Dept. of Mech. Engineering University of Kentucky 1 Two Vacuums Shopvac Bosch.

1Dept. of Mech. EngineeringUniversity of Kentucky

Two Vacuums

Shopvac Bosch


Bosch Vacuum

Exhaust flows through foam


Shopvac Vacuum


Sound Quality

Bosch (original) Bosch (w/o foam)

Shopvac Bosch (w/o 1st peak)


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.”

5.74

7.70 8.00

4.61

0123456789

10

BOSCH Foam BOSCH NoFoam

Other Vacuum BOSCH No 1stPeak

Ave

rage

Rat

ing


Foam Inside Bosch Vacuum


Sound Intensity (Shopvac)


Sound Intensity (Bosch)

Lesson 2Noise Measurements

Slides © 2006 by A. F. Seybert


Instrumentation

PC Spectrumanalyzer

Display

Function Generator

• sine wave• square wave

Loud-speaker

Sound level meter


SLM Calibration

94 dB tone @ 1 kHz

Adjust until display reads 94 dB

Calibrator

Calibration should be checked each time SLM is turned on and before it is turned off.


Pure Tone Sounds

• Pure tones are sounds at a single frequency

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

Time - s

Soun

d Pr

essu

re (P

a)


time

Frequency Spectrum

Fourier Series of a Square Wave

tAtAtAtf 111 5sin543sin

34sin4

1 13 15 17 19


Reference Quantities and Decibel Scales

Example: what is the sound pressure level of a pure tone sound having a peak sound pressure of 1 Pa?

Definition of the decibel:

pa20

)dB(log20log102

10

ref

ref

rms

ref

rmsP

p

pp

ppL

)dB(0.9110202

1log20

2

610

PL


Typical Sound Pressure Levels

Source/Environment LP (dB) prms (Pa)

Launch noise, near payload bay 160 2,000Heavy artillery near gunner’s head 140 200Large jet engine, 30 m 120 20Inside textile factory (uncontrolled) 100 2Shouted male voice, 1 m 80 0.2Two-person conversation 60 0.02Residential neighborhood, night 40 0.002Empty symphony hall 20 0.0002Average threshold of hearing (1 kHz) 0 0.00002


Sound Level Meter Block Diagram

Micro-phone

MeterFilter

CircuitAmp Weighting

Networks RectifierRMSMeter

To oscilloscope or analyzer if desired

pi

Soundpressure

e0 e1

e2

e3 e4

A, B, C, flat Fast/Slow


The Electrodynamic (Moving Coil) Microphone

Sound WavesVibrate Diaphragm Voltage output from coil is

Proportional to velocityOf Diaphragm

(the inverse of a loudspeaker)


The Piezo-electric Microphone


The Condenser (Capacitor) Microphone


Condenser Microphone - Schematic

SoundWaves

Diaphragm (foil, 0.0001” thick)Air gap, 0.001”

Back plate withdamping holes

case

Quartzinsulator

e0

Polarizationvoltage

Eb e0

Polarizationvoltage

Eb

Variable capacitance

Equivalent

Circuit

Protective Grid


Characteristics of the Condenser Microphone

• High sensitivity – 25 to 50 mV/Pa• Requires polarizing voltage of 150-200 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• DelicateB&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


Characteristics of the Condenser Microphone• Sensitivitas tinggi - 25 sampai 50 mV / Pa• Membutuhkan tegangan polarisasi dari 150-200 V dc (diperlukan untuk

awalnya mengisi mikrofon); menentukan sensitivitas• Respon frekuensi yang luas (dibatasi oleh frekuensi resonansi diafragma

pada frekuensi tinggi dan ventilasi pada frekuensi rendah) • linearitas baik pada berbagai tingkat suara• Keluaran impedansi tinggi - memerlukan pengkondisian sinyal dekat dengan

diafragma• Rentan terhadap kelembaban saat suhu mendekati titik embun • Halus

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


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


Octave Band Filters

Frequency (Hz)63125

250 1000500

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


Octave Band Filters (Partial List)

Center Frequency (Hz) Lower Band Limit (Hz) Upper Band Limit (Hz)

63 44 88

125 88 177

250 177 355

500 355 710

1000 710 1420

2000 1420 2840

4000 2840 5680

8000 5680 11360

16000 11360 22720


One-Third Octave Band Filters (Partial List)

Center Frequency (Hz) Lower Band Limit (Hz) Upper Band Limit (Hz)

50 44.7 56.2

63 56.2 70.8

80 70.8 89.1

100 89.1 112

125 112 141

160 141 178

200 178 224

250 224 282

315 282 355


Sound Pressure Level (Overall Level = 119.4 dB)

60

70

80

90

100

110

120

10 100 1000 10000

Frequency (Hz)

SP

L (d

B)

Octave Band

1/3 Octave Band

Narrowband - 5 Hz

Example Same Spectrum, Various Frequency Bands


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.

Tugas 1. dikumpul paling lambat senin, 26 Maret 2012

• Buat makalah tentang : – Piezo electric microphone – Condensor microphone – 1 oktaf, 1/3 oktaf, narrowband

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Dept. of Mech. Engineering University of Kentucky 1 Two Vacuums Shopvac Bosch.

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