I am currently building new enclosures, and did a knock test on both boxes. One has all the bracing, the other has no braces yet. Then I wondered if anyone is aware of any measurements that illustrate the effect of the bracing, if I should take some measurements, and what to measure. Any ideas?
See Toole's book Sound Reproduction, pg. 382, "Interpreting the Data, Exercises in Detection".
He mentions anechoic data at 2 Hz resolution to find loudspeaker resonances.
You could do the same thing if you can do your FR measurements outside on an elevated platform, away from buildings, fences, and hard concrete pavement, taken with slow upsweeps and long time gating.
Chris
He mentions anechoic data at 2 Hz resolution to find loudspeaker resonances.
You could do the same thing if you can do your FR measurements outside on an elevated platform, away from buildings, fences, and hard concrete pavement, taken with slow upsweeps and long time gating.
Chris
system impedance measurements sometimes show up irregularities in the 100 to 1000 hz range that are associated with cabinet resonance.
You need to do your tests after the speaker is completely put together.
Meniscus Audio sells a small microphone that can be placed against a cabinet's surface when a strong pink noise or low freq. signal is being placed. With this device I've typically found FR blips in the 200-600 hz rangel which again, are characteristic of a cabinet sourced resonance. The nice thing about the mic is you can move it around the surface and take multiple meas. to see the changes. The closer to the cabinet edge, FR spikes tend to go away because that is where the cabinet is stiffer.
You need to do your tests after the speaker is completely put together.
Meniscus Audio sells a small microphone that can be placed against a cabinet's surface when a strong pink noise or low freq. signal is being placed. With this device I've typically found FR blips in the 200-600 hz rangel which again, are characteristic of a cabinet sourced resonance. The nice thing about the mic is you can move it around the surface and take multiple meas. to see the changes. The closer to the cabinet edge, FR spikes tend to go away because that is where the cabinet is stiffer.
Iverson's early work:
dave
I see the various bracing schemes Iverson used, but not the effect on FR. Can you share the FR's?
Iverson's early work:
dave
Aehm, this figure does not show any measured data, right?
Found a PDF of the book and found that the figure on p. 382 (and the corresponding text) just discuss this as "a resonance", but it does not say if it's the enclosure, and it does not mention bracing of the enclosure.
I won't do a similar outdoor measurement as I don't want to spend so much time on it; need to proceed with the speakers. But maybe I could do a dedicated experiment later.
Yup, that would be nice, but would require the speaker built completely in two incarnations (with and without bracing).
At this stage I thought of doing a somewhat controlled knock test, record the sounds, and analyse them in the time and frequency domains. But I don't want to delay the cabinet building much, so the test should be easy to implement.
Just let me know if anyone has an idea for a simple controlled knock test (or similar) so I could record the data, continue building the speakers and do the analysis later.
Sure it does. The actual panel resonance in each case.
Typically you want to use bracing to push the panel resonance higher. These clearly show the best ways to brace to get that.
dave
D'Appolito's book Testing Loudspeakers, 1998, Amateur Audio Press, section 3.6, shows the method of looking for impedance irregularities to find enclosure reflections and resonances (which is what a cabinet vibration produces in the output of the loudspeaker). That's a pretty simple test. You could also scan the Klippel procedures online for an equivalent procedure.
I don't believe that "knocking on the cabinet" is going to find what you really seek.
YMMV.
Chris
I don't believe that "knocking on the cabinet" is going to find what you really seek.
YMMV.
Chris
Bare, you are focusing on the wrong info. What we are interested in is how effective each technique is to the others. For instance, with one parallel brace it is more effective to run it the long way than the short way.
You want to target pushing the resonances up above about 500 Hz. Up there they are less likely to ever get excited.
dave
You want to target pushing the resonances up above about 500 Hz. Up there they are less likely to ever get excited.
dave
The behaviour of a single thin panel clamped at all edges is not necessarily going to read across that well to the thicker 6 panels of a speaker box. The lowest box modes are likely to involve all 6 panels moving together which is most effectively stiffened by braces running between the panels. The modes which mainly involve the centre of some of the panels moving in and out are likely to occur at higher frequencies. If bracing between the panels is introduced to stiffen the lowest modes then the ends of the braces will also stiffen the panels and change how they are most effectively stiffened further.
Well, my bad. Each drawing does show where the FR peaks from resonance under each cabinet bracing configuration.
However, what the data does not show and is even more important is the how severe the peak was.
Can the OP of the Iverson data share the FR peak +dB as well?
Well, I don't even know what I was seeking.
I just did the knock test because that's what I (and everyone else) do(es) with speaker enclosures. Then I thought it would be cool to have some objective data rather than just some subjective impression that is hard to interpret.
You could set up a microphone and something like REW or ARTA, and record the knocking, then do a spectral analysis of that data, but to say that the results have anything to do with box resonances due to acoustic reflections or box plate vibrations is more than a stretch.
Myself, I'd first do a series of slow high-resolution sweep indoors using different bands of interest, and see if gating the measurements might reveal any midrange resonances via its frequency response, phase response, constructed impulse response, and waterfall displays. I'd also check the impedance data vs. frequency. Then any irregularities can be identified and the hunting of the source of those resonances begins. I've found that the drivers themselves are responsible for many issues, including in particular cone-surround-to-spider resonances.
Barring getting good results from that, I'd take it outside and repeat the test without nearfield reflectors.
Chris
I don't know that it exists. I scanned that out of the AES Journals 15 years ago…
dave
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