Electric Car & Hybrid Sounds: What They Might Sound Like

Often the only noise those walking or biking by or near electric cars or hybrids can hear is the very quite hum of the electric power train, if that is even heard at all. Concerns exist in some corners this lack of audible noise could cause an increase in accidents. To counter this, the U.S. Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) is now proposing that hybrid and electric vehicles meet minimum sound standards in order to help make all pedestrians more aware of the approaching vehicles.

What’s known as the proposed Federal Motor Vehicle Safety Standard No. 141 would fulfill a requirement put forth by Congress in the bipartisan Pedestrian Safety Enhancement Act of 2010 to make distinct audible cues available. The sounds in reference, according to the NHTSA, “would need to be detectable under a wide range of street noises and other ambient background sounds when the vehicle is traveling under 18 miles per hour. At 18 miles per hour and above, vehicles make sufficient noise to allow pedestrians and bicyclists to detect them without added sound. Each automaker would have a significant range of choices about the sounds it chooses for its vehicles, but the characteristics of those sounds would need to meet certain minimum requirements. In addition, each vehicle of the same make and model would need to emit the same sound or set of sounds.”

2013 Nissan Leaf at 2013 Detroit Auto Show (image copyright EarthTechling)

2013 Nissan Leaf at 2013 Detroit Auto Show (image copyright EarthTechling)

The governmental agency believes if this plan is put into place there would be 2,800 fewer pedestrian and pedalcyclist injuries over the life of each model year of hybrid cars, trucks and vans and low speed vehicles, as compared to vehicles without sound. It is currently soliciting comments over the next 60 days from the public on what it hopes to implement.

“Safety is our highest priority, and this proposal will help keep everyone using our nation’s streets and roadways safe, whether they are motorists, bicyclists or pedestrians, and especially the blind and visually impaired,” said U.S. Transportation Secretary Ray LaHood in a statement.

As for the examples of the specific sounds, listen for yourself below and perhaps give the NHTSA your feedback on this important decision. Note that all of the sounds included here are presented monaurally, that is, when played through headphones, the sound presented to each ear is the same.

Reference Sounds

These are sample sounds of what a typical internal combustion engine vehicle should sound like through speakers on your computer. This will allow you to compare them with the sample sounds of the proposed rulemaking.

  • Sound (A) sound is a monaural recording of an existing ICE economy vehicle (4-cylinder) traveling at a constant speed of 10 km/h, approaching a pedestrian.
  • Sound (B) sound is a monaural recording of an existing ICE SUV (V6) traveling at a constant speed of 10 km/h, approaching a pedestrian.

Sound File Descriptions

  • Sound One (1) sound is a monaural recording of an existing electric vehicle’s alert sound traveling at a constant speed of 10 km/h, approaching a pedestrian. It does not meet the proposed requirements for detection, only exceeding the minimum level in the 2500 Hz one-third octave band. The lowest tone that could be clearly identified was at 2550 Hz, this would not meet the proposed specification for tonal component for recognition since it is above 400 Hz.
  • Sound Two (2) sound is a monaural recording of an existing electric vehicle’s alert sound traveling at a constant speed of 10 km/h, approaching a pedestrian. The level of each one-third octave band has been adjusted so that it just meets the proposed detection requirements in all one-third octave bands. In so doing, all specified one-third octave bands are very close to the proposed minimum proposed detection requirements. The lowest tone that could be clearly identified was at 1100 Hz, this would not meet the proposed tonal requirements since it is above 400 Hz.
  • Sound Three (3) sound is based on low levels of broadband noise and strong tonal components that have been slightly modulated in pitch and amplitude to produce a rich tonal sound. This sound was optimized to just meet the proposed detection requirements in all specified one-third octave bands.
  • Sound Four (4) sound is a synthesized 10 km/h, constant speed, pass-by sound based on the resampling of an ICE vehicle at idle. The sound was processed so that it included level changes and Doppler due to the approach towards the pedestrian. A tone was added to the signal at 290 Hz to meet the proposed tonal requirements. This sound was optimized to just meet the proposed detection requirements in all specified one-third octave bands.
  • Sound Five (5) sound is a synthesized 10 km/h, constant speed, pass-by sound based on an ICE vehicle at idle. The sound is similar to four, except that rather than resampling an idle sound, filtered noise was used to generate the sound. The sound was processed so that it included level changes and Doppler due to the approach towards the pedestrian. A tone was added to the signal at 290 Hz to meet the proposed tonal requirements. This sound was optimized to just meet the proposed detection requirements in all specified one-third octave bands.
  • Sound Six (6) sound is a synthesized 10 km/h, constant speed, pass-by sound based on an ICE vehicle at idle. The sound is similar to five, except that the sound has been adjusted so that its level is 10 dB greater than the minimum proposed requirements.
  • Sound Seven (7) sound is a synthesized 10 km/h, constant speed, pass-by sound based on an ICE vehicle at idle. The sound is similar to five, except that the sound has been adjusted so that its level is 10 dB less than the minimum proposed requirements.
  • Sound Eight (8) sound is 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 315 Hz. The sound was processed so that it included level changes and Doppler due to the approach towards the pedestrian. The sound meets the minimum proposed requirement in the 315 Hz band only and does not contain any tones.
  • Sound Nine (9) sound is 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. The sound was processed so that it included level changes and Doppler due to the approach towards the pedestrian. The sound meets the minimum proposed requirement in the 5000 Hz band only and does not contain any tones.
  • Sound Ten (10) sound is a synthesized 10 km/h, constant speed, pass-by sound that is generated by adding two 45 dB(A) pure tones, one at 315 Hz and one at 2500 Hz. The sound was processed so that it included level changes and Doppler due to the approach towards the pedestrian. The combination of these two tones provide an overall level of 48 dB(A). This sound does not meet the proposed requirements.
  • Sound Eleven (11) sound is a synthesized 10 km/h, constant speed, pass-by sound that is generated from a base, broad band noise that has been optimized to meet the minimum proposed requirement. Added to this is a pure tone at 315 Hz with a level sufficient to produce a Tone-to-Noise Ratio (TNR) of 6 dB. Thus this sound just meets the proposed requirements for TNR. Note, that because the tone is added to a sound that already meets the minimum proposed detection requirements, this sound exceeds the minimum requirements in the 315 Hz one-third octave band by 7 dB.
  • Sound Twelve (12) soundSound Thirteen (13) sound, and Sound Fourteen (14) sound are similar to Sound Eleven (11) except that the TNR is increased to 8, 14, and 20 dB respectively (giving total levels in the 315 Hz band of 9, 15, and 22 dB(A)). Comparing these sounds allows one to appreciate the effect of increasing TNR.

I am the editor-in-chief and founder for EarthTechling. This site is my desire to bring the world of green technology to consumers in a timely and informative matter. Prior to this my previous ventures have included a strong freelance writing career and time spent at Silicon Valley start ups.