Measurement of the inherent noise of a microphone
The measurement of low sound levels is increasingly in demand. The human ear covers an enormous dynamic range from 0 to 120dB. High sound levels can be picked up relatively easily by measurement microphones. Low sound levels close to the threshold of hearing are still a challenge today.
An important feature of a measurement microphone is its self noise. It consists of the noise of the capsule and the electrical noise of the preamplifier.
How do we measure the inherent noise of a measurement microphone ?
The measurement microphone is placed in a sound-insulating box. The noise voltage at the output of the preamplifier is measured. This inherent noise is rated A or CCIR. In addition we measure the noise in 1/3-octave bands
If possible, we replace the capsule with an equivalent capacitance and measure the electrical noise of the preamp separately
Intrinsic noise of the microphone and lowest measuring range of a sound level meter
In the case of acoustic silence, the measuring device shows the self noise. However, the useful level to be measured should be 6 to 10 dB above the inherent noise. If a microphone has an inherent noise of 20dB, the lowest measurable level is 26dB. This lowest measuring range and intrinsic noise are often mixed and leads to confusion.
How high is the inherent noise of different typical measurement microphones?
Simple measurement microphones have an inherent noise level of 34dB(A). Typical 1/2" capsules with 50mV/Pa (the workhorses) are around 15-20dB(A). Specialized high-performance microphones reach 0-5dB(A).
Audio samples from different measurement microphones
Measurement setup: We used a mechanical clock placed 50cm in front of the microphone.
- Behringer ECM8000 , very inexpensive measurement microphone with XLR connector and 48V phantom power. Due to the small capsule size, only noise can be heard. The self-noise is about 34dB(A)
- MiniDSP UMIK-1 , inexpensive USB measurement microphone with a capsule similar to the ECM8000. At least the clock can be heard, but with a lot of noise.
- ATD5-T with 1/2" class 1 capsule (50mV/Pa) intrinsic noise is about 15dB(A) and is caused by the thermal noise of the capsule.
- GRAS 40HL , an extremely low-noise measurement microphone with 200V polarization voltage. This microphone represents the reference class and has an inherent noise level of 5dB(A). Such a complete system costs about 6000 euros. The data sheet can be found here
scope of services
We measure the inherent noise acoustically and electrically (if the capsule can be replaced by a replacement capsule)
We measure in third-octave bands from 5Hz to 20kHz (ultrasonic up to 200kHz on request)
Broadband value A and CCIR rated
