always obstacles. Nevertheless the law can be an useful approxima- may be found which uses the dyne/cm2. tion in circumstances where, for example, the source is a small The relationship between these is: 1Pa = 1N/m2 = 10 dyne/cm2. loudspeaker and obstacles and reflecting surfaces are relatively Other units, used mostly for static pressures, as in meteorology for distant. The wavefronts then approximate to concentric spheres and example, are the bar and torr. 1 bar = 105 Pa and 1 torr = 133.322Pa. are known as spherical waves. (Circumstances with spherical waves Pressure on its own is a very useful unit but it gives no indication where reflecting surfaces and other obstacles are sufficiently far of the available power or energy in a sound wave. For this the term away for their effect to be negligible are often referred to as near intensity is used, the basic unit for which is the Watt/metre2 2 How- field conditions.) Particle Velocity Particle Displacement Pressure Figure 8.1 Sound wave parameters 8/4 Sound, speech and hearing It is important to note that intensity is proportional to (pressure)², For sound waves, the frequency range which can be detected by SO that, denoting pressure by P, Equation 8.1 may be rewritten as in the normal human ear is approximately 16Hz to 16kHz, and Table Equation 8.2. 8.2 shows some values of wavelength in this frequency range. (8.2) Table 8.2 Examples of wavelength and frequency within range of the human ear 8.1.2 The velocity of sound Frequency (Hz) Wavelength (m) Sound velocity (c) depends upon the medium and also, in the case 21.25 of gases, upon the temperature (but not, within normal limits, on pressure). Some typical values for care given in Table 8.1. The large value of c for helium accounts for the raised pitch of speech when an oxygen/helium mixture is breathed. Table 8.1 Examples of sound velocity Substance m/s (approx.) 10000 Water 16000 Concrete Steel 5000 6000 8.1.4 The Doppler effect 3000 4000 This is the apparent change of frequency of a received sound when Helium g