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Science, Maths & Technology

Challenge: Record a Sound or Voice

Updated Monday 28th January 2008

The science behind phonographs and sound recording, part of the BBC/OU's programme website for Rough Science 2

Armed with only the most basic of tools and materials is it possible to capture the sounds of the Rough Science island?

First of all, what is it we are trying to record?

Sound. When we speak, the vocal cords in our throats vibrate. This causes the air near to them to be momentarily compressed and expanded at the same rate. The rhythmic compressions of the air move out and away from the vocal chords, up through the throat and out through the mouth.

If we could see the compressed air it would look as if the sound was travelling outwards from the source, rather like ripples in a pond when a stone is thrown in.

Sound waves travel quite fast but need a medium to do so. Each air molecule knocks against its neighbour and transfers sound energy along the line. The air molecules themselves might not go very far but the energy can go a long way.

sound waves travelling to the ears Copyrighted image Icon Copyright: Used with permission

As sound waves enter the ear tiny hairs deep within it are set in motion. The movement of the hair triggers electrical impulses that go to the brain.

There is an amazing diversity of sounds around us. The number of vibrations per second of a sound gives it its pitch and the strength of the vibrations determines its loudness.

Birdsong is high pitched.

bird Copyrighted image Icon Copyright: Used with permission

A watch ticking is even higher in pitch but much softer.

watch Copyrighted image Icon Copyright: Used with permission

A construction worker's drill is low pitched.

drill Copyrighted image Icon Copyright: Used with permission

An aircraft engine is even lower in pitch but much louder.

plane Copyrighted image Icon Copyright: Used with permission

Although our ears are very sensitive they are limited in their ability to hear all types of sound. Our ears can hear from about 20 to 20,000 vibrations per second. The low notes of bass musical instruments are around 100 vibrations per second while high notes from flutes and tin whistles are 1,000 vibrations per second. Bats use sound waves as high as 50,000 vibrations per second but these are outside of our human range (ultrasonic).

Given the limited materials available to us, we decided to make a recording on a phonograph.

How does a phonograph work?

Sound is collected by a horn that is attached to a diaphragm. The sound causes vibrations in the air that travel down the horn causing the diaphragm to vibrate. The diaphragm is connected to a stylus and pressed into a cylinder covered in wax (or alternatively a thin layer of tin foil).

When a handle is turned, the cylinder rotates and also moves very slowly along. The stylus pushes into the wax and, when the cylinder is rotated, cuts a groove. The stylus also moves up and down very slightly as it vibrates with the sound and so the wax now contains a recording of the sound in the groove.

We play the sound back by using the stylus to translate the groove back into vibrations onto the diaphragm and this in turn to the horn from which the sound can be heard.

Although the phonograph idea is simple enough it was a triumph of engineering. The movement of the cylinder and groove need to be very accurately aligned in order for the machine to work at all. This is a difficult thing to do and requires precision engineering!

Edison's phonograph Copyrighted image Icon Copyright: Used with permission

Thomas Edison's Phonograph

How did we go about making a Rough Science version?

Rough Science phonograph Copyrighted image Icon Copyright: Used with permission

HORN: To collect the sound. We used a large metal funnel.
DIAPHRAGM: To hold the stylus. We used the base of a tin can.
SOUND BOX: Fixed to the horn, it increases the efficiency of the transfer of sound to vibration. We made ours out of a small tin can.
STYLUS: Used to transfer vibrations onto the recording cylinder. Made out of a drawing pin and pushed through the middle of the diaphragm.
RECORDING CYLINDER: The recording cylinder rotates, moves along and supports the wax on which the sound is recorded. Formed of two wooden circles mounted on a piece of studding (long thread). Held apart and fixed in place by wooden blocks.
WAX: Covers the recording cylinder and contains the impression from the stylus. We used many layers of wax and smoothed it down with a razor blade.
HANDLE: Turns the recording cylinder. Placed at one end of the studding (long thread).
HINGE: Used to fix the horn-diaphragm-stylus unit onto the base. It is able to move so that the position of the drawing pin on the wax can be adjusted.

How was a recording made?

To make a recording, or rather an attempt at a recording, the cylinder needed to be turned. The drawing pin stylus was placed onto the wax so that it pushed in about 0.5 to 1 mm. The handle was turned at an even rate and we shouted into the horn.

Then we re-adjusted the drawing pin in the wax and used the last half to have a go at recording under slightly different conditions, hoping that one of these two recordings would have worked.

diagram of stylus pushed into groove Copyrighted image Icon Copyright: Used with permission

Stylus pushed into the groove

How do we play it back?

Assuming that you have something recorded onto the wax cylinder it's now time to try and play it back. After coming to the end of the wax, we carefully removed the drawing pin stylus and made sure that it was well away from the cylinder.

We wound the handle in the other direction so that the wax cylinder was brought back to the start position. The stylus was put back carefully onto the start of the groove but this time slightly less deep than we had it for recording. We turned the handle at the same rate and direction as we did when we recorded the sound and listened. The stylus will pick up a noise from the groove but on top of that noise, if you are very lucky, you will hear your recording.

Jonathan and Kate listening to the recording Copyrighted image Icon Copyright: Used with permission


The BBC and the Open University are not responsible for the content of external websites.

Thomas Edison's Phonograph from the Bizarre Stuff Webpages - dedicated to the preservation of early recorded sounds

Sound Waves and Music - from the Physics Classroom website

Anatomy of the Ear - Learning activity


Advanced Physics by Tom Duncan, John Murray


For further information, take a look at our frequently asked questions which may give you the support you need.

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