Enclosure resonances, not a big deal?

[planet10]

Originally posted by David_Web
However note that a box does not behave like a piston, only the "middle" moves the full amount. The rest moves less and at the edge "none".

Another consideration is that if you can get the panel resonating at a specific frequancy it becomes acoustically transparent.

dave

[gedlee]

Originally posted by panomaniac
Although the resonance point went up with bracing, the amplitude is lower and it decays much faster. That ought to sound different. Maybe less noticable.

It can only sound different if you can hear it above the excitation.

To understand this problem it is essential to understand the concept of radiation efficiency. Take a dipole versus a monopole with identical cone excursion. The differences in output are enormous. Amplitude alone is a very poor indicator of sound radiation.

[gedlee]

Originally posted by planet10


Another consideration is that if you can get the panel resonating at a specific frequancy it becomes acoustically transparent.

dave

Dave, this is true, but only for an infinite panel - its called coincidence. It would occur at a very high frequency in a loudspeaker enclosure.

[planet10]

Originally posted by gedlee
Thats true, but from the standpoint of "stiffening" this is as good as it gets. Forcing the point of maximum excursion to zero excursion is the ideal. Lets face it its impossible to make something that "can't resonate". The idea is to do the best with the least - to do just enough, and not more - its called engineering.

The paper contradicts your 1st point. If we extend the "dowel" cross-section to the point where is extends across the whole cabinet then the panel is stiffer yet. If we extend it in such a way that the aspect rations of the 2 subpanels increase then it gets stiffer than if the aspect ratio shrinks.

Further unequal subpanels is better than equal subpanels (not necessarily in stiffness, but in terms of distribution of enery required to excite panel resonances).

I have been working very hard towards an elegant solution to box bracing that is simple and effective. My latest creations are getting pretty good (matter of fact, you came to my aid in one thread where that structure was being discussed)

dave

[amiklos]

Originally posted by panomaniac
I don't understand the radiating efficiency thing. If a panel of any material - plywood, mdf, steel, glass, plastic, etc is moving a certain amount, why would it not be just as loud as the cone of a speaker of the same area?

What is magic about a speaker cone that makes it better at coupling to the air?

All the box speakers I've ever seen have had a much larger box surface than cone surface. How much less does the large surface of the wall need to vibrate than the cone to make the same amount of noise?

The iPhone thing? That's no big deal. Take a small woofer of the same surface area and feed it with the very low frequency of the vibrator. But don't put it in a box! How much sound do you get out of it? - not much, maybe none. Place that same vibrating iPhone on a wooden or glass table and hear how much sound you get? Can we say "tiny dipole?" Can we say "soundboard?"

Agreed on most all points. I used the iPhone as a comparison to a loudspeaker (in an enclosure) moving with similar energy and noted in a later reply that the folly in the comparison includes the isolation of the backwave from the listener, and the fact that a driver is generally intended to act as a piston.

Originally posted by thadman
Another thing to consider, although the surface is vibrating ... are all of the individual points across the surface vibrating in perfect phase with one another? I think we can agree that they are not, and thus the observation of kinetic energy within the cabinet walls does not translate directly to acoustic energy.

This is a much better articulation of what I was trying to convey when I used "sound at the listener" or some variation thereof.

Originally posted by panomaniac
Exactly my point! It's rather hard to figure out just how much the walls are radiating. For the reasons posted above, as well as others.

But radiate they do. As we can see in the graphs that Rudolph linked to, bracing makes a difference for sure. I did a little overlay of the graphs of those 2 plots just for my own amusement. Will post later, if Dave doesn't beat me to it. Although the resonance point went up with bracing, the amplitude is lower and it decays much faster. That ought to sound different. Maybe less noticable.

There are going to be a number of factors at play in determining what someone in a reasonable listening position (ear not against a cabinet) would hear.

I'm inclined to believe that if a reasonable amount of effort is put into appropriately eliminating panel resonance via typical construction techniques (bracing, cld, etc.), the incremental gain from going further will be overshadowed by the resonances of all the other surfaces aside from the cabinets that are present in any given room.

I know I can get kitchen cabinet doors to start resonating in the next room at higher volumes. The objects in the room with the loudspeakers have to be doing something and it would seem to me that the summation of those items is more likely to be an issue than the relatively minor cabinet vibrations that remain after good cabinet construction.

Now I'm off to go perform a "laying on of hands" on the various objects in my listening space.

[gedlee]

Originally posted by planet10


The paper contradicts your 1st point. If we extend the "dowel" cross-section to the point where is extends across the whole cabinet then the panel is stiffer yet. If we extend it in such a way that the aspect rations of the 2 subpanels increase then it gets stiffer than if the aspect ratio shrinks.

dave

I don't see any contradiction. The paper has nothing to do with what I am doing. And I don't claim that there aren't ways to make the cabinet stiffer - only that there aren't any simpler ways to get such an effective result. "Frugal" is a good word for it.

[pjpoes]

I've posted this before, and I know, without comparison to others, or measurements of the actual sound radiated from the box, it's of little help in such a discussion, but it was informative to me. The above image is from an accelerometer reading on the side of a 5 cubic foot ported subwoofer box. The walls are roughly 2" thick CLD, with 3" thick CLD front baffle. Unfortunately the builder didn't listen to my instructions and used nails to hold the entire thing together while the adhesives set, so each panel isn't necessarily free to move like you would want. None the less, I asked for a flexible adhesive be used to laminate MDF together. The inside has interlocked window pane braces. The walls for the left and right side also have panels attached with numerous 4" and 6" holes cut into it. Don't know what to call it, I stole the idea from Focal. Finally, once I received the finished box, I lined all interior surfaces with a spray on dampening compound, followed up by a multi-layer dampening foam, like black hole, totaling 2"s thick. The box was filled with wool batting, and the port lined with the same foam Dr. Geddes uses for his horns. This was my attempt at an all out assault on minimizing vibrations.

I think it's worth noting that, it's just not that dead. My windowing and range makes it look worse than it really is, but frankly, I expected to measure next to nothing, and that's not what I got. Also keep in mind that, this box, raw materials alone, was almost 300 dollars.

[pjpoes]

This is my build of Dr. Geddes box for the Abbey, measured in the same way, just different software. Worth noting here is that the accelerometer can not be calibrated to eliminate it's own response issues, such as that peak at 20khz, nor does it have a good noise floor. With it attached to nothing, and no sound, the noise floor is, though uneven, around 50-60db's. Also keep in mind, these are not calibrated accelerometer measurements. I have no way of isolating the output of the panels from that of the box to calibrate the device, so I can only use it in relative terms. The output of the box is not 80db's, I have no idea what it really is. Also note, the output voltage was 5.6 volts for the Abbey, and 15.2 for the subwoofer (into an 8 ohm load).

Probably most important, how does any of this relate to how the speakers sounded. What do those measurements, which seem to be pretty industry standard for manufacturers, correlate to some impact on sound. I have no way of of interpreting these. I took them, sat down, studied them, and still am left with no good way to interpret them. With the Abbey box, I can say there are no resonant modes apparent. The vibrations from the box hit the noise floor around 1khz or possibly lower. Adding additional bracing and dampening to the Abbey had almost no effect, as can be seen when comparing the blue and red lines, blue being with added dampening and bracing.

[amiklos]

Interesting data. Thanks for posting it.

[badman]

8-14 dB over the 60-120Hz range seems like a valuable reduction to me.