Originally Posted by michael hurd
I have had the opportunity to hear a sweep of a tower loudspeaker in an anechoic chamber. Standing behind the loudspeaker at a distance of about 2 meters, you can barely perceive any sound with a slow descending sweep, until the sound 'wraps' around the cabinet, and it becomes 'very' loud. Baffle step frequency was approximately +/- 456 hz in this case. Where the horn was dominating the pattern control, there was nil for sound behind the cabinet, but above 500 hz to about 2khz, there was 'some' radiated sound rearward.
The loudspeaker in question has a horn of approximately 8" square, and two 8" woofers, approximately 2.5 cu ft net, with two simple shelf braces. The larger the speaker, the lower in frequency you have problems.
Time domain measurements are helpful in determining problems, too much emphasis is placed on the frequency response in my opinion. With a well braced, stiff enclosure, problems are moved high in frequency, where they are more easily dealt with.
I'm wondering what your suggesting here? Are you trying to suggest that your experience proved for you that sound does not radiate through a speaker cabinet and out again? If that is the case, I think that is well proven to be the case given the studies I linked.
It does make sense that at frequencies covered by a horn/waveguide little sound was traveling back through the cabinet. Very little sound would radiate through the horn or through the compression driver into the cabinet and what little sound does do so would be quickly absorbed by the polyfill. This issue is (as shown in the measurements of the papers I included) happening in the midrange and upper bass range.
You wouldn't be able to hear the difference in delayed sound of that which "wraps" around the speaker and that which travels through the speaker. They are both delayed. 1 foot equals roughly 1 ms. That means that if a speaker cabinet is 12" deep, it would only be delayed 1ms in wrapping around the speaker. You couldn't easily perceive that. Sound delayed through the cabinet of the speaker is actually greater than 1ms. What you heard could just as easily (if not more likely) been sound traveling through the cabinet.
With regard to bracing raising the problem, I think you would need to read the articles I've posted to understand, or reread what I've written. The sound traveling through a cabinet does not happen only at the resonance frequency of the panel. It just happens that the resonant frequency is where the panels become very efficient transducers themselves with high Q peaks with little decay, thus creating audible problems. In this case, yes its easier to deal with those when they are high Q and higher in frequency, but only if you deal with them. That's why I'm suggesting CLD, its a very efficient and effective means of dissipating the energy. The first article I linked to from AES, the study looking at two versions of an NHT speaker cabinet found that the better braced cabinet had higher Q lower amplitude resonances but had more overall radiated sound than the unbraced cabinet. It showed the importance of addressing the energy with damping.
In reality what I'm saying here is you need to have damping, not just bracing and mass. The CLD happens to be a great way of doing that which is known to be more effective than other means of damping, but...any damping is better than nothing. Polyfill inside a cabinet is technically providing some damping, but it's not providing much. Gluing fiberglass, cotton, wool, etc. to the panel walls also provides damping, but still not as much as a proper damping compound.