Measuring sound using a microphone

1. What is sound pressure?     

Sound is pressure fluctuations that travel through a medium (air, liquid, or other medium that can be sensed by the human ear). The pressure oscillations/sounds are converted into electronic signals by the eardrum and transmitted to the human brain, which receives the signals and recognizes different forms of sound based on the characteristics of the signals, such as music, speech, noise, etc. Microphones work in the same way as eardrums. You can then record and analyze these signals to collect information about the characteristics of the sound's propagation path from the sound source to the microphone. For example, during noise, vibration, and harshness testing, engineers often want to reduce unwanted sounds, such as reducing the sound that affects passenger comfort during driving. Noise can be above or below the frequency range that the human ear can hear, or the sound amplitude at a certain resonant frequency. These measurements are critical for design engineers who need to reduce noise to meet emission standards or analyze equipment characteristics such as performance and life.

Humans live in a world full of sound. The human ear can perceive the sound pressure around it, so sound pressure measurement is one of the common types of measurements. The sound pressure level indicates the intensity of the sound perceived by the receiver and is expressed in Pascals (Pa). We can also measure the sound power of a sound source. The sound power level reflects the total amount of energy radiated by the sound source to the surroundings and is expressed in watts (W). It is not affected by environmental factors such as the space, the receiver, or the distance from the sound source. Power is a property of the sound source, while sound pressure is affected by the environment, reflecting surfaces, the distance between the sound source and the receiver, ambient noise, and other characteristics.

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2. How microphones work

When designing a microphone, there are many options to choose from, but externally polarized condenser microphones, prepolarized electret condenser microphones, and piezoelectric microphones are the most commonly used measurement microphones.

Condenser microphone

A condenser microphone is a type of microphone based on a capacitor design. A condenser microphone contains a metal diaphragm as one substrate of the capacitor. A metal sheet next to the diaphragm serves as the other substrate of the capacitor. When the sound field excites the metal diaphragm, the capacitance between the two substrates can change with changes in sound pressure. Applying a stable DC voltage to the substrate through a high resistor retains the charge on the substrate. The change in capacitance produces an AC output that is proportional to the sound pressure. Prepolarized microphones can charge the capacitor through an external polarization voltage or the properties of the material itself. Externally polarized condenser microphones require a supply voltage of 200V. Prepolarized microphones are powered by an IEPE preamplifier that requires a constant current source.

Piezoelectric Microphone

Piezoelectric microphones use a crystal structure to generate a backplate voltage. Many piezoelectric microphones use the same signal conditioning mechanism as accelerometers, and some also use IEPE signal conditioning to provide the polarization voltage. Although this sensor type has low sensitivity, it is durable and can measure high amplitude sound pressure. However, this microphone usually has a high background noise level. This design is suitable for shock pressure and burst pressure measurement applications.

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3. How to choose a suitable microphone?

Response Field

When choosing a microphone, you must consider the type of field it operates in. There are three types of microphones: free field, pressure field, and diffuse field. At low frequencies, these microphones operate similarly, but at high frequencies they are very different.

Free-field microphones measure the sound pressure from a single source directly at the microphone's diaphragm. These sensors measure the sound pressure that existed before the microphone entered the sound field. These microphones are best suited for open areas without hard or reflective surfaces. Anechoic rooms or more open areas are ideal for free-field microphones.

Pressure field microphones are used to measure the sound pressure in front of the diaphragm. Its amplitude and phase are the same at any position in the field. Its wavelength is relatively small and is often found in confined spaces or cavities. Examples of pressure field sensor applications include wall compression testing, wing pressure testing, and pressure testing of internal structures such as pipes, colloids, and cavities.

Sometimes the sound does not come from just one source. Diffuse-field microphones respond uniformly to sounds coming from different directions at the same time. These microphones are useful for measuring sound in churches or other places with hard, reflective walls. However, for most microphones, the pressure-field and diffuse-field responses are similar, so pressure-field microphones are often used for diffuse-field measurements as well.

Dynamic Range

The main criterion for describing sound is based on the amplitude of the sound pressure fluctuations. The lowest sound pressure amplitude that the human ear can perceive is 20 parts per million (20 μPa). The sound pressure expressed in Pascals is usually too small to be handled, so decibels (dB) are often used as a unit of measurement. This logarithmic scale can more accurately describe the human ear's response to sound pressure fluctuations.

Manufacturers specify a maximum decibel level based on the design and physical characteristics of the microphone. The maximum decibel level is the sound pressure at which the diaphragm approaches the back plate, or when the total harmonic distortion (THD) reaches a specified value (usually 3% THD). The maximum decibel level that a microphone can output in a given application depends on the voltage supplied and the sensitivity of the microphone. Before we can calculate the maximum decibel level of a microphone using a specific preamplifier and its corresponding peak voltage, we need to calculate the maximum sound pressure level that the microphone can withstand. The sound pressure level can be calculated using the following formula:

P=Pa, voltage is the peak voltage of the preamplifier.

Once the maximum sound pressure level at the microphone's peak voltage is determined, the sound pressure level can be converted to decibels using the following calculation formula:

Where P is the pressure expressed in Pascals
P0: reference sound pressure (constant, = 0.00002 Pa)

This formula gives the maximum measurable rating for a microphone when used with a specific preamplifier. To determine the minimum noise level or sound pressure required, refer to the module thermal noise rating standard for the microphone. The CTN specification provides the minimum detectable sound pressure value that is above the inherent electrical noise of the microphone. Figure 6 shows the typical noise levels of a microphone and preamplifier at different frequencies.

When selecting a microphone, you must ensure that the pressure value you are measuring is between the microphone's CTN value and the maximum rated decibel value. In general, the smaller the diameter of the microphone, the greater the upper decibel limit. Microphones with larger diameters generally have smaller CTN values ​​and are therefore often used for low-range decibel measurements.

Frequency response

Once you have determined the type of microphone field response and dynamic range you need, you can refer to the microphone specification standard to determine the usable frequency range. Microphones with smaller diameters generally have higher upper frequency limits. Conversely, microphones with larger diameters have higher sensitivity and are better suited for low-frequency detection.

Manufacturers typically set the frequency tolerance to ±2dB. When comparing different microphones, be sure to check the frequency range of the different microphones and the tolerance for a specific frequency range. If the application is not demanding, and the increased decibel tolerance is within the allowable range, it will increase the usable frequency range of the microphone. You can confirm with the manufacturer or refer to the microphone calibration table to determine the actual usable frequency range for a specific decibel tolerance.

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Polarization type

Traditional externally polarized microphones and new pre-polarized microphones are suitable for most applications, but there are differences between the two. The sensitivity of externally polarized microphones is more consistent with the temperature range of 120 ° C to 150 ° C, so it is recommended to use externally polarized microphones in high temperature environments. Pre-polarized microphones are more suitable for humid environments. Sudden changes in temperature can cause the internal capacitor structure of externally polarized microphones to short-circuit.

Since externally polarized microphones require a specific voltage of 200V, only 7-pin cables and LEMO connectors are available for configuration. Newer pre-polarized microphones are more popular because they are powered by a constant current of 2-20mA, which makes them easier to use. In this configuration, you can use standard coaxial cables and BNC or 10-32 connectors to provide current and signals to the reading device.

temperature range

Microphone sensitivity decreases when the ambient temperature reaches the maximum specified temperature of the microphone. It is important to consider the operating and storage temperatures of the microphone. Operation or storage in extreme conditions can negatively affect the microphone and increase its calibration requirements. In most cases, the system preamplifier is the limiting factor in the operating temperature range. Although a high temperature of 120°C has no effect on the sensitivity of most microphones, the required preamplifier is limited to operation in an environment of 60 °C to 80 °C.

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4. Microphone signal conditioning

When preparing a microphone to be measured with a DAQ device, there are a few things to consider to ensure that all of your signal conditioning requirements are met:

• Amplification to improve measurement accuracy and signal-to-noise ratio
• Current excitation to power the preamplifier of the IEPE sensor
• AC coupling to remove DC offset, improve resolution, and utilize the full range of the input device
• Filtering to eliminate external high-frequency noise
• Proper grounding to eliminate noise caused by currents between different ground potentials
• Dynamic range, to measure the full amplitude range of the microphone