Just in case anybody was trying to make sense out of it, I feel obliged to correct one of the statements I made in my post headed "RE: SB Leaks & Dipole Effects -- Was Isolated Air-Movement" of yesterday 2/2/04 at 12:47 p.m. In the middle of the second paragraph of the excerpt from that post below, where it says "We hope either more like the latter or like neither," I meant to say almost the exact opposite, i.e., "We hope more like the FORMER..." Oops. -- Trent "...Going back to the loudspeaker, if we now whack it (or sharply tap it), instead of just pushing it, we'll get a similar puff of air, but if the speaker is relatively undamped (it might be hard to find one undamped enough, but it helps if one of the wires is disconnected) there will also somewhat of a drum sound that sometimes even has a recognizable pitch. Imagine hitting the big gong that's suspended -- the gong swings an inch or a few, but the sound comes from the faster vibrations -- it's the quivering of the gong, not its slow swing back into plumb, that makes the sound. The reason for referring to the ported loudspeaker is that at the quivering frequency, the air going in and out of the port will be delayed so that it is in almost perfect timing to add to the effect of the cone's movement. When the cone goes out, the air goes out, so the acoustic output is reinforced by the "hole" at this point, even though the first puff of air that came out of the port -- corresponding to the swinging of the gong -- represented acoustic canceling. This reversal, phase reversal, is because there is now a reactive path. Now if we excite the cone with a periodic source, say by coupling it to a string, there will be acoustic reinforcement from the hole/port until the frequency of the string used is so low that the port output simply follows the backwave of the cone in phase (non-reactively). Now we don't know yet if at the relevant frequencies the holes act more like the port does at or above its tuning, or more like the way it does below its tuning, as from from a slow push. We hope either more like the latter or like neither. The reactive reinforcing behavior of the port in a speaker happens when the spring of air inside the cabinet aligns through the cross-sectional area of the opening with the mass of the air inside the tube, and we can get a pretty good idea how all this works by just taking a weight and suspending it from a rubber band. Use a weight that's heavy enough or a rubber band that's thin enough so the natural periodic bouncing motion is slow, so that it's easy to observe the timing of it relative to motions of your hand. Hold the end of the rubber band and bob the weight up and down. If you do it very slowly, the weight will go up and down with your hand. As you increase the speed, you'll reach a frequency where the weight will be moving up and down opposite to the motion of your hand. At this frequency, the resonant frequency of the band/weight system, it will take only very small motions of your hand to keep the weight bobbing quite a bit. This corresponds to the tuning of a ported speaker, and represents the lower limit of substantial bass response (and just like your hand, the excursions of the cone are controlled by the opposing forces of reactance of the system, reducing distortion or preventing the speaker from mechanically bottoming out.) Connecting to some other current discussions about soundboard stiffness and mass etc., there is now also a very high impedance relationship of the weight/spring to your hand, and energy is reflected back to your hand and thus dissipated very slowly. If your hand was a string it would have long sustain at this frequency. Now continue to move your hand up and down faster, and the motion of the weight will diminish, and will continue to diminish as you increase the frequency. So at higher frequencies there is neither constructive nor destructive interference coming out of the port; the higher frequencies "see" the port as a sealed hole, and speaker behaves as it would in a sealed box in these frequencies. And if we re-did this whole experiment with the weight in thick oil to add substantial resistance to the system (or by adding big air-brakes made out of cardboard to the weight), we'd have some idea of the resistive or friction-loss component. You'd see that resonant motion is attenuated -- but also that losses from destructive interference below resonance would be substantially reduced. So now we have a couple of common-sense analogies to all three components of the soundboard hole's behavior -- 1) acoustic short circuit; 2) friction losses; and 3) reactive (mass and/or spring)..." EVEN MORE IMPORTANT NOTICE: Never mind the message below. I use this e-mail account for personal e-mail even when I'm not at work, but this message is intended for law-client-related communications, and the computer people at the law firm where I work seem to have made it impossible to keep it from being added to one's e-mails, otherwise I would turn it off and it would not appear below. For purposes of this message it may be considered as meaningless, random clutter. Piano tuners, technicians, rebuilders, makers, designers, scientists, craftsmen, artisans, musicians, engineers, enthusiasts and friends need not trouble themselves. Despite what the notice says, you are hereby notified of nothing (except that you are hereby notified of nothing). ****** IMPORTANT NOTICE ****** This e-mail, and any attachments hereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail, you are hereby notified that any dissemination, distribution or copying of this e-mail, and any attachments hereto, is strictly prohibited. If you have received this e-mail in error, please immediately notify me at (312) 207-1000 and permanently delete the original and any copy of any e-mail and any printout thereof.
This PTG archive page provided courtesy of Moy Piano Service, LLC