STSF Case Study
In this article, we relate a case study where STSF was used to solve an exterior noise problem of a passenger vehicle.
An example of what the system can be used for is illustrated by the case of a passenger vehicle that failed the pass-by test by 1 dB on one side of the vehicle. FFT analysis revealed that the main sound component was somewhere in the 100 Hz region, suggesting engine-related noise. The vehicle was placed in a semi-anechoic chamber on a dynamometer and measurements were taken using a vertical row of 10 microphones positioned using a robot. With the measurement made, it was easy to see the noise "hotspots" occurring at 100 Hz on a contour display of the near field intensity. Two areas, the exhaust outlet and the area directly below the engine compartment, were of especial interest.
Investigating the sound field at the position where the microphones used in the pass-by measurement were placed (7.5 m from the centre line of the vehicle), it was possible to look at the noise contributions of the exhaust outlet and the engine compartment. Here, the main contributor seemed to be the exhaust outlet, although near field measurements (close to the vehicle) showed the engine compartment to be radiating more energy in the frequency range investigated.
To investigate this discrepancy, the intensity vectors for the sound field between the near field and the point corresponding to the pass-by measurement position were calculated. The result showed that a proportion of the sound energy radiating from the engine compartment radiated backwards and combined with noise being radiated from the exhaust outlet.
The next stage involved simulating the effect of attenuating the two sources by 3 dB. Reducing the exhaust noise by 3 dB only produced a 0.8 dB change in the noise levels at the pass-by measurement position. However, a reduction of 3 dB from the engine compartment produced a 1.8 dB reduction, and this was duly implemented by modifying the engine air intake assembly.
This example showed that once the acoustical field radiating from the vehicle had been measured, it was possible to calculate all the relevant parameters, both in the near field and in the far field. The STSF technique provides a complete description of the sound field, where both phase and magnitude are known at all points. This enables it to transform the sound field from the plane in which we have measured to any other plane closer to, or further away from, the source. Modification of the sound field, and calculation of the resulting new sound field in other planes, is also possible.
The STSF System
An STSF system comprises STSF software, a data acquisition system such as IDA (Intelligent Data Acquisition System Type 3561), a stationary microphone array or a row of robot-positioned microphones to sweep an area, and one or more reference microphones. Reference microphones are placed close to the area or object under test. During a measurement cross-spectra between the microphone array and the reference microphones are taken, stored and used to generate information on the sound field.
