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Wide-Area Psychoacoustic Correction for Problematic Room-Modes Using Nonlinear Bass Synthesis

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Small rooms are characterized by low-frequency resonant modes, where traditional equalization systems may not efficiently correct the response at a few narrow frequency bands. A proposed supplementary approach based on virtual bass improves output efficiency while further reducing quality variations over a wide-area. The problematic low frequencies are reduced by the equalization system and are processed to create the illusion of the reduced bass by adding harmonics of those frequencies. Subjective testing showed that the seat-to-seat variation in quality was significantly reduced but the amount of virtual bass must be limited to avoid the perception of distortion.

JAES Volume 59 Issue 11 pp. 825-834; November 2011
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Patrick James Hegarty

Comment posted January 3, 2012 @ 15:39:13 UTC (Comment permalink)

I have a number of questions about the perceptual experiment described in the paper.
Section 4.1
Paragraph 2 & 3
Can the authors provide some more information about the soundfield in the two listening areas before and after the equalisation was applied?
How were the selected modes equalised?
Paragraph 4
Does each of the programs listed in Table 1 excite all of the room modes?
Can the authors provide some more information about the interaction between the program material and the hybrid virtual bass algorithm?
Were the unprocessed and processed pieces of program material loudness aligned? If so, how?
Paragraph 5
What is the experimental design?
Are order effects controlled?
Was a subject free to move between the seats throughout a particular program and enter his/her response at any time?
Paragraph 6
Can the authors provide some more information about how the aspects of sound quality to be rated were defined or explained to the subjects?
Section 4.2
Paragraph 1
Can the authors provide some more information about each subject’s audiometry?
How was the data summarised into Figure 14 considering, for example:
i.                    Subjects may not have heard the same 3 programs?
ii.                  Subject variability is not reported?
Paragraph 2 & 3 + Discussion
Are there any suggestions as to why these phenomena were observed?

Adam J. Hill
Author Response
Adam J. Hill

Comment posted January 13, 2012 @ 14:34:58 UTC (Comment permalink)

Thanks for the interest in the paper. I'll try to address all of your questions:

Section 4.1, Paragraph 2 & 3

- The selected modes were equalized using third-order Butterworth filters in a bandstop configuration. The cutoff frequencies are defined based on the measured spectral widths of each target room-mode

- After equalization the sound field exhibits less spatial variance between the two locations, but with reduced low-frequency content due to the applied equalization.

Section 4.1, Paragraph 4

- The test material was chosen for its rich bass content, which excites the room-modes in question. Of course the specific levels of modal excitation will vary among the signals.

- The algorithm operates consistently, regardless of program material, with the exception of the transient content detector which monitors the transient content of the input signal to dynamically weight the virtual bass algorithms running in parallel.

- The signals are roughly aligned by matching levels in the processed low-frequency band (before the PEQ is applied) to the levels in the unprocessed signal. Ideally, the virtual bass components should compensate for the absence of the removed components.

Section 4.1, Paragraph 5 & 6 + Section 4.2, Paragraph 1

- The subjects were free to move between the seats as many times and as often as necessary.

- Sound quality ratings were based on general impressions of the full-range sound field at each listening location. The scale ranges from 0 (poor) to 100 (excellent) with appropriate descriptors at intermediate intervals.

- The test subjects consisted predominantly of non-experienced listeners with no/minimal hearing loss.

- The subjects were free to choose three programs for testing. This method was chosen to allow the subjects to listen to material they are familiar with.

- The described listening test was designed to provide proof of concept, not absolute subjective performance. If the algorithm were picked up for use in a product a more rigorous subjective evaluation would be necessary.

- The data in Fig. 14 is the mean over the second and third quartile of the test data. As alluded to above, the bulk of time with this work was dedicated to algorithm development where the subjective tests were used for proof of concept. Again, more thorough testing and statistical analysis would be necessary if this were to be implemented beyond the current prototype.

Section 4.2, Paragraphs 2 & 3 + Discussion

- Since the most problematic room-modes (in terms of spatial variance between seats) have been removed, it is expected that seat-to-seat variance will be reduced after the virtual bass processing. The trick is to maintain acceptable sound quality after processing, which is largely dependent on the virtual bass essentially replacing the removed low-frequency components.

- Generally, the virtual bass processing can work in place of the physical low-frequency reproduction with the exception of the physical impact lost with removal of the components. This is what was observed during testing and is something that really can't be addressed with virtual bass alone (although this could be achieved using other means, but is beyond the scope of this work).

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