Making slow and idling electric cars noisier

This is really interesting:

Earlier this week, a federal agency proposed rules to set standards for, and require, noise to be emitted by electric and hybrid cars when traveling at speeds under 18 mph.

According to a Department of Transportation press release, "At 18 miles per hour and above, vehicles make sufficient noise to allow pedestrians and bicyclists to detect them without added sound." But there's a problem when electric and hybrid cars travel more slowly: pedestrians can't hear them.

Here's a graphic from the National Highway Traffic Safety Administration's notice of proposed rulemaking that illustrates how blind pedestrians, in particular, depend on the sounds gas-powered cars make at intersections. The basic concept, as I understand it, is that a pedestrian can place, by hearing, which cars are idling, and which are accelerating or decelerating. From that information, a pedestrian can know the direction of traffic flow and make decisions to walk, or not walk, accordingly.

The proposed rules are suitably technical. Here's a sampling:

"S5.1.2 Backing. For vehicles capable of rearward self-propulsion, whenever the vehicle’s gear selection control is in the reverse position, the vehicle must emit a sound having at least the A-weighted sound pressure level in each of the one-third octave bands according to Table 2 as measured according to the test conditions of S6 and the test procedure of S7.3.

"S5.1.3 Constant 10 km/h pass by. When tested under the conditions of S6 and the procedures of S7.4, the vehicle must emit a sound having at least the A-weighted sound pressure level in each of the one-third octave bands according to Table 3 at any speed greater than or equal to10 km/h, but less than 20 km/h.

"S5.1.6 Pitch shifting to signify acceleration and deceleration. The fundamental frequency of the sound emitted by the vehicle must vary with speed by at least one percent per km/h between 0 and 30 km/h."

And below is a link to a brief audio file of "a synthesized 10 km/h, constant speed, pass-by sound that is generated by passing broad band noise through a single one-third octave band filter centered at 5000 Hz . . . processed so that it included level changes and Doppler due to the approach towards the pedestrian," one that "meets the minimum proposed requirement in the 5000 Hz band only."

Download 9

More information:

• A good article by Emily Badger in The Atlantic Cities, Should Hybrid and Electric Cars Have to Sound Like Regular Cars?, with a comment thread that includes skepticism about the need.
• Reference sounds from the National Highway Traffic Safety Administration.