Chapter One: An Acoustics Primer
5. What is frequency? | page 4
One particularly interesting frequency phenomenon is the Doppler effect or Doppler shift. You've no doubt seen movies where a police siren or train whistle seems to drop in pitch as it passes the listener. In actuality, the wavelength of sound waves from a moving source is compressed ahead of the source and expanded behind the source, creating a sensation of a higher and then lower frequency than is actually being produced by the source. This is the same phenomenon used by astronomers with light wavelengths to calculate the speed and distance of a receding star. The light wavelengths as stars move away are shifted toward the red end of the spectrum, hence the term red shift.
|The formula for an approaching sound source is:||The formula for a receding sound source is:|
fobserved=frequency we hear, fsource=frequency of source,
v=speed of sound, vsource=speed of approaching or receding sound source
|Example: At 20°C the speed of sound (v) is 343.7 m/s. An oboist in a convertible traveling at 29 m/s (vsource ) or 65 mi/hr is tuning to A440. As the convertible approaches your position, you hear 480 Hz and as the car passes and moves away, you hear 405 Hz.|
It is particularly important for musicians to have their hearing tested regularly, where audiograms may depict both the maximum frequency heard and any loss of sensitivity in certain frequency ranges. Many audiologists will only test up to 8kHz, because that is considered the high end of what is necessary for speech perception. Composers should insist on tests for the full hearing spectrum if possible.