Frequency range of a microphone
The frequency range of a microphone is defined as the interval between its upper limiting frequency and its lower limiting frequency. With today’s microphones you can cover a frequency range starting from around 1Hz and reaching up to 140 kHz.
Low frequency measurements require a microphone with a well controlled static pressure equalization with a very slow venting. High frequency measurements are very sensitive to diaphragm stiffness, damping and mass as well as diffraction.
The frequency ranges of various G.R.A.S. microphones are shown in the chart below. Different colours are used to distinguish between pressure (dark grey), free-field (orange) and random incidence (light grey) microphones.
Upper limiting frequency
The upper limiting frequency is linked to the size of the microphone compared with the wavelength of sound. Since wavelength is inversely proportional to frequency, it gets progressively shorter at higher frequencies. The smaller the diameter of the microphone, the higher are the frequencies it can measure. The sensitivity of a microphone is also related to its size which also affects its dynamic range.
Lower limiting frequency
The lower limiting frequency of a microphone is determined by its static pressure equalization system. Basically, a microphone measures the difference between its internal pressure and the ambient pressure. If the microphone was completely airtight, changes in barometric pressure and altitude would result in a static deflection of its diaphragm and, consequently, in a change of frequency response and sensitivity. To avoid this, the microphone is manufactured with a static pressure equalization channel for equalising the internal pressure with ambient pressure. Equalization must be slow enough to avoid affecting the measurement of dynamic signals.