Remember too, that we're talking about full systems. HFs are easier to damp with the driver mounting, and cab stuffing helps keep higher freqs from exciting the walls acoustically. While lower freq stimulii may be able to excite higher freq resonances, I think a well designed stiff cab is my preference.
That's true.
Internal acoustical resonances excite the walls, that's right, but they simply don't exist at low frequencies. Below those resonances, all that matters is mass (not stiffness! in Germany it is called "mass law" or "Bergersches law", I don't know the name in Anglo-American countries).
This is in contradiction to all available data.
In America it is called the "mass law" as well. But unless the Germans have discovered something new or redefined it, it refers to the fact that sound transmission through a panel drops above coincidence as 1/f corresponding to the mass density of the panel. Hence above coincidence the transmission is dependent only on the mass density and below it is governed only by the stiffness. Near coincidence things get very complicated.
To me this equates to a high stiffness to weight ratio material with high internal damping. Just like polyurethane.
isn't that the same as the white packaging material used for electronic devices?
why has nobody used that before?
Well, my preference has no meaningful data associated with it- it stands in isolation.
But I understand that it's not for everybody. It's pretty hard to excite a meaningful resonance on a cab that's got 6" square unbraced areas at max. There may be better, or more efficient, ways, but one CAN "brute force" their way to a neutral cab.
No, we haven't discovered something new, we're not THAT genius.
But I know this law the following way.
- below coincidence, sound transmission drops with 1/f
- below coincidence, the total sound transmission depends on the mass (higher mass, lower transmission)
- at coincidence one has to take the stiffness into account
- above coincidence we're in modal region, with highly variing transmission
But I'll check it when I get hands on the literature (presumably tomorrow at work).
Edit: we should distinguish between sound transmission - a sound wave travelling free through air hits a wall - and the behaviour of the cabinet at very low frequencies (ballooning). The latter one is surely stiffness controlled.
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Have you ever measured the resonances with an accelerometer? I did: http://www.diyaudio.com/forums/multi-way/218981-enclousure-strengthening-needed-2.html#post3151455
Well, what does your chart tell me about the difference between a heavily braced cab and another?
Okay, there's a large amount of HF resonance in your accelerometer data, which is not what I'd expect. That doesn't tell me anything about the efficacy of the various construction methods.
Am I reading it right in that the highest resonance is down about -70dB?
Okay, there's a large amount of HF resonance in your accelerometer data, which is not what I'd expect. That doesn't tell me anything about the efficacy of the various construction methods.
Am I reading it right in that the highest resonance is down about -70dB?
Correct, and completely consistent with what I said. Mass will help the high frequency transmission, but there is little coupling at the HFs and damping is a bigger factor. One needs stiff and light (and damping, of course) to help transmission of the lower frequencies.
I've had reason to study a lot of the architectural tables of various wall constructions. Much of it is enlightening for comparison to cainet issues. Most wall types start out in the mass law region and climb 6 dB per Octave from the lowest frequencies. I think the trial walls are large enogh that the first diaphragmatic resonance is below the measurement range. The intention is to show a wall characteristic largely independent of wall size.
Coincidence effects tend to null TL to near zero (full transparency) unless the two sides of the wall float and have a lot of fiberglass between the layers. Eventually the maximum TL flattens out (this is modeled in the simplified STC curve).
Note that coincidence is not exactly a panel resonance as it is a function of incident angle.
David S
Hi,
The theory is extensively covered in the Fahy books for example
Amazon.com: fahy vibration
I’ll try to explain the best I can my small contribution to acoustical behavior of loudspeakers I did a few years ago.
As an introduction I would say that conclusions are only confirmations of theory that may however go against to audiophile wisdom.
Apparatus:
I use at first a laser interferometer (Polytec) to make cartography of a small bookshelf speaker.
The tight spatial sampling (1500 points) enabled me first to “see actual resonances” up to 3000Hz.
In a second time I to ran a BEM code to calculate acoustical radiation for several frequencies (and combined with driver SPL)
Finally I used BK accelerometers and Matlab driven Stanford analyzer to extend my conclusions over a tower shape speaker.
I also used an Ometron scanner as reference. I have also undergone listening tests.
Insights:
1. Simple vibration measurements cannot account for any precise acoustical results.
There no simple operator to transpose accelerometer out-put into dB SPL.
The geometry of pattern’s deformation is preponderant in is acoustic radiation of each resonance past the first one (some call that balloon mode).
2. Every result is dependent to the case under test. It not trivial to infer result for another case than the one examined.
(i.e. what works in a shoe box might not work on the tower etc.)
Speakers’ behavior, as any structure, is described by global resonances, vibration modes. So treatments focusing only on one side/panel of a speaker are useless.
For example a cross on one panel has no effect both in terms of vibrations and radiation.
Treatments dealing with several panels are far more effective, the idea is to link stiff parts of the enclosure vibrating parts
i.e. a “V shape” brace with its legs on the junction between panel to the center of the opposite panel.
The combination of several effective treatments taken separately often leads to worst results than any one of each treatment used alone.
For example two braces one Front-back + one Left-Right is worst than either a single brace Front – Back or Left – Right, the "cross" enhances the coupling between the high vibrating parts of the panels: bad.
Some treatments can lead to worst results than no treatment at all.
Stiffness and Damping are the main factors. You want something, stiff light and damped (easy…).
If too much damping is added the mass increases, not a good thing.
Constrained layer is best, example thinish ply, 1mm of Wurth mastic + thin sheet of Aluminum and the you bend the wall for a nice shape and much higher stiffness ;-)
The front panel is the key of vibrations. Priority should be to treat it. Mass addition is not the best solution.
Important couplings are: drivers/front panel and front panel/rest of the enclosure.
See the kind of image I obtained with cartography (sorry for the poor quality...)
Also internal cavity acoustic resonances are detrimental and can make the whole cabinet radiate sound.
M
The theory is extensively covered in the Fahy books for example
Amazon.com: fahy vibration
I’ll try to explain the best I can my small contribution to acoustical behavior of loudspeakers I did a few years ago.
As an introduction I would say that conclusions are only confirmations of theory that may however go against to audiophile wisdom.
Apparatus:
I use at first a laser interferometer (Polytec) to make cartography of a small bookshelf speaker.
The tight spatial sampling (1500 points) enabled me first to “see actual resonances” up to 3000Hz.
In a second time I to ran a BEM code to calculate acoustical radiation for several frequencies (and combined with driver SPL)
Finally I used BK accelerometers and Matlab driven Stanford analyzer to extend my conclusions over a tower shape speaker.
I also used an Ometron scanner as reference. I have also undergone listening tests.
Insights:
1. Simple vibration measurements cannot account for any precise acoustical results.
There no simple operator to transpose accelerometer out-put into dB SPL.
The geometry of pattern’s deformation is preponderant in is acoustic radiation of each resonance past the first one (some call that balloon mode).
2. Every result is dependent to the case under test. It not trivial to infer result for another case than the one examined.
(i.e. what works in a shoe box might not work on the tower etc.)
Speakers’ behavior, as any structure, is described by global resonances, vibration modes. So treatments focusing only on one side/panel of a speaker are useless.
For example a cross on one panel has no effect both in terms of vibrations and radiation.
Treatments dealing with several panels are far more effective, the idea is to link stiff parts of the enclosure vibrating parts
i.e. a “V shape” brace with its legs on the junction between panel to the center of the opposite panel.
The combination of several effective treatments taken separately often leads to worst results than any one of each treatment used alone.
For example two braces one Front-back + one Left-Right is worst than either a single brace Front – Back or Left – Right, the "cross" enhances the coupling between the high vibrating parts of the panels: bad.
Some treatments can lead to worst results than no treatment at all.
Stiffness and Damping are the main factors. You want something, stiff light and damped (easy…).
If too much damping is added the mass increases, not a good thing.
Constrained layer is best, example thinish ply, 1mm of Wurth mastic + thin sheet of Aluminum and the you bend the wall for a nice shape and much higher stiffness ;-)
The front panel is the key of vibrations. Priority should be to treat it. Mass addition is not the best solution.
Important couplings are: drivers/front panel and front panel/rest of the enclosure.
See the kind of image I obtained with cartography (sorry for the poor quality...)
Also internal cavity acoustic resonances are detrimental and can make the whole cabinet radiate sound.
M
Dr. Yu Luan from Loudsoft has done work to develop modelling for panel issues
http://www.loudsoft.com/images/artikler/Yu%20Luan's%20PhD%20thesis_v3.pdf
More here Articles
They have also some info about cone resonances etc. LOUDSOFT
http://www.loudsoft.com/images/artikler/Yu%20Luan's%20PhD%20thesis_v3.pdf
More here Articles
They have also some info about cone resonances etc. LOUDSOFT
Have a look at the BBC report to see measurements of an unbraced cabinet. OK, they're not completely comparable, but it gives a good insight in what happens when you brace a cabinet (not that anything is unexpected)
No, the absolute levels of nearfield mic and accelerometer measurements are not comparable. Also the measurements do not say anything about the amount of sound radiation from the side panels.
And be careful about the accelerometer measurements outside the acoustical bands of the drivers (e.g. > 1kHz for the woofer), the normalization may be misleading.
Thinking about it further, of course ballooning is stiffness controlled, but high mass will also help in reducing the bending of the walls.
Hi maiky,
Nice to see some real research. Simulations and modal plotting are great to reveal what is the cause of the output peaks, but what would your criteria of goodness be? Its no different than modal analysis of speaker cones: lots of modes to be found, but which are bothersome?
The acoustical output is the key. If I could kill the direct radiation of a driver and listen to the cabinet output only (some have put two identical cabinets over both sides of a driver), then what construction gives lowest and smoothest output? Are their frequency ranges that are subjectively worse and to be further reduced?
The Harwood study is still the most complete and on point look at that.
David
I am reading all of this technical information and theory but I am still not clear about the answer. Even if it can be measured and calculated the question is can it be heard while the music is playing? Most modern cabinets are mdf. Its pretty solid and dense stuff. Is there any audible benefit in using other materials? We need to put two cabinets side by side, one made of brick (a solid material) and another with unbraced mdf, to decide this. Has anyone actually done it?
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