What is Sound?

Sound is produced by vibrating objects. The object pushes and pulls the air near it creating pressure waves. If these waves enter our ears the make our ear drums vibrate and we can hear the sound. Sound waves cannot travel through a vacuum because there are no air molecules to vibrate and carry the wave.

Human ears can detect a range of frequencies between 20 Hz and 20 kHz. Anything above this is called ultrasound.

Pitch and Loudness

The frequency of a wave is how many complete waves pass a point every second. It is measured in Hertz (Hz)

We perceive the frequency of sound as its pitch. A high pitched note is a sound wave with a high frequency, e.g. from a whistle. A low pitched note has a low frequency, e.g. from a bass guitar.

The amplitude of a wave is how far the vibrating particles move. A loud sound comes from a wave with a large amplitude. The wave has lots of energy.

We can investigate and compare the frequencies and amplitudes of sound waves by producing a trace using a microphone and an oscilloscope.

Waveforms

We can also investigate the waveform of a sound. Most sounds are a mixture of several, sometimes many different frequencies. This mixture gives a sound a particular tone. This is why we can tell the difference between musical instruments even when they playing exactly the same note.

Reflection and Refraction of Sound

Just like light waves sound waves can reflect and refract.

An echo is sound reflecting off a surface. Some animals like bats use this to hunt their prey.

Have you noticed that some sounds seem louder at night? Waves that head into the warmer air higher up are partly refracted downwards. During the day the air higher up is colder and the sound waves are refracted upwards so the same sound would be quieter to the listener in the diagram above.

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Revision Questions

1. What must an object do to produce sound waves?

2. Why can't sound travel through a vacuum?

3. What is meant by the frequency and the amplitude of a sound wave?

4. Sketch the trace that would be produced by a) a wave with a high frequency but small amplitude and b) a wave with a low frequency and big amplitude

5. A flute and a violin play the same note. Explain why you can tell them apart.

6. Find another piece of evidence that sound can refract.