Recording and Reproducing Sound

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Recording and Reproducing Sound

• Unit 4 Physics

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System Components

• Knowledge of how sounds are created and
  transmitted, and how our hearing works forms the
  basis of how sound engineers develop recording
  and playback systems.
• Input can be from a variety of sources
  including microphone, CD, mp3, becoming an
  electrical signal.
• Amplifier increases the size of the input
  signal ideally with minimal distortion and have
  the same gain for all frequencies
• Tone control these increase or suppress certain
  frequencies and can make the sound closer to the
  original.
• Loudspeakers convert the electrical signal to
  soundwaves, with their construction designed to
  most accurately reproduce the original sound, eg
  different sizes for different frequencies.
• A Hi-Fi system (high fidelity) gets its name
  from being accurately able to reproduce a wide
  range of frequencies. They ideally have a flat
  linear response curve.

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Microphones

• Microphones are designed to respond in a
  particular way to the sound waves around it by
  producing a variable voltage
• Microphones are an example of a transducer.
• Transducers receives waves of a particular type
  from a source and produce related waves, not
  necessarily of the same type.
• The role of a microphone is to faithfully produce
  a potential difference (voltage) that is exactly
  proportional to the incident sound.
• There are many ways to do this, and the choice
  will depend on the application.

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Piezoelectric Microphones

• piezo from Greek meaning pressure.
• The transducer element produces differential
  voltage when mechanically deformed.
• Their output voltage can drive a high-impedance
  transistor directly
• Frequency response is uniform up to around 10kHz
  in a range of environmental conditions
• Cheap to make, so used widely until advent of
  modern transistors created demand for mics that
  would work with modern amplifiers.

• Used to be made using an assymetric salt crystal,
  hence the name crystal mic, but now use
  ceramics that are resistant to environmental
  extremes and are called ceramic mics.

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Electrodynamic Microphones

• Popular for high quality recording for a while
  and can respond well to sounds hear by humans
  20Hz-20kHz.
• Faraday discovered that a changing magnetic field
  will induce an EMF and if connected to a circuit
  will create a current.
• In dynamic (or moving coil) mics, a diaphragm is
  attached to a coil of wire which moves between
  the poles of a permanent magnet inducing a
  voltage equal to

• If the velocity of the coil varies with A or f,
  the voltage will vary, and the mechanical
  resonance of the coil can be damped to give a
  uniform frequency response.

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Velocity (Pressure gradient) Mics

• Also use induction, except use a thin metal
  ribbon suspended between two permanent magnets
  instead of a coil.
• The ribbon vibrates in response to sound waves
  and induces an EMF proportional to the speed of
  vibration.
• Magnetic fine mesh metal screens are placed on
  both sides of the ribbon to provide resistance
  dampening from other sources.
• This makes them suitable as directional mics,
  enabling the ribbon to respond to vibrations from
  a particular direction only.
• Tpical response range from 30-15000Hz.

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Electret-condenser Microphones

• Work on the principle that the sound waves
  vibrating the diaphragm, as one side of a
  capacitor will change the dimensions and hence
  its capacitance due to the relative proximity of
  the charges.
• The diaphragm is made of polycarbonate with a
  thin metal film (50nm) on one side, then heated
  and charged with a high DC potential.
• The electret capsule, if well designed can retain
  full charge and hence sensitivity for over 10
  years.
• that foil can be made very cheaply
• - that it cannot withstand high tensions needed
  for high frequencies.

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Loudspeakers

• These are essentially the reverse of a moving
  coil mic.
• A changing current will cause a coil in a
  magnetic field to move, and if the coil is
  attached to a diaphragm, it will create a sound
  wave.
• The larger the current, the larger the force and
  hence amplitude.
• The magnet is usually circular, producing a
  strong radial field.
• The diaphragm is usually cardboard or stiff
  plastic with springy edge suspension to restore
  it to the central rest position after being
  driven forwards and back

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Speaker Design

• Size roughly matches the range of frequencies
  woofers, 30-500Hz, are larger, mid-range
  500-4000Hz and tweeters 4-20kHz, are smaller.
• The ouput signal is filtered and separated and
  the appropriate frequencies sent to the
  appropriate speaker.
• In returning the speaker to rest position,
  damping needs to occur to prevent the speaker
  from oscillating due to the momentum of the
  speaker.
• This is done by shorting the coil terminals. This
  results in a current in the opposite direction,
  and hence a counter EMF and will oppose the
  cones motion. This is called the back EMF.
• Without damping, distortion will occur.
• The quality of a speaker will be determined
  partly by how well it damps unwanted
  oscillations.

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Enclosures

• When a speaker moves to create a compression at
  the front, it will also create a rarefaction at
  the back.
• At low frequencies (gt200Hz), this will diffract
  around the speaker and cause destructive
  interference.
• To prevent this, speakers are generally mounted
  in a box with sound absorbing material on the
  inside, containing the backwave.

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Baffles and Ports
baffle

• Designers go to considerable lengths to prevent
  sound from the back superimposing with the
  desired sound.
• The larger the baffle (panel surrounding the
  speaker) the better, but there are obviously
  physical limitations.
• Rather than absorb and block it, some speakers
  use a port.
• Ports are carefully designed holes in the front
  of the speaker such that when the sound is
  reflected from the back of the enclosure, it acts
  as a second diaphragm to reinforce the sound.
• To work effectively, the enclosure resonance
  needs to match the speaker resonance and to
  reverse the phase of the backwave.
• Properly designed, they can add an extra octave
  below to the speakers frequency response.

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THE END

• Of Sound
• And Unit 4.
• Now comes the study, exams, then the
  electromagnetic wave at the end of the open ended
  tube!!!!!