Topic 11:
P1.5.1 (continued) Sound
Sound is a longitudinal wave
The particles vibrate backwards and forwards along the wave
Sound is produced by vibrating objects.
compressions
rarefactions
The vibrating object pushes the air to make high pressure compressions then pulls the air to make low pressure rarefactions
Sound wave
What the particles do
Re
vi
sio
n
This is the trace on an oscilloscope after a microphone converts sound to voltage
Properties of sound
• sound needs a medium to travel in
Sound travels when the particles of the medium vibrate against each other.
Electric
bell
If you suck most of the air out the bell goes very quiet due to a lack of particles to carry the sound.
Space is a vacuum ­ an absence of particles.
If there are no particles then there is nothing to transmit sound vibrations, so:
"In space, no one can hear you scream"
Properties of sound • the speed of sound depends on the density
of the medium it is travelling in (amongst other factors)
Sound is due to particles striking against each other.
The closer together the particles, the faster the vibration is transmitted.
The closer the particles, the faster the wave can propagate (spread)
• sound can be reflected (ie, it echoes)
We need the distance there, not the distance there and back again
Get the time taken for the sound to bounce back, HALVE IT (why?), and then use:
distance = velocity x time
to calculate how far away the object is.
link
Sound travels at 1500m/s in water.
The whale's sonar pulse comes back in 0.01s.
How far away is the fish?
Time to reach fish
= 0.01/2 = 0.005s
Distance = speed x time
= 1500 x 0.005
Speed = distance/time
= 7.5m
Properties of sound • sound can be refracted
(ie, it bends when it crosses into a medium where it has a different speed)
Sound is faster in water (1500m/s) than it is in air (330m/s). This is because the particles are closer together in water, so the waves can spread (propagate) faster
fast
slo
w
w
slo
AIR
st
fa
WATER
Sound travels better over water at night.
The cool water cools the air sitting on top of it
Without refraction, this sound would not reach the listener
The sound can also bounce off the surface of the water for extra range
Properties of sound • sound can be diffracted
This is when a wave goes through a gap and then spreads out.
We can't explain diffraction at GCSE level
­ look up Huygens principle and constructive and destructive interference if you have a desperate need to know
Diffraction is strongest when the gap is of a similar size to the wavelength.
...and weakest when the gap is much bigger than the wavelength.
Barrier
sound waves
Sound will diffract around corners because it has a large wavelength.
Light will not diffract around corners due to its tiny wavength.
The sound waves diffract around the corner
Short wavelength.
No significant diffraction.
Long wavelength.
Easily diffracts around corners
Musical Sounds
Noise
No regular pattern
Pure tone
Very regular & repetitive
Bigger amplitudes mean that the air is moving a greater distance.
This creates a louder sound.
A higher pitch/greater frequency vibrate the eardrum faster to produce a higher pitched note.
Musical instruments produce many tones/frequencies which blend together to produce complex waveforms and the characteristic sound of that instrument.
Measuring the speed of sound in air
clappers
stopclock
speed = distance/time
Speed of sound in air = 330m/s
clappers
echo