This paper describes two case studies in which multiple microphone processing (beamforming) and microphone location were evaluated to determine their impact on improving embedded automatic speech recognition (ASR) in a vehicle hands-free environment. While each of these case studies was performed using slightly different evaluation set-ups, some specific and general conclusions can be drawn to help guide engineers in selecting the proper microphone location and configuration in a vehicle for the improvement of ASR. There were some outcomes that were common to both dual microphone solutions. When considering both solutions, neither was equally effective across all background noise sources. Both systems appear to be far more effective for noise conditions in which higher frequency energy is present, such as that due to high levels of wind noise and/or HVAC (heating, ventilation and air conditioning) blower noise. Microphone location was also shown to have a substantial effect on the performance of the ASR system. The results from both studies showed that simply moving a single microphone from the overhead console (OHC) to the sun visor near the driver can provide a great benefit in ASR performance without the cost of multiple beamforming microphones. For the sole purposes of speech enhancement of the driver, it is recommended that moving a single microphone from the OHC to the driver’s sun visor position be performed before additional microphones are added. However, beamforming could provide a role if the desire is to include other occupants in a voice session. For instance, a centrally located beamformer could be steered toward the vehicle occupant wishing to issue a voice command. Ultimately, a user-case strategy of the voice control application will determine the microphone configuration and location(s).
