Fibrous Damping Material in Subs, Why Bother?

[Travis Gibby]

The subwoofer I'm building will be 12 ft^3 and it's longest dimension will be 4 ft. The lowest standing wave mode will be just above 140 Hz. If the 24dB/octave xo is at 80 Hz then the response will be ~ 20 dB down @ 140 Hz.

Obviously it's big box, and a smaller one would have it's lowest standing wave mode at an even higher frequency. So knowing all this, why even bother adding fibrous damping material, unless it is being used to make the enclosure perform like a larger one?

Travis

[johnnyx]

I built a smaller sub box than you, and when I measured the frequency response there was a reflection at about 500Hz. This would be way above the passband, and probably inaudible, but it looked so horrible that I put a thick damping sheet of felt on the back wall of the cabinet. The frequency response looked so much better with just a blip at that frequency; I could sleep peacefully after that.

[tade]

my guess is that, some higher frequencies get threough the crossover always, and more likely, if your box rings damping will help that problem. I dont do it because damping material is more expensive than particle board!

[DSP_Geek]

Originally posted by Travis Gibby
So knowing all this, why even bother adding fibrous damping material, unless it is being used to make the enclosure perform like a larger one?

Look at this thread:
Do I Need A Notch Filter?

The guy had a peak in the woofer's response about an octave above the crossover point, and apparently it was quite audible. It was at 4.2 kHz instead of 140 Hz, granted, but I bet a subwoofer resonance might show up as congested low-mids. Besides, fiber fill is relatively cheap compared to your time spent building the cabinets, so why not get some?

You also want to brace that bad boy big-time, because panel resonances will be a problem. That's an awfully large box.

[Travis Gibby]

Thanks everyone for the responses.

The consensus seems to be that cabinet standing wave modes in the stop-band may be audible under some circumstanses. This is a much better explanation than the one that I got on the home theater forum.

However, if this is true, why do so many build cube shaped subwoofers which will have the worst standing wave resonances possible?

Travis

[tade]

The cubic shape is the easiest to build and fit into the average room. When building such a large box, size is a bigger consiteration than standing waves.

[DSP_Geek]

Originally posted by Travis Gibby
However, if this is true, why do so many build cube shaped subwoofers which will have the worst standing wave resonances possible?

Consider that a cube is the paralleliped including the greatest volume for its size. Your 12 cubic foot cabinet is, at a guess, 4 x 2 x 1.5 feet. If it were a cube, the internal dimensions would be closer to 2.3 x 2.3 x 2.3 feet: the longest edge is almost halved, which puts the dominant mode around 240 Hz instead of 140 Hz.

Note there are very few cubic subs that large: most of them are less than 2 feet on the outside, meaning they're smaller on the inside by a couple of inches, so the resonance lands above 300 Hz. At 75 Hz subwoofer cutoff, a 2nd order crossover would be down 24 dB at that peak while a 4th order would have you down 48 dB. Fiber fill also starts being really effective at those frequencies, so the resonance is not nearly so fearsome as it would be otherwise.

[johninCR]

Just use matrix bracing with oddly spaced holes and that will raise the box modes well out of range. My understanding is that you should not have an unbraced panel in excess of 8-10 inches anyway.

[Volenti]

From my testing adding fill to a sealed enclosure reduces Fs by a couple of hz , smooths the F3 corner frequency by 2-3db and halves the driver impedance at resonance.

In simulation doing the same thing to a very large vented enclosure will have a similar effect including the added benifit of reduced group delay by around 20%( the group delay reduction also occurs in sealed enclosures but since they have low GD to begin with it's isn't very noticable).

[ralphs99]

Hi Travis,
Your logic is correct, there is no good reason to add damping if the box is only excited by the sub-woofer operating at a low frequency. Damping material would have little effect at such long wavelengths anyway.
However I'm not sure that you can assume the box will only be excited by the sub-woofer. Your mid/high boxes will also excite the sub. Damping material in the sub will stop the box resonating and re-radiating higher frequencies from other sources.
Cheers,
Ralph