PRELAB 9: TRANSDUCERS
I.
INTRODUCTION
Transduction is a process that changes energy (or information) from one form to another.
Microphones transduce acoustical energy into electrical energy (voltage); loudspeakers
transduce voltage back into sound. Tape deck record and playback heads transduce
electrical signals into magnetized regions on magnetic tape, and vice versa. CD players
transduce the information recorded on a CD back into electrical energy. Thus,
transducers are a crucial part of the study of acoustics.
A microphone is a transducer that converts acoustic energy to electrical energy. There are
five key types of microphone you may use
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Moving coil (dynamic microphone)
Ribbon
Condenser
Electret
Crystal
All employ different mechanisms to convert sound energy to electrical energy. Hence all
have different advantages and disadvantages. Parameters of microphone include
sensitivity, frequency response and directional response. Since the microphone operates
into HIGH ELECTRICAL IMPEDANCE such that there is (approximately) zero current
and the VOLTAGE is the output variable, it is not expected to deliver electrical power.
Consequently, it is conventional to talk of the "Open Circuit" response. The microphone
may be sensitive to any combination of acoustic variables, but the simplest case is a
microphone that is responsive to pressure input.
II Microphone sensitivity
A microphone’s sensitivity (pressure sensitivity) is defined as the voltage generated in
response choosing a microphone to a certain pressure input. The common unit is: M0
(Volts/Pascal). For example: A microphone is exposed to 94dB SPL generates 50mV
output, its sensitivity is 50mV/Pa because a SPL 94dB corresponds to an RMS acoustic
pressure of 1Pa. An alternative unit for a microphone’s sensitivity is expressed in
logarithmic form, relative to a reference, i.e. dB re 1 V/Pa. To converting to this scale, we
have 20log(M0) dB re 1 V/Pa. The above microphone sensitivity is 20log(0.05)=−26 dB
re V/Pa or −26 dBV/Pa.
Question: A microphone is rated −52 dB re 1 V/Pa. If an acoustic pressure of 5×10−2Pa
is incident on the microphone, what is the open-circuit output voltage of the microphone?
PRELAB 09: Transducer; Page 1
III FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION
The magnetic field H, is measured in amperes per meter. However, the commonly called
Magnetic field B is known as magnetic flux density that has the SI units TESLA. One
Tesla is equal to 10000 or 104 Gauss. The magnetic field lines point away from N-pole
(or North Pole) and point toward S-pole (or the South Pole). Typically, the Earth
magnetic field is about 0.5 Gauss pointing toward Earth North Pole. Thus the Earth North
Pole is a magnetic South Pole. Typical magnetic field strength of magnets of speakers
and microphones is 0.1 T to 1 T. Pace makers should avoid magnetic field strength larger
than 5 Gauss, and typical refrigerator magnets are about 50 G.
The operation of most transducers is based on Faraday’s Law, which says that if you
move a wire through a magnetic field (or vary the magnetic field strength through a coil
of wire), a voltage will be produced in the wire. The induced electromotive force (EMF)
is equal to the rate change of the magnetic flux, i.e. E=–∆Φ/Δt, where the negative sign
arises from the Lenz’s law, E is the induced EMF, Φ is the magnetic flux in [Tesla-m2],
given by (magnetic field B) × (area of an electric circuit loop), and t is the time in
[seconds]. The unit of EMF is [V]=[T m2]/[s].
Example: A bar magnet is moved rapidly toward a 40-turn,
circular coil of wire. As the magnet moves, the average value of
magnetic flux density B over the area of the coil increases from
0.0125 T to 0.450 T in 0.25 s. If the radius of the coil is 3.05 cm,
the magnitude of the induced emf can be found as follows:
Answer: The initial and final magnetic fluxes through the coil are
Φ i = Bi A = (0.0125T)π (0.0305m) 2 = 3.65 × 10 −5 Tm 2
Φ f = Bf A = (0.45T)π (0.0305m) 2 = 1.32 × 10 −3 Tm 2
The change of magnetic flux is ΔΦ=Φf–Φi. The induced voltage over the 40-turn coil is
| ε |= N
Φf − Φi
= 40 × (1.28 × 10 −3 Tm 2 ) / 0.25s = 0.205V
tf − ti
In a dynamic microphone, the induced voltage is small, and the
induced voltage is of the order of mV (10–3V) that needs
amplification for an audible sound. Ribbon microphone uses
Faraday’s law for transduction.
IV Condenser microphone
Two conductor sheets can hold charges between two plates. The
amount of charge depends on the distance between these two
sheets. When the sound wave exerts pressure on the condenser
sheet, the distance change and the charge on each plate varies. This
sets up electric current. This process converts sound energy to
electric energy.
What is the difference between a speaker and a microphone?
Acoustic Lab 09: Transducer; Page 2
The microphone is a device that converts sound into electrical energy. They are used in
radio broadcasting, recording, and sound amplifying systems. Its basic component is a
diaphragm that responds to the pressure or velocity of the sound waves. Various forms of
microphone were developed independently in 1877 by Emile Berliner, David E. Hughes,
Thomas A. Edison, and others. It was first used as a telephone transmitter. Variety of
microphones includes into Condenser microphone (including Electret condenser
microphone), Dynamic microphone, Ribbon Microphone, Carbon microphone,
Piezoelectric microphone, Fiber optic microphone, Laser microphone, Liquid
microphone, MEMS microphone, and Speakers as microphones.
In a condenser microphone, also called a capacitor microphone or electrostatic
microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce
changes in the distance between the plates. There are two methods of extracting an audio
output from the transducer thus formed: DC-biased and radio frequency (RF) or high
frequency (HF) condenser microphones. With a DC-biased microphone, the plates are
biased with a fixed charge (Q). The voltage maintained across the capacitor plates
changes with the vibrations in the air, according to the capacitance equation (C = Q / V),
where Q = charge in coulombs, C = capacitance in farads and V = potential difference in
volts. The capacitance of the plates is inversely proportional to the distance between them
for a parallel-plate capacitor.
RF condenser microphones use a comparatively low RF voltage, generated by a lownoise oscillator. The oscillator may either be amplitude modulated by the capacitance
changes produced by the sound waves moving the capsule diaphragm, or the capsule may
be part of a resonant circuit that modulates the frequency of the oscillator signal.
Demodulation yields a low-noise audio frequency signal with very low source
impedance. The absence of a high bias voltage permits the use of a diaphragm with looser
tension, which may be used to achieve wider frequency response due to higher
compliance. The RF biasing process results in a lower electrical impedance capsule, a
useful byproduct of which is that RF condenser microphones can be operated in damp
weather conditions that could create problems in DC-biased microphones with
contaminated insulating surfaces.
An electret microphone is a relatively new type of capacitor microphone invented at Bell
laboratories in 1962 by Gerhard Sessler and Jim West. The externally applied charge
described above under condenser microphones is replaced by a permanent charge in an
electret material. An electret is a ferroelectric material that has been permanently
electrically charged or polarized. The name comes from electrostatic and magnet; a static
charge is embedded in an electret by alignment of the static charges in the material, much
the way a magnet is made by aligning the magnetic domains in a piece of iron.
Due to their good performance and ease of manufacture, hence low cost, the vast majority
of microphones made today are electret microphones; a semiconductor manufacturer
estimates annual production at over one billion units. Nearly all cell-phone, computer,
PDA and headset microphones are electret types. They are used in many applications,
from high-quality recording to built-in microphones in small sound recording devices and
telephones. Though electret microphones were once considered low quality, the best ones
PRELAB 09: Transducer; Page 3
can now rival traditional condenser microphones in every respect and can even offer the
long-term stability and ultra-flat response needed for a measurement microphone. Unlike
other capacitor microphones, they require no polarizing voltage, but often contain an
integrated preamplifier that does require power (often incorrectly called polarizing power
or bias). This preamplifier is frequently phantom powered in sound reinforcement and
studio applications.
Dynamic microphones use electromagnetic induction. They are robust, relatively
inexpensive and resistant to moisture. Coupled with their potentially high gain before
feedback, they are ideal for on-stage use. A small movable induction coil, positioned in
the magnetic field of a permanent magnet, is attached to the diaphragm. When the sound
wave moves the diaphragm, the coil moves in the magnetic field and produces a varying
current in the coil through electromagnetic induction. A single dynamic membrane does
not respond linearly to all audio frequencies. Some microphones for this reason utilize
multiple membranes for the different parts of the audio spectrum and then combine the
resulting signals. Combining the multiple signals correctly is difficult and designs that do
this are rare and tend to be expensive.
Ribbon microphones are similar to moving coil microphones in the sense that both
produce sound by means of magnetic induction. Basic ribbon microphones detect sound
in a bi-directional (also called figure-eight) pattern because the ribbon, which is open to
sound both front and back, responds to the pressure gradient rather than the sound
pressure. Though the symmetrical front and rear pickup can be a nuisance in normal
stereo recording, the high side rejection can be used to advantage by positioning a ribbon
microphone horizontally, for example above cymbals, so that the rear lobe picks up only
sound from the cymbals. Crossed figure 8, or Blumlein pair, stereo recording is gaining
in popularity, and the figure 8 response of a ribbon microphone is ideal for that
application.
A carbon microphone, also known as a carbon button microphone (or sometimes just a
button microphone), use a capsule or button containing carbon granules pressed between
two metal plates like the Berliner and Edison microphones. A voltage is applied across
the metal plates, causing a small current to flow through the carbon. One of the plates, the
diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure
to the carbon. The changing pressure deforms the granules, causing the contact area
between each pair of adjacent granules to change, and this causes the electrical resistance
of the mass of granules to change. The changes in resistance cause a corresponding
change in the current flowing through the microphone, producing the electrical signal.
Carbon microphones were once commonly used in telephones; they have extremely lowquality sound reproduction and a very limited frequency response range, but are very
robust devices.
A crystal microphone or piezo microphone uses the phenomenon of piezoelectricity —
the ability of some materials to produce a voltage when subjected to pressure — to
convert vibrations into an electrical signal. An example of this is Rochelle salt (potassium
sodium tartrate KNaC4H4O6·4H2O), which is a piezoelectric crystal that works as a
transducer, both as a microphone and as a slimline loudspeaker component. Crystal
microphones were once commonly supplied with vacuum tube (valve) equipment, such
as domestic tape recorders. Their high output impedance matched the high input
Acoustic Lab 09: Transducer; Page 4
impedance (typically about 10 MΩ) of the vacuum tube input stage well. They were
difficult to match to early transistor equipment, and were quickly supplanted by dynamic
microphones for a time, and later small electret condenser devices. The high impedance
of the crystal microphone made it very susceptible to handling noise, both from the
microphone itself and from the connecting cable.
A fiber optic microphone converts acoustic waves into electrical signals by sensing
changes in light intensity, instead of sensing changes in capacitance or magnetic fields as
with conventional microphones.
Laser microphones are often portrayed in movies as spy gadgets. A laser beam is aimed
at the surface of a window or other plane surface that is affected by sound. The slight
vibrations of this surface displace the returned beam, causing it to trace the sound wave.
The vibrating laser spot is then converted back to sound. In a more robust and expensive
implementation, the returned light is split and fed to an interferometer, which detects
movement of the surface. The former implementation is a tabletop experiment; the latter
requires an extremely stable laser and precise optics.
Early microphones did not produce intelligible speech, until Alexander Graham Bell
made improvements including a variable resistance microphone/transmitter. Bell's liquid
transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid
added. A sound wave caused the diaphragm to move, forcing a needle to move up and
down in the water. The electrical resistance between the wire and the cup was then
inversely proportional to the size of the water meniscus around the submerged needle.
Elisha Gray filed a caveat for a version using a brass rod instead of the needle. Other
minor variations and improvements were made to the liquid microphone by Majoranna,
Chambers, Vanni, Sykes, and Elisha Gray, and one version was patented by Reginald
Fessenden in 1903. These were the first working microphones, but they were not
practical for commercial application. The famous first phone conversation between Bell
and Watson took place using a liquid microphone.
The MEMS (MicroElectrical-Mechanical System) microphone is also called a
microphone chip or silicon microphone. The pressure-sensitive diaphragm is etched
directly into a silicon chip by MEMS techniques, and is usually accompanied with
integrated preamplifier. Most MEMS microphones are variants of the condenser
microphone design. Often MEMS microphones have built in analog-to-digital converter
(ADC) circuits on the same CMOS chip making the chip a digital microphone and so
more readily integrated with modern digital products.
A loudspeaker, a transducer that turns an electrical signal into sound waves, is the
functional opposite of a microphone. A conventional speaker is constructed much like a
dynamic microphone (with diaphragm, coil and magnet), it can actually work “in
reverse" as microphones. The result, though, is a microphone with poor quality, limited
frequency response (particularly at the high end), and poor sensitivity. In practical use,
speakers are sometimes used as microphones in applications where high quality and
sensitivity are not needed such as intercoms, walkie-talkies or Video game voice chat
peripherals, or when conventional microphones are in short supply.
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Acoustic Lab 09: Transducer; Page 6