Investigating Diffusivity Of Virtual Diffuse Field Acoustic Test (DFAT) Using Boundary Element Modeling And Wavenumber-Frequency Analysis

In an ongoing effort to complement or replace expensive, time-consuming, and potentially damaging (due to transportation) acoustic qualification testing of aerospace systems in reverberant chambers, Direct Field Acoustic Testing® (DFAT®) has been examined more closely in recent years.

In DFAT testing, the structure to be tested is positioned in the center of a set of speaker stacks, which commonly have several input channels that can be independently controlled to obtain an acoustic field that is as uniform as possible. A set of control microphones is used to indicate diffusivity (through levels and cross-correlation) and to provide inputs to the control system to actively adapt the speaker inputs to optimize diffusivity. However, these control microphones can only define the field characteristics at a limited number of points, which are generally not sufficient to evaluate the overall diffusivity inside the testing volume.

To more thoroughly investigate the field diffusivity at significantly more points of interest in the test volume, the test setup may be modeled using numerical methods, particularly the Boundary Elements Method (BEM). BEM models offer flexibility for exploring the effect of different speaker configurations and inputs and for probing the acoustic field at multiple locations to check for “hot spots” or “cancellation” regions.

It is of particular interest to identify DFAT “cancellation” regions where, in contrast to reverberant chamber testing, the assumed fully diffuse acoustic field is not present. As a result, the assumed incident acoustic energy from a range of angles that may cause strong structural excitation is also not present. Potentially missing acoustic energy from important incident angles is one of the principal shortcomings of DFAT testing versus using a reverberant chamber. Being able to predict and improve this aspect is key to using a DFAT test with confidence.

In this paper, a methodology is proposed for analyzing the diffusivity characteristics of representative DFAT configurations using Wavenumber-Frequency Spectra (WFS) computed from a BEM-simulated acoustic field at several planes inside the volume enclosed by the speakers. For different excitation configurations, the resulting WFS are compared to the ideal diffuse acoustic field WFS and to other WFS, including propagating waves, to gather insights from the predicted acoustic field at different angles of orientation. Simulations are also carried out, including a test specimen, to investigate the influence of the structure on the acoustic field.

Recommended citation: Castel,A. et al., (2018). "Investigating Diffusivity Of Virtual Diffuse Field Acoustic Test (DFAT) Using Boundary Element Modeling And Wavenumber-Frequency Analysis." ECSSMET 2018.'
Download Paper