Gordon Ebbitt

Reverberation room for acoustical testing

A reverberation room comprising walls, ceiling and floor having specific dimensions and spatial relationships used in an automotive testing laboratory to analyze sound for the purposes of measuring transmission loss and sound absorption. The room possesses two transmission loss test windows. One window is placed in the ceiling and one is placed in a side wall. Samples for testing can be mounted horizontally in the ceiling window. A rotating microphone boom is used in combination with the room to sample the sound field in the reverbration room and in reception chambers attached thereto. Three loudspeakers are also positioned in the room to generate the sound field in the reverberation room and indirectly in the reception chambers.

NOISE ABSORPTION OF AUTOMOTIVE SEATS

In this study seat covers made from textiles, leather and vinyl were evaluated for noise absorption. The textiles included woven velours, pile knits and flat wovens. The noise absorption of the covers and the corresponding seat assemblies was tested by the reverberation room method per ASTM C423. The effect of different foams was also tested. For the leather and vinyl covers, the effect of perforation was evaluated. Test results showed distinctive differences between textiles and leather/vinyl with cloth seats having superior noise absorption. Even among the textiles, there are significant differences. Core foam densities affect the characteristics as well.For pile fabrics (woven velours and pile knits), the size of the pile fibre does not affect the acoustic characteristics of the seat. Also, no significant difference was observed between a bonded seat and a conventional (cut and sew) seat. (A) For the covering abstract of the conference see IRRD 492369.

A potential link between structural parameters and the radiated sound field of a vibrator

Two powerful techniques available for studying structural vibration and radiation are nearfield acoustical holography (NAH) and modal analysis. The NAH technique measures the pressure field over a surface sound field in the half‐space above the radiator. The modal analysis technique determines the structural parameters of the vibrator. Since both techniques can determine the velocity of the vibrating surface, it seems likely that linking these two techniques via the measured velocity could provide useful insight into how the structural parameters affect the radiated sound field. A modal analysis and a NAH analysis of a simple plate has been performed and the results of attempting to link these two techniques will be discussed.

On the standardization of metrics for sound quality assessment

There is interest in the use of sound quality metrics as aids in the design of the sounds that machines make. Assessment of aspects of sound quality by subjective testing is time consuming and difficult to perform; unintentional biases may be introduced into the test and comparisons of results from different tests are difficult to make, due to the changing criteria that people may use to judge the sounds. Currently available commercial instrumentation calculates many of these metrics, but, due to the absence of standards, different systems produce different results. Many metrics are based on a measure of time‐varying loudness, and its calculation is not standardized either. A summary of the current status of metric calculation is given with examples taken from some commercially available hardware systems. Options for future directions will also be explored.

Mounting and selection of microphones for two‐microphone impedance measurements

The determination of acoustic impedance as specified in ASTM E‐1050 requires the measurement of sound pressure at two positions along a standing wave tube. The choice of microphones and their mounting can have a significant influence on the measurement results. A variety of microphone types (i.e., phase‐matched condensers, probe tube types, condenser microphones fitted with phase correctors, etc.) will be considered and the results from these various types will be compared. The mounting of the microphones (i.e., flush with the tube walls, near the middle of the tube, etc.) will also be considered.

Compensation for phase mismatch in sound intensity measurements

Sound intensity measurements place stringent requirements on the phase matching of the measuring apparatus (including the transducers). In the past this has nearly always been accomplished by relying on the manufacturer to provide phase matched instrumentation and transducers. These days more and more processing power is being built into analyzers. This has opened the door to the possibility of compensating for the phase mismatch between the two measurement channels. The procedure consists of first measuring the phase mismatch between the channels and then correcting for it as a postprocessing operation. In practice, this correction can be included as part of the calculations required to compute the intensity from the two input signals. While there are clear benefits to this technique (matched transducers are not required), there are a number of practical problems. Chief among these is getting an accurate measurement of the phase difference between the channels. The benefits and difficulties associated wi...

Field calibration and verification of sound intensity systems

A two‐microphone sound intensity system is, in general, calibrated by using a pistonphone or a sound level calibrator to set the sensitivities of the two measurement channels. While this is all that is required for a calibration of the system, it does not provide any check of the systems performance. Such checks have traditionally been limited to the laboratory because they require special acoustic environments. A new device for calibrating and checking intensity systems will be considered here. This device consists of an acoustic source and a coupler. The coupler can be configured in two ways. In one configuration both microphones are exposed to the same sound field. This mode can be used to calibrate the channel sensitivities and to measure the residual intensity index of the system. In the other configuration the coupler will simulate free‐field conditions. Either a pistonphone or a broadband source can be used as the acoustic source. The design of the device will be considered and its use illustrated.

A scanning microphone implementation of nearfield acoustical holography

A system that implements the nearfield acoustical holography (NAH) technique in air has been designed and placed into operation at the CBS Technology Center. The system utilizes a scanning microphone that may be positioned within a 3.7×3.7×1.2 m volume by a computer controlled robot. Sound pressure data from the scanning microphone and a reference are stored in a mini computer and subsequently processed using nearfield holographic techniques. The resulting reconstructions of the pressure, velocity, and vector intensity fields may be displayed using either a 3‐D video graphics system or a digital plotter. A detailed discussion of this NAH system will be presented and reconstructions of the sound field generated by a simple source will be shown.

Vibration and decay characteristics of electrical basses

One of the assets of electric musical instruments such as guitars and basses is that they posess very long sustain. Since no acoustic radiation is required from the body of the instrument, very rigid and massive elements may be used in its construction. This will generally ensure that the sustain is long and constant from note to note. Structural resonances, however, occur in all structures with finite stiffnesses and masses and these resonances will influence the sustain of the instrument. Since the resonances occur at discrete frequencies, certain notes may have a more rapid decay than others and this may cause “dead spots” at certain notes. The relation among the structural resonances, the decay rates, and the input inertances at the ends of the speaking lengths of the strings of an electric bass will be discussed. The modal shapes will be shown and their shapes relative to the speaking length of the string will also be considered.

Modal analysis and its application

Modern modal analysis techniques provide a method for the rapid determination of structural mode shapes and frequencies. This analysis utilizes the impulse measurement of the transfer inertance (acceleration/force) between a reference point and points on the surface of a structure. The procedure involved in the analysis will be outlined and potential problems in its implementation will be discussed. These potential problems include the improper location of the reference position, the number of measurement points chosen, the resolution of the processing system, and the spectral content of the impulse. The application of modal analysis to musical instruments will be considered and some results will be presented.