Overcoming Challenges of Measuring Distortion Audibility in Automotive Audio Systems
Standardized measurement of in-car audio systems remains an ongoing challenge, particularly regarding microphone configuration and placement, test signals, and correlation with perception. Current recommendations from the AES Technical Committee on Automotive Audio (TC-AA) advocate spatial averaging using multi-microphone arrays to improve repeatability.
While this method reduces the influence of reflections and standing waves to improve frequency magnitude measurement consistency and repeatability, it limits frequency, phase and amplitude resolution, which help identify the root causes of the distortions and evaluate their perceptual impact. An alternative approach is to apply frequency-normalized distortion analysis, where reflections or standing waves in the frequency response are also echoed in the distortion results, and greatly reduced by direct comparison of the two measurements. In this paper, spatial averaged distortion measurements using the TC-AA recommended 6 microphone array are compared with normalized distortion calculations, and a single microphone capture using the normalized distortion method.
Buzz, Squeak & Rattle (BSR) measurements otherwise known as Rub & Buzz or Impulsive Distortion measurements are also challenging. The TC-AA recommends a crest factor algorithm, available in a few audio measurement systems. This adequately detects transient distortions, but is susceptible to background noise, therefore requires a tightly controlled environment for making measurements. Two other methods, enhanced perceptual Rub & Buzz and enhanced Loose Particles offer improved repeatability in the presence of background noise, and the results are easier to correlate to audibility. The three methods are compared, both in a quiet environment, and with background noise.
Author: Steve Temme, Listen, Inc.
Presented at the AES Automotive Conference (July 2026) Detroit, Michigan
Overview
This paper investigates improved methods for measuring audible distortion in automotive infotainment systems. Traditional Total Harmonic Distortion (THD) measurements are often compromised by vehicle cabin reflections and standing waves, making it difficult to distinguish genuine loudspeaker distortion from acoustic artifacts. The paper demonstrates that Frequency-Normalized Total Harmonic Distortion (nTHD) produces more accurate and perceptually meaningful results than conventional THD, while also reducing measurement complexity and cost.
Key Findings
- Frequency-normalized THD (nTHD) largely removes the effects of cabin reflections by comparing harmonics with the fundamental at the same measured frequency before calculating distortion.
- nTHD produces clearer distortion measurements than conventional THD and more accurately identifies the true source of distortion.
A single microphone using nTHD closely matches measurements obtained from the AES TC-AA recommended six-microphone array, suggesting a practical lower-cost solution for production-line testing. - For Buzz, Squeak and Rattle (BSR) measurements, traditional Crest Factor analysis is susceptible to background noise.
Two alternative techniques—Enhanced Perceptual Rub & Buzz (ePRB) and Enhanced Loose Particles (eLP)—provide better repeatability, improved noise immunity, and stronger correlation with what listeners actually hear.
Why It Matters
Automotive audio systems are measured in highly reflective environments where conventional distortion measurements can misinterpret reflections as loudspeaker defects. Frequency-normalized distortion separates linear acoustic effects from true nonlinear distortion, allowing engineers to identify problems more accurately. The paper also shows that perceptually based distortion measurements better reflect human hearing and remain reliable in noisy environments such as vehicle production facilities.
Applications
The techniques described are particularly valuable for:
- Automotive infotainment system development
- End-of-line manufacturing testing
- Loudspeaker quality assurance
- Diagnosis of Buzz, Squeak and Rattle (BSR)
- Production environments requiring faster, lower-cost acoustic measurements
Conclusion
The paper concludes that frequency-normalized THD provides a more accurate representation of audible distortion than conventional THD in vehicle cabins. Combined with perceptually based Rub & Buzz and Loose Particle measurements, it offers improved correlation with listener perception, greater immunity to background noise, and the potential to replace expensive multi-microphone measurement systems with a single microphone for many automotive production applications.
