Actuator-based sound reproduction is increasingly adopted in automotive audio systems due to its advantages in packaging efficiency, reduced mass, and seamless integration into vehicle structures. This work presents the implementation and tuning of a hybrid automotive sound system combining distributed mode loudspeakers (DMLs) with conventional dynamic drivers in a constrained open-cockpit vehicle platform developed by Morgan Motor Company. Two full-range actuators were integrated into a 2 mm aluminum dashboard panel to reproduce mid and high frequency content, supplemented by conventional woofers, a subwoofer, and low-frequency actuators. A comprehensive measurement methodology was employed, including spatially averaged frequency response measurements using an eight-microphone array and time-domain analysis via centralized impulse response capture. Measurements were conducted under multiple operating conditions, including stationary and driving scenarios with varying roof configurations. Digital signal processing, implemented using tools developed by Sennheiser, was applied to achieve frequency response linearization, time alignment, crossover integration, and final subjective tuning across the hybrid system. Additional tuning was guided by structured critical listening evaluations across representative use cases. Results indicate that actuator-based reproduction can provide improved spatial impression and apparent source width compared to conventional full-range door-mounted loudspeaker configurations, particularly in acoustically challenging environments characterized by high ambient noise. The findings demonstrate the viability of hybrid actuator-conventional systems and highlight the importance of panel behaviour, placement constraints, and robust tuning methodologies in achieving consistent performance.
Automotive OEMs invest significantly in branded audio systems to create differentiated, premium in-vehicle experiences. However, loudness tuning—critical for maintaining consistent perceived sound quality across listening levels—is often underdeveloped or inconsistently implemented. As a result, systems optimized at a single reference level can sound thin, unbalanced, or inconsistent under real-world usage conditions, negatively impacting perceived quality and brand identity. This paper presents a practical and production-ready methodology for loudness tuning based on ISO equal-loudness contours. Starting from a calibrated reference listening level, level-dependent compensation curves are derived to address changes in human auditory sensitivity at reduced playback levels. These compensations are implemented using a small set of efficient filters controlled via a volume-dependent lookup table, enabling scalable integration into automotive audio platforms. To further validate the approach, a benchmark dataset of over 200 production vehicles was analyzed, combining objective measurements with listener preference rankings. High-performing systems were used to derive effective loudness compensation characteristics, which shows a good alignment with ISO-based predictions while revealing systematic deviations due to in-cabin acoustics and playback conditions. The results demonstrate improved spectral consistency and preservation of brand sound signature, providing a high-impact, low-cost opportunity for OEMs to enhance perceived audio quality across all listening conditions.
Immersive object-based spatial audio is now firmly established in the music industry as the standard for production, distribution, and playback. The number of automobiles integrating such content to provide premium entertainment experiences is steadily increasing, driving the development of new audio rendering techniques. While loudspeakers integrated into automotive headrests have been around for more than 50 years, they have not yet achieved status as a standard feature in new cars. However, they represent a powerful tool for reproducing immersive audio by enabling the creation of personal sound zones with reduced passenger distraction while effectively complementing existing cabin speakers. We conduct subjective assessments using paired comparison experiments to measure preference and multiple spatial audio attributes. We plan to model the resulting probability outcomes using probabilistic choice models, such as Bradley-Terry-Luce rank ordering. We expect the findings of this work to provide a technical roadmap for the integration of headrest speakers in next-generation automotive spatial 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, 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 eliminated 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. BSR measurements are also challenging. The TC-AA recommends a crest factor algorithm, available in most 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.
Sound sensing through structural vibrations has emerged as a robust alternative to conventional air conducted microphones for automotive exterior applications, where environmental exposure can degrade microphone performance. This study evaluates the feasibility and performance of sensing airborne speech via vehicle body surface vibrations using surface mounted accelerometers. Three miniature accelerometers with different signal to noise ratios were mounted at seven locations on a vehicle body to sense speech generated by an artificial mouth positioned outside the vehicle. Speech quality was objectively assessed using ETSI TS 103 281 perceptual metrics under various driving conditions. The results demonstrate that acceptable to good quality of speech can be reconstructed from body surface vibrations under relatively low noise conditions such as parked or idling states. Accelerometer SNR is identified as a key performance factor, with higher SNR sensors consistently yielding superior speech quality across all mounting locations and test conditions. Sensor mounting location also plays a significant role, particularly under elevated driving noise, with relatively flexible and noise isolated body panels providing better performance. In addition, increasing the speech level improves performance, consistent with the benefits associated with higher SNR accelerometers. Finally, the results indicate that introducing a high pass characteristic into the accelerometer frequency response does not provide a consistent performance benefit.
Microphones used in automotive hands-free and Automatic Speech Recognition (ASR) systems are typically required to meet wideband or fullband specifications defined by standards such as ITU-T P.1110 and P.1120. In practice, however, compliance with these standards is often challenged by vehicle cabin integration constraints and automotive-grade durability requirements. Moreover, there is limited empirical evidence clarifying how specific microphone characteristics influence perceptual audio quality and ASR performance. This paper presents an experimental investigation into the effects of microphone frequency response and bandwidth variations on system-level performance in automotive environments. Noise signals recorded under real-world driving conditions are used to evaluate perceptual speech quality using ETSI TS 103 281 metrics, including S-MOS, N-MOS, and G-MOS, as well as ASR accuracy quantified by Word Error Rate (WER). In addition, the interaction effects between microphone characteristics and signal processing algorithm is also studied by processing the signals through Noise Reduction (NR) modules. The results aim to identify the most critical frequency response attributes for properly determining microphone specifications in automotive hands-free and ASR applications.
This paper presents a vehicle-scale multichannel automotive audio demonstrator based on piezoelectrically actuated flat-panel loudspeakers (FPLs) integrated into existing cabin surfaces. The proposed architecture combines piezoelectric driven radiators embedded in headrests, front and rear doors, and an OLED center console with a conventional electrodynamic subwoofer for low-frequency extension, enabling full-band reproduction in a hybrid configuration. Operating frequency bands are assigned from electroacoustic characterization of each radiator and implemented through dedicated crossover design and DSP tuning. The system is analyzed through numerical simulation and evaluated experimentally in a prototype vehicle using AES automotive assessment procedures, objective electroacoustic indicators, and perceptual evaluation through Multi-Dimensional Audio Quality Score (MDAQS) metric. Results show that surface geometry, actuator placement, and time alignment improve in-cabin sound field distribution and directivity. After tuning, spectral balance improves and impulsive distortion artifacts are no longer evident, with corresponding enhancements in perceived timbre and overall quality. However, maximum output level remains constrained by the subtractive equalization required to control distortion and spectral balance. Overall, the study demonstrates the feasibility of hybrid piezoelectric--dynamic automotive systems, highlighting their packaging advantages and potential for reduced electrical power consumption while identifying the remaining limitations in low frequency capability, distortion performance, and achievable Sound Pressure Level (SPL).
A White Paper for in-car measurements version 1.0 was officially published in 2023 by the Automotive Audio section of the AES. It comprises basic characteristics of car audio systems. One of the intentions was to make audio systems as well as different cars comparable. This paper is divided into two parts: In the first part, measurements of several cars with different sizes, ages, and audio performance are presented based on version 1. The feasibility of the suggested procedure is evaluated, and improvements are suggested. The first version addresses frequency responses at different levels and at maximum sound pressure level, as well as defect symptoms such as rattling. In a second part, additional measurements are suggested that address compression and non-linear distortion. The suitability of different distortion measurements is discussed and practically demonstrated. Time variant effects, which are typical for DSP-based algorithms, such as limiters, protection systems, and adaptive control, are also investigated, and methods to characterize those are proposed. Stationary behavior is not ensured in such systems; thus, methods to cope with transient effects are required.
