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Soundwaves: The Symphony of Physics. Copyright BBC

Sound Waves: The Symphony of Physics

Dr Helen Czerski takes us on a sonic odyssey through the sounds of the universe.

About the programme

Dr Helen Czerski takes us on a sonic odyssey through the sounds of the universe – to reveal what the physics of sound can tell us about the world and how it works.

The world is awash with sound waves, they are all around us. However sound is so much more than simply a soundtrack to our lives, and the more we’ve discovered about the physics of sound, the more extraordinary the secrets it has revealed.

Full details of the programme and its broadcast can be found on the BBC programme page

Discover the range of qualifications and modules from the OU related to this programme:

Helen Czerski in a sound chamber for soundwaves series BBC/OU

Copyright: BBC

A pink and blue graphic representation of sound waves

What is sound?

We hear it all the time, but what is sound? Discover the science here...

But just what is sound? How is it produced and how does it travel? And, indeed, how is sound heard?

Unlike our eyes, we cannot close our ears, so it can be argued that hearing is one of our most important senses. We are continually subjected to a huge number of different types of sound.

Some are pleasant and desirable, such as speech and music, while others are unwanted and annoying, such as machinery and road noise.

But just what is sound? How is it produced and how does it travel? And, indeed, how is sound heard?

Any sound, whatever it may be, is caused by something vibrating. In other words, by something which is moving back and forth, either in a regular manner or in a random manner, about the position it occupies when at rest. The source of the sound may be a car engine, a burglar alarm or a bird singing. Whatever it is, some part of it must be vibrating for it to produce sound.


A close-up of the audio cone like you would find in a set of speakers

In fact, it is easy to detect the vibrations of many sources of sound. If you touch your throat while singing or speaking, you can feel the vibrations of your vocal cords. Similarly, a hi-fi loudspeaker vibrates strongly especially when the volume is turned up. By lightly touching the speaker cone, you can feel its vibrations as a tingling sensation in your fingertips.

For such vibrations to be heard as sound, there must be a medium through which they can travel from the vibrating source to the ear. For example, sound travels clearly through water, as any swimmer can testify, and it also travels exceptionally well through metal. Most of the sounds we hear, however, are transmitted through air.

The vibrations of a sound source cause the neighbouring air molecules to be alternately squeezed together and pulled apart. These air molecules then push and pull against their neighbours which, in turn, push and pull against their neighbours.


A diagram showing the inner workings of the human ear

By Chittka L, Brockmann - Perception Space—The Final Frontier, A PLoS Biology Vol. 3, No. 4, e137 doi:10.1371/journal.pbio.0030137 (Fig. 1A/Large version), vectorised by Inductiveload, CC BY 2.5 under Creative Commons BY-NC-SA 4.0  license

In this way, a series of compressions (regions of higher pressure) and rarefactions (regions of lower pressure) is generated which travels away from the vibrating source. This sequence of pressure fluctuations is what we refer to as a sound wave.

When we hear a sound, what our ears are actually doing is converting the rapid fluctuations in air pressure that make up a sound wave into neural impulses. The human ear comprises three fairly distinct sections; the outer ear, the middle ear and the inner ear.

The outer ear funnels sound waves through the ear canal to the middle ear. In the middle ear, the sound waves meet the tympanic membrane (eardrum) causing it to vibrate.

Three bones in the middle ear - the malleus, the incus and the stapes - transmit vibrations from the eardrum to the inner ear. In the inner ear, the cochlea converts the vibrations to nerve impulses.

Finally, the auditory nerve receives the messages which have been translated into nerve impulses by the ear and carries them to the brain where they are interpreted as sound.



A woman wearing headphones and enjoying music playing from her phone

What makes a sound musical?

What is it that turns those vibrations into something we can dance to? We look at the defining characteristics that make a sound musical.

Musical sounds can be thought of as having three defining characteristics: pitch (or lack of it), loudness and timbre.

Pitch

A row of violins on display

If a sound source vibrates in a regular manner, it produces a pressure wave which is made up of a periodically repeating pattern of compressions and rarefactions. This is interpreted by the human ear as a note of definite musical pitch. The pitch of the note - that is, how low (bass) or high (treble) the note sounds – is determined by the frequency, or number of times per second, at which the pattern of compressions and rarefactions repeats. Examples of instruments which produce pitched notes include the violin, flute and piano. These instruments tend to be used to play melodies or tunes.

Notes without a definite sense of pitch are produced when a sound source vibrates in a random manner, producing an irregular and chaotic pressure wave. Such notes tend to be produced by percussion instruments like the snare drum, the cymbals and the maracas. Unpitched notes can still be considered to be musical sounds – they tend to be used to emphasise the rhythmical structure in a piece of music.


Loudness

Ripples spread out across water, reminiscent of the way sound waves travel

The loudness of a sound is largely determined by the size, or amplitude, of the vibrations of the source producing it. However, it also depends on the pitch of the sound, with the human ear being less sensitive both at very low frequencies and at very high frequencies. In a musical passage, variations in loudness are used to add extra interest for the listener.


Timbre

Photo by Marius Masalar on Unsplash

The timbre of a musical note is the quality or character of the note. It is the timbre that allows the human ear to distinguish between sounds which have the same pitch and loudness. Differing timbres ensure that we are able to distinguish between notes produced by, say, the flute and the viola.

Most sound sources vibrate at several frequencies simultaneously. The additional frequencies present in the sound wave produced by such a source are called overtones or harmonics. The relative strength of these harmonics plays a large part in determining the timbre of the sound. However, it is not the only factor. For example, time-varying aspects of the sound are also important. In particular, the nature of the attack or onset of a note plays a vital role in defining its timbre.


Meet the OU academics on this series

Professor David Sharp, Professor in Musical Acoustics The Open University
Professor David SharpProfessor in Musical Acoustics, School of Engineering & InnovationVIEW FULL PROFILE
Professor David Sharp, Professor in Musical Acoustics The Open University
Professor David SharpProfessor in Musical Acoustics, School of Engineering & Innovation

My research focusses on investigating exactly how a musical instrument’s playing properties are defined by its physical shape. To this end, I have developed several non-invasive techniques for measuring the internal geometries and resonance properties of musical wind instruments. I combine such measurements with rigorous testing involving musicians and listeners, to identify and correlate physical and musical differences between instruments.

Dr. Shahram Taherzadeh, Lecturer in the department of Engineering and Innovation/Mathematics, Computing, Technology
Dr. Shahram TaherzadehLecturer in the department of Engineering and Innovation/Mathematics, Computing, TechnologyVIEW FULL PROFILE
Dr. Shahram Taherzadeh, Lecturer in the department of Engineering and Innovation/Mathematics, Computing, Technology
Dr. Shahram TaherzadehLecturer in the department of Engineering and Innovation/Mathematics, Computing, Technology

My specialism is on outdoor sound propagation; both mitigations unwanted noise (say from various forms of transport) and also making use of sound waves to extract information about soil and other outdoor surfaces. I have been co-investigator and principal investigator on a number of government and EU funded research projects and co-published over thirty articles in peer-reviewed journals.

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