# How to Lower Noise and Preserve Your Compression Drivers



## Patrick Bateman

_A member of the forum mentioned that he has a friend who keeps blowing up diaphragms.

I do the crossovers on my speakers in an unconventional way which lowers noise and helps you avoid blowing them up.

I've posted some of this info in other threads, just thought I'd put it in one place in case anyone could use it._

90% of the people running horns in a car are using a combination of active EQ and an active crossover. In the 90s the AudioControl units were popular; you would buy one unit to do each function, an investment of about $400 in addition to the $600 or so for horns and compression drivers. So $1000 in all.

I've gone this route, I have USD horns in the garage, along with two AudioControl xovers and an EQ.









Here's the measured response of a compression driver on a waveguide. With an AudioControl, you'd probably cross it over at about 1200hz, then EQ it to flatten it out.

The crossover will protect the compression driver, by reducing it's output at 600hz from 90dB down to 63dB.

So far, so good right?

Here's the tricky part: we have to apply EQ in the midrange, to flatten it out. *And if you apply a 'boost' at 1500hz, to flatten it out, you're also going to counteract the effect of your crossover.*

I hope that makes sense. This will all depend on the 'Q' and gain of the filters, *but the EQ in the midrange can have the effect of boosting the level at 750hz by 3-6dB.*

Six dB might not sound like a lot, but at 750hz that can be the difference between life and death for your compression driver. Compression drivers have very little excursion; at their low end they're very vulnerable to detonation.










It might seem like it's impossible to blow up a compression driver, but the shallow depth of car audio horns means that the power handling drops like a rock as you get closer and closer to 1000hz. So overpowering them at the low end can definitely be fatal.











Here's what I do, which works to save the driver and also reduces noise.
First, I use a passive high pass filter set to a very high frequency. For instance, a 2mfd cap inline with an 8ohm compression driver will create a first order filter at 10khz.

Now I know what you're thinking, *why on earth would you set a crossover point at 10khz?*

The reason is that compression drivers aren't flat. They have a drooping response at high frequency. So when you combine the cap at 10khz with the natural response of the CD, you get a nearly flat response. (How flat will depend on your horn or waveguide mostly.)

In the pic above, here's what we get:
1) At 10khz, output is down by 3dB. *From 100 to 97dB.*
3) At 5khz, output is down 9dB. *From 105 to 96dB.*
4) At 2.5khz, output is down 15dB. *From 105 to 90dB.*
5) At 1250hz, output is down 21dB. *From 97 to 76dB.*
6) At 625hz, output is down 27dB. *From 90 to 63dB.*

Basically, a $1 capacitor carefully chosen can get your response to about +/- 6dB from 2khz to 20khz. +/- 6dB isn't "high fidelity" but it's close enough that we can EQ it flat. It might be tempting to set the xover lower, but if you do that, then you'll have to use something like 10-20dB of EQ above 10khz to make it flat. Moving the xover lower also increases excursion and distortion. About 1-2mfd is a nice "sweet spot" that gets you close to flat, and then you use your EQ to do the rest.

For instance, 2500hz is at 90dB now, and we need it to be about 97dB. *But due to the cap, you can get quite aggressive on the EQ. (Since the cap is reducing excursion from 10khz all the way down.)

When I used active crossovers and active EQ, I couldnt' get away with a boost of much more than about 6dB. With a cap in line, I've been able to do about twice that before things start to sound 'grungey.'

You can also use a cap for passive filtering, an active crossover for filtering, and an EQ.

Basically the capacitor is similar to working with a safety harness. Yes, it limits your motion and you have to be conscious that it is there. But it will also keep you from doing something that might hurt you 



If you choose your cap carefully, you might be able to get away with just a cap and an EQ. I buy 1mfd caps off of eBay, then fiddle with them until I get the right response. (Because two 1mfd caps make a 2mfd cap in parallel and a 0.5mfd cap in series. So a bag of 1mfd caps can make just about any value you might need, and they cost $5)

MiniDSP and a single capacitor is probably the ultimate solution. The cap reduces noise, which is a big problem with the miniDSP. But the miniDSP allows you to 'dial in' the optimium filter response and phase response.

*


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## subterFUSE

Awesome post.

I still haven't had a chance to get my horns on an RTA yet, so I've just been working on the phase and crossover points.

That makes total sense about EQing up and affecting the frequencies below the crossover point as a result. The part that frightens me is that I currently have my horns crossed over at 800 Hz @ 24/dB Linkwitz. I didn't realize how close I might be to damaging them. Fortunately, I have not EQ'd anything up yet.


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## thehatedguy

About 6k on Eric's minihorns at 6dB gets you pretty flat FR and acoustic 24 dB slopes around 1200-1500 hertz...I don't remember which off of the top of my head. But I did play with in lspCAD last year. With the horns reversed polarity (and the offset I had in my car) and 18dB XO point at 1200-1500 on the midbasses got you pretty flat phase too.

"Quasi-optimal" crossover for high-efficiency loudspeaker system - diyAudio

Is what I want to play with next.


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## Patrick Bateman

If anyone has a miniDSP and a microphone, the ability to manipulate phase is shocking.

Twenty years ago, here's how the guys from Dynaudio, Thiel, Vandersteen and Dunlavy did it:

First, you get a midrange and a tweeter with really wide bandwidth. That's why Dynaudio uses those poly cones. It's so that they can get the right filter response. Aluminum won't work.









Once you have that down, you have to adjust the depth of the drivers. It's not as simple as using a tape measure; you have to literally measure the response, and then physically move them forward or backwards.

The reason that you have to do that is that the voice coil of the woofer is an inductor, and that inductor introduces a small delay into the signal. So a tape measure can get you close, but you'll probably be off by a quarter of an inch or so. 

Probably the most miserable part of the process is that you can't simply use a single capacitor, or a single inductor. The natural rolloff of a loudspeaker combined with the filter will always yield a response shape that's imperfect, and that changes the phase. It's not as simple as using a cap; you have to create a filter that mimics the response of a loudspeaker that has perfectly flat response from 20hz to 20khz.









^^ the crossover from a Thiel CS6

Besides spending weeks or even months building and tweaking everything, there are also soul-crushing delays while you wait for parts to arrive. You never have the right inductor when you need it.

As they say, "the devil is in the details."




With MiniDSP? You can literally do all of this in the afternoon. Set the response shape, "move" the drivers forwards and backwards via delay. It's like magic.


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## Canopy

I have been using a 6db slope on my horns and find I need less eq than if I do 24db slope.

I am almost force to do it though because with a 24db slope my ears ring after a few minutes of listening. 

Any idea why that is?


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## Patrick Bateman

Generally I find that tendency to want to turn off a horn is due to higher order modes. To me, the sound of distortion isn't fatiguing, unless there's so much of it that it swamps the music itself. (IE, distortion levels approaching 100%)

It's possible we're talking about two different things though. When I'm listening to horns and I feel like I need to go running for the volume knob, that's usually HOMs.


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## Canopy

Happens also on mids/midbass speakers as well. Occurs on all audio content and is very easy for me to hear playing pink noise and moving the crossover slope on my headunit from 6->12->18->24. At 24db slope it feels like my ear is being tickled but not in a good way. That's the only way I could describe it. It also occurs in the range of 150-10k. I can cross my sub at 24db and it doesn't bother me.


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## subterFUSE

Patrick Bateman said:


> Generally I find that tendency to want to turn off a horn is due to higher order modes. To me, the sound of distortion isn't fatiguing, unless there's so much of it that it swamps the music itself. (IE, distortion levels approaching 100%)
> 
> It's possible we're talking about two different things though. When I'm listening to horns and I feel like I need to go running for the volume knob, that's usually HOMs.


What's the solution to this? I definitely experience some ear fatigue with my new horns, even when I'm not listening that loud. Less than 90 dB.

I haven't EQ'd them yet, however. I'm sure that will have some impact. Just don't have access to an RTA yet.


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## nadcicle

I believe this might be pointed at my friend. I will make sure he sees and reads this. Thanks!


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## Eric Stevens

subterFUSE said:


> What's the solution to this? I definitely experience some ear fatigue with my new horns, even when I'm not listening that loud. Less than 90 dB.
> 
> I haven't EQ'd them yet, however. I'm sure that will have some impact. Just don't have access to an RTA yet.


Most likely you have some peaks in the upper midrange area or maybe elsewhere but typically it's in the 2khz to 4 kHz range.

With L & R Eq linked Set and eq filter to -6dB and a Q of 3 or higher and change the frequency in your processor while listening to a song that is fatiguing. I would suggest a higher Q of 6 and do this a couple of times. With a 1/3rd octave I would try -1 to 4 dB @ 2 kHz and -3 to 6 @2.5 and -3 to 6 @ 3.1 and -1 to 3 @ 4 kHz


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## cajunner

or you could get a piece of reticulated foam, in the 30 PPM or whatever, and stuff the horn mouth until those nasty higher order modes are just mopped all up and nothing but clean, clear tasty waves are coming off the horn exits...


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## subterFUSE

Eric Stevens said:


> Most likely you have some peaks in the upper midrange area or maybe elsewhere but typically it's in the 2khz to 4 kHz range.
> 
> With L & R Eq linked Set and eq filter to -6dB and a Q of 3 or higher and change the frequency in your processor while listening to a song that is fatiguing. I would suggest a higher Q of 6 and do this a couple of times. With a 1/3rd octave I would try -1 to 4 dB @ 2 kHz and -3 to 6 @2.5 and -3 to 6 @ 3.1 and -1 to 3 @ 4 kHz


Thanks Eric. Will try this out soon.

I'm using an Audison BitOne, so I don't think I have the ability to change the Q. But it does have 1/3 octave EQ, so I will try the settings you suggest.

If I had a miniDSP, would that give the ability for Q adjustments?


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## Patrick Bateman

subterFUSE said:


> Thanks Eric. Will try this out soon.
> 
> I'm using an Audison BitOne, so I don't think I have the ability to change the Q. But it does have 1/3 octave EQ, so I will try the settings you suggest.
> 
> If I had a miniDSP, would that give the ability for Q adjustments?


yes, a miniDSP allows the user to dial in both the 'Q' and the frequency of each point.


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## Patrick Bateman

cajunner said:


> or you could get a piece of reticulated foam, in the 30 PPM or whatever, and stuff the horn mouth until those nasty higher order modes are just mopped all up and nothing but clean, clear tasty waves are coming off the horn exits...


"an ounce of prevention is worth a pound of cure."

Basically a whole lot of foam will 'mop up' HOMs, but a carefully designed throat works even better. The first two or three inches of a horn is critical.


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## subterFUSE

Patrick Bateman said:


> yes, a miniDSP allows the user to dial in both the 'Q' and the frequency of each point.


That's pretty sweet!

Would it be difficult incorporating a miniDSP with a BitOne? Probably would have to go between the BitOne and my amp?


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## jpeezy

JL audio has a free app on the app store that has a basic rta feature, use that and get the microphone from parts express,(made for this purpose),and its "value" priced. It is obviously not lab accurate but it will help show where your issues are. I personally would stay away from multiple processors, harder to fully tune, more chance for extraneous noise (engine noise and noise floor), and more chance for phase issues. The mini DSP has a model that is EQ and Crossover, this would work well, except that i think your using factory head unit? In this case then yes you will need a quality interface for your signal chain. Maybe the bitone has a bypass mode for EQ? then use the mini dsp?


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## jpeezy

Dayton Audio iMM-6 15.00


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## subterFUSE

jpeezy said:


> JL audio has a free app on the app store that has a basic rta feature, use that and get the microphone from parts express,(made for this purpose),and its "value" priced. It is obviously not lab accurate but it will help show where your issues are. I personally would stay away from multiple processors, harder to fully tune, more chance for extraneous noise (engine noise and noise floor), and more chance for phase issues. The mini DSP has a model that is EQ and Crossover, this would work well, except that i think your using factory head unit? In this case then yes you will need a quality interface for your signal chain. Maybe the bitone has a bypass mode for EQ? then use the mini dsp?


Thanks, I might give this a try. Although I've also been putting some thought into getting something for the laptop like TrueRTA.


I think I am pretty locked-in to the BitOne because I need the high level inputs to connect the Bose amplifier. I need at least 7 inputs because the Bose stereo has 3-way active front speakers but they are not full range. The front midbass doesn't get any signal in the sub-bass range. So I need the subwoofer connected, which means 7 high level inputs required.

BitOne is one of only a few processors out there that have 8 high level inputs.


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## subterFUSE

jpeezy said:


> Dayton Audio iMM-6 15.00


Ordered.


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## subterFUSE

Resurrecting this thread.... because now I have some measurement equipment and have been learning the ins/outs of REW.

Please have a look at my HLCD response curves below. Left in Blue, Right in Red.

Measured with REW using a UMIK-1.

Each driver was measured independently, with a spacial average of 8 points around my head.

A Linkwitz 24dB High pass is applied at 800Hz. No EQ has been applied yet.

Measured sweep from 500Hz to 20kHz.










I'm most interested in the huge dip at 2.5kHz. Is that common? How would I correct this?


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## Patrick Bateman

subterFUSE said:


> Resurrecting this thread.... because now I have some measurement equipment and have been learning the ins/outs of REW.
> 
> Please have a look at my HLCD response curves below. Left in Blue, Right in Red.
> 
> Measured with REW using a UMIK-1.
> 
> Each driver was measured independently, with a spacial average of 8 points around my head.
> 
> A Linkwitz 24dB High pass is applied at 800Hz. No EQ has been applied yet.
> 
> Measured sweep from 500Hz to 20kHz.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I'm most interested in the huge dip at 2.5kHz. Is that common? How would I correct this?












Here's my measurement of my USD HLCDs. See a similar dip?

Check out my 'homster' thread over at diyaudio, from 2009, for some more info on the issue.

IMHO, the dip is caused by the 90 degree bend in the waveguide. The sound radiates from the compression driver, then a fraction of it hits the bend and is reflected back to the diaphragm. *The reflected sound is delayed by a fraction of a second, and due to that delay, is out of phase at a given frequency.*

Easiest way to confirm is to measure the distance from the diaphragm to the bend. If the dip is at 2500hz I'd expect the distance would be about 3cm:
speed of sound / frequency / 4 = 
34000 cm per second / 2500hz / 4 =
3.4cm


As far as fixing things goes, yikes that's a tough one. I found that a roundover and some reticulated foam in the throat helped. But I personally prefer a horn without a reflector in it. I really and truly tried to get reflectors to work, and I've built easily ten or twenty horns with reflectors in them, but they're tough to get right.

At lower frequencies, it's not a big issue.

If you try the foam trick, put the reticulated foam all the way up to the diaphragm.


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## Patrick Bateman

For comparison's sake, here's a couple of horn measurements. The first horn is the $35 QSC horn that I love so much. Just bought another one this week. *See how smooth it is?* No reflector, oblate spheroidal curve.
The second horn is Art Welter's paraline. A paraline has three ninety degree bends in it. *See how the response is like a sawtooth?* That's due to the reflectors, and the slow expansion.


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## thehatedguy

You sure that dip is real, and not from mic placement?


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## subwoofery

subterFUSE said:


> Resurrecting this thread.... because now I have some measurement equipment and have been learning the ins/outs of REW.
> 
> Please have a look at my HLCD response curves below. Left in Blue, Right in Red.
> 
> Measured with REW using a UMIK-1.
> 
> Each driver was measured independently, with a spacial average of 8 points around my head.
> 
> A Linkwitz 24dB High pass is applied at 800Hz. No EQ has been applied yet.
> 
> Measured sweep from 500Hz to 20kHz.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I'm most interested in the huge dip at 2.5kHz. Is that common? How would I correct this?


Can we have pics of your install? 

Kelvin


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## Patrick Bateman

cajunner said:


> the idea that the horn's reflector causes a dip at 2.4 Khz, doesn't coincide with Eric Stevens' statements that a good reflector should only affect the very top end of the horn's response, somewhere between 16Khz and up and only by a margin of plus/minus 1 db.
> 
> I feel like this huge dip in response is possibly due to something mechanical in the compression driver not being right, it's so big as to not be a reflective surface and it isn't in the range of mid bass null modes.
> 
> Maybe insulation in the cup fell off and is resting on the diaphragm or the ferrofluid has contaminants or something off the wall crazy....
> 
> I can't wait to see what the experts have to say other than doing more averaged testing...


Bruce Edgar documented all of this in Speaker Builder (I believe he was a consultant on these horns too.)










Check out page 2 in particular

http://volvotreter.de/downloads/Edgar-Monolith-Horn.pdf

Here's the entire article. The frequency of the dip will go up in frequency with a large horn, and down in frequency with a small horn. The math to calculate the frequency of the dip is listed a couple of posts ago.









I nearly chopped off my right thumb building Beolab lenses; took off half of it. Functional acoustic lenses are a bit of a Holy Grail.


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## cajunner

Patrick Bateman said:


> Bruce Edgar documented all of this in Speaker Builder (I believe he was a consultant on these horns too.)
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Check out page 2 in particular
> 
> http://volvotreter.de/downloads/Edgar-Monolith-Horn.pdf
> 
> Here's the entire article. The frequency of the dip will go up in frequency with a large horn, and down in frequency with a small horn. The math to calculate the frequency of the dip is listed a couple of posts ago.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I nearly chopped off my right thumb building Beolab lenses; took off half of it. Functional acoustic lenses are a bit of a Holy Grail.


thanks, I wasn't aware of this material.

the thumb, that's what they call the personal touch...


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## Patrick Bateman

You never realize how important thumbs are until you don't have them.

The most depressing part was tying my shoes, it was literally impossible.


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## subterFUSE

cajunner said:


> the idea that the horn's reflector causes a dip at 2.4 Khz, doesn't coincide with Eric Stevens' statements that a good reflector should only affect the very top end of the horn's response, somewhere between 16Khz and up and only by a margin of plus/minus 1 db.
> 
> I feel like this huge dip in response is possibly due to something mechanical in the compression driver not being right, it's so big as to not be a reflective surface and it isn't in the range of mid bass null modes.
> 
> Maybe insulation in the cup fell off and is resting on the diaphragm or the ferrofluid has contaminants or something off the wall crazy....
> 
> I can't wait to see what the experts have to say other than doing more averaged testing...


I doubt this is a problem with the compression driver because, as you can see, the dip is the same for both left and right side. Possible? Perhaps. But unlikely to have the exact same mechanical problem with 2 different drivers in the same car, in the same frequency band.

Eric says it might be something with my measurements, or maybe my source unit. I'm going to re-calibrate and test again.

Will report back soon.


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## subterFUSE

thehatedguy said:


> You sure that dip is real, and not from mic placement?


I used Room EQ Wizard as my sweep source.

UMIK-1 microphone.

I took 8 measurements per driver. Each measurement taken from different points around my head. Average of 8 measurements was created, and that's what you see in my pic above.


I'm going to measure again today and report back.
Maybe I am missing something in my setup?


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## subterFUSE

subwoofery said:


> Can we have pics of your install?
> 
> Kelvin


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## Patrick Bateman

It's really hard to get a clean measurement in the car

For a start, try a ground plane measurement. Literally put the mic on the ground, then put the horn on the ground one meter away. *Be sure to do this outside; it's really hard to get quality measurements indoors, and even harder in the car.*

A ground plane measurement isn't perfect, but it's good, and the alternative is getting the mic *and* the horn a couple of meters off the ground, like this:


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## Eric Stevens

Can you choose a different noise source? Pink noise is better since it is random. 

Also I would suggest comparing near field to listening position. Alot of what you re seeing might be the environment rather than the speaker.

since your signal is going through the Audi stock system There quite possibly is both signal delay and frequency response corrections in the stock system. look at the response directly out of the amp if you can,. Or maybe power the amp directly off the sound card. 

Another possibility is during installation foreign material got into the voice coil gap if you were doing any metal work. You can check drivers if don't find anything else.

I guarantee you that the frequency response you have measured is not typical nor caused by the 90 degree reflector. Take a look at the FR posted by BigRed here in this forum for a measurement of the same outside the car in his garage. Quite different even if you applied 1/3rd octave smoothing to your response curve.

Eric


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## Rybaudio

FWIW I have the full size horns (pro drivers though) and I do not see that dip. The mounting doesn't look like it's a problem and like you said the L/R symmetry makes a problem with the driver unlikely. Something with the measurement system or HU/processor seems likely. 

If it's the HU/processor then you'd hear it - does it sound like there's a big hole there? If you can play tones, trying manually sweeping through that range.

You might check your computer's sound settings. For example on my laptop there was a noise-rejection feature enabled by default - not the best feature when trying to RTA with pink noise.


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## Patrick Bateman

Eric Stevens said:


> Can you choose a different noise source? Pink noise is better since it is random.
> 
> Also I would suggest comparing near field to listening position. Alot of what you re seeing might be the environment rather than the speaker.
> 
> since your signal is going through the Audi stock system There quite possibly is both signal delay and frequency response corrections in the stock system. look at the response directly out of the amp if you can,. Or maybe power the amp directly off the sound card.
> 
> Another possibility is during installation foreign material got into the voice coil gap if you were doing any metal work. You can check drivers if don't find anything else.
> 
> I guarantee you that the frequency response you have measured is not typical nor caused by the 90 degree reflector. Take a look at the FR posted by BigRed here in this forum for a measurement of the same outside the car in his garage. Quite different even if you applied 1/3rd octave smoothing to your response curve.
> 
> Eric












This is BigRed's measurement


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## subterFUSE

OK Guys, I think I found the problem.


So I went back this morning and tested my horns again, and I still had that massive dip @ 2.5kHz.

I tested again with the mic placed directly up to each driver. Still had the response dip.

So I then decided to test my midbass speakers with a sweep that covers that same range, to see if it was just the horns or if other drivers were affected.
My midbass also displayed that same dip.

Next, I bypassed my Car's head unit and plugged directly into the AUX port on my BitOne. Still got the same dip in response with horns and midbass.

Then I swapped out sound cards, and the dip in response @ 2.5kHz went away.

Lastly, I returned to the original sound card and re-calibrated it. Tested the left Horn with the mic placed directly in front of it. Here's the response on this measurement. Notice, the dip @ 2.5kHz is gone.











So I blame pilot error in calibrating the sound card. :blush:
I'm still not sure how or why I messed it up, but now that it's recalibrated I think it's working better.


I'll post some actual listening-position averaged measurements later tonight.


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## subterFUSE

Well, I thought I had figured out the issue but I was wrong.

It's definitely not the horn drivers, though. Now I'm having a problem with my midbass measurements showing huge holes in the response. I'm trying to isolate the issue but it's frustrating me to no end, so I'm going to have to take a break for a while.


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## P_4SPL

* Horns in a vehicle can be difficult to get right, my Veritas Horns sounded Harsh and in your face with Audio Controls analog X-ver and EQ's> I swapped out Crossover modules and tried different crossover freq's, as well I tweaked and dialed in the EQT's EQ settings without getting any results.

*Then Audio control released their Digital Crossovers and Eq's, the Digital equipment helped warm up the Mids and Horns, peaky freq's of the horn @ 1.8khz can be dialed back using the x1 parametric EQ channel available, this reduces micro distortion that can cause bright peaks in and around that freq'y, so choosing a wide Q setting helps reduce those anomaly's, as well the Digital crossover really warms up the blending of the drivers, it creates an natural ambience and sound stage coherance, in the vehicle, with all the drivers.

*I've contacted Audio Control about their Digital stuff and they've informed they've gone in a different direction in terms of DSP products for vehicles, it's too bad because if they came out with another Digital EQ x-over with Time Delay, I would consider picking one up. 

My Veritas horns have a Huge Radeon B950 compression driver fitted to them, the DSP definatly helps improve the transient attack and efficiency of the neodemium magnet.

*I've also experimented with impendance matchng transformers, which I've used with horns, they help Clean up alot of impendance fluctuations that occur in the horns themselves, and produces what I would call a finer sound to the horns. Where at low volumes the horns high freq's would sound too dispersed , the impendance transformer seemed to reduce that static sounding dispersion characteristics of the horns and made them sound more coherent tonally.


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