Sound Sensors

Types of Sound Sensors

Figure 1. A microphone is a transducer that converts acoustical waves into electrical signals.

A microphone is a transducer that converts acoustical waves into electrical signals. The most common instrumentation microphone, a condenser microphone, operates on a capacitive design. The cartridge from the condenser microphone uses basic transduction principles and will transform the sound pressure to capacitance variations, which are then converted to an electrical voltage. This is accomplished by taking a small thin diaphragm and stretching it a small distance away from a stationary metal plate, called a “backplate.” A voltage is applied to the backplate to form a capacitor. In the presence of oscillating pressure, the diaphragm will move, which changes the gap between the diaphragm and the backplate. This produces an oscillating voltage from the capacitor, proportional to the original pressure oscillation.

Figure 2. The most common instrumentation microphone, a condenser microphone, operates on a capacitive design.

Instrumentation microphones can operate on their own or with an integrated amplifier as in IEPE sensors. Piezoelectric microphones use a crystal structure to generate the backplate voltage. In general, IEPE microphones are preferable because they perform the amplification at the signal source reducing the amount of susceptible noise.

See the Sound and Vibration Transducers Guide
View the Microphone Handbook

Designing the Right Measurement System for Sound Sensors

To make sound measurements, you will need amplification, current excitation, and AC coupling. In addition, you should consider the sampling rate and dynamic range of your measurements.

Amplification

The charge produced by piezoelectric microphones is very small, yielding output voltage levels in the millivolt range. The piezoelectric crystal is a very high-impedance source and therefore requires a high-input impedance low-noise detector. So, the sensor must be connected to a charge-sensitive amplifier to decrease noise and reduce the output impedance.

IEPE sensors have a charge-sensitive amplifier built inside them as close as possible to the transducer. This amplifier accepts a constant current source and varies its impedance with respect to a varying charge on the piezoelectric crystal. You can see this change in impedance as a change in voltage across the inputs of the microphone. Thus, the microphone uses only two wires per axis for both sensor excitation (current) and signal output (voltage).

Current Excitation

As mentioned in the section above, microphones require an external current to be supplied to power the amplifier. In choosing a data acquisition system it is important to be aware of the excitation voltage the accelerometer requires. Depending on the sensor some platforms may not provide an adequate amount of excitation to work with a particular sensor.

AC Coupling

The signal acquired from the sensor consists of both DC and AC components, where the DC portion offsets the AC portion from zero. AC coupling removes the DC offset in the system by means of a capacitor in series with the signal. An AC-coupled sensor system eliminates the long-term DC drift that sensors have due to age and temperature effect, dramatically increasing the resolution and the usable dynamic range of the system.

Sampling Rate

When performing audio analysis, the frequency of interest is an important aspect to consider. The human ear can detect sounds up to 20,000 Hz. To provide accurate representation of data at that rate requires a sampling rate 10 times higher.

Filtering

To be sure that you are sampling the correct range of frequencies, add a lowpass filter before the sampler and ADC. This ensures that you attenuate higher-frequency noise and that these aliasing components above the sampling rate do not distort the measurement.

Dynamic Range

Dynamic range is the most important consideration when choosing a data acquisition system for IEPE microphones. Dynamic range is a measure of how small you can measure a signal relative to the maximum input signal the device can measure. Expressed in decibels, the dynamic range is 20 log (Vmax/Vmin). For example, a device with an input range of ±10 V and a dynamic range greater than 110 dB may have a voltage ratio of 106. Thus, with a maximum signal of 10 V, the smallest signal that you can see on the device is 10 µV. To get appropriate readings, it is important that the dynamic range on the data acquisition system be higher than the dynamic range of the sensor.

Simultaneous Sampling

When measuring multiple microphones in an array it is crucial that they all be sampled at the same time. Simultaneous sampling provides phase values that can be used to calculate the position where the sound occurs. To provide the best phase accuracy, each channel must have its own ADC.

View the Sound and Vibration Measurements How-To Guide
Learn 10 questions to ask when selecting your sound and vibration measurement system

NI Measurement Systems for Sound Sensors

PXI

Figure 3. The PXI platform provides a complete range of functionality for sound monitoring and analysis with high-performance signal conditioning and up 272 synchronized channels per chassis.

The PXI platform provides a complete range of functionality for sound monitoring and analysis with high-performance signal conditioning and up 272 synchronized channels per chassis. National Instruments offers a variety of PXI modules that provide IEPE signal conditioning, 24-bit resolution, antialiasing filters, up to 118 dB dynamic range, and up to 204.8 kS/s simultaneously sampled channels. In addition, PXI modules offer either two inputs and two outputs or up to 16 inputs making the platform ideal for high-channel-count microphone array applications and applications that require simultaneous generation and acquisition of signals.

Learn more about PXI hardware for sound measurements
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NI USB-4431

Figure 4. The USB-4431 is recommended as a rugged and portable low-cost solution for sound measurements.

The USB-4431 recommended as a rugged and portable low-cost solution for sound measurements includes four analog inputs and one analog output channel. The USB-4431 sound module offers 24-bit resolution, ±10 V input range, up to 100 dB dynamic range, and up to 102.4 kS/s simultaneously sampled channels. This high-speed USB solution incorporates IEPE signal conditioning for microphones, antialiasing filters, and software-selectable AC/DC coupling.

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Sound and Vibration Software

National Instruments sound and vibration software provides a complete software solution for all acoustic; electroacoustic; noise, vibration, and harshness (NVH); and machine condition monitoring applications. Based on an open analysis capability and a flexible measurement library, the NI Sound and Vibration Measurement Suite and NI Sound and Vibration Toolkit present a unique software-based measurement approach to creating customized applications.

Learn more about the Sound and Vibration Analysis Software

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