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

44 CHAPTER 3. MEASURING SOUNDfactors. In general one can state that with a Type 1 meter the error will beless than 1 dB. The accuracy of a Type 2 device is approximately 2 dB.3.5 Calibration of measurement systemsA microphone is usually accompanied with a calibration chart upon delivery.On this map the frequency sensitivity is mapped (see Figure 3.11). Thereexist numerous calibration methods for microphones or measuring systemsa a whole. One has recorded that when measuring sound pressure levels,the best results are obtained when a pistonphone is used for the calibrationof the system (see Figure 3.12). A pistonphone (piston calibrator) consistsof an engine that moves a few pistons back and forth. The calibration isdone by placing the junction piece of the pistonphone over the microphoneand switching on the device. A pistonphone will typically generate a signalof 250 Hz at 124(±0,5) dB. Oscillator based soundlevelcalibrators are light,small and battery-backed. The electrical oscillator controls a piezoelectricalelement that causes a membrane to move. The sound pressure level thatis generated is 94 ± 0.3 dB. The resonance frequency typically is 1000Hzsince at this frequency possible weighting has no influence. his frequency isstabilized by the Helmholtz resonator with a natural frequency of 1000Hz,that is formed by a cavity behind the membrane.3.6 Presence of the observerThe presence of the instruments and the observer in the sound field willperturb the measurement. If measurements in a diffuse field are executed,the error will usually be small. If one wants to measure in the proximity ofa source, the observer shall obviously not be in the direct line between thesource and measurement device. Preferably the instrument is placed on atripod and the observer steps back at least half a meter behind and sidewaysof the instrument. Octave band or small band measurements are much moresensitive to the presence of the observer. Usually in this case the microphoneis mounted on a tripod and connected with a cable of at least half a meter tothe other equipment. When measuring in an anechoic room, all equipment(except for the microphone) and the observer shall be placed outside theroom.