Basically Speaking --Understanding sound and sound reproduction.
Audio concepts and terms explained, by Michael Riggs
Sound and Sound Reproduction
IF A TREE CRASHES in a forest with no one there to hear it, does it make a sound? This little conundrum, familiar to most, goes to the heart of sound reproduction. Although apparently unanswerable or subject to endless dispute, it actually has two fully satisfactory solutions, each of which depends on the interpretation of the word "sound."
The more common answer says that sound is what we hear: It is a form of perception and, therefore, a mental process. In that interpretation, sound doesn't exist if there's no one there to hear it. The other alternative is the scientific view, which says that sound is air in motion-it's a physical event that causes the sensation of hearing. By this second definition, sound can exist whether or not anyone actually hears it.
The purpose of sound re production, whether by a simple radio or phonograph or by an elaborate stereo system, is to give you a sensation similar to the one you might have experienced had you been present where and when the broadcast or recording originated. To do this, it is necessary to re-create the physical cause of that sensation. And to do that, it is necessary to create a very precise pattern of vibrations in the air.
These vibrations are called sound waves, and their characteristics are similar in many ways to those of other kinds of waves encountered in nature. Waves in water are perhaps the most familiar example. If you drop a pebble into a still pond, ripples expand outward concentrically from the point at which the stone hits the water. If you look closely, you will see that these ripples form a regular pattern of peaks and troughs perpendicular to their direction of travel: they are called transverse waves.
Sound waves differ from transverse waves in one important respect: Their peaks and troughs are in line with the direction of travel. They are therefore called longitudinal waves. As with waves in water, however, sound waves are formed by the displacement of some volume of matter-in this case, by the alternate compression and rarefaction of the air in the vicinity of the sound source, like the pleats of a bellows as it is squeezed closed and pulled open again.
With a very sensitive barometer, you could translate sound waves directly into local air-pressure readings. In effect, this is essentially what the ear does.
From the ear's "readings," the brain extracts two vital pieces of information about the sound: its loudness and its pitch. A sound's loudness is determined by the amplitude of the changes in air pressure (analogous to the height of a wave in water). The larger the variation in pressure, the louder the sound.
This correspondence is not a simple arithmetical one, however. Doubling the physical amplitude of a sound wave does not double its perceived loudness: the wave's actual amplitude would have to increase by a factor of ten to produce a sound that's roughly twice the level of the original, and by a factor of one hundred (ten times ten) to sound four times as loud (two times two). This characteristic of the human hearing mechanism, however peculiar it may seem at first glance, is its saving grace. Without it, we would either not be able to hear soft sounds at all, or loud sounds would be unbearable. As it is, the ratio of the amplitude of the loudest sound we can hear without pain to the softest sound we can hear at all (i.e., the dynamic range of our ears) is greater than a million to one on a linear scale.
But as we've already noted, a linear scale is next to useless for conveying any sense of subjective loudness. For this reason, a scale based on powers of ten has been developed. This logarithmic scale takes as its fundamental unit of measure the smallest difference in level that can be distinguished as a change in loudness. It is called a decibel (dB). Every 10-dB increase in the amplitude of an acoustic wave is equivalent to a doubling of subjective loudness. Hence, 20 dB is twice as loud as 10 dB, 30 dB is twice as loud as 20 dB (and four times as loud as 10 dB). But measured linearly, the wave is ten times larger at 20 dB than at 10 dB, and ten times larger at 30 dB than at 20 dB--so it is a hundred times (ten times ten) larger at 30 dB than at 10 dB.
This scheme can also be used to approximate the increase in subjective loudness as the amplitude of a sound wave is progressively doubled. Each such doubling results in a 3-dB increase in loudness. This means that increasing the amplitude by a factor of four (twice two) will result in a 6-dB increase, that increasing it by a factor of eight (twice four) will yield 9 dB more loudness, a factor of sixteen (twice eight) 12 dB more, and so on.
So much for loudness and its measurement. What about pitch? Perceptually, pitch is how high or low a sound is: it's what distinguishes a squeak from a rumble. And it, too, is related to a certain physical property of waves-namely, the number of complete waves that pass a fixed point in the line of travel in a fixed period of time.
This is the frequency of the vibration and is expressed in Hertz (Hz), or--as it used to be called--cycles per second (cps).
A single wave, or cycle, constitutes the interval from the base level (e.g., the level of undisturbed water or atmospheric pressure), through a peak and a trough, and back to neutral. Because the rate at which a wave travels is fixed by the medium in which it moves-the speed of sound in air at sea level is, for example, about 750 miles per hour--the only way to alter its frequency is to com press it or stretch it, so that there are more or fewer cycles occupying the same space. In short, for the frequency to change, the length of the wave must change, and the two are mathematically related. This means that pitch can be specified either in terms of frequency (with higher pitches at higher frequencies) or in terms of wavelength (with higher pitches corresponding to shorter wavelengths) and that if you know one, you can always figure out the other.
-HF
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Also see:
1982 Speaker Designs: Closer to Perfection? by Michael Riggs and Peter Dobbin-- A look at 100 new speakers; plus, how four speaker designers view their work.
Two Digital Mahler Tenths Reviewed by Derrick Henry-- Deryck Cooke's final completion of what may be Mahler's greatest symphony