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Physics Notes: SOUND 3. Hearing and displaying frequencies with a CRO

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.


diagram image CRO traces of sound waves frequency amplitude gcse physics igcse

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.


CROpics2.gif
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.


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