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Laboratory 1 Measurement of Sound

Laboratory 1 Measurement of Sound. I. Introduction to Sound Level Meter (SLM) A. Functioning of SLM 1. Microphone 2. Pre-Amplifier 3. Weighting 4. Voltmeter B. Controls 1. Gain 2. Weighting 3. Meter “Speed” 4. Other. Laboratory 1 Measurement of Sound. II. Exercises A. SLM

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Laboratory 1 Measurement of Sound

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  1. Laboratory 1Measurement of Sound I. Introduction to Sound Level Meter (SLM) A. Functioning of SLM 1. Microphone 2. Pre-Amplifier 3. Weighting 4. Voltmeter B. Controls 1. Gain 2. Weighting 3. Meter “Speed” 4. Other

  2. Laboratory 1Measurement of Sound II. Exercises A. SLM 1. Controls 2. Steady Tone, Tone Burst 3. Wavelength Effect 4. Weighting Effect B. Shop Signals a. Effect of Source Doubling (3dB) b. Effect of Distance Doubling (6dB) c. Direct vs. Reverberant d. Near vs. Far

  3. The Sound Level Meter Microphone Weighting RMS Pre-Amp Network Voltmeter

  4. Microphone From B&K

  5. The Sound Level Meter Microphone - Converts sound pressure fluctuations into proportional voltage fluctuations Pre-Amplifier - Conditions the weak voltage signal  Boosts so that ordinary voltmeter can measure it  Reduces output impedance so that measurement does not alter the signal

  6. The Sound Level Meter Weighting Network - Modifies signal to correspond with the appropriate human response to the signal  Excludes signals with frequency <20Hz or > 20kHz  Boosts signals with frequency 500-3000Hz, since the corresponding pressure fluctuations are more audible  Nature of modification depends on the response being evaluated RMS Voltmeter - Provides a numerical readout of the weighted signal  Logarithmic scale, dB re. 20X10-6N/m2  Meter movement damped to reduce excessive fluctuation, or averaged to slow down response

  7. 20 10 0 ) -10 B d ( -20 n i a G -30 -40 -50 -60 Frequency Response Characteristics of the A, B, C and D Weighting Networks D A C B and C D B A 4 1 2 3 10 10 10 10 Frequency (Hz)

  8. dBA

  9. Meter Response Controls FAST - Meter responds quickly to step changes in continuous sound level. Approximately 0.2 sec required SLOW - Meter responds slowly to step changes in continuous sound level. Approximately 1.0 sec required IMPULSE - Meter responds to maximum RMS value of repetitive impulsive sounds PEAK - Meter responds to maximum peak value of impulsive sound, even of single responses

  10. Actual Signal Fast Response Slow t Controls 1.Gain Control - Sets amplifier. Usually tied to the meter display 2.Weighting Selector - A, C, Linear 3.Meter “Speed” Selector - Controls meter response Peak 50ms Impulse 35ms Fast 200ms Slow 1000ms

  11. Controls 4.Other Controls - added features SEL / LEQ / SPL / MAX / MIN Sound Incidence - Frontal / Random Peak - RMS AC Out / DC Out External Filter In / Out

  12. The Microphone in the Sound Field dB +5 Free - Field Response 0 Pressure Response - 5 Random Incidence Response Hz 50 200 1000 5000

  13. Exercises 0. CALIBRATION 1. STEADY TONE : a. Compare dBA vs dBC b. Compare dBA(fast) vs dBA(slow) 2. FLUCTUATING TONE : a. Compare dBA vs dBC b. Compare dBA(fast) vs dBA(slow) 3. RANDOM SIGNAL : a. Compare dBA vs dBC

  14. Exercises 4. WAVELENGTH EFFECT : a. With Position in Environment b. With Proximity of Obstacles c. With Microphone Orientation d. With Position of Operator 5. SHOP SIGNALS : a. Effect of Doubling Numbers of Sources b. Effect of Doubling Distance From Source c. Direct vs. Reverberant Field d. Near vs. Far Field e. Noise Control Concepts  Reduce Major Source First  Reduce Equivalent Sources Together  Do Not Overkill the Major Source

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