VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

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