Microphones and speakers
- describe in terms of electrical and electromagnetic effects, the operation of
–microphones, including electret-condenser, crystal, dynamic and velocity microphones
–dynamic loudspeakers;
•describe the effects of baffles and enclosures for loudspeakers in terms of the interference of soundwaves due to phase difference;
•interpret frequency response curves of microphones, speakers, simple sound systems and hearing,including loudness (phon);
•evaluate the fidelity of microphones and loudspeakers in terms of
–the intended purpose of the device
–the frequency response of the system
–physical construction (qualitative);
1.[2 marks](2004 Q1)
Ryan and Lee are investigating some properties of sound, and sound systems. They have been given a signal generator that provides frequencies in the audible range, a small loudspeaker, and an oscilloscope that they use to observe the amplitude of the signal from a microphone.
They first adjust the signal generator so that sound is emitted from the loudspeaker.
The figure below shows the oscilloscope trace of the signal from the microphone.
What is the frequency of the signal from the microphone?
Choose one of the microphones labelled M1 or M2 above.
Using the microphone of your choice, answer the following two questions.
2.[2 marks](2004 Sample Q1)
Identify the type of microphone which you have chosen.
A. / electret condensor / B. / crystalC. / velocity / D. / dynamic
3.[4 marks](2004 Sample Q2)
A sound pressure wave is incident on the microphone. Describe how the microphone of your choice detects the wave and produces the signal output.
4.[2 marks](2004 Sample Q3)
One of the world’s most popular microphones is the Shure SM58. The microphone is shown below, together with a graph detailing its frequency response.
Which one of the following (A–D) best describes the range of frequencies that can be reliably detected by this microphone?
A. frequencies below 200 Hz
B. frequencies above 10 000 Hz
C. frequencies below 200 Hz and above 10 000 Hz
D. frequencies between 200 Hz and 10 000 Hz
5.[2 marks](2004 Sample Q4)
A synthesiser produces two pure sine tones, one at a frequency of about 1000 Hz, and another at a higher frequency of 4000 Hz. The SM58 microphone is used to capture these sounds. The synthesiser has been adjusted to ensure both tones have the same amplitude at the microphone. An oscilloscope is used to observe the signal output of the microphone.
Which one of the following diagrams (A–D) best represents the waveform observed?
6.[3 marks](2004 Pilot Q1)
Complete thetable below by writing the type of microphone which best matches the description in the spaces provided.
Choices – crystal, dynamic, electret-condensor, velocity
Microphone type / DescriptionThe sound pressure moves the cone which is attached to a coil of wire. This coil moves in the field of a magnet and produces a voltage.
The sound pressure moves the metallic strip in the magnetic field, generating a voltage between the ends of the strip.
The sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. This causes a change in capacitance and produces a current.
7.[2 marks](2004 Pilot Q4)
The frequency response of four different speaker systems is being investigated. The figure below shows the individual frequency response curves of each speaker system. A test CD is used to compare the performance of each system.
• Track 1 is a series of tones between 100 Hz and 1 000 Hz.
• Track 2 is musical recording of several instruments and sounds in the frequency range
500–10 000 Hz.
In answering the following, you may assume that the CD player and amplifier shown below do not distort the sounds.
Which speaker system could produce the loudest sound for CD Track 1?
A. / Speaker 1 / B. / Speaker 2 / C. / Speaker 3 / D. / Speaker 48.[2 marks](2004 Pilot Q5)
Which speaker system could reproduce CD Track 2 with the best fidelity?
A. / Speaker 1 / B. / Speaker 2 / C. / Speaker 3 / D. / Speaker 49.[2 marks](2004 Pilot Q6)
A test of each speaker is performed. In each case, the volume of sound produced by the speaker increased when the speaker was mounted on a wooden board as shown above. Why does this increase occur?
10.[4 marks](2004 Pilot Q7)
The combination of multiple speakers in an enclosure can be used to optimise the overall frequency response of a system. Identify the best two of the speakers (1–4) shown on previous page, for inclusion in an infinite baffle enclosure. Explain the reasoning behind your choice.
11.[3 marks](2005 Q2)
Listed below are three types of microphones.
. crystal
. dynamic
. electret-condenser
Complete the following table by choosing, from the choices above, the type of microphone that matches the physical effect on which its operation depends.
Microphone type / Principle of operationelectromagnetic induction
piezoelectric effect
capacitance
12.[2 marks](2005 Q3)
A high fidelity loudspeaker system comprising individual speakers mounted on a baffle board is shown in thediagram below.
Explain the role of the baffle board in improving the performance of the loudspeaker system above.
13.[2 marks](2005 Q4)
The frequency response curve for one of the speakers is shown below.
Which type of speaker is most likely to have a response curve similar to that shown above?
A. / sub-woofer / B. / wooferC. / mid-range speaker / D. / tweeter
14.[3 marks](2005 Q5)
A system uses a single, wide-frequency response speaker. Explain why the quality (fidelity) will deteriorate asthe listener moves off the centreline. Hence explain why a multiple-loudspeaker system, as shown above, would be more satisfactory.
15.[2 marks](2006 Q6)
Rachel and Bruce have assembled some laboratory equipment and are planning a series of sound-related experiments.
An audio-signal generator is used to drive a small loudspeaker, which emits sound uniformly in all directions. The audio power from the loudspeaker is kept constant at all frequencies used in the experiments. A sound-levelmeter is used to measure sound intensity.
The sound intensity is measured 8.0 m directly in front of the loudspeaker.
Bruce then places the loudspeaker in a box as shown below. Rachel observes that the sound intensity level at 8.0 m directly in front of the loudspeaker has now increased.
In terms of wave theory, explain how the box increases the sound intensity level. Make sure that your answer includes references to both interference and phase.
16.[2 marks](2006 Q10)
Which one of the following (A - D below) best describes the physical operating principle of the electret-condensor microphone?
A. / electromagnetic induction / B. / piezo-electric effectC. / capacitance / D. / electrical resistance
17.[2 marks](2006 Q11)
Briefly explain the operation of a dynamic microphone.
18.[3 marks](2007 Q1)
In the paragraph below, options to complete each sentence are given within the brackets. Circle the correct option in each case.
A loudspeaker is removed from its enclosure box. When an audio signal is connected,the loudspeaker produces sound waves at both its front and rear surfaces. The sound wavesfrom the front of the loudspeaker are[ in phase with / out of phase with / of much higher intensity than ] the waves from therear. For a listener in front of the speaker the waves from the front[interfere constructively / interfere destructively / diffract destructively ] with thosegenerated from the rear surface. This affects the [ frequency / intensity / directional spread ]of the resulting sound.
19.[2 marks](VCE 2007 Q6)
Explain the operation of a dynamic microphone.
20.[2 marks](VCE 2007 Q7)
The figure below shows the frequency response curve for a dynamic microphone.
From the data on the graph, what makes this microphone particularly suitable for use by a singer?
21.[2 marks](VCE 2008 Q1)
Three types of microphone are
• electret-condenser
• crystal
• dynamic.
The physical properties on which the operation of these microphones depend are listed below (not in order).
• electromagnetic induction
• piezo-electric effect
• capacitance
Which one of the boxes correctly matches the microphone type to the relevant physical property?
electret-condenser / crystal / dynamicA. / piezo-electric effect / electromagnetic induction / capacitance
B. / capacitance / piezo-electric effect / electromagnetic induction
C. / capacitance / electromagnetic induction / piezo-electric effect
D. / electromagnetic induction / piezo-electric effect / capacitance