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

Bosch Vacuum Exhaust flows through foam Dept. of Mech. Engineering 2 University of Kentucky 2

Shopvac Vacuum Dept. of Mech. Engineering University of Kentucky 3

Sound Quality Bosch (original) Bosch (w/o foam) Shopvac Bosch (w/o 1st peak) Dept. of Mech. Engineering University of Kentucky 4

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.” Dept. of Mech. Engineering University of Kentucky 5

Foam Inside Bosch Vacuum Dept. of Mech. Engineering University of Kentucky 6

Sound Intensity (Shopvac) Dept. of Mech. Engineering University of Kentucky 7

Sound Intensity (Bosch) Dept. of Mech. Engineering University of Kentucky 8

Sound Intensity (Bosch) Dept. of Mech. Engineering University of Kentucky 9

Lesson 2 Noise Measurements Slides © 2006 by A. F. Seybert

Instrumentation Function Generator Loud- Sound level meter sine wave square wave Loud- speaker Sound level meter PC Spectrum analyzer Display 11 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

Pure Tone Sounds Pure tones are sounds at a single frequency 13 Dept. of Mech. Engineering University of Kentucky

Fourier Series of a Square Wave time Frequency Spectrum 14 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

Typical Sound Pressure Levels Source/Environment LP (dB) prms (Pa) Launch noise, near payload bay 160 2,000 Heavy artillery near gunner’s head 140 200 Large jet engine, 30 m 120 20 Inside textile factory (uncontrolled) 100 2 Shouted male voice, 1 m 80 0.2 Two-person conversation 60 0.02 Residential neighborhood, night 40 0.002 Empty symphony hall 20 0.0002 Average threshold of hearing (1 kHz) 0 0.00002 16 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

The Piezo-electric Microphone 19 Dept. of Mech. Engineering University of Kentucky

The Condenser (Capacitor) Microphone 20 Dept. of Mech. Engineering University of Kentucky

Condenser Microphone - Schematic Sound Waves Diaphragm (foil, 0.0001” 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 Dept. of Mech. Engineering University of Kentucky

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 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 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky

Example Same Spectrum, Various Frequency Bands 27 Dept. of Mech. Engineering University of Kentucky

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 Dept. of Mech. Engineering University of Kentucky