Analysis of acoustic data for hybrid and electric vehicles measured on hemi-anechoic chambers

Hastings, Aaron; Shumway, Meghan Ahearn; McInnis, Catherine · 2015 · ROSA P / United States. Department of Transportation. National Highway Traffic Safety Administration

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Summary

This report analyzes acoustic data for hybrid, electric, and internal combustion engine vehicles to support the National Highway Traffic Safety Administration’s (NHTSA) rulemaking under the Pedestrian Safety Enhancement Act of 2010. The study aims to establish performance requirements for alert sounds that allow pedestrians, particularly those with visual impairments, to detect nearby quiet vehicles. Specifically, the research evaluates the reliability of acoustic measurements by examining ambient noise levels, measurement repeatability (run-to-run consistency), and measurement reproducibility (consistency across different test sites and environments). The analysis utilized data from two indoor hemi-anechoic chambers equipped with chassis dynamometers—General Motors Milford Proving Grounds (MPG) and International Automotive Components (IAC)—and one outdoor facility, the Transportation Research Center (TRC). Seven vehicle models, including the Nissan Leaf, Tesla Roadster, and Toyota Camry Hybrid, were tested at constant speeds of 10, 20, and 30 km/h. The study compared ambient noise conditions, assessed repeatability within each indoor site, evaluated reproducibility between the two indoor sites, and compared indoor results against outdoor data. Ambient corrections were applied based on NHTSA’s proposed rulemaking procedures, though indoor ambient levels were generally low enough to require minimal correction. The findings indicate that measurement repeatability within each indoor facility was high, with standard errors ranging from 0.5 to 0.75 dB, meaning estimated mean values were within this range of the true mean with 95% confidence. Reproducibility between the two indoor sites was moderate, with average differences of approximately 2 dB, though individual measurements varied significantly, resulting in 95% confidence intervals of ±2.5 to ±6.7 dB. Reproducibility between indoor and outdoor sites was poorer, with 95% confidence intervals ranging from ±9.2 to ±9.8 dB. The authors attribute the larger indoor-outdoor discrepancies partly to the use of non-identical vehicles (different model years) and partly to tire-dynamometer interactions, which produced higher sound pressure levels indoors, particularly at 20 and 30 km/h. The study concludes that while indoor hemi-anechoic chambers provide consistent and repeatable acoustic measurements, significant variability exists when comparing different facilities and environments. The authors suggest that the tire-dynamometer interaction requires further study to ensure confidence in test procedures utilizing dynamometers. These results inform the development of standardized testing protocols for quiet vehicle alert sounds, highlighting the challenges in achieving consistent acoustic data across different testing conditions.

Key finding

Reproducibility between indoor and outdoor test facilities resulted in 95 percent confidence intervals of +/- 9.2 dB to +/- 9.8 dB, while indoor repeatability remained within 0.5 to 0.75 dB of the true mean.

Methodology

dataset

Sample size: 7

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

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