When hybrid cars first appeared on the scene, manufacturers touted their silent engines. But soon it was clear that cyclists and pedestrians — especially the visually impaired — relied on car sounds to help them navigate streets and intersections. So U.S. lawmakers are mulling the automotive equivalent of adding odorant to odorless gas: They want car makers to put the sound back into electric cars. Here’s how it works:

“We’re not talking about customized “car tones” that would enable a Prius driver to make her engine rev like a Porsche’s (though technically that’s possible). Proponents want synthetic car noises to be standardized across models. Audio supplier Harman International has developed technology that synthesizes digital sounds that mimic an engine’s hum, tuning them to match the car’s state — idling or accelerating, say. The audio system is then linked to the car’s electronic controls so that the correlating sound will be triggered by pressure on the car’s pedals or the shifting of gears. The sounds are then broadcast through speakers that are strategically mounted to the outside of the vehicle so that pedestrians can hear the vehicle up to 10 feet away.