Enclosure resonances, not a big deal?

[thadman]

Originally posted by gedlee



Well if that is the point, the answer is yes and no. The internal modes that affect the cone as a lumped mass can be exactly controlled, but any enclosure modes in the enclosure itself cannot. Basically any EQ is a single dimensional control scheme and can only ever control things that occur in only one dimension -"lumped parameter" effects. Box internal modes are basically lumped parameter, but the box stuctural modes are distributed. If EQ is used on any structural modes then it will affect the results differently at different locations. It can be shown that some locations will get better but some will get worse and that no EQ control can correct the problem globally.

I'm not interested in equalizing structural modes. I don't even see how that would be possible unless you incorporated active damping elements into the enclosures (possible, but probably excessive and very expensive). I'm interested only in equalizing the frequency response (impulse response) so that the modal resonances affect on cone motion is reduced to zero.

If an enclosure resonance is imparting a force on the cone, wouldn't it be possible to feed the driver coil an electrical signal equivalent to the resonance in amplitude but of opposite phase. The forces cancel and the effects of the resonance on the frequency response are gone.

[PLB]

Hi Thadman,

I think this paper is relevant to the topic.

http://www.fesb.hr/~mateljan/arta/pa...m-aaaa2007.pdf

As far as standing waves are concerned, I think a 1.5”- 2” layer of open cell foam covering the internal surfaces of the enclosure will reduce the magnitude/Q sufficiently for them to be considered inaudible. On the other hand, untreated panel resonances in my opinion can be more of a problem, because the surface area of an enclosure is many times greater than the area of the driver’s cone, so panels only need a very small excursion for their acoustic output to equal that from the driver itself. For this reason, I always treat large enclosure panels with bracing.
To treat these resonances with some form of processing could help, but personally, I would opt for the solutions above before I resorted to processing techniques, simply because they are so inexpensive to implement.
There may be a greater opportunity to limit resonances due to standing waves by processing, because in its simplest form, standing waves exit through the loudspeakers cone. So in spatial terms, they are in a single dimension, however, panel resonances are in three-dimensional space, so processing cannot effectively treat them in my opinion.

Regards

Peter

[thadman]

Excellent link

[gedlee]

Interesting paper, nothing new really, but it doesn't say anything about the subject at hand.

[xpert]

Originally posted by thadman

If an enclosure resonance is imparting a force on the cone, wouldn't it be possible to feed the driver coil an electrical signal equivalent to the resonance in amplitude but of opposite phase. The forces cancel and the effects of the resonance on the frequency response are gone.

yes

[planet10]

Originally posted by thadman
If an enclosure resonance is imparting a force on the cone, wouldn't it be possible to feed the driver coil an electrical signal equivalent to the resonance in amplitude but of opposite phase. The forces cancel and the effects of the resonance on the frequency response are gone.

On the gross frequency response. But that is trying to judge the ocean just by looking at its surface.

dave

[sdclc126]

I have A/B listened to pairs of speakers where one had an external vibration damper attached. The differences WERE audible AND significant.

Despite some of what I've read here, if an enclosure vibrates past some threshold it is audible and undesirable. Vibrating surfaces can make sound - if that surface is the speaker cabinet and not the transducer, artifacts are being added to what reaches the listener's ears that were not in the original recording.

At the least I would prefer to overbuild the cabinets and err on the side of caution.

[gedlee]

Originally posted by sdclc126
I have A/B listened to pairs of speakers where one had an external vibration damper attached. The differences WERE audible AND significant.

Of course this was a blind test!

Originally posted by sdclc126
Despite some of what I've read here, if an enclosure vibrates past some threshold it is audible and undesirable.

Past some threshold it will, agreed. But I claim that threshold is a lot higher than what typically gets thrown about.

Originally posted by sdclc126

Vibrating surfaces can make sound - if that surface is the speaker cabinet and not the transducer, artifacts are being added to what reaches the listener's ears that were not in the original recording.

First, the "radiation efficiency" of the cabinet is going to be very low (if you don't understand that term, it means the level of radiated sound to the level of the vibration). Loudspeakers are designed to radiate sound and so they tend to have high levels of radiation efficiency. Second, the actual level of the vibration of the cabinet to that of the speaker is miniscule. The net efect is a very low level of sound radiated from the enclosure itself.

Doing nothing to reduce enclosure vibrations is going to be a problem, but a few key techniques will reduce the sound levels radiated to below the threshold of annoyance (if not audibility).

I have tried to measure the amount of sound radiated by the cabinet and found it to be extremely difficult. That makes it, at best, a very small effect.

[claudio]

Originally posted by gedlee

I have tried to measure the amount of sound radiated by the cabinet and found it to be extremely difficult. That makes it, at best, a very small effect.

Good evening Earl,
I am very much interested in your founding, since they seems in contrast with what Barlow and Stevens found out in theirs 1975 AES preprints: if I remember well, Stevens measured a 10dB difference between the back pannel radiation and the drive output, using an undamped 50 liters box.
How much did you measured, and which setup did you used?

[DonA]

Interesting thread...

If I had to pick a single item which has caused me some of the most difficult-to-solve problems in DIY speaker building, it is cabinet resonances. I find I am incredibly sensitive to them. Maybe it's because I owned Magneplanars for many years and I am used to a dipole sound. Despite this, I am not a total torch carrier for the dipole crowd. I also like speakers in cabinets for reasons too numerous to go into here... Both have their strong points. BTW, dipoles DO have undesirable structure resonances. Even Sigfried Linkwitz found this out.

Let's talk about the issue of cabinet resonances. It has been proposed that if a cabinet contributed no nonlinearities but had resonances, that with proper filtering, you could avoid exciting these resonances and therefore avoid their audible effects. If a cabinet has a resonance at 300 Hz which augments the output from the speaker at that frequency, one must simply reduce the magnitude of the signal to the speaker by the proper proportion at that frequency and end up with a "flat" frequency response despite the cabinet resonance. Let me start by saying that I have actually used this scheme with some significant measure of success, but it is by no means perfect. Here's why... With the non-linearities of speakers, let's say you put in 2 signals; one at 700 Hz, and one at 1000 Hz. Just one of the most likely of distortion products to come from the speaker is the difference of 1000 - 700 = 300 Hz. This signal is an unplanned-for signal and we did not put it in. There are innumerable combinations of frequencies which, when you put them into our real world speaker driver, give distortion products at 300 Hz. This is why, although you may not be putting IN any frequencies at 300 Hz, intermodulation distortion products will exist at or around 300 Hz which will be augmented by the 300 Hz resonance in our cabinet. This will produce the "resonant signature" which is so immediately identifiable. And remember, the more complex the music is, the more likely it is that the distortion byproducts will coincide with that resonance. That is why using music sources with many voices and/or instruments will so mercilessly reveal cabinet coloration, even when you filter the input signal.