Essential step in every precision noise measurement to establishes the relationship between the sound pressure acting on a microphone and the resulting electrical output of the microphone.
Calibration is an essential step in every precision noise measurement. It establishes the relationship between the sound pressure acting on a microphone and the resulting electrical output of the microphone. There are basically two properties of a measurement microphone requiring calibration, these are: level calibration and a frequency-response calibration. Level calibration determines the absolute sensitivity of the measurement microphone. Various methods can be used, e.g. reciprocity, comparison, pistonphone or calibrator.
a) Reciprocity is normally considered the most accurate of methods but is elaborate and expensive.
b) Comparison is where the sensitivity of the microphone under test is compared with the known sensitivity of a reference microphone. It is simple and can be done with commonly-available equipment and requires minor investment.
c) A pistonphone, with a precision barometer for applying static pressure corrections, is a robust and highly reliable method of level calibration at 250Hz.
• At 250Hz, the frequency response of most microphones is flat and will give a more accurate result.
d) A calibrator is a convenient way of calibrating a microphone at 1000Hz but does not have the same precision as a pistonphone. Neither does it require static-pressure corrections.
• At 1000Hz, weighting filters have 0dB attenuation and will therefore not affect the calibration. In these cases it might be an advantage to use a 1000Hz calibration tone.
A frequency-response calibration describes the response of the microphone over a range of frequencies. Frequency-response measurements can be presented in various ways, i.e. pressure response, free field response and diffuse-field response.
Generally, pressure response is determined by using an electrostatic actuator which simulates purely an oscillating pressure exerted on the microphone’s diaphragm. Free-field and diffuse-field responses can then be arrived at by adding predetermined correction values to the measured actuator (pressure) response of the microphone.
Electrostatic actuators require no special acoustic laboratory facilities since background noise is not too critical a factor.
An electrostatic actuator consists of an electrically conductive rigid plate mounted close to, and parallel with, the microphone’s diaphragm. When an oscillating voltage is applied between the microphone’s housing and the electrostatic actuator, an oscillating force will be exerted on the diaphragm. This oscillating force simulates an oscillating sound pressure, thus making it possible to determine the response of the microphone to pressure alone. This means that the frequency response of microphones can be measured under normal circumstances, not requiring special sound insulated test chambers, as long as the background noise levels are reasonable low.
The pistonphone works on the principle of a pair of similar opposing, reciprocating pistons actuated by a precision-machined cam disc with a sinusoidal profile. The profile of the cam disc is such that the pistons follow a sinusoidal movement at a frequency equal to four times the speed of rotation. This results in a corresponding sinusoidal variation in the effective volume of
The closed coupler and, consequently, an acoustic signal within it.
The mechanical structure of the pistonphone makes this generated acoustic pressure signal very reliable and stable. By careful control of the atmospheric pressure conditions and the calibration temperature, the calibration far exceeds the requirements for class LS calibrators. Absolute calibration accuracy has been determined to be within ±0.05dB at reference conditions for the pistonphone.