Sounds introduction

Imagine you're working at your desk upstairs. You become distracted by a news programme from a downstairs radio: starting to pay attention you realise that you can clearly hear every single word.

Not surprising in some ways… but then again there's something rather fundamental to notice here. The sound starts from the radio's speaker downstairs and reaches you at the top of the house. The sound travels up the stairs, back along the landing and then up along another set of stairs to your lair. Not only could you hear the news up-stairs and down-stairs, but you could probably hear it outside as well.

In day-to-day living we are surrounded by all kinds of different sounds. Just stop and listen. The lived-in world is awash with sounds: noise; music; speech; birdsong. Some of these, such as the noise from traffic or the sound of people talking or the radio playing are just there and we often don't even notice them. Of course, sounds don't just exist as a kind of backdrop to our lives. They also provide us with information. For example, you quickly learn to recognise, by the sounds, when bacon is cooking too quickly, when the kettle is coming to the boil or when the car is misfiring.

The human ear

The way in which humans hear is rather complicated, and explaining it fully involves aspects of physiology, psychology and acoustics. A simpler description concentrates on the action of the ear as it converts sounds to a sequence of nerve impulses which are transmitted to the brain. The ear's ability to do this allows you to detect, all at once, a range of sounds, from loud to soft and from a high-pitched squeak to a low-pitched growl.

The ear consists of three parts: the outer ear, the middle ear and the inner ear. Each part serves a specific purpose in the task of detecting and interpreting sound.

The outer ear collects and channels sound to the middle ear. The middle ear transfers the to and fro motion of the air (which is the sound) into matching vibrations of the bone structure of the middle ear. These vibrations are transmitted into a to and fro vibration in the fluid of the inner ear. This vibration is changed into nerve impulses, which are then transmitted to the brain as electrical signals.

Teacher Tip: The board game Mouse Trap provides a good basis for imagining how the ear works.

Range of sounds

The range of sounds that humans are able to hear is impressively large, taking us from the loud roar of an aircraft as it makes its final approach on landing to the quiet murmur of the wind as it passes through the trees. Alternatively we might think in terms of moving from the low pitched rumble of distant traffic to the high-pitched screech of the dentist's drill.

We can consider these two ranges of our hearing as forming two sides of a box stretching:

• From very quiet to extreme loudness.
• From a low-pitched rumble to a high-pitched squeak.

Hearing different notes

People vary in their ability to detect the pitch of a sound. For example, in tuning a guitar, some are able to distinguish easily between two notes of similar pitch while others can't hear the difference. A more objective measure than pitch is frequency, measured in hertz (Hz).

The frequency of any vibration is defined as the number of complete vibrations made each second: the frequency (hertz) is equal to the number of vibrations per second.

Sometimes a complete vibration is referred to as a cycle: the frequency (hertz) is equal to the number of cycles per second.

You can picture one vibration or cycle of the speaker cone being completed as it starts from its most forward position, moves backwards, and then moves forwards to return to its original position.

The frequency value not only specifies the number of complete vibrations made by the source each second, but also the number of regions of high (or low) density produced in the medium each second. In other words, it gives the frequency of the actual sound.

High pitch corresponds to high frequency, low pitch to low frequency. There is a whole spectrum of sounds arranged along the frequency axis.

More on frequency

Quite often the numbers get large, and the frequency is referred to in terms of kilohertz (kHz): 1000 hertz is the same as 1 kilohertz.

For example, a hi-fi loudspeaker might have a range of 20 hertz to 20 kilohertz and so be capable of producing sounds over our whole range of hearing.

In 1939 musicians moved to an agreement on standard pitch. Middle C (called that because it is a note around the middle of the piano keyboard) is 256 Hz and concert A (the note the orchestra tunes to before the show begins) is 440 Hz.

The frequency doubles for every octave upwards.

What does the loudness of sound depend on?

Sounds vary not only in terms of the pitch or frequency, but also in their relative loudness.

As you might expect, louder sounds are produced by larger vibrations of the source. If the loudspeaker cone moves backwards and forwards over a greater distance, a bigger disturbance is produced in the air and a louder sound results.

As with pitch, the loudness of a sound is a subjective measure – it depends on the person. You may have an elderly relative who listens to the TV at what seems to be ear-shattering volume to you, and yet is comfortable for their ears. The same sound is judged to be of different loudness by two people.

A more objective measurement of loudness is the amplitude of the vibration.

The human ear is sensitive to sounds over a huge range of loudness. For example, the loudest sound that the ear can safely detect without suffering any physical damage is more that one billion times more intense than the threshold of hearing (where sounds can just be heard).

The ear responds to sounds of different loudness in such a way that huge increases in amplitude are registered as small increases in loudness.

The decibel scale

To match this way in which the ear responds, the relative loudness of sounds is usually measured in decibels.

The decibel scale, which is a logarithmic scale, stretches from 0 dB at the threshold of hearing to 140 dB at the threshold of pain (values vary, as this is subjective – pain is not a precisely defined experience). Each time you go up one decibel, the loudness of the sound increases by a constant factor and you can just about hear this change.

Logarithmic intensity scales are also used to report on the perceived brightness of light (the sensitivity of the eye is also logarithmic) and for measuring the strength of earthquakes (the Richter scale).

Echoes, imaging and measurement

Clap your hands not too far from a big wall and you can hear both the original clap and, shortly afterwards, its echo. What is happening here is that the sound takes one path straight from your clapping hands to your ear (a short distance), while another path involves the sound starting off from your hands, reflecting off the wall and then travelling back to your ears (a somewhat greater distance).

In fact, these are just two of the many paths followed by the sound as it travels out in all directions through the surrounding air.

By measuring the total distance travelled (from your hands to the wall and back) and timing the gap between the original clap and you hearing its echo, it is easy enough to get a rough value of the speed of sound in air.

Depending on the accuracy of your timing, you are likely to find a value of about 300 metre / second. In textbooks the value of the speed of sound in air is often quoted as 340 metre / second.

Under normal conditions, this speed remains pretty well constant. However, since the motion of the sound depends on the movement of air particles, the speed of sound does change with temperature because this determines how fast the particles are moving.

If we assume that the speed of sound is 340 m s^-1, we have a way of measuring distances. This is used in sonar measurements.

Timing lots of pulses in a cunningly planned pattern can give an ultrasound scan. With really careful and fast processing of these signals you can get a real-time image, of, for example, a baby in a womb.

Bats use ultra-sonic (too high a frequency for us to hear, but good for the bats) pulses to navigate with. It's possible to imagine that they see the world very differently from us.

Sounds in solids, liquids and space

Sound can travel through any medium, so long as there are sufficient particles in each cubic metre to allow the to and fro motion of one block of particles to be passed on to their neighbours.

Solids, liquids and gases differ in the arrangements of their particles and in the forces between the particles. So you might expect variations in both the speed and range of sounds travelling through them, since both depend on how the disturbance passes on from one block of particles to the next.

Sounds travel faster and farther in solids. Presumably this fact must have been well known to all of those who put their ear to a rail track to detect whether or not a train was coming.

In liquids sounds also travel over great distances at considerable speed. This is apparent from listening to whales as they communicate with one another. The complexity of the paths which these vibrations follow is exploited by Tom Clancy in his book The Hunt for Red October, where he makes much of the difficulties involved in interpreting sonar signals while tracking submarines.

In deep space there are only about 3 particles of hydrogen per cubic metre and this is not enough to sustain a sound. Disappointingly the booms of space movies are a fiction! All explosions in outer space take place in eerie silence. As the Alien movie tells us: In space, no-one can hear you scream.

The speed of sound varies in different media. Here are some typical examples: in air, 340 m s^-1 ; in water, 1500 m s^-1 ; in steel, 6000 m s^-1.

All around you there is plenty of evidence that different frequencies of sound travel at the same speed. The very existence of music depends on this. If the notes of differing frequencies, from different instruments, arrived at your ears all at different points in time, the effects would probably be rather unpleasant!

A recap

Sound consists of audible vibrations, which travel from a source to a detector. These can be of a whole range of frequencies and a whole range of amplitudes. We hear these as sounds of different pitches and different loudnesses. Sometimes one sound will be made up of many of these frequencies at once, each with their own amplitude. All of these contribute to the amplitude of the sound, as well as its characteristic quality.

These vibrations travel at one quite constant speed, about 340 metre / second in air, whatever the frequency or amplitude. Sound travels through solids and liquids at higher speeds.