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

3.2. THE MEASUREMENT MICROPHONE 39sound power caused by the wind in function of the wind speed. This figuredisplays a very strong sound signal caused by wind speeds above 40 km/u,even when using windscreens. The energy in the noise signal caused by thewind is the highest for low frequencies. At those wind speeds measurementsoutside are better postponed. Moreover, most norms (including the Vlaremlegislation for community noise that is discussed later) prohibit the executionof measurements at speeds higher than 5 m/s.Figure 3.6: Foam windscreens and other accessories for the microphone.Source: Bruël&Kjær.3.2.4 Division of sound in frequency bandsNoise that surrounds us is usually made up out of frequencies spread over thehearing range (20 Hz - 20 kHz). The sound pressure level of this broadbandsignal can be measured in a number of consecutive frequency intervals thatare called frequency bands. Sometimes one prefers frequency bands witha fixed width, bust mostly one uses octave or fractional octave bands. Animportant reason for this choice is that a change of frequency of one octavein the hearing range always causes the same impression of change. Thedifference in frequency between 40 and 50 Hz will be sensed similar as thedifference between 4000and 5000Hzfor example. The standard octave bandsin the audible frequency range have central frequencies (f c ) that equal 31.5