The Effect of Whole-body Vibrations on Preferred Bass Equalizations of Automotive Audio Systems

Binaural Room Scanning (BRS) is a technology that allows Harman scientists to binaurally capture, store, and later reproduce sound fields through a headphone-based auditory display that includes head tracking for accurate localization of sound sources [1]. BRS enables us to do controlled, double blind comparative evaluations of different automotive audio systems, home theatre systems or sound reinforcement systems that would otherwise not be practical or possible to do. However, current BRS systems do not typically capture and reproduce the whole-body vibrations that are associated with low frequencies reproduced by the audio system. Therefore, an important question related to the accuracy and ecological validity of BRS-based evaluations is whether whole-body vibrations play an important role in our perception of the quality and realism of the automotive audio system.

We recently presented a paper at the 127th Audio Engineering Society Convention that addressed this research question [2]. Three experiments were reported that measured the effects of both real and simulated whole-body vibration associated with the low frequency sounds reproduced by an automotive audio system on listeners’ preferred bass equalizations. A PDF of the side presentation can be found here.

In all three experiments, the same automotive audio system was used: a high-quality 17-channel audio system installed in a 2004 Toyota Avalon. The car was parked in our automotive audio research lab with the engine turned off so that the effects of the road and engine noise and vibration were not part of the experiment. The BRS system was calibrated for each individual listener to minimize errors related to headphone fit, etc. The differences in magnitude response measured at the listeners’ ear in situ versus through the BRS system were very small indeed (see slide 9).

The Effect of Real Vibration on Preferred Bass Levels

In the first experiment, listeners adjusted the level of bass equalization (see slide 10) while experiencing real whole-body vibration produced by the car audio system. The same task was also repeated via the BRS headphone-based system without the vibration present. This was repeated three times using three different music programs (see slide 7). On average, listeners adjusted the bass equalization 1.5 dB higher for the BRS playback condition where the low frequency vibration was not present (see slide 11). The preferred bass level was found to be program dependent due to the amount of bass present in the signal, and the resulting vibration it produced(see slides 12 and 13).

The Effect of Simulated Vibration on Preferred Bass Levels

In the second experiment, we simulated the whole-body vibration produced by the audio system by attaching an actuator to the driver's seat of the car. The actuator was driven by the low frequency portion of the audio signal below 100 Hz. Comprehensive whole-body vibration measurements performed prior to this experiment found that most of the whole-body vibration produced by the audio system occurs below 100 Hz (see slides 15 and 16) at the seat and floor. The level and frequency of the whole-body vibration varies with music program, and the weight of the listener.

Each listener sat in the driver’s seat of the car listening a virtual BRS rendering of the automotive audio system reproduced through headphones. Listeners adjusted the bass level in their headphones while experiencing four different levels of simulated whole-body vibration that varied from none, low (0 dB) medium (+4 dB) and high (+8 dB). The medium level corresponded to the measured vibration in experiment one, when the bass equalization of the automotive audio system was adjusted to its preferred level.

The experimental results indicated that the preferred level (dB) of bass equalization decreased 3 dB when the level of whole-body vibration increased 8 dB. (see slide 21), and varied with program. At the low vibration level, there was no effect on the preferred bass level, since the vibration level was near or below detection thresholds reported in the literature. At the highest level (8 dB), the vibration tended to be annoying, and listeners tended to turn the bass level down with the hope that the vibration would also be reduced. The effect of vibration on preferred bass level was somewhat dependent on the listener, which could be related to their weight (see slide 24).

The experimental results confirm those of a previous experiment conducted by Martens et al. where the vibration was simulated via a platform, and both head-tracking and individualized BRS calibrations were not employed [3]. The results from the Martens' et al. experiments and this one are plotted above in Figure 1. In spite of the methodological differences between the two experiments, there is good agreement between the two studies. This suggests that the effect of whole-body vibration on the preferred level of bass equalization is quite robust.

The Effect of Whole-body Vibration on the Similarity in Sound Quality between BRS and In Situ Reproductions

The third experiment, listeners sat in the car and rated the overall similarity in sound quality between the BRS headphone-based reproduction with and without the simulated vibration compared to the same audio system experienced in situ. Listeners could switch at will between the in situ and two BRS reproductions (with and without shaker). The two BRS treatments were presented double-blind, and repeated two times with four music programs (see slide 27).

The results (see slide 29) show that sound quality of the BRS reproduction system was significantly improved with the presence of whole-body vibration (shaker on).

Conclusions

From these experiments, it is clear that the whole-body vibration associated with the low frequency sounds of an audio system influences listeners’ perception of the quality and quantity of bass. When the vibration is absent from a stereo or binaural recording of music reproduced through headphones there may be a perceived lack of bass. A 4 dB increase in whole-body vibration produces about a 1.5 dB decrease in preferred level of bass equalization. However, there appears to be upper and lower threshold limits beyond which a change in vibration level will have no effect. Moreover, the amount of vibration and its effect on preferred levels of bass equalization will depend on the low frequency characteristics of the music and the individual listener (and possibly their weight).

Finally, adding simulated whole-body vibration to BRS reproductions can greatly enhance their perceived realism and fidelity when compared to the in situ experience, as long as the vibration levels are above the listener's detection threshold.

References

[1] Sean E. Olive and Todd Welti, “Validation of a Binaural Car Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems,” presented at the 36th International AES Automotive Audio Conference, (June 2-4, 2009).

[2] Germain Simon, Sean E. Olive, and Todd Welti, “The Effect of Whole-body Vibration on Preferred Bass Equalization in Automotive Audio Systems,” presented at the 127th Audio Eng. Soc. Convention, preprint 7956, (October 2009).

[3] William Martens, Wieslaw Woszczyk, Hideki Sakanashi, and Sean E. Olive, “Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Vibration,” presented at the AES 36th International Conference, Dearborn, Michigan (June 2-4, 2009).

Validation of a Binaural Room Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems

In a previous posting on Audio Musings, I described Harman’s binaural room scanning (BRS) measurement and playback system. BRS is a powerful audio research and testing tool that allows Harman scientists to capture, store and later reproduce through a head-tracking headphone-based auditory display the acoustical signature of one or more audio systems situated in the same or different listening spaces. BRS makes it practical to conduct double-blind listening evaluations of different loudspeakers, listening rooms, and automotive audio systems in a very controlled and efficient way.

I also pointed out that all binaural recording/playback systems contain errors that require proper calibration for their removal. However, removing all BRS errors can become very expensive and impractical, so some compromise is necessary. This precipitates the need to experimentally validate the performance of the BRS system to ensure that the remaining errors after calibration do not significantly change listeners’ perceptual ratings of audio systems evaluated through the BRS system as compared to in situ evaluations.

To this end, Todd Welti, Research Acoustician at Harman International, and I recently presented the results of a series of BRS validation tests performed using different equalizations of a high quality automotive audio system [1]. You can view the Powerpoint presentation of the conference paper here. For more detailed information on this experiment, you can view the proceedings from the recent 36th AES Automotive Conference in Dearborn, Michigan, when they become available in the AES e-library .

To assess the accuracy of the BRS system, a group of trained listeners gave double-blind preference ratings for different equalizations of the audio system evaluated under both in situ (in the car) and BRS playback conditions. For the BRS playback condition, the listener sat in the same car listening to a virtual headphone-based reproduction of the car's audio system. The purpose of the experiment was to determine whether the BRS and in situ methods produced the same preference ratings for different equalizations of the car's audio system.

Listeners gave preference ratings for five different equalizations using 4 different music programs reproduced in mono (left front speaker), stereo (left and right front channels) and surround sound (7.1 channels). The three playback modes were tested separately to isolate potential issues related to differences in how the BRS system reproduced front versus rear, and hard versus phantom-based, auditory images.

The listening test results showed there were no statistically significant differences in equalization preferences between the in situ and BRS playback methods. This was true for mono, stereo and multichannel playback modes (see slides 21-23). An interesting finding was that these results were achieved using a BRS calibration based on a single listener whose calibration tended to work well for the other listeners on the panel. This suggests that individualized listener calibrations for BRS-based listening tests may not be necessary, so long as the calibration and listeners are carefully selected.

In conclusion, this validation experiment provides experimental evidence that a properly calibrated BRS measurement and playback system can produce similar preferences in automotive audio equalization as measured using in situ listening tests.

Reference

[1] Sean E. Olive, Todd Welti, “Validation of a Binaural Car Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems,” presented at the 36th International AES Automotive Audio Conference, (June 2-4, 2009).

Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Equalization

Last week I attended the AES Automotive Audio Conference in Dearborn, Michigan where about 70-odd (pun intended) audio scientists and engineers gathered to discuss the latest scientific and technological developments in automotive audio. A detailed description of the program can be found here.

This article focuses on a paper I co-authored and presented called “Whole-body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Equalization" [1]. The work was a joint effort between three researchers, Drs. William Martens, Wieslaw Woszcczyk, and Hideki Sakanashi, from the CIRMMT at McGill University in Montreal, and myself, at Harman International. A copy of our Powerpoint presentation given at the conference can be viewed here.

It is well established that human perception is a multimodal sensory experience [2]. For example, both auditory and visual cues associated with a sound source and its acoustic space are integrated and interrogated by high level cognitive processes that determine our spatial perception of the source based on the plausibility, strength and agreement between the visual and auditory cues. Bimodal sensory interactions have been reported in studies where the video quality of the picture influences listeners’ judgment of the audio system’s sound quality and vice versa (although the audio quality has much less influence on the perceived quality of video than vice versa) [3].

However, little is known regarding how low frequency (below 100 Hz) whole-body vibration produced by the audio system influences our perception of the quality and quantity of bass. Perhaps the most related study is Rudmose’s “case of the missing 6 dB” where the perceived loudness of low frequency signals reproduced through headphones was reported to be approximately 6 dB lower than that of loudspeakers producing the equivalent sound pressure levels at the ears [4]. Rudmose showed that the absence of tactile stimulus in headphone reproduction could, in part, account for why headphones sound less loud than loudspeakers when producing equivalent sound pressure level at the ears (the rest of the missing 6 dB was due to experimental factors, and the increased physiological noise in the ear canal introduced by the coupling of the headphone to the ear).

A Tactile-Auditory Bimodal Sensory Experiment

To shed more light on this mystery, an experiment was conducted at McGill University. A total of 6 trained tonmeisters listened through calibrated headphones to binaural recordings of a virtual high-quality automotive audio system. Each listener adjusted the low frequency boost applied to different multichannel music reproductions according to their taste while experiencing a high and low level whole-body vibration. This was generated by a programmable motion platform driven by the low frequency portion (below 80 Hz) of the music signal. In this way, vibration was delivered to both the feet and body of the listener through the chair (see slide 5). The virtual automotive audio system was based on a binaural room scan (BRS) of the audio system installed in our research vehicle located at the Harman International Automotive Audio Research Lab in Northridge, California (see slide 3). For more information on how BRS works, please refer to my previous BRS blog postings, Part 1 and Part 2.

Whole-Body Vibration Influences Preferred Equalization of the Audio System

The researchers found that the preferred bass equalization of music reproduced through the virtual automotive audio system was significantly influenced by the level of whole-body vibration experienced. While the amount of preferred bass boost varied with music program and listeners, the listeners always preferred less bass for the high vibration condition than for the low vibration one, which was 12 dB lower: on average, listeners preferred 6 dB less bass boost in their headphones moving from the low to high vibration conditions (see slide 10). In other words, there was a bimodal sensory interaction effect between the auditory and tactile senses that influenced listeners' preferred bass equalization of music reproduced through the headphones.

It is important to note that the 6 dB effect reported here may not be the same as observed in an automobile where the level and other physical characteristics of the vibration observed may be different from what was tested here. Under driving conditions, listeners experience additional sources of vibration (and acoustic noise) from the road and engine of the vehicle that may partially mask the whole-body vibration effects produced by the audio system. More research is currently underway to study how real and simulated whole-body vibration in vehicles influences listeners' perception of the audio system and its sound quality.

References
[1] William Martens, Wieslaw Woszczyk, Hideki Sakanashi, and Sean E. Olive, “Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Vibration,” presented at the AES 36th International Conference, Dearborn, Michigan (June 2-4, 2009).

[2] Multimodal Integration, Wikipedia.
[3] Beerends, John G; De Caluew, Frank E. “The Influence of Video Quality on Perceived Audio Quality and Vice Versa,” JAES, Vol. 45, (5), pp. 355-362 (May 1999).
[4] Rudmose, Wayne, “The case of the missing 6 dB,” J. Acoust. Soc. Am. Volume 71, Issue 3, pp. 650-659 (March 1982)

The Harman International Reference Listening Room

Last week I returned from the AES Munich Convention where I gave a paper entitled ”A New Reference Listening Room for Consumer, Professional, and Automotive Audio Research.” It describes the features, scientific rationale, and acoustical performance of a new reference listening room designed and built for the purposes of conducting controlled listening tests and psychoacoustic research for consumer, professional, and automotive audio products. The main features of the room include quiet and adjustable room acoustics, a high-quality calibrated playback system, an in-wall loudspeaker mover, and complete automated control of listening tests performed in the room. A copy of my Munich AES presentation is available here.

The first prototype reference room was built at the Harman Northridge campus in 2007. Additional reference listening rooms have since been built at Harman locations in the UK, Germany, with the fourth one being constructed in Farmington Hills, Michigan. We are in the process of measuring and calibrating the performances of the different rooms using acoustical measurements and binaural room scans, which will be evaluated for their perceptual similarity in sound quality.

With a standardized listening room and playback system, Harman scientists can conduct listener training, psychoacoustic research and product testing at different Harman locations throughout the world. The results from these different locations can be compared or pooled together since the room, playback system, and trained listeners are a constant variable. With this brings greater testing efficiency, flexibility, and new opportunities in the kinds of product research and listening tests Harman is able to do in the future. Already, we are using the unique features of these rooms to conduct very controlled listening tests on consumer in-wall speakers, and to research and benchmark the performance of various commercial and prototype loudspeaker-room correction devices.

You will hear a lot more about the Harman International reference listening rooms in the near future because of the pivotal role they will play in the research, testing and subjective benchmarking of new Harman consumer, professional and automotive audio products. Just thinking about these research possibilities makes me truly excited!

Binaural Room Scanning - A Powerful Tool For Audio Research & Testing

Binaural Room Scanning (BRS) is a powerful audio technology being used by Harman scientists to conduct innovative psychoacoustic research and listening tests that were previously not practical, or even possible. The roots of BRS are traced back to Studer (a Harman International company) who in the late 1990‘s developed a BRS processor that allowed recording engineers to remotely monitor their recordings via headphones through a virtual copy of their control room [1].

Unlike auralization methods, BRS provides an auditory display based on actual acoustical measurements of the loudspeakers and listening environment - not simulations based on a model of the loudspeakers and room. For this reason, BRS reproductions are significantly more accurate and realistic than model-based auralizations.

BRS measurements of the loudspeakers and listening space are made with an anthropomorphically accurate binaural mannequin equipped with microphones in each ear (see top photo above). Measurements are made at every 1-2 degrees over a range of ±60 degrees by precisely rotating the mannequin's head via a stepper motor controlled by the BRS measurement computer. Each measurement is stored as a set of binaural room impulse responses (BRIR) that provide the filters through which music is convolved and sent to a calibrated pair of high quality headphones (see bottom photo above). A key feature of the BRS playback system is its ultrasonic head-tracker: it constantly monitors the position of the listener's head, sending the angular coordinates to the playback engine, which in turn switches to the corresponding set of measured BRIRs. In this way, the BRS playback preserves the natural dynamic interaural cues, used by humans to localize sound in rooms. Without these dynamic cues, headphones tend to produce sound images localized inside or near the head with front-to-back reversals being quite common. Head-tracking is therefore necessary for accurate assessment of the true spatial qualities of the audio reproduction.

Current and Future Applications For BRS

As a research tool, BRS offers greater efficiencies and opportunities in how audio scientists research, develop and test audio products within home, professional and automotive listening spaces. BRS allows an unlimited number of acoustical variables to be manipulated, sequentially captured, and later evaluated in a highly repeatable and controlled manner. Using BRS, Harman researchers can do perceptual experiments and product evaluations that would otherwise be impractical or impossible using conventional in situ listening tests. This includes double-blind, controlled comparisons of different audio systems in different automobiles, concert halls or arenas, and home theaters.

BRS has already been used at Harman to study how the acoustical properties of the loudspeaker and listening room interact with each other, how these interactions affect the sound quality of the music reproduction, and the extent to which listeners’ adapt to the room acoustics when listening to multichannel audio systems [2],[3]. Over the next few years, BRS will help expand our current scientific understanding of how listeners perceive sound in rooms, so that we can optimize the sound quality of loudspeakers, acoustic spaces, and room-correction devices used to tame loudspeaker-room interactions. A BRS auditory display connected over the internet to a BRS database could even allow consumers to compare and select their most preferred loudspeaker model, concert hall seat, or automotive audio system configuration, without ever leaving the privacy of their home.

Finally, BRS brings enormous efficiencies, flexibility, and cost savings to psychoacoustic research and testing. The acoustical complexity of an automotive audio system can be captured and stored as a relatively small 10 MB file, which can then be emailed and evaluated anywhere in the world using a relatively inexpensive auditory display. The high costs associated with building expensive ITU-R listening rooms, transporting listeners, automobiles, and loudspeakers around the world for evaluation may soon be a thing of the past.

In the next installment, I will discuss some of the inherent errors found in all BRS systems, and how they can be removed through proper calibration. Some recent listening experiments will be described that validate the perceptual accuracy and performance of our BRS system.

References

[1] Horbach, Ulrich, Karamustafaoglu, Attila, Pellegrini, Renato, Mackensen, Philip, Theile, Günther, “Design and Applications of a Data-Based Auralization System for Surround Sound,” presented at the 106th Audio Eng. Soc. Convention, preprint 4976, (May 1999). Download here.

[2] Olive, Sean and Martens William L. “Interaction between Loudspeakers and Room Acoustics Influences Loudspeaker Preferences in Multichannel Audio,” presented at the 123rd Audio Eng. Soc., Convention, preprint 7196 (October 2007). Download here.

[3] Olive, Sean and Welti Todd, “Validation of a Binaural Car Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems,” to be presented at the 36th International Audio Eng. Conference, Dearborn, Michigan, USA (June 2-4, 2009).