Why not high compliance subs?

Davide82 · 21st May 2006, 10:43 AM

Lately I had a try and built a subwoofer for my car.

I used a CHEAP 8" driver with dual voice coils.
Then I powered it with a very small amp... 2x20 watt (no need for SMPS!)

It REALLY shaked the car and make everyone amazed..... you could feel 30 Hz easily and not boomy sound.

the magic? I used an extremely large enclosure, so that efficiency was high at low frequencies.... Alignment was very good and transient respone superb.

Now my question....
I would like not to use my hole trunk as subwoofer.
BUT, if I build a smaller box, the sound gets of course boomy because the driver gets harder to move at all: this is a drawback!
This all happens because the stiffness of suspension adds to the stiffnes of the small air trapped in the enclosure. Efficiency shifts to higher frequencies and we get bad sound.

Do they exist drivers with a very low stiffness so that they can work in very small enclosures while maintining good low end efficiency?

The only one I saw were 20 years old drivers with really soft suspension... I guess they work good there.. but I can't see new drivers (and cheap) similar to these ones!
Why all new drivers are quite stiff?

lndm · 21st May 2006, 12:10 PM

As far as I can fathom with a driver that has stiff suspension, the mechanical damping remains dominant until the box behind it is quite small and the air spring is therefore stiff also.

If this is correct, a low compliance driver will have a low V_as and will therefore not respond to a large box.

In my experience, car drivers are low compliance, and produce small box sizes and a high F_s.

Davide82 · 21st May 2006, 12:32 PM

Yes, low compliance drivers in small enclosure lead to high Fs, but this IS BAD!

Why are not out there high compliance drivers? And why don't people use them in cars allowing for a smaller enclosure?

lndm · 21st May 2006, 01:00 PM

What I suggested is that a low compliance (stiff) driver will only work in a relatively small enclosure. ie, for two drivers with the same Sd, a box that appears as an acoustic suspension to a high compliance driver will look like an infinite baffle to a low compliance driver.

A high compliance driver would lead to a lower Fs, and a larger box. I am not an expert here, but I think you would need more mass to produce a lower Fs whilst keeping the small box. The thing for me is, with cabin gain, and the inability to produce the deepest bass in a small space, I find a high Fs satisfactory.

I have a 12" car sub that has a Vas=107L, Fs=27Hz and Qts=0.24. If I put it in a 14L enclosure, it has a Butterworth rolloff at about 80Hz. My main car listening is FM radio, and I don't do comps. I am satisfied.

RobWells · 21st May 2006, 01:33 PM

Peerless XLS series give low Q in small boxes.

Rob.

Volenti · 21st May 2006, 03:36 PM

Have you considered using a transmission line?

I've been developing TL designs for in car use for some time, the loading in the vehicle lets you get away with very short line lengths (as little as 3.5'-4' for 8'' and 10'' subs with heavy fill)

I recently made a folded TL to suit a 15'' driver that takes up only 2.7cuft, (5' line length), it has a very good response in the test car (has yet to be installed in the clients vehicle)

I'll try and get the pictures off the work camera tomorrow.

21st May 2006, 10:43 AM	#1
Davide82 diyAudio Member Join Date: Dec 2003 Location: xxxxx	Why not high compliance subs? Lately I had a try and built a subwoofer for my car. I used a CHEAP 8" driver with dual voice coils. Then I powered it with a very small amp... 2x20 watt (no need for SMPS!) It REALLY shaked the car and make everyone amazed..... you could feel 30 Hz easily and not boomy sound. the magic? I used an extremely large enclosure, so that efficiency was high at low frequencies.... Alignment was very good and transient respone superb. Now my question.... I would like not to use my hole trunk as subwoofer. BUT, if I build a smaller box, the sound gets of course boomy because the driver gets harder to move at all: this is a drawback! This all happens because the stiffness of suspension adds to the stiffnes of the small air trapped in the enclosure. Efficiency shifts to higher frequencies and we get bad sound. Do they exist drivers with a very low stiffness so that they can work in very small enclosures while maintining good low end efficiency? The only one I saw were 20 years old drivers with really soft suspension... I guess they work good there.. but I can't see new drivers (and cheap) similar to these ones! Why all new drivers are quite stiff?

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21st May 2006, 12:10 PM	#2
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	As far as I can fathom with a driver that has stiff suspension, the mechanical damping remains dominant until the box behind it is quite small and the air spring is therefore stiff also. If this is correct, a low compliance driver will have a low V_as and will therefore not respond to a large box. In my experience, car drivers are low compliance, and produce small box sizes and a high F_s.

21st May 2006, 12:32 PM	#3
Davide82 diyAudio Member Join Date: Dec 2003 Location: xxxxx	Yes, low compliance drivers in small enclosure lead to high Fs, but this IS BAD! Why are not out there high compliance drivers? And why don't people use them in cars allowing for a smaller enclosure?

21st May 2006, 01:00 PM	#4
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	What I suggested is that a low compliance (stiff) driver will only work in a relatively small enclosure. ie, for two drivers with the same Sd, a box that appears as an acoustic suspension to a high compliance driver will look like an infinite baffle to a low compliance driver. A high compliance driver would lead to a lower Fs, and a larger box. I am not an expert here, but I think you would need more mass to produce a lower Fs whilst keeping the small box. The thing for me is, with cabin gain, and the inability to produce the deepest bass in a small space, I find a high Fs satisfactory. I have a 12" car sub that has a Vas=107L, Fs=27Hz and Qts=0.24. If I put it in a 14L enclosure, it has a Butterworth rolloff at about 80Hz. My main car listening is FM radio, and I don't do comps. I am satisfied.

21st May 2006, 01:33 PM	#5
RobWells diyAudio Member Join Date: Aug 2002 Location: UK	Peerless XLS series give low Q in small boxes. Rob.

21st May 2006, 03:36 PM	#6
Volenti diyAudio Member Join Date: Aug 2003 Location: Australia	Have you considered using a transmission line? I've been developing TL designs for in car use for some time, the loading in the vehicle lets you get away with very short line lengths (as little as 3.5'-4' for 8'' and 10'' subs with heavy fill) I recently made a folded TL to suit a 15'' driver that takes up only 2.7cuft, (5' line length), it has a very good response in the test car (has yet to be installed in the clients vehicle) I'll try and get the pictures off the work camera tomorrow.

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