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Speech communication from the front seat to the rear seat in a passenger vehicle can be difficult. This is particularly true in a vehicle with an acoustically absorptive interior. Speech Transmission Index (STI) measurements can quantify the speech intelligibility, but they require specialized signal processing. The STI calculations can be simplified if it is assumed that reverberation and echoes play an insignificant role in an automobile. A simplification of a STI measurement is described that uses a stationary reference speech signal from a talker mannequin in the driver's seat to create a signal at the rear passenger positions. On-road noise measurements are used for the noise level and the calculated signal to noise ratio is used to calculate a simplified STI value that tracks closely to a full implementation of the STI method for sedans. In fact, this method is very similar to the techniques described in the Articulation Index (AI) and Speech Interference Index (SII) standards.

The automotive industry is often interested in controlling noise radiated from trim pieces in the passenger cabin. In general, there is a small air gap that separates these trim pieces from the sheet metal that is the actual source of the noise. It is common practice to place an acoustically absorbent material in this space to reduce radiated noise. In this paper the in situ noise control performance of a variety of materials is examined by placing them in a test fixture that simulates the sound field in the vicinity of vehicle pillar trim. In this fixture a noise source is positioned behind a piece of sheet metal. A flat plastic sheet that is similar in composition to pillar trim is placed a small distance away from the sheet metal. The sides and rear of the fixture are sealed so that the plastic sheet is the only significant radiator of the sound radiated from the sheet metal.