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Sound recording & playback

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VCE Physics.com Sound recording & playback - Sound recording & playback Dynamic microphone Condenser microphone Carbon microphone Frequency response curves Sound recording • Amplifiers • Loudspeakers 1
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Page 1: Sound recording & playback

VCE Physics.comSound recording & playback -

Sound recording & playback

• Dynamic microphone

• Condenser microphone

• Carbon microphone

• Frequency response curves

• Sound recording

• Amplifiers

• Loudspeakers

1

Page 2: Sound recording & playback

VCE Physics.comSound recording & playback -

Dynamic microphone

2

• Dynamic microphones have a coil attached to a diaphragm suspended in the magnetic field of a magnet.

• Pressure changes cause the diaphragm to vibrate, changing the amount of magnetic flux threading the coil – inducing a current in the coil to produce a signal.

Page 3: Sound recording & playback

VCE Physics.comSound recording & playback -

Condenser microphone

3

• Condenser microphones have two charged plates, the front one of which is very thin, and usually covered in a very fine gold layer.

• Pressure changes cause the front plate vibrate, changing the distance between the plates. This changes the capacitance of the system.

• A voltage is supplied to plates, and thus the amount of charge on the plates varies, causing a current to produce the signal.

Page 4: Sound recording & playback

VCE Physics.comSound recording & playback -

Carbon microphone

4

• A carbon microphone uses a capsule containing carbon granules pressed between two metal plates.

• A voltage is applied across the metal plates.

• The diaphragm vibrates with sound waves, applying a varying pressure to the carbon. This changes the resistance and the current through the microphone.

• Carbon microphones were once commonly used in telephones; they have low-quality sound reproduction and a very limited frequency response range.

Page 5: Sound recording & playback

VCE Physics.comSound recording & playback -

Frequency response curves microphones

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• The response curve of a microphone shows how well a microphone records sound at different frequencies.

• Frequencies that have higher db values on the chart are reproduced at a greater intensity.

• A perfect microphone would have a flat curve over the whole range of frequencies.

• The response curve is determined by a number of factors, such as the dimensions of the microphone & the type of transducer used.

Page 6: Sound recording & playback

VCE Physics.comSound recording & playback -

Frequency response curves microphones

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Dynamic microphones have peaks designed to gain clarity with stage

vocals. (1 - 5 kHz)

Condenser microphones tend tohave a fairly !at response curve.

The reproduce well over a wide range of frequencies.

Page 7: Sound recording & playback

VCE Physics.comSound recording & playback -

Sound recording

7

• There are a number of different methods that have been used to store recordings of sound.

• Vinyl records have a groove with sideways variations that corresponds to the sound wave.

• Magnetic tape has magnetic iron oxide particles aligned to represent the sound wave.

• Digital recordings (such as CDs and mp3s) are created by converting sound intensity into a digital number, at a defined rate.

• CD audio uses 16 bit quantisation (allowing for over 65,000 possible values), with the intensity sampled 44,100 times per second. The data is recorded as a series of 0s and 1s.

• MP3 files are converted to a lower bit rate and have some inaudible frequency combinations compressed to reduce the file size.

• Digital signals need to be converted through a Digital to Analogue Converter (DAC) for playback through a loudspeaker.

Page 8: Sound recording & playback

VCE Physics.comSound recording & playback -

Ampli!ers

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• Audio amplifiers are designed to increase the electric current that will be sent to the loudspeakers.

• Amplifiers are generally measured by their power output. Typical home stereo systems will be rated between 20W and 100W per channel.

• Clipping of the signal through the amplifier can cause distortion and overheating in speakers, so the amplifier is best chosen to be a higher power rating.

• Perfect amplifiers will increase the intensity of electric signals uniformly across all frequencies.

• For audio systems, an equaliser is also used so that selected frequencies can be made louder or quieter to suit the music or listener’s tastes.

• A “loudness” button will increase the intensity of the lowest frequencies, to accommodate the ears reduced perception at low volumes. (Not so useful at higher volumes)

Page 9: Sound recording & playback

VCE Physics.comSound recording & playback -

Loudspeakers

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• A loudspeaker works like a dynamic microphone in reverse.

• A coil of wire surrounds a magnet attached to the diaphragm.

• Changes in the current through the wire cause motion of the diaphragm following the same pattern.

• The movement of the diaphragm causes movement of air.

Page 10: Sound recording & playback

VCE Physics.comSound recording & playback -

Loudspeaker design

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• The basic principle of a loudspeaker is that needs to move as much air as possible, in phase with the electric signal.

• Speaker baffles are designed to stop the destructive interference of waves in front and behind the speaker.

• Ports allow for low very frequencies to resonate and be reproduced louder.

• Two-way speakers have two individual speakers:- a small tweeter for high frequencies- a woofer for low frequencies- the crossover separates the signals

• The impedance of a speaker is a measure of how the speaker opposes the variations in alternating current produced. Most speakers are between 4 and 8 ohms.


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