1. Two Vacuums Shopvac Bosch Dept. of Mech. Engineering 1
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2. Bosch Vacuum Exhaust flows through foam Dept. of Mech. Engineering 2
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3. Shopvac Vacuum
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4. Sound Quality Bosch (original) Bosch (w/o foam) Shopvac
Bosch (w/o 1st peak)
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5. Sound Quality: Jury Test
Note: Rate each vacuum on a scale from 1 to 10 where 1 is “very quiet” and 10 is “very loud.”
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6. Foam Inside Bosch Vacuum
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7. Sound Intensity (Shopvac)
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8. Sound Intensity (Bosch)
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9. Sound Intensity (Bosch)
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10. Lesson 2 Noise Measurements
Slides © 2006 by A. F. Seybert

11. Instrumentation Function Generator Loud- Sound level meter sine wave
square wave
Loud-
speaker
Sound level meter
PC Spectrum
analyzer
Display 11
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12. SLM Calibration 94 dB tone @ 1 kHz Calibrator
Adjust until display reads 94 dB
Calibration should be checked each time SLM is turned on and before it is turned off. 12
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13. Pure Tone Sounds Pure tones are sounds at a single frequency13
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14. Fourier Series of a Square Wave
time
Frequency Spectrum 14
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15. Reference Quantities and Decibel Scales
Definition of the decibel:
Example: what is the sound pressure level of a pure tone sound having a peak sound pressure of 1 Pa? 15
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University of Kentucky

16. Typical Sound Pressure Levels
Source/Environment LP (dB) prms (Pa)
Launch noise, near payload bay ,000
Heavy artillery near gunner’s head
Large jet engine, 30 m
Inside textile factory (uncontrolled)
Shouted male voice, 1 m
Two-person conversation
Residential neighborhood, night
Empty symphony hall
Average threshold of hearing (1 kHz) 16
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17. Sound Level Meter Block Diagram
Micro-
phone
Meter
Filter
Circuit
Amp
Weighting
Networks
Rectifier
RMS
To oscilloscope or analyzer if desired pi
Sound
pressure e0 e1 e2 e3 e4
A, B, C, flat
Fast/Slow 17
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18. The Electrodynamic (Moving Coil) Microphone
Sound Waves
Vibrate Diaphragm
Voltage output from coil is
Proportional to velocity
Of Diaphragm
(the inverse of a loudspeaker) 18
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19. The Piezo-electric Microphone19
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20. The Condenser (Capacitor) Microphone20
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21. Condenser Microphone - Schematic
Sound
Waves
Diaphragm (foil, ” thick)
Air gap, 0.001”
Protective Grid
Quartz
insulator
Back plate with
damping holes e0
Polarization
voltage Eb
Variable capacitance
case e0
Polarization
voltage Eb
Equivalent
Circuit 21
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22. Characteristics of the Condenser Microphone
High sensitivity – 25 to 50 mV/Pa
Requires polarizing voltage of V dc (needed to initially charge the microphone); determines sensitivity
Wide frequency response (limited by diaphragm resonance frequency at high frequency and venting at low frequency)
Good linearity over a wide range of sound levels
High output impedance – requires signal conditioning close to diaphragm
Susceptible to humidity when temperature is close to the dew point
Delicate
B&K 4190 ½” microphone:
Sensitivity: 50 mV/Pa
Polarization: 200 V dc
Frequency range: 5 Hz – 10 kHz (+/- 1 dB); resonance freq. = 14 kHz
SPL range: 15 dB – 148 dB (3% distortion)
Operating temperature range: -30 to 150 degrees C 22
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23. Frequency Analysis of Noise
Uses of frequency analysis:
Noise source diagnostics (resonances, etc.)
Qualification testing of prototypes
Conformance to company, industry, or international standards
Other
Types of frequency analysis:
Narrowband (FFT based) – best frequency resolution (reveals narrow peaks due to resonances, harmonics); lots of data
Octave band – best for broadband noise – limited data
One-third, one-twelfth octave bands – narrower versions of octave band filters 23
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24. Octave Band Filters etc. 0.707 x center frequency Frequency (Hz)63
125
250
1000
500
Each octave band filter has a fixed center frequency and is twice as wide as the one before it. Each octave band filter may be divided into three one-third octave band filters for more frequency resolution.
etc.
0.707 x center frequency 24
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25. Octave Band Filters (Partial List)
Center Frequency (Hz)
Lower Band Limit (Hz)
Upper Band Limit (Hz) 63 44 88
125
177
250
355
500
710
1000
1420
2000
2840
4000
5680
8000
11360
16000
22720 25
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University of Kentucky

26. One-Third Octave Band Filters (Partial List)
Center Frequency (Hz)
Lower Band Limit (Hz)
Upper Band Limit (Hz) 50
44.7
56.2 63
70.8 80
89.1
100
112
125
141
160
178
200
224
250
282
315
355 26
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27. Example Same Spectrum, Various Frequency Bands27
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28. Equal Loudness Contours
Experiment: ask subjects to compare a second tone to one at 1 kHz by increasing or decreasing the level of the second tone until it the two tones have equal loudness. Sounds at lower frequency must have a higher SPL to be judged equivalent in loudness to the tone at 1 kHz. The hearing mechanism does not have a flat frequency response. 28
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University of Kentucky