Sound is the sensation produced when longitudinal vibrations of the molecules in the external environment—that is, alter natephases of condensation and rarefaction of the molecules— strike the tympanic membrane. A plot of these movements as changes in pressure on the tympanic membrane per unit of time is a series of waves (Figure 1); such movements in the environment are generally called sound waves. The waves travel through air at a speed of approximately 344 m/s (770 mph) at 20°C at sea level. The speed of sound increases with temperature and with altitude. Other media can also con duct sound waves, but at a different speed. For example, the speed of sound is 1450 m/s at 20°C in fresh water and is even greater in salt water. It is said that the whistle of the blue whale is as loud as 188 dB and is audible for 500 miles.
Fig1. Characteristics of sound waves. A is the record of a pure tone. B has a greater amplitude and is louder than A. C has the same amplitude as A but a greater frequency, and its pitch is higher. D is a complex wave form that is regularly repeated. Such patterns are perceived as musical sounds, whereas waves like that shown in E, which have no regular pattern, are perceived as noise.
In general, the loudness of a sound is directly correlated with the amplitude of a sound wave. The pitch of a sound is directly correlated with the frequency (number of waves per unit of time) of the sound wave. Sound waves that have repeating patterns, even though the individual waves are com plex, are perceived as musical sounds; aperiodic nonrepeating vibrations cause a sensation of noise. Most musical sounds are made up of a wave with a primary frequency that determines the pitch of the sound plus a number of harmonic vibrations (overtones) that give the sound its characteristic timbre (quality). Variations in timbre permit us to identify the sounds of the various musical instruments even though they are playing notes of the same pitch.
Although the pitch of a sound depends primarily on the frequency of the sound wave, loudness also plays a part; low tones (below 500 Hz) seem lower and high tones (above 4000 Hz) seem higher as their loudness increases. Duration also affects pitch to a minor degree. The pitch of a tone cannot be perceived unless it lasts for more than 0.01 s, and with durations between 0.01 and 0.1 s, pitch rises as duration increases. Finally, the pitch of complex sounds that include harmonics of a given frequency is still perceived even when the primary frequency (missing fundamental) is absent.
The amplitude of a sound wave can be expressed in terms of the maximum pressure change at the eardrum, but a relative scale is more convenient. The decibel scale is such a scale. The intensity of a sound in bels is the logarithm of the ratio of the intensity of that sound and a standard sound. A decibel (dB) is 0.1 bel. The standard sound reference level adopted by the Acoustical Society of America corresponds to 0 dB at a pressure level of 0.000204 × dyne/cm2, a value that is just at the auditory threshold for the average human. A value of 0 dB does not mean the absence of sound but a sound level of an intensity equal to that of the standard. The 0–140 dB range from threshold pressure to a pressure that is potentially damaging to the organ of Corti actually represents a 107 (10 million)-fold variation in sound pressure. Put another way, atmospheric pressure at sea level is 15 lb/in2 or 1 bar, and the range from the threshold of hearing to potential damage to the cochlea is 0.0002–2000 μbar.
A range of 120–160 dB (eg, firearms, jackhammer, and jet plane on takeoff) is classified as painful; 90–110 dB (eg, subway, bass drum, chain saw, and lawn mower) is classified as extremely high; 60–80 dB (eg, alarm clock, busy traffic, dish washer, and conversation) is classified as very loud; 40–50 dB (eg, moderate rainfall and normal room noise) is moderate; and 30 dB (eg, whisper and library) is faint. Prolonged or frequent exposure to sounds greater than 85 dB can cause hearing loss.
The sound frequencies audible to humans range from about 20 to a maximum of 20,000 cycles per second (cps, Hz). In bats and dogs, much higher frequencies are audible. The threshold of the human ear varies with the pitch of the sound (Figure2), the greatest sensitivity being in the 1000- to 4000-Hz range. The pitch of the average male voice in con versation is about 120 Hz and that of the average female voice about 250 Hz. The number of pitches that can be distinguished by an average individual is about 2000, but trained musicians can improve on this figure considerably. Pitch discrimination is best in the 1000- to 3000-Hz range and is poor at high and low pitches.
Fig2. Human audibility curve. The middle curve is that obtained by audiometry under the usual conditions. The lower curve is that obtained under ideal conditions. At about 140 dB (top curve), sounds are felt as well as heard.
The presence of one sound decreases an individual’s ability to hear other sounds, a phenomenon known as masking. It is believed to be due to the relative or absolute refractoriness of previously stimulated auditory receptors and nerve fibers to other stimuli. The degree to which a given tone masks others is related to its pitch. The masking effect of the background noise in all but the most carefully soundproofed environments raises the auditory threshold by a definite and measurable amount.
