Sunday, November 22, 2009

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).


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.


[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).

Sunday, November 1, 2009

The Subjective and Objective Evaluation of Room Correction Products

In a recent article, I discussed audio’s circle of confusion that exists within the audio industry due to the lack of performance standards in the loudspeakers and rooms through which recordings are monitored. As a result, the quality and consistency of recordings remain highly variable. A significant source of variation in the playback chain occurs from acoustical interactions between the loudspeaker and room, which can produce >18 dB variations in the in-room response below 300-500 Hz.

In recent years, audio manufacturers have begun to offer so-called “room correction” products that measure the in-room response of the loudspeakers at different seating locations, and then automatically equalize them to a target curve defined by the manufacturer. The sonic benefits of these room correction products are generally not well known since, to my knowledge, no one has yet published the results of a well-controlled, double-blind listening test on room correction products. To what degree do room correction products improve or possibly degrade the sound quality of the loudspeaker/room compared to the uncorrected version of the loudspeaker/room? Can the sound quality ratings of the different room correction products be explained by acoustical measurements performed at the listening location?

A Listening Experiment on Commercial Room Correction Products

To answer these questions, we conducted some double-blind listening tests on several commercial room correction products [1]. I recently presented the results of those tests at the 127th Audio Engineering Society Convention in New York. A copy of my AES Keynote presentation can be found here.

A total of three different commercial products were compared to two versions of a Harman prototype room correction that will find its way into future Harman consumer and professional audio products. The products included the Anthem Statement D1, the Audyssey Room Equalizer, the Lyngdorf DPA1, and two versions of the Harman prototype product (see slide 7). Included in the test was a hidden anchor: the same loudspeaker and subwoofer without room correction. In this way, we could directly compare how much each room correction improved or degraded the quality of sound reproduction.

Each room correction device was tested in the Harman International Reference Room using a high quality loudspeaker (B&W 802N) and subwoofer (JBL HB5000) (slides 8 and 9). A calibration was performed for each room correction over the six listening seats according to the manufacturer’s instructions. Two different calibrations were performed with the Harman prototype: one based on a multipoint six-seat average, while the second calibration used a six-microphone spatial average focused on the primary listening seat. The different room corrections were level matched for equal loudness at the listening seat.

The Listener's Task

A total of eight trained listeners with normal hearing participated in the tests. Using a multiple comparison method, the listener could switch at will between the six different room corrections, and rate them according to overall preference, spectral balance, as well as give comments (see slide 14). The administration of the test, including the design, switching, collection and storage of listener responses, was computer automated via Harman’s proprietary Listening Test Software. A total of nine trials were completed using three different programs repeated three times. The presentation order of the program and room corrections was randomized.

Results: Significant Preferences For Different Room Corrections

The mean preference ratings and 95% confidence intervals are shown above in Figure 1 (or slide 17). The room correction products are coded from R1 through R6 in descending order of preference. The identities of the products associated with the results are not relevant for the purpose of this article. Three of the five room corrections (RC1-RC3) were strongly preferred over no room correction (RC4). However, one of the room corrections (RC5) was equally rated to the no correction treatment (RC4), and one of the room corrections (RC6) was rated much worse. Overall, the sound quality of R6 was rated "very poor" based on the semantic definitions of the preference scale.

Perceived Spectral Balance of Room Corrections

Listeners rated the perceived spectral balance of each room correction across seven equal logarithmically spaced frequency bands. The mean spectral balance ratings averaged across all listeners and programs are shown in slide 18. The more preferred room corrections were perceived to have a flatter, smoother spectral balance with extended bass. The less preferred room correction products (R5 and R6) were perceived to have too little bass, which made them sound thin and bright.

Listener Comments on Room Corrections

Listeners also gave comments related to the spectral balance of the different room correction products. Slide 19 shows the number of times a particular comment was used to describe each room correction. The bottom row indicates the correlation between preference rating and the frequency of the comment. The most preferred room corrections were described as "neutral" and "full," which corresponded to flatter, smoother and more bass extended spectral balance ratings. The least preferred room corrections (R4-R6) were described as colored, harsh, thin, and muffled, which corresponded to less flat, less smooth, and less bass extended spectral balance ratings. Slide 20 graphically illustrates the same information in slide 19.

Correlation Between Subjective and Objective Measurements

In-room acoustical measurements were made at the six listening seats using a proprietary 12-channel audio measurement system developed by the Harman R&D Group. Slides 23 and 24 show the amplitude response of the different room corrections spatially averaged for the six seats (slide 23), and at the primary listening seat (slide 24). The measurements are plotted from top to bottom in descending order of preference, each vertically offset to more clearly delineate the differences. A few observations can be made:

  1. The six-seat spatially averaged curves (slide 23) of the room corrections do not explain listeners' room correction preferences as well as the spatially averaged curves taken at the primary seat (slide 24). This makes perfect sense since all of the listening was done in the primary listening seat.
  2. Looking at slide 24, the most preferred room corrections produced the smoothest, most extended amplitude responses measured at the primary listening seat. The largest measured differences among the different room corrections occur below 100 Hz and around 2 kHz where the loudspeaker had a significant hole in its sound power response. The room corrections that were able to fill in this sound power dip received higher preference and spectral balance ratings.
  3. A flat in-room target response is clearly not the optimal target curve for room equalization. The preferred room corrections have a target response that has a smooth downward slope with increasing frequency. This tells us that listeners prefer a certain amount of natural room gain. Removing the rom gain, makes the reproduced music sound unnatural, and too thin, according to these listeners. This also makes perfect sense since the recording was likely mixed in room where the room gain was also not removed; therefore, to remove it from the consumers' listening room would destroy spectral balance of the music as intended by the artist.


There are significant differences in the subjective and objective performance of current commercial room correction products as illustrated in these listening test results. When done properly, room correction can lead to significant improvements in the overall quality of sound reproduction. However, not all room correction products are equal, and two of the tested products produced results that were no better, or much worse, than the unequalized loudspeaker. Room correction preferences are strongly correlated to their perceived spectral balance and related attributes (coloration, full/thin, bright/dull). The most preferred room corrections produced the smoothest, most extended in-room responses measured around the primary listening seat.

More tests are underway to better understand and, if necessary, optimize the performance of Harman's room correction algorithms for different acoustical aspects of the room and loudspeaker.


[1] Sean E. Olive, John Jackson, Allan Devantier, David Hunt, and Sean Hess, “The Subjective and Objective Evaluation of Room Correction Products,” presented at the 127th AES Convention, New York, preprint 7960 (October 2009).