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

34 CHAPTER 3. MEASURING SOUNDconverted in an electric signal. This is amplified and optionally filtered in thefrequency domain. The electrical signal is then read out by the measuringdevice. This reading can be done in several ways (digital display, computerscreen, analog display.The microphone is the critical element in every measuring system forsound. The microphone detects the sound pressure variations and convertsthem into electrical signals. This can be done in several ways:Ceramic or piezoelectrical microphones. The working principle ofceramic or piezoelectrical microphones is based on the properties of thepiezoelectrical material. This material generates an electrical voltagewhen mechanical pressure is applied on it. Ceramic microphones arerobust and not sensitive to moisture and other environmental impacts.Other advantages are their relatively low cost and the fact that noexternal voltage source is necessary.Condenser microphones. The condenser microphone is used to executeprecise measurements. Condenser microphones make use of twoelectrically chargedplates withanair gapinbetween. Oneoftheplatesis a light membrane that moves under influence of the incoming soundwaves. Figure 3.1 displays a construction of such a microphone. Betweenthe membrane and the base plate an electrical charge is createdby a voltage supply. Due to the incoming sound wave the distancebetween the base plate and membrane changes, casing the capacityto change. This results in variations on the voltage over the microphonethat that proportional with the incoming pressure (see Figure3.2). Condenser microphones can be designed to have a sensitivitythat does not change much over time and to have a frequency responsethat is very flat (the sensitivity is the ratio of the measured tensionover the sound pressure, this value is of the order of a few mV/Pa).Moreover they are very insensitive to temperature changes. Due tothis stability condenser microphones are designated to use for precisionmeasurements. Since an notable polarisation tension needs to be appliedover the capacitor, a to high humidity can give problems. The useof a heating element can present a solution to this problem, if lengthymeasurements need to be conducted.Electret microphones an electret is a polymer film with an electricalcharge bound to the molecules. An electret condenser microphone ismade by applying the electret on a perforated metal plate, and shieldit off on the front side with a plastic membrane on which a thin metal