Recognising Sound revision summary


Here are some notes on recognising sounds, should include all the important points (without much waffle!)


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  • Created on: 10-10-09 10:02
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Recognising Sounds
1.1 Music and sound
Wave speed = Frequency x Wavelength
Ultrasound = 20+ kHz
Infrasound = -20 Hz
Any two notes of frequency ratio 2:1 (i.e. one frequency is double the
other) are said to be separated by an octave.
Any two notes of frequency ratio 5:4 are said to be separated by an
interval of a third.
The timbre of a wave is made up of three aspects:
The harmonics
The attack
The decay
The vibrato
Quality/timbre gives a musical instrument its unique quality, when they
are playing the same pitch/note the waves will look different.
Harmonics are the multiples of the fundamental wave frequency that are
The attack of a sound is a measure of how quickly it reaches its peak
amplitude or loudness
The decay is a measure of how long it takes to die away.
Vibrato is a periodic change in the pitch of the waveform as the sound
continues to be produced.
1.2 Listening to sounds
Sound waves consist of a series of compressions and rarefactions that
travel through a substance, usually air.
Amplitude ­ the maximum displacement from its rest position of one of
the particles of the medium that carries the wave.
Polarisation of a transverse wave restricts the direction of oscillation so
that it occurs in one plane perpendicular to the direction of travel. It is
sometimes used in windscreens to reduce sun light.
Longitudinal waves, such as sound, cannot be polarised.
Measuring speed of sound ­ hammer, metal plate, two microphones,
ruler, and timer.
Speed of sound = distance between speaker and microphone
Time read from oscilloscope trace
1.3 Sound production

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Progressive/travelling waves cause energy to move from the source to
the surroundings.
When a travelling wave is restricted (e.g. a string with fixed ends), it
reflects at each end and forms a standing wave.
Principle of Superposition ­ when 2 or more waves meet their TOTAL
displacement and are the sum of their individual displacements.
String instruments
String plucked or bowed and a standing wave is set up.
The vibration is transferred to the body of the instrument
which vibrates as a whole.…read more


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