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SOUND
3.
Sound waves: The ear, human hearing, displaying sound wave frequencies on a
cathode
ray oscilloscope screen (CRO),
recognising relative differences in pitch-frequency and amplitude-loudness -
human voices or musical notes
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3.
Sound waves and human hearing and displaying frequencies on a CRO
Your ear is designed to collect sound waves and
cause the eardrum to vibrate.
Your ear drum resonates with a
sound wave hitting it and via some bones and nerve receptors, the 'sound
impulses' are transmitted to the brain.
Your ear is designed to
collect sound waves - the outer part is a bit like a misshapen
convex mirror!
When sound waves funnel down
and hit your eardrums, the pressure variations cause them to
vibrate and the vibrations are
transferred to tiny bones in your ears called ossicles,
then through the semicircular canals on to the cochlea.
The cochlea converts the vibrations into
electrical nerve
signals which transmitted to the brain.
The brain
interprets the nerve signals (your sense of hearing)
relating to the different frequencies ('pitches') and amplitudes
('volumes').
In some ways the effect is
similar to a microphone works!
Human hearing is limited by the size and
shape of the eardrum and the structural features of the parts
that make the ear's vibration sensing mechanism.
A typical frequency range of human hearing is 20 Hz to 20 kHz, frequencies
outside this range would be beyond many people's hearing.
This is clear example of yourself
appreciating energy transfer by sound waves - greatly appreciated by
somebody who is deaf in one ear!
A higher frequency sound is perceived as a higher pitch
(lower frequency = lower pitch).
A sound wave of greater amplitude is perceived as a louder
sound (lower amplitude is a softer sound).
We experience longitudinal waves as
sound, but we can only hear a relatively narrow range of frequencies.
What sound frequencies can we hear and
why?
What we can hear as human beings is limited
by the size and shape of the eardrum and anything else that is connected and
vibrates - resonating with the eardrum. The ossicles, the bones of the middle
ear only function well over a limited frequency range. We cannot hear very low
pitched or very high pitched sounds.
The bones are most efficient at
transmitting frequencies of around 1000 Hz to 3000 Hz (1-3 kHz)
Younger people have a much greater
hearing range which can be as wide as 20 Hz (0.02 kHz) to 20 000 Hz (20
kHz).
Unfortunately, as you get older, the
upper frequency limit decreases AND your sensitivity decreases - you become
harder at hearing - sounds like speech need to be louder.
This is often due to unavoidable wear and tear of the cochlea or
auditory nerve.
A personal note (if you pardon the pun!)
The cochlea of my left ear never
developed correctly, and so, although all the bones are there and
presumably vibrate, no nerve signals are generated, so I've always been
deaf in my left ear. My deafness was spotted by a primary school teacher
when I was 10 and duly tested to confirm I was indeed deaf in my left
ear. I didn't know anything different to monophonic sound, so I've never
known what stereophonic sound sounds like! My loving parents didn't seem
to realise it either, even though my deafness got me into trouble! One
line in my school report, as regards homework, read "plays on his
deafness", brilliant eh! It has had very amusing consequences for my
classroom teaching (many years ago!). If there was a bit of nonsense on
the left at the back of the lab, I always enquired to the right and
entirely blamed the wrong group. The students thought this most amusing
with many giggles and sniggers and I was regarded as a bit eccentric.
Since I couldn't resolve the problem, I once more 'played on my
deafness' and accepted at times I'd never find the culprits and sought
'diplomatic' and 'amicable' solutions and survived to teach in
comprehensive schools for over 28 years!
Sound is important to humans - a means of communication via speech, enjoyment of music etc.
Another good example, which I'm glad to say I
have not encountered, is the enormous power of
earthquake waves - a huge
amounts of energy can be conveyed many miles through the Earth's crust, mantle
and even through the core.
Echo sounding is important to bats,
they can generate and listen to sounds from 30 Hz to 20 kHz.
Sound waves are produced by
mechanical vibrations (e.g. musical instruments) and travel through any medium, gas, liquid or solid,
but not vacuum, where there is nothing to vibrate!
In music, if a middle C tuning
fork is struck, the two prongs vibrate from side to side 262 times every
second ie middle C has a frequency or pitch of 262 Hz.
The pitch of a sound is
determined by its frequency and loudness by its amplitude.
The rest line is represented by
the horizontal red line on the CRO diagrams below.
The four pictures could represent the sound
waves of musical notes recorded by a microphone, converted to an electronic
signal and displayed in wave form
on an cathode ray oscilloscope screen (CRO).
You can produce a wide range of frequencies
using a signal generator and they can be converted into sound waves.
Note that ...
The shorter the wavelength the higher the
frequency (or pitch) of the sound.
The higher the waveform (greater the
amplitude) at the point of maximum compression, the louder the sound - and
conveying more energy.
So, we can interpret the four signals as
follows:
(1) has the smallest amplitude,
the softest note (opposite of loudest) - just a whisper!
(2) has the largest amplitude,
the loudest note - a good shout out loud!
(3) has the longest wavelength,
lowest frequency, lowest pitch e.g. a low note sung by a base singer.
(4) has the shortest wavelength,
highest frequency, highest pitch, e.g. a treble note or a squeaky animal.
Some more examples - imagine some
musical sounds from a microphone
displayed on a CRO
The height of the wave above the baseline
(0) gives the amplitude.
In this case, two of the amplitudes,
for waves A and D are double that of waves B and C, in other words the
height of the wave at the peak is double when measured from the zero horizontal base line.
Wave A will transfer more energy than
wave B, and, wave D will transfer more energy than wave C.
(You don't need to know the maths,
but the energy in a wave is proportional to the amplitude squared).
Convert to Q on musical
instruments?
I've made the time frame 0.02 seconds so
that we can do some simple calculations
Frequency = oscillations per second (Hz)
For waves A and B there are 10
complete cycles of the wave in 0.02 s. Frequency = 10 / 0.02 = 500
Hz.
For waves C and D there are 5
complete cycles of the wave in 0.02 s. Frequency = 5 / 0.02 = 250 Hz.
Assume sketched of the same width of a CRO screen and note the number
of waves and their height.
A class of students were listening to single music notes
played into a microphone and the result displayed on a cathode ray
oscilloscope (CRO).
The students where asked to sketch pictures for each sound
produced.
A selection of their sketches is shown above and interpreted
below.
FRED is producing a loud highly pitched note (many
waves, big amplitude).
JO is producing a soft low pitched note (few waves,
small amplitude).
TANYA is producing a loud low pitched note (few
waves, large amplitude).
RICKY is producing a highly pitched soft note (many
waves, small amplitude)
INDEX of physics notes
on SOUND
Keywords, phrases and learning objectives for
sound waves
Know that sound wave frequencies (pitch) can be displayed on cathode ray
oscilloscope screen (CRO).
Be able to recognise from a CRO trace the relative differences in
pitch/frequency or amplitude/loudness from human voices and musical notes
- singing or instruments.
Use your
mobile phone in 'landscape'?
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