Legal news : Minimum Sound Requirements for Hybrid and Electric Vehicles

Sample Sounds and How They Relate to the Proposed Rulemaking

These sounds files are examples of sounds that could be proposed for use as alerting sounds. Some of these sounds would meet the proposed specifications; some would not. With the exception of the first three sounds, all sounds are based on an ICE vehicle sound. Other sounds that are not based on ICE vehicle sounds could also be proposed. It is expected that all alert sounds meeting the proposed specifications will be recognizable as a vehicle in operation due to requirements in the proposal for increasing sound level and pitch shifting with vehicle speed, as well as the innate characteristics associated with a moving vehicle.

All of the sounds included here are presented monaurally, that is, when played through headphones, the sound presented to each ear is the same; therefore pass-by events will not be perceived as, for example, moving from the left to right as one will experience in the field. With the exception of sounds 1 and 2, the sounds do not include tire noise; however at 10 km/h, tire noise is not a major contributor. Finally, the relative loudness of low frequency components to high frequency components will depend on individual playback equipment and settings. When the sound files are played at high levels, the low frequency sound will be greater relative to the high frequency sound than when the sound files are played at low levels. If headphones or speakers are not “flat”, they will accentuate particular regions of the sound. It is recommended that these sounds be listened to through high quality “monitor type” headphones at moderate level.

In order to provide context, alert sounds are combined with an ambient (background sound). The same ambient is used for all alert sounds with the exception of sounds 11 through 14, which focus on the tone-to-noise ratio of the alert sound itself. The ambient consists of pink noise filtered such that the spectral shape conforms to the average of several typical ambient conditions as described in White Paper on External Warning Sounds for Electric Cars: Recommendations and Guidelines. DELTA SenseLab, Pedersen, Torben et al (2011). The overall A-weighted level of the ambient was set to 55 dB(A), which is the level that was used to determine minimum one-third octave band levels for the alert sounds. Real ambient conditions generally contain some slow variations in level, which are not present in the simulated ambient, however, this ambient provides a consistent, documentable condition that has the same masking effect at all times.

With the exception of Sounds 1, 2, and 3, all sounds are based on an idling ICE vehicle that is modified. Broadband spectra are adjusted; tones are added, overall levels are adjusted to account for the vehicle approaching a pedestrian, and Doppler shifts are added. Each sound contains six seconds of approach. At time t = 6 seconds, the sound is representative of the sound as the vehicle arrives at a pedestrian’s location. Sounds 3 through 14 also include 6 seconds of departure.

Sound File Descriptions...

( Source NHTSA )