Frequency and Sound Waves

Matthew Reeve

Frequency is a characteristic of sound that can be measured objectively. Our perception of pitch is closely related to the measurement of frequency. However, our pitch perception is also affected by the intensity of the tone and is less objective. Frequency is measured in hertz (Hz) or cycles per second (cps), 1 Hz = 1 cycle per second. Hertz are named after the German physicist Heinrich Hertz (1957-94) who help to confirm the electromagnetic theory by showing that electrical waves had light-like properties. In the simplest terms, frequency refers to the number of vibrations, cycles or recurrences per unit of time. Human beings can hear sounds from objects vibrating from about 20 Hz to 15,000 Hz. Frequencies above 15,000 Hz are known as ultrasonic frequencies.

Sound Waves

When an object is vibrating it affects on the surrounding air particles. The surrounding particles are forced to compress and then rarefy (expand) in sync with the object. This pattern of compressions and rarefactions cause waves of moving particles to radiate from the object. These longitudinal waves are known as sound waves.
When the sound waves reach the ear the moving air particles bang onto the ear drum. The brain decodes this information as sound. When a sound wave is regular it is said to be periodic. The information contained within sound waves can be plotted on to a graph. The wavelength is the distance a wave travels in the time it takes to complete one cycle.
The amplitude of the sound wave relates to the intensity of the compressions and rarefactions of the air particles. The greater the intensity is, the more energetically the particles move within the sound wave. When the sound waves reach the ear, the brain decodes this information as loudness.

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