Well, the box is a "pressure zone" inasmuch that, at low frequencies it enclosed air acts as a spring, against which the cone must be moved. As the cone has mass, we have a mass/spring system which therefore exhibits simple harmonic motion at a resonant frequency. And that's the end of the story for a subwoofer.
It's the higher frequencies which complicate things - we have sound waves propagating within the box, and which bounce around internally. It would be a relatively simple test to put a small "bookshelf" speaker within a box intended for a (somewhat larger) speaker. Any sound emanating from the hole in the front baffle would provide a useful demonstration of how much of a problem the rear radiation from a driver would be - and the target driver could be fitted to demonstrate the opaqueness or otherwise of its cone. Don't expect much attenuation though.
Unless the box's walls are especially inert, there will be sound, coloured by panel resonances, emanating from here too (no doubt mostly caused by vibrational transmission from the driver's chassis, but also via the air within the box).
It's the higher frequencies which complicate things - we have sound waves propagating within the box, and which bounce around internally. It would be a relatively simple test to put a small "bookshelf" speaker within a box intended for a (somewhat larger) speaker. Any sound emanating from the hole in the front baffle would provide a useful demonstration of how much of a problem the rear radiation from a driver would be - and the target driver could be fitted to demonstrate the opaqueness or otherwise of its cone. Don't expect much attenuation though.
Unless the box's walls are especially inert, there will be sound, coloured by panel resonances, emanating from here too (no doubt mostly caused by vibrational transmission from the driver's chassis, but also via the air within the box).
Interesting GIF, but it describes a transverse wave through a long, thin pair of joined media. A cone is a thin membrane, far, far thinner than the wavelengths of any sounds of interest, and which is subjected to sound waves (largely) normal to its surface.
Imagine if the walls of the box were made of cone material of the same thickness. Would anyone expect any great attenuation through the walls?
Nice .gif Galu but how does that pertain to the inside of an enclosure? Are the sound waves inside the enclosure even audible? If I put myself inside of a sealed enclosure with a woofer firing .. what will I hear exactly? .. or will I just experience the pressure pulses?
There is often a fair amount of sound reflecting off the driver's basket/frame (depending on the basket). You can improve this by using absorptive material glued onto the basket - though again, you can also negatively effect air losses like this.
For the fullrange driver I used (taped-on) felt (all over) but it was too much for the driver, and eventually I used modeling clay on both the inside and exterior of the frame (which also damped the basket).
Materials-wise, diaphragms that are sandwiched (with foam in the center) are probably best, and something like my low Mms fullrange drivers are the absolute worst.
For the fullrange driver I used (taped-on) felt (all over) but it was too much for the driver, and eventually I used modeling clay on both the inside and exterior of the frame (which also damped the basket).
Materials-wise, diaphragms that are sandwiched (with foam in the center) are probably best, and something like my low Mms fullrange drivers are the absolute worst.
That's a very impressive degree of sound absorption and attention to detail there.
So there's no use of felt lining the internal walls? I mention that because (if it's dense enough) the felt will soften the air to wall transition, with the idea of reducing the amplitude of reflected waves.
Wow, you really went to town on this!
The treatment of the cones themselves is interesting, as the results seem to demonstrate the usefulness of distributed damping. I think manufacturers like ATC and Harbeth(?) follow this kind of thinking too. Particularly useful for bass/mid drivers which need to accommodate large excursions by using low-loss surrounds...
Nothing on the walls, and in fact I waxed the walls. I've found over time that surface material effects very small signal/amplitude results of drivers that have low loss suspensions (in the sub mm range). It also effects a bass reflex vent (..and can increase non-linear distortion, in fact you can marginally lower non-linear distortion by waxing the inside of a BR Vent depending on how smooth/low friction the vent already is).
Basically you can stuff the box as much as you want so long as the fiber isn't on or near the drivers/panels/vent. Of course some drivers are relatively insensitive to this so it isn't particularly important with them.
Clarification: I didn't treat the diaphragm, just the basket.
I have tried treating the diaphragm of several drivers before with various results (though not the ES-R). Most of the time it's more of a change than an improvement. Really, the improvements are treating/damping just the driver-to-surround connection. Even then though - you almost always take a "hit" in efficiency.
When a longitudinal pressure wave strikes a plane surface between two media, part of the wave will be transmitted and part reflected.
The mathematics is the same as for a "long, thin" string carrying a transverse wave, except that the variable that is varying sinusoidally is the pressure.
Yes, but the cone is about 1mm thick - far, far less than 1 wavelength. It offers next to no resistance to an incident sound wave - and even if it did, it would retransmit, having been deflected bodily.
If the boundary cannot be regarded as fixed, that would certainly introduce more variables!
It’s easy to measure how much is reflected from the back of the cabinet and back through the cone
Feed a high-passed low frequency square wave into the amplifier so that you just get a series of clicks at say 1Hz
Connect a mic to a scope and you’ll see a large peak from the front of the cone, and the reflected click from the rear
Brian
Feed a high-passed low frequency square wave into the amplifier so that you just get a series of clicks at say 1Hz
Connect a mic to a scope and you’ll see a large peak from the front of the cone, and the reflected click from the rear
Brian
Yes, and the entire back wave is a good thing, supporting the driver resonance even over a wide band. Damping material would not be necessary.. except that the box also goes modal. So talking about backwaves is probably one step removed from the problem, which is box modes.
Full-range users may have the additional problem of higher frequencies bouncing off the cut-out.
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So far these differences have only been alluded to. What does velocity do if it finds itself in a room? It creates pressure.
While this happens in theory, it doesn't find it's way into real listening rooms for either type of speaker.
Well, I was asking about sound leakage through the cone that some say is an issue and/or is audible.
Indeed. Put simply, in the case where the box is holding pressure the cone holds against that pressure and there is no such issue other than what can be described in regular resonance terms.
As far as modal behaviour yes, you have sound which is not even with frequency and which is delayed. You damp that in the box so there isn't an issue.
I'm not sure what others are saying is the issue (ok, the backwave but what mechanism?), so I have doubts.
As far as modal behaviour yes, you have sound which is not even with frequency and which is delayed. You damp that in the box so there isn't an issue.
I'm not sure what others are saying is the issue (ok, the backwave but what mechanism?), so I have doubts.
Yes. It was quite clear last time I tried it being about 10% the size of the first peak. I could also see a smaller peak which was the reflection off the magnet
you can see it clearly on this post
https://www.diyaudio.com/community/threads/bms-10c262-coaxial-build.376450/post-6820071
Brian
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Maybe you lack some imagination? I was magically transported to see "The Graduate", even with the worst sound system!
I was Dustin and Mrs. Robinson loved me passionately. 😉
Well of course.
Actually (having a dipole subwoofer in my HT room) it most certainly does *NOT activate the room's lowest mode. (..if you can close a non-sealed door to a room with a subwoofer that isn't causing booming in the room next to it (with very low freq. content at higher spl) - then it very obviously isn't activating the room's lowest mode. Of course having had a sealed sub in the same room does: nauseating people in the next room with the door closed with very low freq. sound. Hell, even the small 6" sub to my sound bar in the adjoining room will boom into the HT room, and its output is very low in spl at very low freq.s.)
The differences are obvious, so I'm really not sure what sort of "real" listening room you have.
*or at least not significantly.
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