Download Microphones - Vicphysics

Unit 4 Physics 2007
Chapter 12
Sound Systems 1
Sound Systems
Human ears do not respond equally to all frequencies. Generally they respond best to
frequencies around 3000 Hz. The graph shows the sound intensity that is required for a person
with normal hearing to just hear each frequency. This means that the low point on the graph
corresponds to the frequency that the ear is most sensitive to. Because the ear is sensitive to
this frequency, it will appear louder. (Have you ever noticed that some ads on TV are very loud.
This is partly because they have a large component of around 3000 HZ, so the ear is very
sensitive to them).
The curved line is a line representing all the frequencies that
sound as loud as each other. So on this graph, a 100 Hz
sound will seem equally as loud as a 1000 Hz sound. This
indicates that our ears are less sensitive to 100 Hz sounds,
because we need more energy/m 2 to hear the 100 Hz sound.
You need to have a very good understanding of this graph
Phon
The phon is the unit of equivalent loudness. It measures how loud a sound is perceived to be
compared to a reference sound – normally at 1kHz (1000Hz). Graphs can be drawn for an
average human ear/brain combination. The phon level is read from the graph below. Hence 60
phons means “as loud as a 60dB, 1000Hz tone”.
To find what intensity level is required for a sound of frequency 5kHz to have a loudness of
80phons, read along the 80-phon curve until it intersects with 5kHz.
80dB
The differences in loudness depend
on the sensitivity of each individual
ear, which obviously varies from
person to person. What may be too
loud for your parents is perfectly OK
for you. The loudness also
depends on the frequency. The
"perceived loudness" of a sound
depends on the response of a
particular ear to a particular pitch.
Loudness and intensity are not the
same, though they are related.
Intensity is measurable, but
loudness is subjective.
Unit 4 Physics 2007
Chapter 12
Sound Systems 2
Audibility range for the human ear
Example
1997 Exam
The figure below represents the sensitivity curve for hearing for an average person; that is, the
lowest intensity sound that can be heard at a given frequency.
To test the sensitivity of hearing at different frequencies, a student with normal hearing sits near a
small speaker which is connected to a frequency generator. The frequency generator is tuned to
200 Hz. The intensity is increased slowly from zero until the student can just hear the sound.
The generator is now tuned to a frequency of 600 Hz without changing its output level.
Question 8
By how many dB must the sound intensity level be decreased so that the student can just hear
the sound?
Unit 4 Physics 2007
Chapter 12
Sound Systems 3
2000 Exam
In an experiment to determine hearing sensitivity, a student uses a set of earphones and a signal
generator. For a range of frequencies, he determines the sound intensity (measured at the ear) at
which the sound just becomes inaudible. Figure 2 shows the sensitivity of hearing of the student.
Figure 2
Question 7
At what frequency is the student's hearing most sensitive?
Question 8
What range of frequencies can the student hear if the sound intensity at the student's ear is
1.0 x 10-11 W m-2?
Microphones
A device that converts a mechanical vibration into another type of signal is called a transducer.
Microphones are transducers that transform sound energy into electrical energy that can be
magnified and then changed back to sound. The electrical signal has the same frequency as the
sound. The amplitude of the electrical signal should ideally be proportional to the amplitude of
the variations in air pressure of the sound. A good microphone must be able to produce an
electrical output that is proportional to the amplitude of the sound over the complete frequency
range.
The diaphragm of a microphone vibrates when sound waves are incident on it. The conversion of
sound waves into electrical signals in microphones is by means of either electrostatic or
electromagnetic techniques.
Microphones also need to have some directional characteristics.
A wide range of microphones have been developed, each with a specific purpose and capable of
responding in a particular way to the sound waves around it. These include electret-condenser,
crystal, dynamic microphones.
The choice of microphone will depend on its application—simple voice recording, high-fidelity
studio recording or directional recording of particular sounds without background noise. It will also
be affected by price constraints, as high-fidelity microphones cost several hundred dollars.
Unit 4 Physics 2007
Chapter 12
Sound Systems 4
Electret condenser microphone
This type of microphone is made with the diaphragm as one plate of a parallel plate capacitor and
can also be referred to as an electrostatic microphone.
An electret is a piece of dielectric material that is
permanently polarised. One side of an electret is
permanently positive, the other side is permanently
negative. The front plate is very thin and usually covered
in a very fine layer of gold.
The sound pressure causes the front plate to vibrate, this
changes the spacing between the diaphragm and the
stationary back plate. This causes a change in
capacitance. A voltage is supplied to the plates and thus
the amount of charge on the plates varies, producing a
current.
The diaphragm of a condenser microphone can be very light compared with the dynamic
microphone and, consequently, can respond more quickly and at higher frequencies. Condenser
microphones usually have a greater frequency range and a better linearity than the dynamic
microphones.
Unit 4 Physics 2007
Chapter 12
Sound Systems 5
Crystal microphone
In the crystal microphone, the diaphragm is attached to
a thin piece of piezoelectric crystal such as quartz.
Piezoelectric crystals produce a current when subjected
to changes in pressure. Sound causes the diaphragm to
vibrate. This produces a changing pressure on the
crystal. This produces an electric signal current whose
size is proportional to the air pressure variations of the
sound.
Dynamic microphone
Dynamic microphones have a coil attached to a
diaphragm suspended in the magnetic field of a magnet.
Pressure changes (sound) causes the diaphragm to
vibrate and the coil to move backwards and forwards
over the pole of the magnet. This, in turn, causes the
magnetic flux in the coil to change, inducing a current in
the coil. The size, frequency and waveform of the
induced current are proportional to the pressure
variations of the sound.
The dynamic microphone has good
frequency linearity and is relatively
strong. Good quality dynamic
microphones are used for recording
purposes.
Dynamic microphones have peaks
designed to gain clarity with stage
vocals
Examples
2004 sample paper
Choose one of the microphones labelled M1 or M2 in Figure 1 above and write your choice in the
box provided. Using the microphone of your choice, answer Questions 1 and 2.
Unit 4 Physics 2007
Chapter 12
Sound Systems 6
Question 1
Identify the type of microphone which you have chosen.
A. electret condensor
B. crystal
C. velocity
D. dynamic
Question 2
A sound pressure wave is incident on the microphone. Describe how the microphone of your
choice detects the wave and produces the signal output.
Dynamic loudspeakers
The principle of the loudspeaker is based on the induced
magnetic field that is created when a current moves
through a wire. If a current moving through a coil in one
direction makes a magnet move to the left, reversing the
current will cause it to move to the right.
The alternating current makes the coil around the fixed
magnet move backwards and forwards, creating
compressions and rarefactions in the air in front of the
cone. The larger the current that’s moving through the
wire, the larger the induced magnetic field will be, and
hence a greater force of magnetic attraction or repulsion
will be exerted by the permanent magnet. The speaker
cone will move in and out through a greater amplitude,
transferring more energy to the surrounding air
molecules and thus creating a louder sound.
The diaphragm and cone are generally composed of paper or stiff plastic. Flexible edge
suspensions, or springs, surround the outer edge and the central diaphragm. These springs
resist the force of the speaker’s movement and provide a restoring force to the cone: they return
the cone to a central rest position.
Today almost all loudspeakers are moving coil speakers. They come in a wide variety of sizes,
roughly matching the range of frequencies they have been designed to best produce: ‘woofers’
for frequencies from 30–500 Hz, mid-range loudspeakers for 500–4000 Hz, and the aptly named
small ‘tweeters’ for the high frequencies from 4–20 kHz.
Enclosures
When a speaker cone moves forward, the front surface sends out a compression wave. But at
the same time the rear of the cone is creating a rarefaction. At low frequencies (less than 200
Hz), diffraction effects cause the sound waves from the back of the loudspeaker to bend around
the outer rim of the speaker and cancel out the sound waves from the front surface. To remedy
this, the speaker is mounted in a box filled with some absorbent material. Sounds coming from
the back surface of the cone are thus contained and absorbed, so they cannot interfere with the
sound waves from the front surface.
Usually the various speakers making up the left channel of a stereo output are mounted together
in one enclosure, and those making up the right in another, although the higher frequencies don’t
suffer as much from diffraction effects. Some modern systems are now keeping speakers
separate to allow more ‘tuning’ of the listening environment. Either way, the supplied frequencies
are filtered and each range is directed to the appropriate speaker: the high-frequency signals are
sent to the tweeters and the low frequencies to the woofer or sub-woofer.
Unit 4 Physics 2007
Chapter 12
Sound Systems 7
Baffles and ports
Designers aim to stop the unwanted sound from the back of the speaker superimposing with that
from the front. Placing the loudspeaker in a large baffle will always improve the production of low
frequencies, because of the increased distance from the back to the front of the speaker. The
‘doof-doof’ of car stereo systems heard way down the street is a good example. A large, or even
infinite, distance—termed an infinite baffle—is desirable but hardly practical.
In a ported enclosure, the closed box is modified by the inclusion of a carefully designed opening
in the front. Through the ‘vented’ or ‘ported’ enclosure (also referred to as a bass-reflex monitor),
sound from the back of the speaker can be added to that from the front without cancelling it. If
the port is carefully designed, it acts like a second diaphragm driven by the backside of the
speaker. It can add an octave or more to the system’s low end frequency response.
One key to the successful design of a port is to make sure the enclosure resonance matches that
of the speaker itself. The process reverses the phase of the backwave, resulting in radiated
sound that is in phase with the sound from the front of the speaker.
Fidelity of microphones and loudspeakers
Fidelity is the degree to which a sound reproduction system accurately reproduces the original
recorded sound. A microphone is high fidelity if it responds equally well to most frequencies in
the human range (approximately 20 Hz to 20 kHz). The electrical signal produced by the
microphone should be accurately proportional to the original sound.
High fidelity
Low fidelity
Frequency response curves
Microphone frequency response curves (or characteristics) show how well microphones
respond to sounds of the same sound intensity level at different frequencies. They usually have a
vertical axis in decibels (dB) and a horizontal axis in hertz (Hz). The graph enables you to read
the variations in the power gain or loss of the microphone with the frequency of the sound.
There are different ways of representing this response. Some manufacturers set the 0 dB level at
the highest point on the curve. The graph then enables you to read off the power loss (in dB) of
the device at a particular frequency.
The diaphragm or ribbon of a microphone will have natural resonant frequencies. It will vibrate
best at these frequencies and therefore produce the strongest output electric signals. There will
be other frequencies at which it produces poor output signals. For example, the microphone
might not respond quickly enough to register high frequency sounds.
Frequency response curve for (a) a woofer and (b) a tweeter
Unit 4 Physics 2007
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Unit 4 Physics 2007
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Sound Systems 9
Unit 4 Physics 2007
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Sound Systems 10
Solutions
1997 Question 8 solution
At 200 Hz the signal generator needed to produce a sound that was 20 dB. If the frequency was
changed to 600 Hz, then the listener would be able to hear a 600 Hz signal quite clearly. The
listener only needs a 600 Hz signal to be at 10 dB for them to be able to hear it.
So the sound intensity level can be decreased by 10 dB (from 20 to 10) and the listener will still
be able to just hear it. Be very careful reading the graph, on the exam you will draw on it to avoid
any confusion.
2000 Question 7 solution
2000 Hz
Hearing is most sensitive when the ear can hear the sound with the lowest sound intensity. This
occurs at 2000 Hz according to this graph.
2000 Question 8 solution
Lowest Frequency
Highest Frequency
600Hz
10 000Hz
If the sound level is at 1.0  10-11 Wm-2, then the range of sounds that can be heard by the
student will be when the minimum sound intensity that the student can hear is either equal
to or less than this value.
 The lowest frequency that they can hear is 600 Hz, and the highest that they can hear is
10,000 Hz.
You need to use common sense to answer this, you should know that humans can’t hear
lower frequencies or higher frequencies very well. So the range of frequencies that you
can hear will be in the middle of the possibilities.
So this question is asking you to read of the two points of intersection with the line at 1.0 
10-11 Wm-2. Note, as usual, the graph is quite easy to read at these points, expect this to
happen regularly.
2004 sample exam Question 1 solution
M1 is a dynamic (moving coil) microphone.
M2 is an electret-condenser microphone, as this is the only microphone that has a permanently
charged backing plate.
2004 sample exam Question 2 solution
M1
The pressure changes on the coil move the coil in and out. This induces an EMF because the
coil is in a magnetic field, and so there is a change in flux.
M2
The pressure changes on the diaphragm at the front of the microphone moves it in and out. This
changes the space between the diaphragm and the charged backing plate. This changes the
capacitance which leads to a voltage signal that leads to a current through the resistor, and
hence a signal out.