Audio Basics: A Is For Ampere Page 9
The frequency of the object's natural vibration rate is called its resonant frequency, and when a body with resonant properties is struck or otherwise set into vibration, the regular series of soundwaves from it gives the sound a recognizable pitch.
Resonances are the basis of pitch in musical instruments, either the resonance of a taut, stretched string or diaphragm or of a column of air. It is also the source of most loudspeaker colorations, because as long as anything is bent out of shape, it is storing energy whose release (when the object unbends) helps to sustain the resonance. Consider a tuning fork (fig.18). This is a piece of steel shaped like a long U, which produces an almost perfect sinewave when tapped on a tabletop or knee to start it ringing. When one of its tines is struck, it bends inward, while inertia causes the other tine to bend inward too. Because they are now flexed and under spring tension, energy is stored in them. Some is released when they spring back to their original positions, but the rest is now in the form of inertial energy, which causes both tines to overshoot their at-rest positions and become tensioned in the other direction. They continue to vibrate---at a frequency determined by the weight and stiffness of the tines---until all the stored energy has been converted into motion, which is why resonances can be such a problem in audio transducers. Not only do they cause frequency-response peaks, they also smear the sound because the resonances continue to "speak" for a period of time after the signal which started them has passed.
Resonances can be suppressed, or damped, by adding friction to the moving system. Damping works like automobile brakes, slowing the motion and dissipating the stored energy in the form of heat. But there are times when a little resonance is a good thing, as when a woofer's surface area is too small to move enough air at low frequencies. Allowing it to resonate a bit can give the system a little more body, although at the expense of some low-frequency detail. (There will be some smearing, which is called hangover in a woofer.)
Soundwaves are also dissipated by absorption, when they penetrate a porous surface. Again it is friction that does the trick, but the effect is very much frequency-dependent. If the waves are longer (farther apart) than the thickness of the porous material, there is little absorption. At very high frequencies, all non-polished surfaces will absorb some acoustical energy.
Soundwaves which are not absorbed are reflected from a large surface much the way light is: at an angle equal to its angle of incidence. But if the reflecting surface is smaller than the sound's wavelength, the soundwaves will tend to flow around the surface rather than being reflected. This is called diffraction.
The ear is a transducer that converts soundwaves into nerve impulses (fig.19). The eardrum is coupled to a snail-shell-shaped nerve-lined chamber (the cochlea) through three tiny bones called ossicles, the largest of which is a mere 1/4" long. These act as levers to convert the feeble but relatively large eardrum motions into stronger but smaller motions to drive the fluid in the cochlea. (Muscles attached to these bones inhibit their motion when sounds are very loud, to protect the ears, but they tire rapidly, which is one reason why prolonged exposure to loud sounds causes permanent ear damage.)