1. Signal Conditioning Requirements for Measuring Sound Pressure
To make sound measurements, you need amplification, current excitation, and AC coupling. In addition, you should consider the sampling rate and dynamic range of your measurements.
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. This means that 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).
As mentioned in the section above, microphones require an external current to be supplied to power the amplifier. When choosing a DAQ system, you should 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.
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, which dramatically increases the resolution and usable dynamic range of the system.
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 an accurate representation of data at that rate requires a sampling rate 10 times higher.
To be sure that you are sampling the correct range of frequencies, add a lowpass filter before the sampler and analog-to-digital converter (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 is the most important consideration when choosing a DAQ 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 DAQ system be higher than the dynamic range of the sensor.
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.
Transducer Electronic Data Sheet (TEDS) Sensors
TEDS-capable sensors carry a built-in, self-identification EEPROM that stores a table of parameters and sensor information. A TEDS contains information about the sensor such as calibration, sensitivity, and manufacturer information. TEDS sensors have two modes of operation: an analog mode, which allows the sensors to operate as transducers measuring physical phenomena, and a digital mode, which allows the user to write and read information to and from the TEDS. Many DSA devices support modes for Class I TEDS sensors without any additional hardware. Some devices require an accessory such as the BNC-2096 to allow the user to digitally communicate with the EEPROM on Class I TEDS sensors.
2. NI-Compatible Microphones
Included here is a small selection of microphones from G.R.A.S. Sound & Vibration. This manufacturer and others offer a much wider selection of transducers for your specific applications.
National Instruments recommends prepolarized microphones and constant current powered (CCP) preamplifiers for use with NI devices that provide Integrated Electronic Piezoelectric (IEPE) signal conditioning.
- G.R.A.S. half-inch free-field prepolarized microphone and CCP preamplifier, 50 mV/Pa, 3.15 Hz to 20 kHz, TEDS, Model 46AE
- G.R.A.S. quarter-inch integrated array microphone and preamplifier, 50 mV/Pa, TEDS
3. NI Platforms for Measuring Acoustics
Benchtop or Field Measurements
An NI CompactDAQ system consists of a chassis, NI C Series I/O modules, and software. Chassis can connect to a host computer over USB, Ethernet, or 802.11 WiFi or operate stand-alone with a built-in controller. With over 50 measurement-specific modules and 1-, 4-, and 8-slot chassis, NI CompactDAQ provides a flexible, expandable platform to meet the needs of any electrical or sensor measurement system.
Embedded Data-Logging Measurements
NI CompactDAQ stand-alone chassis offer a high-performance platform for embedded measurements and data logging. The 8-slot chassis feature an integrated computer and nonvolatile storage, so that NI CompactDAQ can be deployed without an external computer.
Extreme Ruggedness and Advanced Control
NI CompactRIO is a reconfigurable embedded control and acquisition system. The CompactRIO hardware architecture includes a reconfigurable FPGA chassis and an embedded controller. CompactRIO can be used in a variety of embedded control and monitoring applications.
C Series I/O Modules With Integrated Signal Conditioning for Measuring Sound
NI 9234 24-Bit Universal Analog Input
High-Performance and High-Channel-Count Systems
The PXI platform provides a variety of chassis, controller, and module options to create a measurement system that meets your specific application needs. NI SC Express strain modules can provide up to 0.02 percent accuracy at up to 102.4 kS/s. PXI can scale to thousands of strain and bridge-based measurement channels for large applications, and provides both wired and wireless synchronization options.
- Configurable desktop, rack-mount, and distributed multichassis solutions
- Compatibility with a wide range of sensors and transducers
- More than 1,500 PXI modules available, including CAN, vision, motion, and GPS
PXI I/O Modules With Integrated Signal Conditioning for Measuring Sound
The NI SC Express family features PXI Express DAQ modules with integrated signal conditioning for sensor measurements, such as microphones.
NI PXIe-4499 16 Ch Simultaneously Sampled, 24-Bit
The NI PXIe-4499 is a high-accuracy DAQ module specifically designed for high-channel-count sound and vibration applications. With 16 simultaneous channels in a single PXI Express module, it has twice the density of any other simultaneously sampled device from NI. InfiniBand to BNC connectors allow for seamless connectivity, and each analog input has software-selectable AC/DC coupling. The module features software-configurable 4 mA IEPE and TEDS for microphones and accelerometers and 24-bit resolution ADCs with 114 dB dynamic range. It also has four gain settings up to +30 dB for input ranges from ±316 mV to 10 V.
The NI PXIe-4492 is a high-accuracy DAQ module specifically designed for sound and vibration applications. Featuring eight simultaneous channels in a single PXI Express module, the NI PXIe-4492 has flexible channel configuration. This module also uses the InfiniBand to BNC connector, but it has only two gain settings up to +20 dB for input ranges from ±1 V to 10 V.
Learn more about Making Sound Pressure Measurements With LabVIEW.
Plug-In Measurement System for Measuring Acceleration
National Instruments industry-leading USB and PCI DAQ devices deliver true high-performance measurements while benefiting from the simplicity and portability of USB or plug-in PCI modules. They range from low-cost, single-function devices to high-performance modular systems, and are ideal for a variety of applications from simple data logging to embedded OEM systems.
NI USB-4431 Portable Input/Output IEPE Module
The NI 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.
The NI PCI-4462 high-accuracy DAQ board is specifically designed for sound and vibration applications. With 24-bit sigma-delta ADCs, it is ideal for applications including noise emission test and acoustic holography. It has six gain settings for input ranges from ±316 mV to 42.4 V. The PCI-4462 features four simultaneously sampled analog inputs at up to 204.8 kS/s and software-configurable AC/DC coupling and IEPE conditioning.