If there is one thing that seems to be a passion of most secondary age students, that thing is music. If you can spark some interest in finding out how some music is produced and recorded, then your students may never listen to a CD in quite the same way again, with the added bonus of finding out quite a lot about physics. Music is a good hook to make physics more relevant to all ages, particularly teenagers.

As a starting point, nothing beats the experience of finding out something new about yourself, and one particularly popular class exercise is to test each other’s hearing. Hairs of the inner ear can be damaged by too much loud music. You can introduce this idea in your classroom by explaining how high levels of sound can damage your ears permanently or bring on conditions such as tinnitus (a non-stop ringing sensation in the ears). If the hairs of the inner ear are flattened, they can persistently send signals to the brain even when no sound is present.

Normal conversation measures around 60dB. In industry, exposure to levels in excess of 110dB is not allowed for more than 30 seconds without ear protection, yet many students attend gigs where levels exceed 120dB for long periods. Even some films have soundtracks at dangerously high levels.

Damage caused to our hearing by loud music is usually first noticeable in a diminished ability to hear the higher notes as clearly as we should. Bearing this in mind, we can measure the range of hearing in students by using a signal generator and speaker to produce a sound at around 8kHz (8,000 vibrations per second). Most will be able to hear the sound at this pitch, so ask the students to all raise their hands. Now increase the frequency or pitch of the note and ask people to lower their hands when they can no longer hear the sound. The drop-off starts around 11kHz and upwards.

When we are born we have a range of hearing from 20Hz up to 20kHz, but however well we look after our hearing, the higher range usually drops off with each year of life. Generally you could expect to find that older people have the smaller hearing range. Students tend to look smug as their teachers’ hands go down at around 11kHz, but most students cannot hear much above 14kHz. It shows that there is a range though, and the introduction of a competitive element (who has the “best” hearing, for example) usually results in lively group discussions. While we may no longer be able to hear the high frequency sound, it is worthwhile pointing out that any dogs in the area may be suffering auditory distress. Dogs can hear up to 45kHz and cats up to 64kHz. At the other end of the scale, elephants can hear sounds as low as 4Hz.

Getting right back to basics, it can be challenging to imagine a sound wave travelling through the air. We are used to seeing pictures representing sound waves on oscilloscopes or in advertisements for new hi-fi systems, but this could be misleading because they really show transverse waves rather than the longitudinal pressure waves that actually make up sound. Holding a Slinky spring stretched across a stage with a volunteer holding the other end, you can illustrate what really happens as sound travels through the air. By grabbing a portion of the spring and pulling it towards yourself - compressing it - you can show how when the student lets go of their end, the compression is transmitted along the length of the spring.

Also try getting six or seven students in a line standing shoulder to shoulder. They represent the molecules of air and you can show how a sound (which is effectively a change in air pressure) travels through them. A quiet sound would be a small nudge with your shoulder against the student at the end of the line. The nudge is passed along the line until the person at the other end is affected. A large sound would be a large impact with the first student in the line, resulting in the last student perhaps being overbalanced at the other end. Also point out that the molecules of air themselves have not carried the information from the sound source to the sound receiver, but have simply passed the change in pressure on to the next molecule.

Resonance is another vital ingredient in making music, yet it is tricky to explain. The analogy of pushing someone on a swing makes most sense to students. If you push someone on a swing, you can stand at just the right distance and put in very little energy to keep the swing going. If you stand in the wrong place or put in energy at the wrong time, you will not maintain the swinging and may get knocked flat on your back.

You can show resonance happening by using a wine glass with water in it. The friction of your finger, dampened slightly with water, rubbing against the rim of the glass at just the right speed will result in an ethereal musical note.

Now challenge your students to guess what will happen if you remove some water from the glass. The result may surprise them. You can then go on to show that smaller things tend to resonate at higher frequencies and therefore give a higher note, using sticks of different lengths mounted in a block of wood. They can see that the shorter sticks vibrate most noticeably at higher frequencies. This applies to the organs of the human body as well, and different parts of the body can be made to vibrate using different frequencies.

Technological developments can create new instruments. One such was the theremin, invented in 1921 and used, most famously, by the Beach Boys in Good Vibrations. The theremin is played by moving your hands in mid-air and interfering with the electro-magnetic fields created by the circuits of the instrument. If you go to the BBC’s science website you can turn your desktop into one of the earliest known electronic instruments.

Among the latest digital musical phenomena are MP3s, which are compressed format files for digital music. A CD has about five miles of track information on it, and one track of a CD has about 32MB of information. On a 56K modem this would take around five hours to download. MP3s are only about 3MB and basically they represent a fairly small proportion of the original information of the recording. With the music on an MP3, any frequencies that your ear can’t hear won’t be stored. Likewise if a loud sound happens at any point in the music, subtler background sounds won’t be stored as the MP3 “knows” your ears wouldn’t pick them up.

Try out an online Fourier Synthesizer to show how the addition of waves can create an almost infinite number of sounds. Challenge your students to create anything remotely resembling a real acoustic instrument and they will appreciate the complexities of sound synthesis.

Wendy Sadler is public programmes manager at Techniquest, Cardiff. She delivers a talk, “Music to your ears”, for students aged 13-15. For details write to: Catherine Wilson, education manager, The Institute of Physics, 76 Portland Place, London W1N 3DH.Email: catherine.wilson@iop.org Tel: 020 7470 4800http:physics.iop.orgSchoolssupteachvenues.html For an online Virtual theremin: www.bbc.co.ukscienceplaygroundtheremin1.shtml For an online Fourier Synthesizer: www.phy.ntnu.edu. twjavasoundsound.html For more information on Techniquest’s Musiquest project: www.techniquest.org