subwoofers - lining

[kimbo1]

I have almost finished my subwoofer project. I am having trouble verifying the usefulness of internally lining a ported box, which is done for a different reason to stuffing sealed boxes. the aim is to absorb standing waves, yet the maximum dimension of my box is 550 mm, I am using a cut-off frequency of 100 hZ which equates to a wavelength of 3400 mm. So even half this wavelength is 1700 mm. I can't see how lining would perfrom a significant function at freqencies below 300 Hz?
Anyone have an informed answer?

 

[MikeyP]

I think you are right. If your design is a simple first order one (1 driver, 1 port) then additional absorption inside the cavity will have little effect if there are no standing waves. The lining may have structural damping properties which may lower the vibration levels of the box, but this is a secondary effect and would only be of use if the box has structural modes below say 150 Hz (If these structural modes are a problem, then proper structural damping treatment such as constrained layer damping would be more appropriate).

Some form of wadding inside the port tube itself could be used to "tune" the speaker. A first order design is effectively a forced Helmholtz resonator and extra acoustic resistance in the "neck" of the resonator would alter the acoustical Q factor and could be used if the response of the speaker is too "peaky".

M

 

[Paulism]

Kimbo,
You may also want to consider a method of fragmenting the soundwaves by using soft, cast urethane pads with texture. These materials will fragment the soundwave without converting the sound energy into any other form (i.e. heat.

 

[GregLocock]

Well that's an interesting theory. So when did we discard the law of conservation of energy exactly? I must have missed the headlines.

Cheers

Greg Locock

 

[kimbo1]

Thanks Guys,
I eventually made sense of it. The box is a reverberent room so I need to absorb as much sound as i can to minimise the amount of sound exiting out the port. Also there will apparantly be higher order harmonics generated which need to be absorbed.
Anyway it worked out really well, I used 75 mm thick fibreglass insulation and I re-iterate the advice I was given to not underestimate the importance of bracing the box - these things move!!