hearing 1

hearing 2

sound intensity






Sound as we usually experience it is a low energy phenomenon. We can assess how low intensity the energy is by placing a hand in front of the mouth as we shout. We can barely detect any change. A change in pressure of only one in a million (equivalent to pressing on a balloon gently with a single finger) is detectable by the ear. A simple experiment can be done to test which frequencies can be heard by humans using the sound output from sound editor software. The school science of sound is well represented on this site. This page represents sound intensity very well and has a nice table of sounds and their intensities.

The intensity of a sound depends on the energy emitted each second and the area over which the sound is spread. For a sound spreading out equally in all directions rom a point source, at any distance from the energy source the energy wil be spread over the surface area of a sphere. A sphere surface area is 4 pi r 2 and so the sound intensity will vary as 1 / r 2 : i.e. an inverse square law. Design an experiment to test this. The least intense sound humans can hear is 10 -12 W m -2

This is a sound which will displace particles by one hundred billionth of a metre. This is called the Threshold of Audibility. Sounds are perceived by humans relative to those already present and so a relative scale of Sound Intensity Level (SIL) is useful. A natural log scale is used to cover the vast range of audible intensities. SIL in decibels (dB) is 10 * ln( (sound intensity) / (threshold of audibilty)). Bels are not used because decibel (dB) is a more appropriate size for comparing audible sounds.

Dog whistles demonstrate that dogs can hear higher frequencies than humans. The sound of a dog whistle can be captured with sound editor software to confirm this. Other species utilise sound in various ways. Bats whales and dolphins are noteable examples of biosonar in action. Medical uses of ultrasound are well known especially in embryology and obstetrics.