Infrasonic Acoustic Modal Survey Testing Using the ARS Neutron™

Author: Dale Schick with information from ATA Engineering and Northrop Grumman Corporation.

Introduction

In our previous application note Leveraging long-duration, low-level acoustic excitation for modal testing: from the NASA VIPER acoustic modal experiment, we described an endeavor by Acoustic Research Systems (ARS) and ATA Engineering to extract mode shapes from a test article using acoustic excitation on the NASA VIPER. Since this test of opportunity, ARS with ATA engineering and Northrop Grumman Corporation have published experimental data that pushes the possibility of true Experimental Modal Analysis via acoustic excitation. Though any commercially available speaker is capable of exciting a test article from 20 Hz and above, modal extraction efforts have only been able to perform Operational Modal Analysis with the given acoustic input. The primary reason for this distinction is that traditional speakers generate noise in two or more separate band passes emanating from different locations, such as low frequencies from a stack of subwoofers and high frequencies from a line-array speaker. This application details how the ARS Neutron™ Acoustic Device is able to overcome these limitations to excite test articles in a way previously only possible with mechanically coupled shakers all the way down to 2 Hz.

Acoustic Excitation with the ARS Neutron™

Programs can save cost and schedule by performing DFA testing at a customer’s facility in lieu of performing traditional acoustic testing at remote facilities. Figure 1 resents a typical DFA test setup using the ARS Neutron™ acoustic exciters. The test article is surrounded by a large number of independent acoustic sources while it is mounted on a either a transportation cart or another boundary condition—perhaps mounted directly to aseismic mass suitable for modal survey tests. Each acoustic exciter is similar to a typical electrodynamic modal shaker in that they are both driven by magnetic voice coil actuators. The difference is how the force is transmitted to a test article. The force generated by a modal shaker is transmitted directly to a test article through a stinger rod and is directly measured with a load cell, whereas the pressure from an acoustic exciter is transmitted to the test article through air and cannot be directly measured. Further savings can be realized if modal survey tests can be performed while a test article is configured for a DFA test.

Image 1: Typical Acoustic Qualification Test Setup

Acoustic excitation is beneficial in exciting acoustically sensitive modes; and some acoustic devices like the Neutron™ exciters are capable of a wider bandwidth, especially in the lower frequencies, than is typical. Two major factors that contribute to this capability are:

1. High initial efficiency through acoustic impedance matching (horn loading) and drivers with very high field strength to coil resistance ratios, and,
2. Low mechanical losses through extremely robust construction – any flexure in the structure of the loudspeaker will be somewhat parasitic to the output unless the construction is very rigid.

The second point provides the added benefit of allowing their use for low-frequency modal testing above 2 Hz.

As such, recent research has been performed to explore how modal parameters (shape, frequency, and damping) can be extracted from either test-measured or frequency response function (FRF) synthesized cross-spectrum matrices that were created using acoustic excitation. The test article in this research is a flight-like mass model of a spacecraft built by Northrop Grumman Corporation in Dulles, VA. The spacecraft was configured to emulate an in-situ modal test performed alongside a traditional DFA test, with the spacecraft situated on a transportation cart which was isolated from the floor by using four rubber isolators under floor-jacks.

Figure 1: Spacecraft Mass Model on a Transportation Cart

Extending Modal Response Below 20 Hz

The “long and low” method described in our previous application is still applicable in this experiment, but is most effective from 30 Hz to 10,000 Hz. To extend response below 30 Hz an ATA Engineering-patented multi-sine sweep excitation was used with eight separate sine sweeps at a rate of ten minutes per decade. The sine sweeps are more effective in these frequencies because they are better at increasing the test-article response above noise floor. To explore the efficacy of this type of excitation, excitation of the test article was performed with both acoustic and modal shakers, creating a baseline set of Frequency Response Functions (FRF’s) for which to compare.

Figure 3 reveals the comparison between modes characterized by the Neutron™ system and the shakers. The lowest mode excited, in this case, was the rigid-body mode of the entire system which occurred at 2.175 Hz. Figure 4 shows a comparison between several excitation methods, with the swept-sine method producing the best coherence with the test article below 20 Hz.

Figure 2: Comparison of Acoustic Sources vs. Shaker Mode Indicator Functions

Figure 3: Coherence Comparison of Different Excitation Methods

Figure 4:  2.175 Hz Rigid X-Axis Mode

The Acoustic Modal Method

In this experiment, a non-contact method for modal excitation was made possible for the usable range of almost any sized test article. The result is being called The Acoustic Modal Method due to its unique ability to move beyond simple Operational Modal for aerospace applications. Further work has been done, and lessons have been learned about applying the method, and we are excited to share more in the coming months.

In our next Application Note, we will explore how ARS and ATA utilized Fixed Base Correction in the experiment to correlate spacecraft mode shapes with a Finite Element Model from Acoustic Excitation.