Spatial Variability Caused by Acoustic Wave Interference in Single-Drive Direct Field Acoustic Testing

Abstract

A significant amount of acoustic spatial variability is characteristic of direct field acoustic testing using a single random signal to drive the loudspeaker sources. This has raised concerns about the potential for under- and over-testing of hardware, particularly because of the consequential high sensitivity of the test environment upon the configuration of the test equipment and test hardware. Meanwhile, little is known about the specific spatial characteristics of the test acoustic field. A recent demonstration single-drive direct field acoustic test afforded the opportunity to map various planes of the acoustic field with a 36 microphone grid, offering unprecedented physical insight into the field characteristics. Analysis of the data reveals that the field is predominantly characterized by wave interference patterns at any given frequency. These data correlate well with 3-dimensional numerical models. The presence of wave interference patterns created by the fully correlated array of sources also explains the occurrence of frequency bands where the acoustic levels were significantly overdriven as a consequence of the control microphone locations coinciding with acoustic nodes associated with the corresponding drive frequencies. For comparison, the same mapping was performed in a conventional reverberant chamber test with the microphone grid, showing detailed contrast in spatial characteristics between the two test methods. This paper presents the highlights of this study and discusses ramifications for the use of this methodology.