the THX or Dolby Digital AC-3 theater surround sound setup. If you have a separate subwoofer, band-limit the signal going to the rear speakers to approximately between 200Hz and 3kHz. You don't need complex crossovers for this, just something like a 6dB per octave first-order high-pass filter at 300Hz and a 6dB per octave first-order low-pass filter at 2kHz. We're talking about one coil and one non-polarized electrolytic capacitor ($20 if you get the real snazzy stuff like polypropylene capacitors and low gauge air core inductors). Minimizing the high frequency content of the signal fed to the rear speakers is much more important than the low frequency content. In fact, if you do not have a dedicated subwoofer, you can do without the 300Hz high-pass filter and let the rear speakers produce the bass frequencies. But keep in mind that rear speakers should have a lower relative volume than the front so the effectiveness of the rear speaker to double as a subwoofer is severely limited.
> 
> To wring out the last bit of negative side effects, the rear speakers should be in mono. This can be done only if you have an external amplifier. In other words, this is impossible with a stock stereo system. The simplest way to do this is if the amplifier has a stereo/mono switch built-in, or to use an amp that is bridgeable. Then just put the two speakers in series and bridge them across the amplifier. If the amp is not bridgeable, you will have to find a crossover or some other signal processor that has a mono output. However, most of the crossovers and signal processors only have mono output for subwoofers (music below 200Hz) and thus are not suitable for this purpose. So the simplest way to do this is to get an inexpensive bridgeable amplifier.
> 
> If you do decide to get rear speakers, you would have to decide which type or brand of speakers to get. After reading what's written above, if you think all this rear speaker negative side effect crap is hog wash, just pick out your rear speakers. But if you are now a faithful believer in low-volume band-limited mono rear speakers, read the next paragraph
> 
> Your cheapest and simplest solution is not to have ANY rear speakers. The next cheapest solution is to keep your stock rear speakers. The reasoning behind this is that most stock speakers are quite OK in the 200Hz to 3kHz region. It is in the bass and high frequency region where they run into trouble. Besides, you are going to be running it low-volume, band-limited and in mono, so the difference between a pair of stock rear speakers and say a $250 pair of high quality mid-bass drivers is not going to be all that noticeable. But again, it is important that you keep it low-volume, band-limited and in mono. If you have a lot of money (send me some) and really want a fancy spare-no-expense type of system, then go ahead and find a good solid pair of mid-bass speakers. DO NOT, I repeat DO NOT, I'll say it again DO NOT buy co-axials or component speakers to use as rear fill since you are going to be wasting a lot of money on the useless tweeter and crossovers. You might be rich but you don't have to be stupid. A $200 set of components or co-axials might be just so-so but a pair of $200 mid-bass drivers is going to be killer. A lot of companies make really good mid-bass drivers. Look into Kicker, JL, Boston Acoustics, MB Quart, Audax, or Morel just to mention a few. There are also a few high dollar brands such as Dynaudio, USD Audio, Image Dynamics and Focal. This is not an inclusive list because there are a LOT of good mid-bass drivers out there.


That’s a long discussion.
For me, I listen to Techno. Techno typically produces a feeling of being a part of the music. There is less a desire for “Stage” effect. 
With Techno, the goal is to be immersed in the music, which can only occur with the addition of rear speakers. Rear speakers jn rear doors—yes—but additionally rear speakers added as far back, high as possible. Like in SUV at the rearmost back left and right panels. Obviously needs to be built custom—fiberglass. They should be designed to augment the front sound and offset it enough that while listening to the music one cannot positively identify where the music is coming from, which is pretty hard in a car, but I have done it over and over and over again.


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