# Lowest Safe Sweep Range for Unprotected Compression Driver?



## subterFUSE (Sep 21, 2009)

Hi all,

One of the things that I have been cautious about is not crossing over my horns too low, for fear of damaging the compression drivers.

I have been using LR 24dB slopes and previously have been crossing around 900-1000 Hz, and never gone below 800 Hz.

One thing which I have wanted to test for but been too afraid to try is to sweep the horns with REW with no crossover applied, just to see how they perform at the bottom end.

You see, when I use an electronic 800 Hz LR 24dB filter the measured acoustic slope of the response ends up being much steeper than 24dB. Closer to 48dB slope. I have been curious about trying a shallower slope Xover setting, or possibly even trying no Xover and a limited range sweep. But I am terrified of damaging the compression drivers. (Eric Stevens Ultra drivers)

Do you think it would be safe to disable the high pass temporarily and then set REW to run a sweep on the horns with a limited low-end range? Perhaps limit to 800Hz and higher?

Or is this too risky to even bother trying?


----------



## Eric Stevens (Dec 29, 2007)

Keep the level very low (less than 90 dB) and you dont need to limit the bandwidth really but you wont get an useful information below 250 Hz. 1 watt sweep above 250 Hz is not hard on the driver at all.


----------



## subterFUSE (Sep 21, 2009)

Great. I wouldn't care about anything as low as 250Hz. I am mostly after 600Hz and up.

I'm interested to see if I can shallow out my highpass curve.

I want to try and get an acoustic crossover of about 800-900 Hz with 24dB slope, but when I use an electronic 24dB slope filter my measured responses have been much steeper than 24dB. Take a look at these measurements before applying EQ. I'm concerned with the slope of the highpass. It seems very steep to me.

Ignore the crossover frequency on these, I was testing higher crossovers to see if that made a difference. 1200Hz, I think. I'm just focusing on the slope at the moment.











Here is a picture of the left side measurement against a simulated 24dB LR xover.
This shows how steep my measured response looks compared to what's expected.


----------



## Eric Stevens (Dec 29, 2007)

If you do damage the drivers the replacement diaphragms are not expensive at all.

MH or Full Size?

Below the LF cutoff of the horn they will roll off rather sharply, combined with the filter response this will yield a very steep acoustic roll off.

You could run a response curve without high pass filters to see the unfiltered response, I suggest measuring near field within 1" of horn mouth and at listening position to see if the environment is contributing.


----------



## Patrick Bateman (Sep 11, 2006)

It's really hard to get much output below 1200hz or so. The output of a compression driver drops like a rock.

The back chamber is tiny, and that raises the Q and the F3.

In the pro sound world, nobody is looking to buy a compression driver with an efficiency of 100dB. And in order to generate more output below 1200hz you have three options:

1) Find something with a bigger diaphragm
2) Use the same compression driver but enlarge the back chamber, and take the efficiency hit that comes with that option
3) Build a Synergy horn

EQ won't do it, because there's just no output below the low corner.

BTW, this is why TADs are so big:










I'm kinda biased, but I think the only practical way to do 600hz-20khz on a horn with high efficiency, without buying a TAD, is to build a Synergy Horn. If you can live with an efficiency hit you can use a full range, but it comes with a serious efficiency penalty.


----------



## subterFUSE (Sep 21, 2009)

Patrick,
That was exactly what I was taking into consideration when I decided to increase my crossover frequency and measure again. I was curious if the horn body or maybe the driver were restricting my low end range. But what I found was that my acoustic crossover slope was still very steep, even when crossed higher. Much steeper than the 24 dB setting I was applying in the DSP. 


Sent from my iPhone using Tapatalk


----------



## Patrick Bateman (Sep 11, 2006)

Yep. Even with no crossover whatsoever they're running out of steam around 1200hz.

They're not like conventional drivers, where the Q is under 1 and the rolloff is modest. Compression drivers have a Q that's very high, and that makes the rolloff steeper.


----------



## subterFUSE (Sep 21, 2009)

I'll try to sweep them up close and also try some low volume sweeps without crossovers. But I would say my initial impression is like you said. There must be something about the horn, the driver or the install that is limiting the low end rather sharply.

So should I try a steeper low pass xover curve with my midbass? Try to match the natural roll off of the horns? I think my DSP will allow asymmetric xover slopes for LP vs HP. So I could do a 24 slope on the sub but a 48 on the midbass to HLCD transition.


Sent from my iPhone using Tapatalk


----------



## Eric Stevens (Dec 29, 2007)

measure the response and I will give some direction after that.


----------



## Patrick Bateman (Sep 11, 2006)

subterFUSE said:


> I'll try to sweep them up close and also try some low volume sweeps without crossovers. But I would say my initial impression is like you said. There must be something about the horn, the driver or the install that is limiting the low end rather sharply.
> 
> So should I try a steeper low pass xover curve with my midbass? Try to match the natural roll off of the horns? I think my DSP will allow asymmetric xover slopes for LP vs HP. So I could do a 24 slope on the sub but a 48 on the midbass to HLCD transition.
> 
> ...











It's the compression driver. Even with a huge horn and unlimited EQ you can't increase output when the driver doesn't have any. (Pictured above is the frequency response of a B&C DE250 with no xover or EQ.)

That's why I use Synergy Horns, they give me an F3 that's two octaves lower than you can get with a conventional horn.


----------



## subterFUSE (Sep 21, 2009)

Thanks. That's good information.

I am going to measure again soon just to confirm, and will post my results.


----------



## subterFUSE (Sep 21, 2009)

Sorry for the delay guys. I've been traveling for work and not had time to mess with the car.


Here are some nearfield measurements I just took of my drivers.


These are the horns:












And the Midbass:












I swept the horns and midbass with no crossovers applied. No EQ. No gain adjustments in the DSP. Amplifier gains were set immediately beforehand using an oscilloscope.

Midbass got a full 20Hz - 20kHz sweep, while I limited the horns to 600Hz-20kHz.



So it looks to me like the compression drivers have a very steep low-end rolloff starting at about 1100 Hz.

Here is a pic of the Left HLCD nearfield measurement vs. a simulated 24dB slope crossover at 1100 Hz:











So that rolloff looks a lot steeper than 24 dB/octave. Maybe 48dB?



If the compression drivers have a steep rolloff, should I consider using a steeper slope on the midbass to match up with the horns? I don't think I am going to get anywhere close to an acoustic 24dB slope with the horns unless I cross way higher than 1100Hz.

Thoughts or suggestions?


----------



## thehatedguy (May 4, 2007)

I would try mixed slopes. Horns as you see roll off much faster at a different rate than direct radiators. Sometimes a mixed slope like odd order on the midbasses and even order on the horns can get good results.

See what a shallow slope on the horns up high will get you. Try something like 6 dB between 6 and 6.5k hz to start with. It will also flatten out the FR some. 

Try it on paper first if you can. Or try it in the car, just be careful with the volume at first.


----------



## cajunner (Apr 13, 2007)

thehatedguy said:


> I would try mixed slopes. Horns as you see roll off much faster at a different rate than direct radiators. Sometimes a mixed slope like odd order on the midbasses and even order on the horns can get good results.
> 
> See what a shallow slope on the horns up high will get you. Try something like 6 dB between 6 and 6.5k hz to start with. It will also flatten out the FR some.
> 
> Try it on paper first if you can. Or try it in the car, just be careful with the volume at first.


question:

is the use of a DSP generated 6 db/oct slope, going to react the same way as a capacitor in the final acoustic response of the system?

and for that matter, would any electronic means of generating that 6 db/oct slope, passive or active then generate a combined crossover slope of approximately 24 db/oct at the compression driver's natural roll off of 1100 hz?

can we generalize that most compression drivers using a 1" exit and ~2" diaphragm/voice coil, exhibit this interesting quality?

and can this capacitor crossover work on compression drivers using 4" voice coils too?

I'd like to run a JBL 2446J driver on the same amplifier channel as a 6.5" mid bass, without a good chance of damaging the CD, do you believe this is safe? I would want to use a transformer for my sensitivity matching, so the CD won't be getting much regular watts, but I wonder if amp clipping into the 6.5" woofer won't be reflected as direct current/square waves in the CD, or top end of the spectrum...

if you could PM me so as not clutter this thread, that would be perfect!


----------



## subterFUSE (Sep 21, 2009)

thehatedguy said:


> I would try mixed slopes. Horns as you see roll off much faster at a different rate than direct radiators. Sometimes a mixed slope like odd order on the midbasses and even order on the horns can get good results.
> 
> See what a shallow slope on the horns up high will get you. Try something like 6 dB between 6 and 6.5k hz to start with. It will also flatten out the FR some.
> 
> Try it on paper first if you can. Or try it in the car, just be careful with the volume at first.


I'm using a new Helix DSP Pro, so I do have some flexible options for crossovers.

There is a Butterworth that can have a slope of -6dB, -12, -18, -24, -30, -36 or -42dB highpass.

Bessel offers the same slopes as Butterworth.

Tschebyc offers the same.

Linkwitz offers -12, -24, and -36dB slopes. (I'm shocked, but 48 not available?)

Self-Define is also available. Select a Freq. and Q and shape your own slope. Q adjustable from 0.5 to 2.0, in .1 increments.


----------



## SQram (Aug 17, 2007)

A couple more options you could try:

-A high shelf on the horns (would be similar to a 6db slope depending on how steep you set it).

-A passive power response compensator network, it's a low level network you insert between the processor and amplifier on the horn channels, the Team Image Dynamics guys used to use it and have posted schematics for it here in the past.

I use both on my horns and it can really help smooth out the response,


----------



## thehatedguy (May 4, 2007)

Yeap, that passive line level filter is really nice...have to give Veritas credit for that though.


----------



## SQram (Aug 17, 2007)

Indeed.


----------



## SQram (Aug 17, 2007)

Patrick Bateman said:


> It's really hard to get much output below 1200hz or so. The output of a compression driver drops like a rock.
> 
> The back chamber is tiny, and that raises the Q and the F3.
> 
> ...


I'm really curious about point #2 you made above. 

It would be interesting to 3D print some rear caps of various sizes for a given compression driver and measure the effects they have on efficiency and FR.


----------



## cajunner (Apr 13, 2007)

what about using a D-250X phenolic compression driver up to 5K, then slipping a little supertweeter into the corner for the top end?

Or that D-2 JBL dual ring-radiator CD, isn't that thing capable of 550 hz at low power, say under 25 watts?

seems like we could push the envelope if we try, I mean the Veritas full-size is supposed to be able to run 630 hz before breaking the bank, right?


----------



## thehatedguy (May 4, 2007)

That D250X really isn't suited that well to audio reproduction.

Eric's horns can run that low too...I've run them to 600. With the right driver you could squeeze 500 out on both of them, but it would be at a low level situation. Back in the day it was pretty common to run them to 600. But back in the day the compression drivers were larger with larger back chambers- better low end at the expense of the treble. The modern drivers have much better treble, but they are so much smaller and have no back chamber, so they don't play as low as well.

And playing low like that, you trade extension for efficiency.


----------



## Eric Stevens (Dec 29, 2007)

Something looks wrong with the response curve. They do not roll off on the low end like that when everything is correct with no processing of any sort. 

I have attached a recent measurement with a similar driver to the Ultra that was taken using LMS. This is a near field measurement. Big Red posted a measured response he took and it has a similar LF response to what I am posting, you can look through the HLCD forum for his measurement.

I would suggest you check a few things:

1- look at the signal coming out of the amplifier and processor separately using an RTA and pink noise, or measure it with the same equipment used previously.

2- inspect and clean the compression drivers, it would be unusual for both to measure the same as it appears they did.

3- Bypass all of the signal chain and run the sweep with only an amp and the horns, making sure the amplifier is operating full range. 

Eric



subterFUSE said:


> Sorry for the delay guys. I've been traveling for work and not had time to mess with the car.
> 
> 
> Here are some nearfield measurements I just took of my drivers.
> ...


----------



## subterFUSE (Sep 21, 2009)

Thanks for the response chart and suggestions. I will do more testing soon.





Sent from my iPhone using Tapatalk


----------



## Eric Stevens (Dec 29, 2007)

Pink noise and an rta app on your phone to make sure the other measurement methods are correct would also be useful. 

That sharp cut-off on the bottom end is not normal.


----------



## Eric Stevens (Dec 29, 2007)

http://www.diymobileaudio.com/forum/1602686-post15.html

This a measurement of an Ultra Driver on a FS horn.


----------



## subterFUSE (Sep 21, 2009)

Right you are, Eric!

I measured again, and here are the results.













I think I know what was causing the steep rolloff. It's REW.

I think that when you set a sweep range in REW it causes a fade effect on the lower range of the sweep, kind of like a built-in crossover to protect your drivers. If you want an accurate sweep of a subwoofer's response from 20-200Hz, you have to sweep from 10Hz or else it will roll off the response slightly above 20.

I was being so careful not to push my compression drivers too low that I was limiting the sweep range in REW to 600 Hz. This was cause the sweep to roll off the response slightly above 600.


----------



## Patrick Bateman (Sep 11, 2006)

Ugh isn't that the worst? I have literally YEARS of measurements that are suspect, due to idiosyncrasies in Arta.

I've had solid repeatable results with HOLM Impulse.

Basically Arta seems to do some type of "scaling", it seems to shift the measurement so that the decibel level is always the same. So you get a response curve that's accurate, but the SPL level is off. Which sucks when you're trying to squeeze maximum SPL out of a box.


----------



## Eric Stevens (Dec 29, 2007)

Glad to help out.


----------

