# What is sound

Sound is a small change in air pressure and can be generated by a vibrating object for instance. The speed with which the acoustic waves move through the air is 343 meter per second. An acoustic wave has a wavelength and an amplitude. The wavelength has a direct relation to the frequency. We hear the difference between frequencies by the pitch. The frequency is the number of waves that are measured per second. The frequency is expressed in Hertz (Hz). The amplitude is the maximum of the pressure and is perceived as the volume. This is expressed in Decibel (dB).

A human being hears sounds between 20 and 20,000 Hz. But as we get older this gradually reduces to approx. 15,000 Hz. The human ear is most sensitive to sound with a frequency between 1 and 4 KHz. A volume of about 50 dB corresponds to a normal conversation. Upwards of 85 dB can cause damage to your hearing.

The dB-scale is not linear but logarithmic. This means that a decrease in sound by 10 dB is experienced as a halving in volume. This also means that two identical sources of sound of 50 dB each, together reach a sound level of 53 dB.

A sound we hear usually consists of a spectrum of frequencies and volumes. Because the human ear is more sensitive to certain frequencies, the combination of frequency and volume determines the harmfulness. Sound measurements usually take this into consideration by counting certain frequencies less heavily in the measured level. This is generally done by using the so-called A-filter. The result is then expressed in dB(A).