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36 CHAPTER 3. MEASURING SOUNDFigure 3.2: Working principle of a condensator microphone. Source:Bruël&Kjær.is only 12,5mV/Pa. 1/8 inch microphones can be used for measurementsof ultrasonic sound, or when measuring impulse sounds or very loud noises.Themanufacturergivesprecisedataconcerningthesensitivity andmeasuringrange for each type of microphone.3.2.2 The sound field where measurements take placeThe sound field has influence on the measured sound pressure (see Figure3.4). To keep the influence of the sound field as low as possible, differenttypes of microphone were designed :The free field microphone. The freefield is defined asanarea whereno reflected sound waves arepresent. This microphone will compensatethe influence of the microphone on the free field. The highest accuracyis obtained when pointing the microphone to the source.Random incidence microphone for measurements in a diffuse fieldanother microphone is developed that compensates incoming soundpressure from all directions.Pressure microphone A pressure microphone gives a constant frequencyresponse of the sound field, the way it exists, including theinfluence of the microphone itself (no compensation is carried out).These microphones are useful i.a. when the sound pressure on the sideof a cavity is to be measured (e.g. exhaust systems).
3.2. THE MEASUREMENT MICROPHONE 37Figure 3.3: Frequency response of the different standard microphone sizes.Source: Bruël&Kjær.The three types of microphones can also be used in another field thanthe one they are developed for : pressure microphones can be used in diffusefields. If a free field microphone is used in a diffuse field, an electronic correctionmust be carried out. When a random incidence microphone is usedin a free field, the microphone must be turned 70 circ to 80 circ relative to thesource of the sound. A pressure microphone must be placed at a 90 circ anglerelative to the direction of the source. The National American Standards Institute(ANSI) relies on the use of random incidence microphones to composeits standards, the International Electrotechnical Commision (IEC) relies onfree field microphones (see Figure 3.5). In Belgium, norms legislation andregulations prescribe apparatus that comply with the IEC-guideline.3.2.3 Influence of the wind speedIt is common knowledge that wind in the vicinity a microphone producesadditional noise. This pollutes the signal. For measurements in open air, itis therefore advised to use a windscreen (this is a soft foam rubber sphere,as can be seen in Figure 3.6). Windscreens are essentialy transparent in theinteresting frequency range. Typical values are a weakening of 0.5 dB at 5kHz, this increases to 2 dB weakening at 12 kHz. Figure 3.7 displays the
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Chapter 6Noise control6.1 Origin of
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6.1. ORIGIN OF NOISE 101Figure 6.2:
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6.3. TACKLING NOISE TRANSMISSION 11
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6.4. RADIATION NOISE 115Figure 6.17
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6.4. RADIATION NOISE 117Figure 6.20
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Part IIINoise directives119
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124CHAPTER 7. DIRECTIVE 2000/14/EG:
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128 CHAPTER 8. NOISE ON THE WORK FL
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140 CHAPTER 9. COMMUNITY NOISE2. No
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142 CHAPTER 9. COMMUNITY NOISE
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144 APPENDIX A. MATERIAL PROPERTIES
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152 BIBLIOGRAPHY[12] Bruel and Kjae
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