5.16 Physical Sound Wave Detection
147
A speaker constructed with a stiff paper cone glued to a coil of wire put in a
magnetic field has no intrinsic need to produce entropy (in this case, heat released
divided by the background temperature), and so can also act as a microphone. As
such, the air vibrations wiggle the cone. This wiggles the coil in the magnetic field,
causing charges to be pushed through the wire of the coil. If the wires are attached
to another speaker, that second speaker will respond and produce a sound. (More
commonly, the first coil is attached to an amplifier to raise the sound level of the
second speaker.)
5.16.1 Microphones
A device which converts one form of energy into another form is called a
‘transducer’. A microphone is any device which can convert acoustical intensity into
a proportional electric current. A faithful conversion will preserve the information
content in the sound.
The first microphone made by Alexander Graham Bell’s assistant used a
conductive wire whose depth in an acid bath varied by the vibrations of an attached
sound diaphragm. Current in the wire then changed according to the sound.
Early telephones used Bell’s electromagnetic speaker and Thomas Edison’s idea
for a microphone: a metal diaphragm gently pressed against grains of carbon, with
another stiff conductive surface on the other side of the carbon. When the carbon
grains are squeezed, their overall electrical resistance drops. Sound impinging on
the diaphragm produces a changing pressure, which varies the current between the
two conducting surfaces.
‘Voice-coil’ microphones, as we have described, take advantage of the induction
of a current by moving a conducting coil (usually copper wire) next to a permanent
magnet. The coil is attached to a cone in order to better match the impedance of air.
This is the reverse technology of a voice-coil speaker.
A ribbon microphone also uses Faraday induction, but in a conductive flexible
ribbon placed in the field of a magnet. In this case, the surface of the ribbon itself
provides impedance matching to the air. The frequency response in the range of
human hearing can be adjusted by how flex lines are imprinted on the ribbon. Simple
corrugation on a thin aluminum sheet is a popular design.
A condenser microphone uses a small parallel-plate capacitor, one side being thin
and flexible enough to respond to sound by slight displacements. As the capacitance
depends on the plate separation distance, a sound will cause variations in the
capacitance, which can be detected by a suitable electronic circuit.
An electret microphone is a special kind of condenser microphone. It uses a
dielectric material, called an electret (such as polytetrafluoroethylene), on which
a ‘permanent’ electric charge has been induced. (The charge decays away in 2–
10 years). Sound moves a conducting diaphragm a short distance in front of the
electret material. The charges on the electret push and pull charges on and off the
diaphragm, causing a small current. Because these microphones can be constructed
147
A speaker constructed with a stiff paper cone glued to a coil of wire put in a
magnetic field has no intrinsic need to produce entropy (in this case, heat released
divided by the background temperature), and so can also act as a microphone. As
such, the air vibrations wiggle the cone. This wiggles the coil in the magnetic field,
causing charges to be pushed through the wire of the coil. If the wires are attached
to another speaker, that second speaker will respond and produce a sound. (More
commonly, the first coil is attached to an amplifier to raise the sound level of the
second speaker.)
5.16.1 Microphones
A device which converts one form of energy into another form is called a
‘transducer’. A microphone is any device which can convert acoustical intensity into
a proportional electric current. A faithful conversion will preserve the information
content in the sound.
The first microphone made by Alexander Graham Bell’s assistant used a
conductive wire whose depth in an acid bath varied by the vibrations of an attached
sound diaphragm. Current in the wire then changed according to the sound.
Early telephones used Bell’s electromagnetic speaker and Thomas Edison’s idea
for a microphone: a metal diaphragm gently pressed against grains of carbon, with
another stiff conductive surface on the other side of the carbon. When the carbon
grains are squeezed, their overall electrical resistance drops. Sound impinging on
the diaphragm produces a changing pressure, which varies the current between the
two conducting surfaces.
‘Voice-coil’ microphones, as we have described, take advantage of the induction
of a current by moving a conducting coil (usually copper wire) next to a permanent
magnet. The coil is attached to a cone in order to better match the impedance of air.
This is the reverse technology of a voice-coil speaker.
A ribbon microphone also uses Faraday induction, but in a conductive flexible
ribbon placed in the field of a magnet. In this case, the surface of the ribbon itself
provides impedance matching to the air. The frequency response in the range of
human hearing can be adjusted by how flex lines are imprinted on the ribbon. Simple
corrugation on a thin aluminum sheet is a popular design.
A condenser microphone uses a small parallel-plate capacitor, one side being thin
and flexible enough to respond to sound by slight displacements. As the capacitance
depends on the plate separation distance, a sound will cause variations in the
capacitance, which can be detected by a suitable electronic circuit.
An electret microphone is a special kind of condenser microphone. It uses a
dielectric material, called an electret (such as polytetrafluoroethylene), on which
a ‘permanent’ electric charge has been induced. (The charge decays away in 2–
10 years). Sound moves a conducting diaphragm a short distance in front of the
electret material. The charges on the electret push and pull charges on and off the
diaphragm, causing a small current. Because these microphones can be constructed