Frequency of Sound
Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically (it kind of vibrates). The number of pressure variations per second is called the frequency of sound, and it is measured in Hertz (Hz) which is defined as the number of cycles per second.
Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of a sound wave; (B) compressions and rarefactions that constitute a sound wave; (C) transverse representation of the wave, showing amplitude (A) and wavelength (λ). Source: Encyclopaedia Britannica.
The higher the frequency, the more high-pitched a sound is perceived. Sounds produced by drums have much lower frequencies than those produced by a whistle.
The unit of frequency is the Hertz (Hz). For a sound vibration to be audible to human beings the object must vibrate between 20 and 20,000 times per second. In other words the audible sound has a frequency of between 20 and 20,000 Hz.
High-pitched sounds (treble) have a frequency much greater than bass sounds. The treble frequency ranges between 2,000 and 4,000 Hz while the bass range from 125 to 250 Hz.
Above: measure of loudness (wave height). The higher the louder.
Below: measure of frequency (wave length). Bass sound has long waves, treble has short waves.
Frequency and loudness are interrelated in the human ear. The range of 20 Hz to 20,000 Hz is called the audible frequency range – we know this already. But the sounds we hear are a mixture of various frequencies, and we don’t perceive all of them with the same clarity.
The entire audible frequency range can be divided into 8 or 24 frequency bands known as octave bands or 1/3 octave bands respectively for analysis. An octave band is the band of frequencies in which the upper limit of the band is twice the frequency of the lower limit. Any particular sound or noise can be represented as a number of 8 (or 24) sound pressure levels in the frequency bands, as illustrated by the diagram below.
A real sound shown as a combination of different sound presure levels, one per each of the 24 frequency bands. Column width: 1/3 octave band (24 in total). Column height: SLP at each frequency band, measured in dB.
The response of the human ear to sound is dependent on the frequency of the sound. The human ear has its peak response around 2,500 to 3,000 Hz and has a relatively low response at low frequencies. Hence, the single sound pressure level obtained by simply adding the contribution from all 1/3 octave bands together will not correlate well with the non-linear frequency response of the human ear.
This has led to the concept of weighting scales. The following diagram shows the “A-weighting” scale:
Reduction of SPL (in dB) at frequencies below and above 2000 to 3000 Hz to reflect the frequency response of the human ear.
In the “A-weighting” scale, the sound pressure levels for the lower frequency bands and high frequency bands are reduced by certain amounts before they are being combined together to give one single sound pressure level value. This value is designated as dB(A). The dB(A) is often used as it reflects more accurately the frequency response of the human ear.
Other, less used weighting scales, are dB(B) and dB(C). The decibel C filter is practically linear over several octaves and is suitable for subjective measurements at very high sound pressure levels. The decibel B filter is between C and A. The three filters are compared below: