Acoustical Society of America
157th Meeting Lay Language Papers

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Are Hybrid Cars Too Quiet?

Lawrence D. Rosenblum
Ryan L. Robart
University of California, Riverside

Popular version of paper 5aNS8
Presented Friday morning, May 22, 2009
157th ASA Meeting, Portland, OR

Hybrid cars are fuel-efficient, clean, and quiet. They may in fact, be so quiet that they pose a danger to visually-impaired pedestrians who depend on car sounds to navigate parking lots and intersections. A congressional bill has recently been proposed to study this issue.

We have conducted research to examine the relative audibility of hybrid and combustion engine cars moving at slow speeds in a quiet, empty parking lot. The first experiments used stereo (binaural) recordings of a hybrid Toyota Prius and a combustion engine Honda Accord driving on a straight path of 110 feet at a speed of 5 mph. This speed was chosen to ensure that the Prius stayed in its electric mode. To make the recordings, small microphones were placed in the ears of an experimenter, who stood 10 feet away and facing the middle of the cars path (55 feet). This recording technique allows for good reproduction of the spatial aspects of sounds. The cars were recorded moving from left to right and right to left relative to the position of the experimenter.

LISTEN: A Toyota Prius at 5 mph

LISTEN: A Honda Accord at 5 mph

These recordings were played to (sighted) subjects over headphones in a laboratory. Subjects were asked to press a button indicating whether they heard the cars approaching from their left or right. They were asked to make these judgments quickly and accurately. While subjects were generally quite accurate at the task, the speed with which they could make the judgments was dependent on which car they heard. On average, subjects could determine the approach direction of the Accord when it was 4.9 seconds (36 feet) away from when it passed in front of them (passed the microphones). For the Prius, on the other hand, approach direction could not be determined until it was 1.4 seconds (11 feet) away.

In another experiment, the background sounds of two quiet, idling car engines were added to the recordings to simulate a normal parking lot. These background sounds made the task more difficult overall, but differentially hindered response accuracy for the Prius. Moreover, with the background sounds added, the approach direction for the Prius could not be determined until after it passed the position of the listener by .2 seconds (1.4 feet). The approach direction of the Accord, on the other hand, could be determined when it was 3 seconds (22 feet) away.

LISTEN: A Toyota Prius at 5 mph (with noise added)

LISTEN: A Honda Accord at 5 mph (with noise added)

Follow-up experiments using different model Prius and other combustion engine cars replicated this general pattern of results, as did studies using blind subjects.

A final, field study tested subject responses as they listened live to the cars, in the same quiet parking lot used for the recordings. The same car path, speed and approach parameters were used, and no background sounds were added. Blindfolded subjects stood 10 feet away from the middle of the cars path. As before, they were asked to press buttons indicating the cars approach direction as soon as the could. These subjects were encouraged to move their heads to help them hear the cars direction. Results revealed that subjects could, on average, determine the combustion engine cars approach direction when it was 4.6 (33.8 feet) away, and the Prius when it was 2.7 seconds (20 feet) away.

Together, the experiments suggest that a hybrid Prius, in its electric mode and approaching at 5 mph, is harder to hear than a combustion engine car. Further, with the presence of normal background sounds, a Prius may be especially hard to hear.

Acknowledgments: This research was supported by a grant from the National Federation of the Blind.

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