Technical Insights > Benchtop

Acoustic Measurement Using a Dynamic DAQ

2020-08-30  |  10 min read 

Recently, while driving my car, I heard a low-tone snapping sound every time I made a sharp turn. It sounded like it was coming from my back tires. I did not feel confident driving my car, so I brought it to my trusted auto shop to investigate the problem. My mechanic drove the car by himself and then took me out with him. While I was in the car with my mechanic, he explained that the problem was likely worn rear-wheel bearings. His intuition came from years of experience listening to different types of sounds and abrupt sound changes while driving. After he took out the wheels and checked the wheel bearing assemblies, he confirmed that it was the rear-wheel bearings. I was a happy customer.

You can think of my mechanic’s intuition as a measurement science. He remembers the specific signatures of various sounds and can interpret their respective mechanical faults. It works in other occupations too. For example, why do doctors use stethoscopes?

Acoustic analysis is an important measurement field. It uses a microphone to capture sound waves of machinery or an engine in operation, or sound pressure changes on a structure as wind or water flows; analyzes the dynamic data; and diagnoses whether a mechanical failure is about to happen.

Acoustic analysis is common in industries that use a lot of heavy machinery. Unscheduled downtime can cost a company millions of dollars if the failure is catastrophic and takes longer to fix than scheduled downtime for preventive maintenance would have.

Acoustic analysis is similar to physical vibration analysis but with a different focus. Instead of finding the cause of an equipment failure by measuring vibrations at a frequency bandwidth of interest, acoustic analysis measures and monitors contact sounds such as ball bearings to determine the health of equipment. Acoustic analysis includes preventive measures taken during scheduled downtime, such as lubricating parts and changing worn-out components.

acoustic-analysis-on-industrial-machinery
Figure 1. Acoustic analysis in progress on industrial machinery

What Do You Need for Audio and Acoustic Analysis?

Design and test engineers in automotive, aerospace and defense, consumer electronics, and power distribution industries require noise, vibration, and harshness test solutions. You need industrial acoustic sensors that comply with the Integrated Electronics Piezo-Electric (IEPE) standard. This will simplify the setup of data acquisition systems (DAQs) that comply with IEPE standards.

In most situations, sound waves converted into electrical signals require some signal conditioning, such as preamplification and filtering, before you can digitize and store them in a dynamic DAQ system.

You can choose acoustic sensors with built-in preamplifiers. An IEPE acoustic sensor likely will have built-in preamplifiers. Such sensors let you avoid having an extra standalone preamplifier, with its associated wiring and costs. DAQs that comply with IEPE standards also provide constant current power directly to source the preamplifiers of your acoustic sensors.

Many types of acoustic microphones can interface with a dynamic DAQ system. Using the correct microphone sensor will allow you to capture high-quality dynamic acoustic signals. For example, air pressure or velocity microphones are useful for picking up sound waves traveling through the air. Contact pickups are sensors that pick up sound waves from a dense physical medium such as wood or metal.

Another key consideration when choosing the right acoustic sensor for your application is frequency response. There are three types of acoustic sensors on the market:

  • The first type is the pressure field acoustic sensor. It has a flat frequency response and uses a diaphragm to capture sound pressure. It is best suited for measuring surface pressures.
  • The second type is a free-field acoustic sensor. It generally has a flat frequency response when measuring acoustics at a 0-degree incidence or directly pointing to the sound source. Its frequency response compensates for higher pressures at high frequencies. A free-field sensor measures sound pressure as if it existed before you introduced the sensor to the sound field. It is best suited for outdoor measurements or in anechoic chambers.
  • The third sensor is a random-incidence field acoustic sensor. It is best suited for measuring a sound field in which sound comes from many directions.

A modern DAQ system supports industrial-standard IEPE sensors by providing an excitation source to the sensors. It can also provide event triggers for capturing dynamic data, preprocessing and storing data, and transferring formatted data to a PC for post-analysis work.

The picture on the top right-hand side of Figure 2 shows a digitized signal displayed on a DAQ system. The picture on the bottom right shows data exported to MATLAB and then post-analysis work done showing the sound pressure level of the dynamic sound data.

Audio-measurement-setup
Figure 2. Audio measurement analysis

How Do You Make an Audio Measurement Using a Dynamic DAQ System?

Here are the hardware and software setups to make acoustic measurements with the ability to perform post-analysis work.

Hardware setup

To measure the source of a sound, we are using a free-field constant current power-sourced microphone — the GRAS 40PP CCP free-field QC microphone. This microphone, from a company called GRAS, has a built-in preamplifier that requires a constant current power source.

The microphone connects to a DAQ system with a 3-meter cable. The DAQ we are using is Keysight’s compact DAQ973A, with up to three slots available for module expansion flexibility. In this setup, we need a Keysight DAQM909A four-channel simultaneous sampling digitizer module.

The DAQM909A digitizer module has two main functions. The first is to provide constant current power to the microphone’s preamplifier. The second function is to digitize the microphone’s sensor output for further data processing, storage, and display in the DAQ system.

Figure 3 shows the hardware setup from left to right: microphone, 3-meter BNC-BNC cable, DAQM909A digitizer module, DAQ973A system, and Keysight PathWave BenchVue software on a laptop controlling the DAQ system.

Audio-setup-with-Benchvue
Figure 3. Hardware setup for acoustic measurement and analysis work

The DAQ973A has a large graphical screen and intuitive soft buttons and hard buttons to configure channels, set up triggering, execute measurements, and display the data in time-domain or frequency-domain charts for analysis. You can quickly zoom, pan, set markers, get set up to collect statistical data measurements, and more using the front-panel menus. You can also store data in a USB thumb drive for further analysis on your PC.

Software setup

If you prefer to remotely control and automate your test sequences, you can use SCPI commands to program the DAQ973A. For this blog post, we are using Keysight’s PathWave BenchVue software to show how easy it is to configure, take measurements, and display those measurements in a time- and frequency-domain chart without all the programming.

Please view the video below on how the hardware and software setup come together using the GRAS microphone sensor, DAQM909A digitizer module, DAQ973A mainframe, and PathWave BenchVue software to make audio and acoustic measurements.

 

Video: How to make an audio measurement using a DAQ system and PathWave BenchVue software

Summary

Acoustic analysis is a measurement science that benefits many industries worldwide. It is especially useful in automotive, aerospace and defense, consumer electronics, and power distribution companies that require noise, vibration, and harshness test solutions.

Choosing the right hardware setup — microphone, digitizer module, DAQ system, and appropriate software control system — provides you with the tools you need to analyze and monitor for mechanical failures that are about to happen. You can potentially save a lot of money by preventing unscheduled downtime of heavy machinery.

For more information on Keysight’s data acquisition system, please visit www.keysight.com/find/DAQ.

The remote PathWave BenchVue DAQ software comes with the purchase of the DAQ hardware. You can check out the software here: Data Acquisition Control & Analysis.

To learn more about dynamic data acquisition, please click and download the Dynamic Data Acquisition System white paper.