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Microphones and Acoustics

Frequently Asked Questions

Get your most commonly asked acoustics questions answered.

Section I: Definitions and Terminology
Section II: Microphone Recommendations
Section III: Calibration and Testing
Section IV: Specification Clarifications
Section V: Specialty Microphone Applications
Section VI: Maintenance and Handling

If you don’t see the answer to your question, call our 24/7 SensorLineSM 716-684-0001 to speak with an application engineer or visit Ask the Acoustics Experts.

What is the minimum sound pressure level that a PCB® microphone can measure?

The microphone system noise floor is dependent upon not only the components of the system, but also the frequency that the end user wishes to measure. The inherent noise of microphone, or commonly called noise floor of a microphone, is defined on the PCB® specification sheet in dBA (how the human ear perceives the noise floor) and in an un-weighted linear output in dB, sometimes referred to as “Linear” or “Z-weighted”. When a preamplifier is added to a microphone, there is additional noise introduced to the system due to electrical noise. At low frequencies the noise floor is impacted more by the preamplifier and at high frequencies the dominant noise source stems from the microphone. It is important to look at the combination of all significant noise sources.

For the noise floor level, or minimum amount of measureable pressure, you need to review the Cartridge Thermal Noise (CTN) rating of the microphone, and the noise floor of the electronics (preamplifier + signal conditioner + DAQ). The cartridge thermal noise specification provides the lowest measurable sound pressure level that can be detected above the inherent noise floor of the microphone. Each microphone will have its own noise characteristics, and the diameter of the microphone will have a major impact on the frequencies and noise levels it can detect. Depending upon the size and design, the microphone can dominate as the inherent noise source at higher frequencies, while the electrical noise of the preamplifier will dominate at lower frequencies. Below is a typical representation of the noise effect at different frequencies for a 1/2” diameter microphone when used in conjunction with a preamplifier.



The overall total noise level is the total area under the curve of all of the frequencies in the specification range of that microphone model. Individual frequencies, as in an FFT, or 1/3rd band octaves, have smaller bands and less area under a curve, and thus, will have lower noise floor measuring capabilities. Below is an example of PCB’s new industry exclusive low noise prepolarized ½” microphone, model 378A04. The noise floor specification for the 378A04 is listed at < 6.5 dBA guaranteed and 5.5 dBA typical. The teal blue bars below represent the A-weighted results of an individually tested microphone and preamplifier system.



Even though the specification is < 6.5 dBA, none of the teal blue bars pass the -5 dBA (yes, that is minus 5 dBA) for the microphone and preamplifer combination tested and shown above. That is because the 1/3 band octaves shown have smaller band widths and thus, a smaller area under the curve and lower noise floor number.

The red outline bars are linear and not weighted and expressed in dB. These are higher because A-weighting represents signals where our human ears attenuate sounds. The human ear is most sensitive in the 1 kHz to 5 kHz range and attenuates signals above and below that. This is why the red lines are higher in certain areas.

Proper microphone selection requires that the sound pressure levels to be tested fall within the dynamic range of the microphone. The limits of the dynamic range are specified by the inherent noise on the low end and the harmonic distortion limit on the high end. In general, the smaller the microphone diameter, the greater the high-end sound pressure level capability. Larger diameter microphones are recommended for low range sound pressure measurements, since the inherent noise or cartridge thermal noise specifications are typically lower.

What is the minimum sound pressure level that a PCB® microphone can measure?

The microphone system noise floor is dependent upon not only the components of the system, but also the frequency that the end user wishes to measure. The inherent noise of microphone, or commonly called noise floor of a microphone, is defined on the PCB® specification sheet in dBA (how the human ear perceives the noise floor) and in an un-weighted linear output in dB, sometimes referred to as “Linear” or “Z-weighted”. When a preamplifier is added to a microphone, there is additional noise introduced to the system due to electrical noise. At low frequencies the noise floor is impacted more by the preamplifier and at high frequencies the dominant noise source stems from the microphone. It is important to look at the combination of all significant noise sources.

For the noise floor level, or minimum amount of measureable pressure, you need to review the Cartridge Thermal Noise (CTN) rating of the microphone, and the noise floor of the electronics (preamplifier + signal conditioner + DAQ). The cartridge thermal noise specification provides the lowest measurable sound pressure level that can be detected above the inherent noise floor of the microphone. Each microphone will have its own noise characteristics, and the diameter of the microphone will have a major impact on the frequencies and noise levels it can detect. Depending upon the size and design, the microphone can dominate as the inherent noise source at higher frequencies, while the electrical noise of the preamplifier will dominate at lower frequencies. Below is a typical representation of the noise effect at different frequencies for a 1/2” diameter microphone when used in conjunction with a preamplifier.



The overall total noise level is the total area under the curve of all of the frequencies in the specification range of that microphone model. Individual frequencies, as in an FFT, or 1/3rd band octaves, have smaller bands and less area under a curve, and thus, will have lower noise floor measuring capabilities. Below is an example of PCB’s new industry exclusive low noise prepolarized ½” microphone, model 378A04. The noise floor specification for the 378A04 is listed at < 6.5 dBA guaranteed and 5.5 dBA typical. The teal blue bars below represent the A-weighted results of an individually tested microphone and preamplifier system.



Even though the specification is < 6.5 dBA, none of the teal blue bars pass the -5 dBA (yes, that is minus 5 dBA) for the microphone and preamplifer combination tested and shown above. That is because the 1/3 band octaves shown have smaller band widths and thus, a smaller area under the curve and lower noise floor number.

The red outline bars are linear and not weighted and expressed in dB. These are higher because A-weighting represents signals where our human ears attenuate sounds. The human ear is most sensitive in the 1 kHz to 5 kHz range and attenuates signals above and below that. This is why the red lines are higher in certain areas.

Proper microphone selection requires that the sound pressure levels to be tested fall within the dynamic range of the microphone. The limits of the dynamic range are specified by the inherent noise on the low end and the harmonic distortion limit on the high end. In general, the smaller the microphone diameter, the greater the high-end sound pressure level capability. Larger diameter microphones are recommended for low range sound pressure measurements, since the inherent noise or cartridge thermal noise specifications are typically lower.