Cyberphysics

Sound

I would prefer you to think of the density of air molecules being altered as sound travels through the air rather than you thinking individual molecules get stretched and squeezed... this video has some good bits and some not so good... see if you can spot them all!

See the waves section for details on wave terms.

Sound is the name we give to energy that we can detect using our ears. Different species have different hearing ranges.

The human range of hearing is 20Hz to 20kHz (You need to know that!)
Sound of frequencies below 20Hz are called sub-sonic and
sound of frequencies higher than 20kHz are termed ultra-sonic.

Click here to try a hearing test

Sound is a mechanical wave - it propogates (moves along) by using the vibrations of particles in matter (as opposed to electromagnetic waves that don't use particles in their propogation!).

The sound energy is transferred through matter by vibrations of particles in that matter in a longitudinal wave. The vibrations made in a sound wave are parallel to the direction in which the energy is travelling.

Wavelength (symbol - lamda a Greek letter 'l') is the shortest distance between two corresponding parts of the waveform. For a soundwave that means the shortest distance between two particles of the medium the sound is travelling in that are oscillating in phase. (Shortest distance between two particles that are moving in step with each other). It is measured in metres (m)
Period (T)is the time taken for one complete oscillation of a particle in the wave. It is measured in seconds (s).
Amplitude (A) is the maximum displacement from the mean position.
Frequency (f) is the number of oscillations per second. It cannot be read directly off a graph. You need to find (T) from the displacement/time graph and then find its reciprocal (f =1/T)

You need to UNDERSTAND that:-

An oscilloscope (see diagram below) is a voltmeter that shows how electrical signals vary with time on a screen. It can show sound waveforms if a transducer such as a microphone is connected to it. This changes sound energy into electrical energy and shows the vibrations on the screen.
In sound : high frequency means high pitch of note (lots of waveforms on the screen), low frequency (few waveforems) means low pitch of note (deep note).
In sound: big amplitude means loud note, small amplitude means quiet (not low as this can be used for pitch too) note.
to make high pitched sounds you use short, thin strings or short pipes and vice versa.
to make loud notes you blow hard into wind instruments and pluck hard at string instruments.
that sound travels much slower than light does.

Question: If there is a ten second gap between you seeing a flash of lightening and hearing the thunderclap. How far away is the storm centre?

Light travels so fast that the time interval between the lightening happening and you seeing the flash is negligible.

Sound travels at 330 m/s therefore in 10 seconds it travels 3300 metres. The storm is therefore 3.3km away.

The time taken for the light to reach you is actually :-

Speed = distance/time

time = distance/speed

= 3300/300,000,000

= 0.000011s

(rather small , eh?
THAT's why we can ignore it !!!)

In an examination you may be expected to:

Interpret oscilloscope traces :-

Perform 'echo' calculations

Question: If the time between the horn sounding and the people on the ship hearing the echo from the ship is 4 seconds, how far away is the ship from the cliff ?

speed = distance travelled by the sound
time taken

distance travelled by the sound = speed x time taken = 330 x 4 = 1320 m

this is 'distance from ship to cliff' + 'distance from cliff to ship'

\ distance between ship and cliff is 1320/2 = 660 m

Use the wave equation in calculations (Make sure you know those SI prefixes! )

If wavelength () = 3 cm and frequency is 60 Hz. What is the speed of the wave ?

Change the wavelength into metres: 3 cm = 0.030m (remember for an equation to work the units have to be correct).
Use the equation:

speed = f

= 0.03 x 60 = 1. 8 m/s

Describe the experiment that shows that sound cannot travel through a vacuum.

The Decibel Scale

This is used to measure how loud a sound is.

140 dB is loud enough to cause you pain - 0 dB is the threshold of hearing - quieter sounds than this cannot be heard.

A typical classroom of students 'actively engaged' in a lesson - discussing with each other is about 70 dB and a typical library is about 30 dB.

A rock concert is usually at the threshold of pain and can cause damage to your ears. Rock musicians wear ear protectors when they play!

Corn starch is a shear thickening non-Newtonian fluid meaning that it becomes more viscous when it is disturbed. When it's hit repeatedly by something like a speaker cone it forms weird tendrils. The video below was shot at 30 fps and the speaker cone was vibrating at 30 Hz which is why there is no blur. This is the original video with the actual sound of the speaker.

Click here to see a video on amplitude of sound waves and loudness

Click here to see a video on frequency and amplitude

Click here to see a video on frequency and pitch

Click here to download an interactive Java simulation

The following site covers sound really well - take a look at it.

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Cyberphysics.co.uk is written and maintained by a teacher who has more than 20 years experience of teaching in a British selective girls' Grammar School