badman said:
This seems to be a gross oversimplification and ignores the radiation efficiency noted earlier.
Allow me to make an oversimplified counterpoint: my iPhone, vibrating by itself, makes almost no noise. The wideband drivers in the speakers on the dresser make an awful lot of noise for a similar perceived level of vibration (read: loud). The iPhone has considerably more radiating area.
badman said:
On cross bracing:
planet10 said:
Hi both of You,
Did You do it yourself. I mean, a valid examination of matters using objective probes, example given a microphone? Could You reproduce Your findings on an other object, so that You could generalize them?
Thanks to the AES paper we know now that papers are not that much on a complicated topic. I know an other one, not AES but free (VISATON, Germany, F. Hausdorff ~1980), that supports Earls conclusions.
I just remember a younger guy in Germany who had a quite simple idea. Put a (very) small box into a bigger one. Isolate them mechanically against each other. Let the inner box play and record the sound out of the container. Then relate that to the sound what is measurable from the inner box without container. (You may take a spectrogramm of harmoonic distortion too). May I await Your both personal results on the topic? As far as I remember the result reflected exactly what Earl wrote above.
Without any data and with missing own engagement the debate is totally useless, sorry
That experiment is badly flawed. Amongst other things, it ignores the acoustic loading of the driver by the bigger box. How much bigger? If there's not a lot of airspace, then it's a pretty useless comparison. Better is to mount a driver in a rigid infinite baffle firing outwards from a house/building (and gasketed) to maximize isolation from the front wave. You can then attach a box to the back, measure the vibes from the box, then compare with measures from the driver which is outside.
Again, I don't need an accelerometer to tell me when a box is vibrating. I don't need objective data to prove that the inside of my oven, when on, is hotter than ambient air. We're not talking about unknown effects. We're talking about boxes we know to be vibrating, you're arguing that the effects are irrelevant....
Most of this analysis relies upon thresholds. Enclosure resonances are not instantaneous, they have a decay time. When this decay time overlays the quiet passages in music, there's a lot less driver noise relative to the enclosure, so what may be -40dB with steady-state excitation is now only -25 or -20, or even more. But there are those of us who, instead of bothering with a complicated dynamic input gated measuring setup, just do it the right way.
Steady state signals are NOT music signals and microphones are NOT ears.
Again, I don't need an accelerometer to tell me when a box is vibrating. I don't need objective data to prove that the inside of my oven, when on, is hotter than ambient air. We're not talking about unknown effects. We're talking about boxes we know to be vibrating, you're arguing that the effects are irrelevant....
Most of this analysis relies upon thresholds. Enclosure resonances are not instantaneous, they have a decay time. When this decay time overlays the quiet passages in music, there's a lot less driver noise relative to the enclosure, so what may be -40dB with steady-state excitation is now only -25 or -20, or even more. But there are those of us who, instead of bothering with a complicated dynamic input gated measuring setup, just do it the right way.
Steady state signals are NOT music signals and microphones are NOT ears.
gedlee said:
As i understand your description (here & elsewhere) your bracing scheme is the top-left illustration. A dowel of some cross-section, extending from one side of the enclosure to the opposite. Especially if placed in the centre of the cabinet, this turns the panel into an anular radiator.
dave
badman said:
Ballpark 140hz. Have you measured the frequency at which your cabinet vibrates?
Really the frequency of my iPhone vibration is rather irrelevant as you seem to have ignored the critical point about radiation efficiency. The iPhone example was purposefully poor, as it was intended to be analogous.
Perhaps a reasonable experiment would be to somehow cause your speaker cabinet, or some other hunk of wood, to oscillate at a frequency and amplitude you deem similar to what you observe of the cabinet under listening conditions. Then take a measurement (or just use your ears).
I think the above is pretty different than looking at a driver mounted in an enclosure (with it's backwave isolated and the driver itself design to produce sound) and proclaiming that a equal given level of perceived vibration is going to result in a similar amount of perceived sound from each. Certainly it is erroneous to claim that every object vibrating at any given frequency is producing an equal level of sound at the listener.
badman said:
That's a reasonable position if your stated goal is to observe "energy". My opposition is rooted in the disconnect between observed kinetic energy and sound.
I can't engage in much more than a regurgitation of Linkwitz's work on decoupling drivers w/ regard to your comment about baffle vibrations and impacts on radiated sound from the driver as I have not done anything more than the most basic experiments on my own.
planet10 said:
A comparison of different enclosure materials and strategies. They built a very sturdy 400 l bandpass enclosure (sandwich with sand filling) and closed one side with different test baffles of 40x100 cm.
All SPL diagrams show the measurement of the open enclosure (top line for comparison) and the closed enclosure (bottom line) in the anechoic room at the Visaton factory. All measurements at 50 cm distance from the test baffle.
Bild 3 is for plain 19 mm particle board, Bild 16 for the same particle board with braces as in Bild 17. Note that the SPL response in 16 was only bettered by 2 cm thick marble (Bild 12).
A diagonal brace along the baffle (Bild 13) didn´t help much.
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?"
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?"
"How much less does the large surface of the wall need to vibrate than the cone to make the same amount of noise?"
You can calculate it here.
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".
http://www.baudline.com/erik/bass/xmaxer.html
There is also directivity when the side wall vibrates so it might just "play" for the walls.
You can calculate it here.
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".
http://www.baudline.com/erik/bass/xmaxer.html
There is also directivity when the side wall vibrates so it might just "play" for the walls.
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.
To accurately determine the acoustic energy emitted, it would be beneficial to model the phenomenon in a Physics modeling package (such as Ansys). Although I'm not sure if going to these lengths to mitigate enclosure vibration is as significant as other aspects of loudspeaker design (as Dr. Geddes explained).
To accurately determine the acoustic energy emitted, it would be beneficial to model the phenomenon in a Physics modeling package (such as Ansys). Although I'm not sure if going to these lengths to mitigate enclosure vibration is as significant as other aspects of loudspeaker design (as Dr. Geddes explained).
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.
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.
planet10 said:
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.
Rudolf said:
Thanx for that, i'll have a closer look in a bit. It is too made they used a ms axis in the CSD instead of periods (something i've been hammering a few vendors for), it makes resonance magnitudes VERY hard to interpret, since you have to scale the ridges in your head.
The paper on resonance audibility shows why very clearly.
dave
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