Measuring sound

Sound is a pressure wave caused when something vibrates, making particles bump into each other and then apart. The particles vibrate back and forth in the direction that the wave travels but do not get carried along with the wave.

When you clap your hands, you force air particles together and then apart. This effect ripples out and away from your hands as a small group of sound waves. The particles close to your hands are pushed outwards and bump into neighbouring particles, and these then move and bump into more particles. The effect is very much like dropping a stone into a pool of water and causing a ripple pattern (sound waves) extending outwards from the original source (your clapping hands).

Similar to water ripples, pressure waves move outwards from the sound source. These changes in particle spacing are also changes in pressure. Pressure increases when particles are squeezed together and reduces when they move apart. It is these changes in pressure that can be detected by organs such as the human ear and are sensed as sound.

We describe the sounds that we hear using several different terms and measure them in different ways.

Volume

Volume (also called loudness) relates to the maximum pressure produced as particles are squeezed together as they are made to vibrate. This is also related to the maximum distance particles are moved from their normal position as they vibrate, much like how tall the ripples are in the pool mentioned before. When you show sound waves on a graph, the amplitude is the height of the waves from their middle position and reflects how loud the waves are.

Loudness of sound is measured in decibels (dB). This is actually a measure of intensity, which relates to how much energy the pressure wave has. Decibels are a relative measurement. They relate the intensity of a pressure wave to a normal or standard pressure.

For the human ear in air, the quietest noises we hear are around 10dB whereas sounds of 130dB are considered painful.

Graphs of sound waves

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Sound has both volume and pitch. Volume is seen as an increase in amplitude of the sound wave. Pitch is seen as a change in the frequency of the sound wave.

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Water is much more dense than air, so the standard pressure is different. This means that you cannot directly convert decibel levels from air to water. (To convert from a decibel reading in air to a decibel reading in water, you should add 61.5dB.)

Pitch

Pitch relates to the frequency, or how many times a second the particles vibrate. The distance between one wave and the next gives the wavelength. For sounds all travelling at the same speed, high-frequency (high-pitched) sounds have waves very close together. Low-frequency sounds have a greater distance between each wave. An extreme example is the low-pitch calls made by humpback whales, which can have up to 100 metres between the pressure peaks of their sound waves.

Frequency

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This graph shows different frequencies of sound waves. As the frequency increases, the distance between the peaks of the sine wave graph decrease.

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Frequency is measured in hertz (Hz). For sound, this means the number of pressure waves per second that would move past a fixed point. It is also the same as the number of vibrations per second the particles are making as they transmit the sound.

A sound of 10Hz means that 10 waves would pass a fixed point in 1 second. (Sound travels at a speed of 343 metres per second in air or 1,484 metres per second in water.) Humans can normally hear sounds between 20Hz and 20,000Hz (20kHz).

Noise

Noise is a very subjective term. It can refer to any unwanted sound but is more correctly used to describe sound that isn’t rhythmic or pure.

When the sound waves form a single sine-shaped wave on a graph, we hear the sound as a pure note. Tuning forks produce a pure sound, one note (a single frequency) and a very smooth line on a graph. When we combine pure notes, we can create harmonics. Harmonics are the basis of all musical instruments and result from overlaying pure notes.

Noise is produced when the notes aren’t pure. The trace on the graph is bumpy and random. Our ears detect this as a less pleasant sensation and often try to screen it out. In terms of listening under water, what we mainly hear is noise – a jumbled mess of sounds with no repeating pattern or clear pure notes.