Enclosure resonances, not a big deal?

[thadman]

One of the recent trends in loudspeaker design has been to incorporate dipole or open baffle alignments into the design. This has the obvious benefit of removing resonances within the body of air behind the diaphragm, but can this same result be achieved through another technique?

As far as I understand it, the behavior of the air within the enclosure and its effects on the loudspeaker can be thought of as a mass-spring system. The resonance or ringing due to the presence of modes within the enclosure space can be equalized out (by applying equal amplitude inversion signal processing of the resonances) because it is a linear phenomenon. The non-linear effects however can not. But where do the non-linear effects caused by the resonances arise? Is it because of nonhomogeneous pressure distribution over the surface of the diaphragm (ie higher damping in certain positions relative to lower damping in other positions) leading to erratic cone motion and thus non-linear distortion? If we managed to make the damping uniform over the surface of the diaphragm (by forcing the diaphragm to launch a 1 dimensional plane wave whose attributes only varied with distance from the diaphragm) wouldn't the observed non-linear effects due to the air vanish and become a linear phenomenon?

Thanks,
Thadman

[xpert]

Originally posted by thadman
The non-linear effects however can not. But where do the non-linear effects caused by the resonances arise?
[/B]

For the most of it they do not arise. Non linear effects are rare, as rattling or the like. On resonance forces are multiplied easily. That for such effects come along with modes. But they are not inherent to them.

[thadman]

Originally posted by xpert


For the most of it they do not arise. Non linear effects are rare, as rattling or the like. On resonance forces are multiplied easily. That for such effects come along with modes. But they are not inherent to them.

With this in mind, is the degradation of sound quality due to enclosure resonances mainly a linear phenomenon that is solvable by signal processing?

[xpert]

Originally posted by thadman


With this in mind, is the degradation of sound quality due to enclosure resonances mainly a linear phenomenon that is solvable by signal processing?

I think so, I didn't experience a degradation since. My boxes are damped - reflex too - and modes were never measurable in the resulting soundfield. Of course non linear distortion was often measured but I never found anything else but rattleing from soft screws etc.

so long

[thadman]

Originally posted by xpert


I think so, I didn't experience a degradation since. My boxes are damped - reflex too - and modes were never measurable in the resulting soundfield. Of course non linear distortion was often measured but I never found anything else but rattleing from soft screws etc.

so long

What if pressure nodes were located along the surface of the diaphragm? Would it not be possible for the forces imposed by them to excite modes present in the diaphragm? I'm sort of thinking along the lines of aerodynamic flutter:

"Flutter is a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion."

Would it be possible for pressure along the diaphragm to do something similar? Amplify structural modes that propagate non-linear vibration?

[dangus]

I don't think enclosure resonances are easily solved by filtering. They're a form of ringing, so they'll keep going longer than whatever excited them. Better to build an enclosure that minimizes it.

Or be like the whackos that designed a speaker called the Adiabat, that made a "feature" of the lack of internal damping. The resonances could be clearly seen on the impedance vs frequency plot when a magazine (SGHT?) reviewed it. I'm surprised the reviewer didn't just slash open the nearest pillow and see how the speakers sounded with some stuffing.

[planet10]

Originally posted by thadman
The resonance or ringing due to the presence of modes within the enclosure space can be equalized out

Only be removing that part of the music that would excite those resonances. The cure would be worse than the problem.

dave

[xpert]

Originally posted by thadman


What if pressure nodes were located along the surface of the diaphragm? Would it not be possible for the forces imposed by them to excite modes present in the diaphragm? I'm sort of thinking along the lines of aerodynamic flutter:

"Flutter is a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion."

Would it be possible for pressure along the diaphragm to do something similar? Amplify structural modes that propagate non-linear vibration?

I've never seen things like that with speakers.

BTW, prefiltering does the job. I'm amazed again every time when people try to tear others into doubt about such. Yes we can! Prefiltering is absolutely O/K against spurious resonances. There is nothing behind.

[planet10]

Originally posted by xpert
BTW, prefiltering does the job... Prefiltering is absolutely O/K against spurious resonances

It does nothing to remove the resonance. The problem is still there. If there was energy in the music to excite it, you have now removed it from the music by pre-filtering. A band-aid at best,

Much much better to just cure the problem at the source and the only way to do it properly.

dave

[gedlee]

Guys, when can argue about the effect a butterfly flapping its wings in South America has on the sound, but is it really worth it? You have to scale these effects to put them in context or the real problems. These resonances DO occur, sure, but are they a significant factor to the sound quality - NO!. Move on!