Physical Principles
Measurement microphones are based on a simple physical principle: capacitance. The capacitance of a microphone is inversely proportional to the distance between the diaphragm (a taut, thin metallic foil) and the backplate (the hard plate). When the diaphragm is affected by sound pressure, it deforms, moving closer to or farther away from the backplate, which changes the system's capacitance.
The change in capacitance is then converted into a change in voltage. This principle forms the foundation for most measurement microphones.
Sizes
Measurement microphones are categorized by size, typically into 1-inch, 1/2-inch, 1/4-inch, and 1/8-inch variants. The larger the diaphragm, the smaller its stiffness, which makes it more sensitive to small variations in sound pressure. However, a larger diaphragm also limits the frequency response because it can only detect wavelengths equal to or larger than its size. Larger diaphragms produce less noise, but smaller diaphragms allow for higher frequency detection. Small microphones tend to have better omnidirectional characteristics at high frequencies.
Did You Know?
- What does a 5 nanometer diaphragm displacement mean? For a typical 1/2-inch microphone, a diaphragm displacement of just 5 nanometers occurs under a pressure of 1 pascal. Under the same pressure, a diaphragm with the size of Earth would only move 5 meters.
- How long is a 20kHz sound wave? A sound wave at 20kHz (the upper limit of the audio range) has a wavelength of 1.7cm, about the width of your index finger.
- How do errors occur? When operating a free-field microphone at 20kHz in a pressure field, errors can approach 9dB due to the difference in sound wave behavior.
- From a strand of hair to three Eiffel Towers: The Brüel & Kjær 4180 and 4160 laboratory standard microphones are incredibly stable, and their sensitivity is used as a benchmark for acoustic measurements worldwide. No sound source can produce sound pressure with sufficiently low uncertainty over a wide frequency range. The dynamic range of Brüel & Kjær's 4191 1/2-inch free-field sensor is from 20dBA to 162dB (142dB). This range is equivalent to measuring from the thickness of a single strand of hair to the height of three stacked Eiffel Towers.
- How stable are these microphones? Since 1984, we have closely monitored the sensitivity of the 4160 and 4180 standard microphones. Their stability remains within a ±0.02dB range, which means the sensitivity changes by less than 0.2%. These microphones are widely used in calibration systems worldwide.
Ensuring Stability
Brüel & Kjær's measurement sensors are highly stable, unaffected by time, temperature, humidity, or environmental pressure changes. To ensure optimal stability, we carefully select high-quality materials, employ controlled high-temperature aging simulations, relieve internal tension within the microphones, and test each microphone at every stage of production.
During manufacturing, the microphones are kept in a "Class 10" clean room, which means there are fewer than 10 particles per cubic foot, with particle sizes no larger than 0.5μm. For comparison, a typical room is around "Class 1,000,000." Given that the gap between the backplate and diaphragm is approximately 20μm, any particles larger than this size could cause stability issues, especially when condensation or temperature changes occur.
