Customer SolutionsCreating a Tube Impedance Measurement System for Acoustical Material Testing Using NI LabVIEW and Hardware
Author(s):Nick Borgerding, MTS Systems Corporation
Industry:Consumer Goods
Product:LabVIEW
The Challenge:Developing a cost-effective solution for the measurement of sound absorption properties using an impedance tube conforming to the ASTM E1050 test standard.
The Solution:Using the MTS Model 50 impedance tube developed in National Instruments LabVIEW as a low-cost, stand-alone solution to automate calculation of acoustic material sound absorption properties.
A Quieter Vacuum Cleaner A residential and commercial vacuum cleaner manufacturer that promotes low noise emissions contacted MTS Software and Consulting to develop a test that measures sound absorption properties. Because most consumers consider noisier vacuums more powerful, vacuum cleaner noise emissions is a complicated issue. Nonetheless, studies show a demand for noise reduction and regulatory trends toward labeling for noise emissions. Most vacuum cleaners on the market today have sound power emissions between 72 and 77 dBA, considered loud and annoying by most users. The manufacturer expects to gain a competitive advantage by offering a quiet product. One of the techniques this manufacturer uses to lower noise emissions is applying sound-absorbing foam inside the covering of its canister models. Acoustic foam absorbs airborne noise by dissipating it into heat, causing a reduction of energy by weakening reflected vibrations. The manufacturer elected to use an established standard, ASTM E1050, to compare how well candidate materials absorb sound over a range of frequencies. ASTM E1050 is the standard test method for impedance and absorption of acoustical materials using a tube, two microphones, and a digital frequency analysis system. The manufacturer contracted MTS to develop an easy-to-operate system for in-house use that would not require a noise, vibration, and harshness (NVH) expert to set up or run. Hardware An impedance tube is one of the most cost-effective and widely used methods of measuring sound absorption. The impedance tube is composed of a speaker (acoustic driver), tube, microphones, and a material sample holder. A sample of the material to be tested is placed in a fixture with a sample holder mounted to one end of a straight tube. A rigid plunger that the manufacturer can adjust for depth is placed behind the sample to provide a reflecting surface. The speaker generates broadband noise, which travels down the tube where it encounters the material. Some of the sound is absorbed (dissipated as heat) and some is reflected. At MTS, we implemented our system with three microphones to extend the frequency range using one tube. In this approach, we mounted three microphones, rather than two, flush with the inner wall of the tube near the sample end of the tube. Having two pairs of microphones in one tube with different spacing facilitates simultaneous measurement of two frequency ranges spanning 100 to 7,100 Hz with one tube with a 25.4 mm diameter. We used a National Instruments PCI-6713 high-speed analog output board to provide a signal to the sound source and an NI PCI-4472 dynamic signal acquisition board to obtain the frequency-response functions (FRFs) among the three microphones. The lower frequency range is a function of microphone spacing (larger spacing for lower frequencies). The upper frequency range is a function of microphone spacing and tube diameter (closer spacing and smaller tube diameter). The PCI-4472 is a simultaneous-sampling dynamic signal acquisition device with 110 dB of dynamic range, making it a good fit for this application because it is available in PCI or PXI versions. Input channels incorporate integrated electronic piezoelectric (IEPE) signal conditioning for the microphones used. We used the microphone closest to the source as the reference channel. From the FRFs, we calculated the pressure reflection coefficient, which can be used to determine the qualities of the material under test. We chose the NI LabVIEW software platform to develop the standardized test. NI and MTS signed an agreement in 2004 to deliver a low-cost framework for standardized noise and vibration testing. This new framework is designed to help noise and vibration experts quickly define and inexpensively deploy standardized tests for repetitive evaluation against targets, benchmarking to set targets, and variability analysis of products in final configurations. System Operation Our goal was to develop a standardized testing framework that was easy for a variety of users to acquire data immediately and generate reports. We designed a user interface with tab controls in NI LabVIEW to guide the acquisition, analysis, and reporting during the material test. With menu-based configuration routines and imported graphics, the vacuum cleaner manufacturer can configure instrumentation and measurement parameters. Configuration settings include microphone calibration, source setup, amplitude and phase correction, test setup, acquisition, and reporting. The MTS Model 50 software provides routines for documenting the material under test. The vacuum cleaner manufacturer can export all test documentation along with the test data into a Microsoft Excel spreadsheet and use Microsoft Excel and other Microsoft Office tools to generate customized reports. The manufacturer also can select multiple files for data comparison and averaging. Results and Benefits MTS developed an easy-to-use, cost-effective product for a vacuum cleaner manufacturer that needed to test samples in house according to an established standard rather than depend on data from suppliers or outside testing services. This saved both time and money, provided more consistent results, and facilitated much more extensive testing. For more information, contact: Nick Borgerding MTS Systems Corporation Software & Consulting 800 East Whitcomb Avenue Madison Heights, MI 48071 Tel: (248) 585-5000 |
