L3 - Characteristics of Sound Waves

objectives

Sound waves are caused by vibrations - for example, the rapid, backwards- and-forwards oscillations of a loudspeaker cone.
The number of oscillations per second is called the frequency. It is measured in hertz (Hz). If a loudspeaker cone has a frequency of 100 Hz, it is oscillating 100 times per second and giving out 100 sound waves per second.
Different frequencies sound different to the ear. You hear high frequencies as high notes: musicians say that they have a high pitch.
You hear low frequencies as low notes: they have a low pitch.
The human ear can detect frequencies ranging from about 20 Hz up to 20 000 Hz, although the ability to hear high frequencies decreases with age.

Octaves* Musical scales are based on these. If the pitch of a note increases by one octave, the frequency doubles, as shown on the keyboard above. This keyboard is tuned to scientific pitch.
Bands and orchestras normally use frequencies that differ slightly from those shown.
The diagrams below show what happens if two steady notes, an octave apart, are picked up by a microphone and displayed on the screen of an oscilloscope. As the higher note has double the frequency of the lower note, the peaks occur twice as often and are only half as far apart.

The waveform on each screen is a graph showing how the air pressure varies with time as the sound waves enter the microphone. The horizontal line is the time axis.

The sounds displayed on the oscilloscope screens above have the same frequency, but one is loader than the other.
The oscillations in the air are bigger and the amplitude of the waveform is greater.
Sound waves carry energy. Doubling the amplitude means that four times as much energy is delivered per second.

Middle C on a guitar' does not sound quite the same as middle C on a piano, and its waveform looks different. The two sounds have a different quality or timbre. Each sound has a strong fundamental frequency, giving middle C. But other weaker frequencies are mixed in as well, as shown on the right. These are called overtones, and they differ from one instrument to another.
With a synthesizer, you can select which frequencies you mix together, and produce the sound of a guitar, piano, or any other instrument.

6.05---Characteristics-of-Sound-Waves.docx

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