10 Questions to Ask When Selecting Your Portable Sound and Vibration Measurement System

A lot of sound and vibration measurements need to take place in the field, onboard a vehicle, or simply away from the lab. Some of these applications include automotive pass-by noise testing, aircraft cabin acoustics, and environmental noise monitoring. For these applications, the portability of the systems must be balanced with the right measurement performance. Traditional measurement systems force trade-offs between portability and performance. Here you can see how you can configure a system that meets your needs for your application.

Example Configurations

• Ultra-portable: USB-9233 with a laptop – A bus-powered, 24-bit, 4-channel dynamic signal acquisition module acquisition with rates up to 50 kS/s, weighing less than a pound, with built-in IEPE for accelerometers and microphones. Ideal for applications requiring maximum portability.

• Up to 32 Channels with Flexible I/O: CompactRIO or CompactDAQ-9233 with a laptop – These systems are DC powered and offer silent operation, yet are still extremely portable. Connect to your laptop with either USB (CompactDAQ) or Ethernet (CompactRIO) and select modules for temperature, pressure, strain, and others.

• Maximum Performance: PXI-446x or 447x with ExpressCard MXI laptop link – NI 446x and 446x devices offer best-in-class dynamic range and input signal bandwidth and ultra fast ExpressCard for extremely high throughput and data logging.

National Instruments provides a variety of tools for creating portable sound and vibration measurement systems. You can use many different combinations of hardware and software, depending on the requirements of your system. The following is a list of 10 questions that you should ask in order to select the best NI hardware and software for your portable measurement system. For information on larger channel count systems, please look at this list of 10 Questions to Ask When Selecting Your Sound and Vibration Measurement System.

1. Which sensor(s) are you using?

In most sound and vibration applications, the commonly measured signals are acceleration, displacement, and sound pressure. Typically, the sensors used to measure these parameters are accelerometers, proximity probes, and microphones, respectively. Obviously, the type of sensor largely governs your choice of hardware and lead to the following questions for your hardware:

• What kind of signal conditioning is required?
• What is the frequency range of the sensor?
• How much dynamic range does the sensor require?

Does your application also require additional non-sound and vibration measurements, such as temperature, pressure, or strain? If so, you should look at platforms that have a diverse I/O offering so that you can acquire all of your data in a single system.

2. What signal conditioning is needed?

Before it is digitized by the data acquisition hardware, the signal from a sensor nearly always requires some type of conditioning, such as amplification, filtering, sensor excitation, and input configuration. NI has many hardware products that have built-in excitation for voltage mode or Integrated Electronic Piezoelectric (IEPE), constant current power, microphones and accelerometers. Also the ability to read data from smart sensors or Transducer Electronic Data Sheets (TEDS) may be necessary for your application. Below is a table that contains the specific values for the various signal conditioning options that are provided on each NI product.

Product	IEPE	Alias Rejection	AC Coupling	Input Configuration	Gain	TEDS
4461	4 or 10 mA	-120 dBc	3.4 Hz	Differential / Pseudo-differential	-20 to +30 dB in 10 dB increments	Yes (PCI only)
4462	4 or 10 mA	-120 dBc	3.4 Hz	Differential / Pseudo-differential	-20 to +30 dB in 10 dB increments	Yes
4472	4 mA	-110 dBc	3.4 Hz or 0.5 Hz	Pseudo-differential	-	-
4474	4 mA	-110 dBc	3.4 Hz	Pseudo-differential	-	-
USB-9233	2 mA*	-92 dBc	0.5 Hz	Pseudo-differential	-	-
9233 for CompactDAQ / CompactRIO	2 mA*	-92 dBc	0.5 Hz	Pseudo-differential	-	Yes (CompactRIO only)
USB-9234	2 mA	-92 dBc	0.5 Hz	Pseudo-differential	-	-
9234 for CompactDAQ / CompactRIO	2 mA	-92 dBc	0.5 Hz	Pseudo-differential	-	Yes (CompactRIO only)

^{*IEPE cannot be turned off}

3. What is your frequency range of interest?

All sensors have a frequency range over which they are designed to operate. Your sensor should have a frequency range large enough to cover the frequency range of interest. Likewise your digitizing hardware should have a large enough frequency range to cover the signals of interest. To prevent aliasing, NI products come with antialiasing filters, which cut the maximum frequency range of the device to a little less than one-half the maximum sampling rate, as prescribed by the Nyquist sampling theorem. Below is a table of the NI devices and their maximum frequency ranges.

Product	Analog Input Sampling Rate	Input Signal Bandwidth
4461	204.8 kS/s	92 kHz
4462	204.8 kS/s	92 kHz
4472	102.4 kS/s	47 kHz
4474	102.4 kS/s	47 kHz
USB-9233	50 kS/s	21 kHz
9233 for CompactDAQ / CompactRIO	50 kS/s	21 kHz
USB-9234	51.2 kS/s	21 kHz
9234 for CompactDAQ / CompactRIO	51.2 kS/s	21 kHz

4. What is the required dynamic range?

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, the NI 4472 has an input range of ±10 V and a dynamic range greater than 110 dB, for example a voltage ratio of 106. Thus, with a maximum signal of 10 V, the smallest signal that you can see on the NI 4472 is 10 µV. Thus, the input range and the specified dynamic range are important for determining the needs of your system. Below is a table of NI devices, their input ranges, and dynamic ranges.

Product	Dynamic Range	Input Range
4461	118 dB	+/- 316 mV to 42 V
4462	118 dB	+/- 316 mV to 42 V
4472	110 dB	+/- 10 V
4474	110 dB	+/- 10 V
USB-9233	102 dB	+/- 5 V
9233 for CompactDAQ / CompactRIO	102 dB	+/- 5 V
USB-9234	102 dB	+/- 5 V
9234 for CompactDAQ / CompactRIO	102 dB	+/- 5 V

[+] Enlarge Image
Note that the 446x devices have multiple input range settings (gain and attenuation stages) that give it different dynamic ranges at different levels.

5. How many analog input and output channels do you need?

Sound and vibration applications require various numbers of analog inputs and outputs depending on the application. With tools from NI, you can expand your system to the number of channels of your choice by simply adding more devices. For instance, using the NI 4472, with its eight input channels, you can fill an 18-slot PXI chassis with 14 modules and achieve 112 analog input channels. In addition, you can synchronize multiple-chassis up to 5,000 channels. The tables below show the number of channels each NI device offers.

Product	Synchronized Analog Input Channels			Analog Output Channels (Single Module)
	Single Module	Single Chassis	System
4461	2	34	1250	2
4462	4	68	2500	-
4472	8	112	5000	-
4474	4	20	-	-
USB-9233	4	-	-	-
9233 for CompactDAQ / CompactRIO	4	32	-	-
USB-9234	4	-	-	-
9234 for CompactDAQ / CompactRIO	4	32	-	-

Note that it is recommended that you use no more than three PCI devices in a standard desktop computer because of heating and cooling issues. You can use more than three devices in an industrial computer where proper cooling and power requirements are met.

In addition to inputs for microphones and accelerometers, PXI, CompactDAQ, and CompactRIO platforms also offer a suite of measurement modules for non-sound and vibration I/O. Some of the measurement types include:

• Temperature – thermocouple, RTD, thermistor
• Pressure
• Strain – with bridge completion
• RF – signals up to 2.7 GHz
• DC, resistance – 7 ½ digit DMMs
• CAN and other automotive communications

6. Do you need the system to operate silently?

The USB-923x, CompactDAQ, and CompactRIO systems all have no moving parts which mean no additional noise from cooling fans or any other items. PXI systems, however, utilize cooling fans to meet the power requirements of the higher performance analog I/O.

7. How will you power your instrumentation?

The USB-923x devices are powered directly from the USB port on your computer, requiring no external power for the measurement device or IEPE sensors. Sound and vibration applications that require in-vehicle measurements often have the availability of DC power. Both the CompactDAQ and CompactRIO platforms are DC powered (11-30 VDC) and PXI systems are available with AC/DC selectable power.

8. Which measurements and analysis do you need?

NI provides a number of software tools for analysis of acquired signals. Your application may require analysis such as power spectrum, octave analysis, or order analysis. LabVIEW provides a number of frequency measurements and analysis tools for various applications. The LabVIEW Sound and Vibration Toolkit and the LabVIEW Order Analysis Toolkit, which are add-on software packages for LabVIEW, provide application-specific analysis. The following chart indicates the type of analysis you can perform with the indicated software.

Measurements	LabVIEW PDS & FDS	Sound & Vibration Toolkit 4.0	Order Analysis Toolkit 2.1
FFT analysis	ü	þ
Zoom FFT		þ
Subset FFT		þ
Short-time Fourier transform (STFT)		þ
Frequency Response (FRF)	ü	þ
Pink Noise		þ
Shock Response Spectrum (SRS)		þ
Peak search		þ	ü
Sound Level		þ
Vibration Level		þ	ü
1/n Octave		þ
Weighting Filters		þ
Integration	ü	þ	ü
Harmonics	ü	þ
Distortion (THD, SINAD)	ü	þ
Intermodulation Distortion (IMD)		þ
THD + N		þ
SNR	ü	þ
Digital Filters	þ	þ
Tone Detection	ü	þ
Swept Sine		þ
Limit Testing	þ	þ	ü
Spurious Free Dynamic Range (SFDR)		þ
Crosstalk		þ
Tachometer Processing			þ
Order Power Spectrum			þ
Order Tracking			þ
Order Extraction			þ
Angular Resampling			þ
Waterfall Plot		þ	þ
Orbit & Shaft Centerline Plot			þ
Polar & Bode plots	ü	þ	þ

You evaluate the Sound and Vibration Toolkit and the Order Analysis Toolkit on-line directly from your web browser at www.ni.com/trysv.

9. Will you connect to a computer (desktop or laptop) or do you need to log data in a stand alone configuration and retrieve the data later?

PXI and CompactRIO systems have on-board storage and processing capabilities so that they can be used without connecting to a laptop or other computer. PXI systems with embedded controllers have Pentium class processors and 40 GB or larger hard drives for long data storage. However, USB and CompactDAQ systems must have a laptop connected for data storage and instrument control. PXI and CompactRIO systems can be configured with ExpressCard, Cardbus, or Ethernet links to also serve as laptop controlled instrumentation systems.

Whether or not you are connecting to a computer, you’ll also need to answer the question of how much processing power and disk capacity you need. For example, some applications may require real-time analysis, while others need continuous streaming data to disk. The performance capabilities of laptops have increased dramatically over the past few years. You can now order off-the-shelf laptops with 100+ GB serial ATA (SATA) hard drives with ultra-fast write speeds and dual-core processors for the most demanding applications. To get a better idea of the computer requirements you’ll need, check out these benchmarks for common sound and vibration applications:

• Benchmarking Data for High Performance Dynamic Signal Acquisition Applications
• System Architectures for High-Channel-Count or High-Performance Applications
• High-Channel-Count/High-Performance Application Examples

10. How much does it cost?

This is a very important question in selecting any type of sound and vibration measurement system. We provide tools that you can configure in systems for a fraction of the cost of traditional sound and vibration measurement systems. The cost of your individual system depends on the components you use. Visit ni.com/soundandvibration for the price of each product. We provide a large number of tools for your sound and vibration measurement system. Your system can be expandable, flexible, and customizable to your application using existing hardware and application software.
Related Links:
NI Sound and Vibration Portal

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