Waves - Sound

A compression is where the particles of the medium are pushed closer together by the wave and rarefactions are the opposite. The vibrations occur in the same direction as the wave is traveling. As the wave travels through the medium, the particles are displaced, but after the wave has passed, they are in the same position as they started. It is the energy carried by the wave that moves, not the particles. Sound is therefore a mechanical wave and cannot travel through a vacuum. This is why we can see the Sun but we can't hear it and is also partly why double and triple glazing in homes and offices reduces the sound coming from outside.

Sound waves have a frequency, wavelength, amplitude and period just like all waves. The frequency is the number of compressions (or rarefactions) that pass a given point in one second, the SI units are hertz (Hz). The wavelength is the distance from one point of a compression (or rarefaction) to the equivalent point on the next. The SI units are meters (m). The amplitude is really a measure of how much energy the wave is carrying, in other words its loudness. The period is the reciprocal of the frequency. Louder sounds carry more energy than quiet ones. There is another important term that you need to know about sound waves - pitch. This is how high or how low the note sounds and is determined by the frequency. A high-pitched note has a high frequency and vise versa. Sound whose pitch is so high that it cannot be heard by humans (above about 20 kHz) is called ultrasound and has several uses such as scanning developing babies in the womb and breaking up kidney stones. Sound that is too low-pitched to hear (below about 20 Hz) is called infrasound and is used for studying the heart and the structure of the Earth.

Sound waves can be reflected and diffracted quite easily. When sound is reflected, we call it an echo. One of the favorite echo questions in senior high school is to get you to calculate the distance from an object using data about an echo. What you must remember is that to hear an echo, the sound has traveled there and back. The biggest mistake that students make is not to divide the distance by two - but do pay attention to the wording of the question as sometimes you may be asked to calculate the distance the sound has traveled, there and back. The speed of sound in air is normally about 330 m/s but is much faster in denser media like water and solids.

Diffraction is one reason why you can hear sound from round a corner. When any wave touches an object that is about the same size at its wavelength, or passes through a gap that is about as wide as its wavelength, it diffracts - spreads out in a circular pattern from the edge of the object.