Sound and Vibration Measurements: How-To Guide

This document is part of the How-To Guide for Most Common Measurements centralized resource portal.

Sound and Vibration and Piezoelectric (IEPE) Sensor Overview

Vibration occurs when a mass oscillates mechanically about an equilibrium point. A common example of a vibrating mechanical system is a spring-mass-damper system, illustrated in Figure 1. Vibrations also occur in surfaces, such as the wing of an airplane, or a gong. In many cases, vibrations are unwanted because they waste energy and cause fatigue stress and noise, and systems are usually designed to minimize these types of vibrations. Meanwhile, vibrating structures generate pressure waves, or sound, which can be desirable in the case of musical instruments.

View a 60-second video about how to take a Sound and Vibration Measurement

Figure 1. Spring-Mass-Damper System

Sound and vibration are essentially oscillations in different media, and just like vibrations can create sound, acoustic waves traveling through air can generate oscillations in solid materials as well. Because the theory behind the two is correlated, measuring sound and vibration is also similar in nature.

You can represent both sound and vibration as oscillations, and the simplest oscillations are sinusoidal waveforms expressed in terms of time as, with angular frequency ω and phase φ as constants. Angular frequency ω is represented in radians per second (rad/sec) and is related to frequency ƒ (Hz or s^-1) by the following formula: ω =2πƒ. Angular frequency is always spoken of in correlation to a phase φ, which describes an offset of the wave from a specified reference point at the initial time t₀, and is usually given in degrees or radians.

Analysis of Sound and Vibration Measurements

In real applications, the measured voltage signals are complex waveforms that contain multiple frequency components. Sound and vibration analysis usually involves identifying and examining these frequency components. To do so, you must convert the signals from the time domain to the frequency domain mathematically using Laplace, Z-, or Fourier transforms. Fourier analysis is the most common for this application because it obtains the magnitude in decibels (dB) and associated phase ω (degrees or radian) for each frequency component in a signal.

IEPE Sensors

Typical indicators for making sound and vibration measurements are acceleration and sound pressure level, respectively. These indicators are commonly measured using devices such as accelerometers (shock and vibration) and microphones (sound).

Many sensors for measuring acceleration and pressure are based on the principle of piezoelectric generation. The piezoelectric effect denotes the ability of ceramic or quartz crystals to generate electric potential upon experiencing compressive stresses. These mechanical stresses are triggered by forces such as acceleration, strain, or pressure.  In the case of microphones, acoustic pressure waves cause a diaphragm, or thin membrane, to vibrate and transfer stresses into the surrounding piezoelectric crystals. Accelerometers, on the other hand, contain a seismic mass that directly applies forces to the surrounding crystals in response to shock and vibrations. The voltage generated is proportional to the internal stresses in the crystals.

A particular class of piezoelectric sensors, known by the term integral electronic piezoelectric (IEPE), incorporates an amplifier in its design next to the piezoelectric crystals. Because the charge produced by a piezoelectric transducer is very small, the electrical signal produced by the sensor is susceptible to noise, and you must use sensitive electronics to amplify and condition the signal and reduce the output impedance. IEPE therefore makes the logical step of integrating the sensitive electronics as close as possible to the transducer to ensure better noise immunity and more convenient packaging. A typical IEPE sensor is powered by an external constant current source and modulates its output voltage with respect to the varying charge on the piezoelectric crystal. The IEPE sensor uses only one or two wires for both sensor excitation (current) and signal output (voltage).

How to Make a Sound and Vibration Measurement

The signal conditioning circuitry for measuring sound and vibration is fairly straightforward. A typical system for measuring acceleration or sound pressure level includes the following components:

•  Sensor
•  Current source to excite the sensor
•  Proper grounding to eliminate noise pick-up
•  AC coupling to remove DC offsets in the system
•  An instrumentation amplifier to boost the sensor’s signal level
•  A lowpass filter to reduce noise and prevent aliasing in the data acquisition system
•  Simultaneous sample and hold circuitry to keep multiple signals properly timed with respect to each other

As mentioned in the above section, sound and vibration measurements are highly susceptible to noise. You can reduce this effect, however, by properly grounding the system. You can avoid improper grounding resulting from ground loops or floating nodes by ensuring that either the signal conditioning input or the sensor is grounded but not both. If the sensor is grounded, you must connect it differentially. If the sensor is floating, you should connect the signal conditioning system’s inverting input to ground.

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.

For accurate measurements, the sampling rate of the system should be at least twice the frequency of the signals being acquired. To be sure that you are sampling the correct range of frequencies, add a lowpass filter before the sampler and the analog-to-digital converter. This ensures that you attenuate higher-frequency noise and that these aliasing components above the sampling rate do not distort the measurement.

Connecting Your Sensor to an Instrument

As an example, consider the NI 9234 C Series module that is designed for accelerometer and microphone measurements (see Figure 2). The NI 9234 can simultaneously sample four analog inputs at 51.2 kS/s while offering software-selectable IEPE signal conditioning, AC/DC coupling, and antialiasing filtering.  The NI 9234 can be used in an NI cDAQ-9172 chassis.

 
Figure 2. NI 9234 C Series Module with NI CompactDAQ Chassis

The module has four BNC connectors that can each connect to an IEPE sensor (see Figure 3). The center pin of the connector, AI+, provides the DC excitation and AC signal connection. The shell of the connector, AI–, provides the excitation return path and AC signal ground reference.

Figure 3. NI 9234 BNC Connector Assignments

An IEPE sensor needs an appropriate cable and/or connector to hook into the BNC inputs of the C Series module. Triaxial accelerometers have three outputs, one axis to one acquisition channel, each requiring its own signal conditioning.

You can connect both ground-referenced or floating IEPE sensors to the NI 9234, but you must use a floating connection to prevent ground noise from being picked up. Typical IEPE sensors have a case that is electrically isolated from the IEPE electronics, so connecting the sensor to the NI 9234 results in a floating connection even though the case of the sensor is grounded.

Getting to See Your Measurement: NI LabVIEW

Once you have configured the system properly, you can acquire and visualize data using the LabVIEW graphical programming environment (see Figure 4).

In software, you can covert the acquired voltage into frequency data through spectral (frequency-domain) analysis functions. A simple example is a fast Fourier transform, or FFT function. You can conduct more advanced software processing of the data using one of the many tools that National Instruments has to offer, such as the NI Sound and Vibration Measurement Suite.

Figure 4. Power Spectrum with NI Sound and Vibration Toolkit

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