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
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
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.😀
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!
Travis Gibby said:
Look at this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=66893
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.
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
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
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.
Travis Gibby said:
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.
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.
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).
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).
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
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
Damping in a sub-woofer box is a very inyersting subject.
On the one hand one could argue that it is not needed if the first standing wave is sufficiently above the crossover frequency.
However, that assumes that the drive force to the cone from the motor system is perfectly linear.
It is not. Any distortion generated by the driver is not by definition filtered by the xover, and therefore the harmonics will excite the standing waves.
Depending upon the severity of the standing wave and the amount of driver distortion, the result may be audible.
So, this would suggest that damping should be used, just in case.
BUT.....any damping in the box will reduce system Q, and especially in a reflex enclosure will reduce the peak output from the vent and so reduce the bass response.
Also, loose fibre filling will move around at highlevels, so the amount of damping or added mass will change with level, impacting the accuracy of the bass dynamics.
The answer....experiment, listen, then choose
Andrew
On the one hand one could argue that it is not needed if the first standing wave is sufficiently above the crossover frequency.
However, that assumes that the drive force to the cone from the motor system is perfectly linear.
It is not. Any distortion generated by the driver is not by definition filtered by the xover, and therefore the harmonics will excite the standing waves.
Depending upon the severity of the standing wave and the amount of driver distortion, the result may be audible.
So, this would suggest that damping should be used, just in case.
BUT.....any damping in the box will reduce system Q, and especially in a reflex enclosure will reduce the peak output from the vent and so reduce the bass response.
Also, loose fibre filling will move around at highlevels, so the amount of damping or added mass will change with level, impacting the accuracy of the bass dynamics.
The answer....experiment, listen, then choose
Andrew
Sealed Box Subs
Hello Andrew
Do you think that fibre loading of sealed box is needed , you make mention about reducing system Q and its affect on a vented box design and subjective sound performance , its affect is not so pronounced in undesized small sealed box subwoofers.
Regards
Arthur
AndrewJ said:
Hello Andrew
Do you think that fibre loading of sealed box is needed , you make mention about reducing system Q and its affect on a vented box design and subjective sound performance , its affect is not so pronounced in undesized small sealed box subwoofers.
Regards
Arthur
I'm stuffing the 12 cf box I'm building more for the effective volume increase.
In Dickason's LDC in a 1 cf box the response at 20 Hz was increased by about 4 dB with a 50% fill of foam.
That's like 2.5X the amp power for free w/o power compression.
But I read somewhere that the effect is less in large boxes, though I don't know why that would be.
In Dickason's LDC in a 1 cf box the response at 20 Hz was increased by about 4 dB with a 50% fill of foam.
That's like 2.5X the amp power for free w/o power compression.
But I read somewhere that the effect is less in large boxes, though I don't know why that would be.
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