AQA Physics B Revision Cards - Recognising Sound

Notes for the AQA Physics B course, section 1, unit 1

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RECOGNISING SOUND
Music and Sound
Sounds are formed by vibrations and require a medium to travel through
Sounds travel as longitudinal waves (or compression waves)
A series of compressions (regions of high pressure) and rarefactions
(regions of low pressure), similar to vibrating a slinky
Wavelength: distance between two successive crests or compressions
Frequency: the number of cycles of waves that pass a point per second,
the number of complete oscillations that occur per second
c=f so wave speed = frequency x wavelength
Sounds from an instrument is caused by the instrument vibrating, the
greater the number of vibrations, the higher the frequency
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The typical human ear can detect frequencies between 20Hz and 20kHz
Frequencies below this are infrasound, and above this are ultrasound
Animals can detect frequencies much higher than humans
When sounds are turned into an electrical signal with a detector, the
frequency can be detected with an oscilloscope
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Pitch: a subjective quality relating to the position of a note on a musical
scale, determined by its frequency
Consonant: two or more noted which when played together produce a
pleasing sound
Harmonic: frequencies that are a multiple of the fundamental frequency
of an oscillator.…read more

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Listening to Sound
Sound waves have amplitude and frequency. The amplitude is the
maximum displacement of a particle and the frequency is the number of
oscillations per second
Light waves do not need a medium to travel through. Light slows down
when travelling though a medium. The speed of light is about 3.00 x 108
ms-1. Light waves are transverse waves.…read more

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The surround sound of a home theatre arrangement is initially set up
assuming that the listener is equidistant from each of the speakers. If not,
then there will be a delay, and the effect is spoilt.
Large orchestras and choirs need to watch the conductor to make sure
they all play simultaneously
Echoes (sound reflections) from the walls and ceilings of a building can add
to the overall sound.…read more

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Sound production
Production of sounds from musical instruments involves
standing/stationary waves
When two or more waves meet, their total displacement is the sum of
their individual displacements, i.e. amplitudes added together
For a consistent pattern, waves need the same frequency and speed
Plucking or bowing a string causes a standing wave on the string.…read more

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In string instruments travelling waves move from the point where the
string is plucked or bowed to the end of the string where they reflect and
undergo a phase change of 180o, where a crest becomes a trough and vice
versa.
Points where the total displacement remains zero are nodes and points
where the total displacement varies between a maximum displacement
and a minimum displacement are antinodes.
A string has a number of frequencies at which it will naturally vibrate.…read more

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In wind instruments air is blown in at one end and out at the other end.
At the end of the pipe the pressure is always atmospheric, so does not
change. At the close end, the pressure varies by the greatest amount so a
pressure antinode is formed.
There is a displacement node at the closed end and a displacement
antinode at the open end.…read more

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