Group delay and Linkwitz transform

[sobazz]

Today I've been playing around with the Lamda SB10 i WinISD. I do not intend to design a speaker or subwoofer at the moment - just doing it for education and fun. To get a q value of 0.707 the box volume should be 88L. Without loosing maximum SPL I can reduce the box volume to only 18L, and use a Linkwitz Transform to get linear response (f3) down to 30hz.

But reducing the box volume alters the group delay. Not significantly, but I get a peak at 44hz of 7.5ms (10hz higher than with q = 0.707, but still a very low GD). Will this in any way way influence on the impression of "tight bass"? Will the LF transform influence on the group delay?

I hope you can help me with this too. How would pushing the SB10 moderately beyond it's Xmax (linear) affect the distortion? Pushing to around 33.5mm p-p will give me more power headroom (500w throughout the entire frekvens response). I know you should probably not calculate on this, but designing with this in mind cannot hurt.

Any help is greatly appreciated.


Soeren

[phase_accurate]

The group delay of a closed-box speaker, that has been EQed by the use of an LTF, will theoretically show the same group-delay as a speaker having the same frequency response without equalizing.
I.e. if you EQ a speaker with QTC=1.0 and fc = 60 Hz to a QTC of 0.5 and an fc of 30 Hz, it will behave like a speaker having a QTC of 0.5 and an fc of 30 Hz.

Regards

Charles

[sobazz]

Thank you Charles. Then you certainly shouldn't worry about group delay in sealed systems under any circumstance.

Do you have any experience in pushing woofers slightly beyond their Xmax?

Altering the parameters of a sealed LF system with linkwitz transform doesn't affect the drivers power handling, does it?

[mikee12345]

i would not push a speaker past its xmax
as then it WILL SOUND VERY BAD

speakers within xmax will generally sound 'good.'

i have a basshorn,it unloads the woofer at a frequency,the woofer moves at 2x XMAX, it sounds so disgusting ,like air puffing out a tiny port with high power applied.

it is a cheap woofer also.

see www.diysubwoofers.org for what dumax-xmax is exactly

Cheers!

[sobazz]

I know what the dumax-xmax is. And please note, that I'm not talking about pushing the woofer anywhere near the Xsus. It seems I must get my hands onto the Loudspeaker design cookbook. I remember a graph showing a typical perfomance curve for a woofer with excursion as the input parametre.

As I remember it, you should be able to push the driver 5-10% without an extraordinary increase in distortion. In an actual design situation I would probably aim for an LF response with f3 at 30hz.

[Ian Millar]

According to the Loudspeaker Design Cookbook, you can go past Xmax by up to about 15% in a sealed subwoofer enclosure without real trouble. My own sims don't go beyond Xmax, but sometimes lick it at around 20-30 Hz, and 4th order Butterworth subsonic filter at F3 around 12 to 15 Hz limits excursion below 20 Hz well. According to Dickason, the slight non-linearity at these low frequencies caused by exceeding Xmax by up to 15% is practicallty inaudible due to human hearing being quite insensitive down there. Incidentally the group delay curve rise caused by the subsonic filter can be very steep from about 30 Hz down (figure not important but slope important), but under 30 Hz who's gonna notice it? Not me.

Addendum: If you look at the Group Delay graph here: http://www.linkwitzlab.com/filters.htm#9 you can see that the LT slightly increases the group delay at the very low frequencies (where I reckon it doesn't matter), but actually reduces it where it counts - i.e. in the area where the speaker/box response's natural peak is cancelled by the transform.

[chris661]

Originally Posted by sobazz

Today I've been playing around with the Lamda SB10 i WinISD. I do not intend to design a speaker or subwoofer at the moment - just doing it for education and fun. To get a q value of 0.707 the box volume should be 88L. Without loosing maximum SPL I can reduce the box volume to only 18L, and use a Linkwitz Transform to get linear response (f3) down to 30hz.

But reducing the box volume alters the group delay. Not significantly, but I get a peak at 44hz of 7.5ms (10hz higher than with q = 0.707, but still a very low GD). Will this in any way way influence on the impression of "tight bass"? Will the LF transform influence on the group delay?

I hope you can help me with this too. How would pushing the SB10 moderately beyond it's Xmax (linear) affect the distortion? Pushing to around 33.5mm p-p will give me more power headroom (500w throughout the entire frekvens response). I know you should probably not calculate on this, but designing with this in mind cannot hurt.

Any help is greatly appreciated.


Soeren

Hi, with the LT applied, go to the Apparent Load (VA) on the drop-down menu. You may find that the extra gain from the eq translates into huge power requirements.

[Ian Millar]

Originally Posted by chris661

Hi, with the LT applied, go to the Apparent Load (VA) on the drop-down menu. You may find that the extra gain from the eq translates into huge power requirements.

He's right. Some people might think that LTing provides an LF "boost" which it "does but doesn't". But I think that "extra gain" is perhaps the wrong way to look at it. Yes it is extra gain at line level, but...

An LTed system needs "over-powered" amps, "over-sized" drivers - high electical limits and high cone area/Xmax to achieve the desired SPL at the selected new F3 frequency.

In WinISD, I'd be simulating a system that gave the required SPL at 20 Hz naturally and without exceeding Xmax (i.e. big drivers, big amps etc.) and LTing it DOWN ("extra attenuation") from there on up to the crossover point. A single 10 inch driver is not a good starting point as it probably can't generate decent SPL at 20 Hz to start with. About the only thing that an LT might do for a single small driver is probably to fry it "trying".

I'm currently building an LT system with two 12 inch drivers per stereo speaker cab for 101dB per channel at 20 Hz with 420W system input. The speakers would raise the ceiling at 70 -80Hz up without something like an LT to ATTENUATE them by up to about 13dB. Rod Elliott (ESP) recommends 20dB as an absolute maximum "gain" for his "Project 71" LT.

Lting will also allow you to reduce box volume to limit excursion with the resultant SPL peak being removed by the transform.

Incidentally, Dickason in an otherwise highly detailed text book, dismisses in just a short paragraph the type of EQ (that the LT is) as stupid (my word - not his), but he too seems to miss the point altogether. The point being that it is only really suited to a system that can produce the desired SPL at 20 Hz (or other chosen F3 frequiency) naturally to start with.

[454Casull]

LT (sometimes) requires significantly more power, so power handling may become an issue. sound.westhost.com had a good article on it.

[Dave Jones]

Reducing the box size diminishes power-handling capacity and can result in thermal compression. Whether you can get the volume you want at low frequencies is in the numbers. Must calculate. To make bass, one must move air and dissipate heat. No way around it.