UNIVERSITY OF SALFORD CRN: 23457
SCHOOL OF COMPUTING, SCIENCE AND ENGINEERING
SEMESTER TWO EXAMINATION
PROGRAMMES:
BSc (Hons) Acoustics
BSc (Hons) Audio Technology
BSc (Hons) Digital Broadcast Technology
BLOCK CODES:
S/AT/F1
S/AU/F1
S/DBT/F1
AUDIO SYSTEMS
Tuesday 6 May 2008 09:30 – 11:30
Instructions to Candidates
This paper contains six questions.
Answer FOUR questions.
Each question is worth 25 marks, making 100 marks in total.
This examination is worth 50% of the marks for the module.
There is a formula sheet at the end of the paper.
The speed of sound for all questions is 340 ms-1
1
1.
(a) With the aid of a diagram, describe the basic operation of a moving coil microphone.
(4 Marks)
(b) On a clearly labelled diagram, sketch the ideal response of a cardioid microphone.
(4 Marks)
(c) With the aid of a diagram, describe the basic construction of a microphone with a
cardioid response.
(4 Marks)
(d) A microphone with a cardioid response has an on-axis sensitivity of –45dB re 1V/Pa
(i) Calculate the on-axis sensitivity in mV/Pa
(ii) The microphone is subject to an on-axis sound source that generates a sound
pressure level at the microphone of 80dB re 20Pa. Calculate the r.m.s. open
circuit voltage generated by the microphone.
(iii) The microphone has an output impedance of 150 and is connected to a
mixing desk with an input impedance of 1000 Calculate the r.m.s. voltage
at the input to the mixing desk.
(iv) Calculate the r.m.s. voltage at the input to the mixing desk when the
microphone is subject to a sound pressure level of 80dB re 20Pa from a
source positioned at 45 degrees to the microphone axis.
(4 + 3 + 3 +3 = 13 Marks)
2
2.
(a) Describe how two directional microphones could be used as a coincident pair to
make a stereo recording and explain how this works.
(5 Marks)
(b) Two matched microphone capsules, directivity K(1+2cos) are used as a coincident
pair with an angular separation of 100 degrees. Calculate the difference, in dB,
between the two microphone outputs for a source that is on-axis for one of the
microphones.
(5 Marks)
(c) Describe how two omnidirectional microphones could be used to make a stereo
recording and explain how this works.
(5 Marks)
(d) Two omnidirectional microphones are placed in front of a 10m wide stage. The
microphones are 500mm apart and each microphone is placed an equal distance from
the centre line of the stage. Both microphones are 5m in front of the stage. Calculate
both the frequencies and the levels of the maxima and minima of the comb filter
response when a performer stands at the extreme left of the stage.
(10 Marks)
10m
Stage
5m
5m
Microphones
500mm
3
3.
(a) With reference to analogue to digital conversion of audio signals, explain the
following terms:
(i) Sample frequency
(ii) Quantisation noise
(iii) Anti-aliasing filter
(iv) Clock jitter
(v) Oversampling
(3 + 3 + 3 + 3 + 3 = 15 Marks)
(b) In an analogue to digital conversion system, the sample value is represented using 8
bits in 2’s complement format. The decimal number 120 represents an analogue
voltage of 1V. Calculate the binary representation of the following analogue
voltages:
(i) 0.5V
(ii) -0.5V
(5 + 3 = 8 Marks)
(c) For the analogue to digital conversion system in part (b), calculate the maximum
voltage that the system can handle before clipping occurs.
(2 Marks)
4. This question relates to perceptual coding schemes.
(a) With the aid of a diagram, describe how a masking tone changes the threshold of
human hearing.
(5 Marks)
(b) Hence, explain why perceptual encoders can use different numbers of bits to
represent different parts of the audio signal.
(5 Marks)
(c) Draw a block diagram of a general perceptual encoder and describe how it works.
(10 Marks)
(d) Discuss the advantages and disadvantages of coding audio data using a perceptual
encoding system.
(5 Marks)
4
5.
(a) Analogue audio signals may be transmitted using radio frequency carriers either by
amplitude modulation of the carrier (AM) or frequency modulation of the carrier
(FM). With the aid of diagrams, describe:
(i) amplitude modulation;
(ii) frequency modulation.
(4 + 4 = 8 Marks)
(b) Discuss the relative merits of amplitude modulation and frequency modulation.
(4 Marks)
(c) Explain how stereo information is conveyed in frequency modulation transmissions.
(4 Marks)
(d) Explain how Digital Audio Broadcast is fundamentally different from analogue radio
transmissions. Describe the advantages that digital audio broadcast holds over
analogue broadcast systems.
(9 Marks)
5
6.
(a) Describe the factors that affect the choice of cone size for a low frequency, moving
coil loudspeaker.
(5 Marks)
(b) Explain the term passive first order cross-over network and draw a circuit diagram
showing how and where such a network would be used.
(5 Marks)
(c) The sensitivity of a particular loudspeaker is 90dB re 20Pa at 1m for 1W input
power. A pair of these loudspeakers is connected to the stereo outputs of a power
amplifier that delivers 100W into each channel. Calculate the sound pressure level at
a listening position 5m from each loudspeaker. State any assumptions that you make
and discuss whether these are valid.
(8 Marks)
(d) A pure tone is fed to the loudspeaker and the output from the speaker measured on a
spectrum analyser. In addition to the fundamental, the analyser shows two significant
harmonics with levels of –50 and –55 dB re the fundamental.
(i) Calculate the percentage harmonic distortion for each of the harmonics.
(4 Marks)
(ii) Calculate the total harmonic distortion as a percentage.
(3 Marks)
6
Audio Systems
Equation Sheet
Note: there is no significance in the order that the equations are listed.
Voff axis 1 cos
Cardioid off-axis response
Vonaxis 1 cos0
1
First Order Cross-over Network R 2fL
2fC
S
Microphone Sensitivity Level = 20Log10
S
o
SPL1 SPL2
Multiple Sources SPLTotal 10Log10 10
10
10 10 ....
R1
R R .Vin
Potential divider voltage =
1 2
P
Power Level = 10 Log10
P
0
r
Reduction in SPL from r1 to r2 = 20 Log10 2
r
1
S/N Ratio = 6.02n + 1.76 dB
P
Sound Pressure Level = 20Log10
P
0
V 2 V 2
THD = 20Log 1 2
V1
V
Voltage Level = 20Log10
V
0
7
8
9
10