Space

Characterization of reverberant room global sound field

Space2JPG Space4JPG Space3JPG Space5JPG Click to enlarge

All reverberant rooms used for space structure testing try to create a homogeneous sound field around the test object in order to simulate as good as possible the free field outdoor rocket launch conditions.

The reverberant rooms are often characterized by their volume and their reverberation time.

In practice, there are limitations to the reverberant room testing because of eigen modes and shortcomings in sound field generation and sound field control.

For sound field control, the current practice is to measure the mean squared sound pressure (which is also denoted as the potential energy density), using an array of an arbitrary number of sound pressure microphones placed at random discrete positions in the reverberant room.

But sound pressure is a place dependent quantity.

The concept of using the total acoustic energy instead of using the acoustic potential energy only overcomes this issue, taking also into account the kinetic acoustic energy as it can be measured with acoustic particle velocity sensors.

Total acoustic energy consists of both potential acoustic energy and kinetic acoustic energy.

Ideally, in an entirely homogeneous sound field the total amount of acoustic energy would be the same at all geometric measurement positions.

In literature, it has been reported since the 1930’s that the measurement of the total acoustic energy is far less susceptible to placement errors than mean squared sound pressure.

Total acoustic energy can be measured using at least four phase and amplitude matched sound pressure transducers, the finite difference between the sound pressure signals used to compute acoustic particle velocity.

The method is above all extremely time consuming and has a poor signal to noise ratio, probably the most important reason why the concept of measuring total energy was never really tested.

With Microflown enabled 3 D sound probes that measure both the sound pressure scalar value and the acoustic particle velocity vector, the computation of both the kinetic acoustic energy and the potential acoustic energy has become a straightforward operation with good signal to noise ratios.

This allows more efficient and more reliable monitoring method of the sound fields in reverberant rooms where extremely costly space structures are tested in the non linear range f acoustics.

The concept of using Microflown enabled relevant methods is described in recent literature.

In Europe, over the recent years, professor Finn Jacobson from the Danish Technical University has published upon the idea of 3 D sound probes to characterize the sound fields in reverberant rooms. This has resulted in several peer reviewed and approved publications in the Journal of the Acoustic Society of America.

It was concluded that, in terms of the number of measurement points to be taken, it is always more efficient to measure either acoustic kinetic or acoustic total energy than to measure sheer acoustic potential energy, favoring the use of 3 D sound probes rather than sheer sound pressure microphones.

In case of high modal overlap, it is three times more efficient to use 3 D sound probes, as the relative variance between kinetic and total energy density approaches 1/3.

In cases of lower modal overlap, the relative variance of both kinetic and total energy density increases, the use of 3 D sound probes still 50 % more efficient in terms of number of measurement points taken.

In the USA, professor Scott Sommerfeldt from Brigham Young University in Utah has published on the use of Microflown based acoustic vector sensors to measure acoustic energy density.

Regarding the more reliable monitoring, it has to be kept in mind that the outdoor launch is above all a very low frequency problem in the range of 5-10 Hz that needs to be tested in a reverberant room.

In practice all reverberant rooms have a lower frequency cut off frequency (“ Schroeder frequency”) that puts a limit on the relevance of the data obtained.

The verification of the lower frequency “Schroeder frequency “ is difficult with a set of sound pressure transducers, as they have a poor signal/noise ratio in the low frequency range.

As Microflown enabled 3 D sound probes do have favorable characteristics in the low frequency range, new opportunities arise to validate the “Schroeder frequency” in practice.