The Pedestrian Safety Enhancement Act of 2010 requires the National Highway Traffic Safety Administration (NHTSA) to establish a Federal Motor Vehicle Safety Standard (FMVSS 141) mandating minimum sound requirements for electric vehicles (EVs), and hybrid electric vehicles (HEVs). As part of FMVSS 141 development, NHTSA needed to select one sound analysis code (a software program) for sound data processing so that methods used to evaluate vehicle sounds for compliance testing or other purposes would be consistent. Two candidate sound analysis codes, the B&K Code and the Volpe Code, have been used by NHTSA. This paper documents NHTSA’s selection of one of these two for its future use.

Criteria for selecting a sound analysis code were that the code:​ (1) must give correct results for mathematically-generated test cases, (2) must meet all filter requirements for one-third octave band Class 1 filters contained in ANSI S1.11-2004:​ Specification of Octave, Half-Octave, and Third-Octave Band Filter Sets, [1], and (3) could be made available outside the Federal government to allow others to perform sound data analyses using NHTSA’s software.

The B&K and Volpe Codes both did an excellent job of calculating one-third octave band levels when pure tones were input. Both sound analysis codes correctly performed A-weighting. When a composite signal consisting of superimposed pure tones, one at the mid-band frequency of each of 13 one-third octave bands, was input, calculated levels exceeded input amplitudes by a small but acceptable amount.

The one-third octave band filters used by the B&K Code did not fully comply with one-third octave band Class 1 filter specifications contained in ANSI S1.11-2004. S1.11-2004 specifies that Class 1 filters asymptote to an attenuation of 70 dB for both high and low frequencies. For low frequencies, the B&K Code is asymptotic to between 55- and 60-dB attenuation for all one-third octave bands. For some one-third octave bands, there was also a region above the specified one-third octave pass band but below the high frequency region that also did not meet S1.11-2004 specifications. The Volpe Code filters complied with all S1.11-2004 Class 1 filter specifications for all one-third octave bands. For this, and other reasons, the Volpe code has been selected for future NHTSA analyses of vehicle-emitted sound.

Additional details about this research are contained in the NHTSA Technical Report “Selecting a Sound Analysis Code for use with NHTSA Test Procedures to Characterize Vehicle Sounds,” [2].