Journal of the Audio Engineering Society

2005 December - Volume 53 Number 12


Spatial impulse response rendering (SIRR) is a recent technique for the reproduction of room acoustics with a multichannel loudspeaker system. SIRR analyzes the time-dependent direction of arrival and diffuseness of measured room responses within frequency bands. Based on the analysis data, a multichannel response suitable for reproduction with any chosen surround loudspeaker setup is synthesized. When loaded to a convolving reverberator, the synthesized responses create a very natural perception of space corresponding to the measured room. A technical description of the analysis–synthesis method is provided. Results of formal subjective evaluation and further analysis of SIRR are presented in a companion paper to be published in JAES in 2006 Jan./Feb.

The subjective significance of two general types of metrics used to describe the “quality” of a room based on its aspect ratio are compared. Tests were carried out to evaluate differences between three virtual rooms that score extreme classifications in each of the metrics. The results of the tests indicate that room aspect ratios do have some effect on the perception of the modal distribution, but the effect is very much dependent on the frequency content of the original signal. This indicates that a room that scores well using a certain metric may still suffer from problems if the frequency content of the driving signal matches one particularly strong modal artifact. More significantly, the results of these tests imply that attempts to rank critical listening spaces based on modal distribution metrics are likely to be highly misleading; and the derivation of difference limen, which would be required for useful comparison, has been shown in practice to be highly problematic and perhaps meaningless.

Modifying the reverberations rendered from original impulse-response records is an essential three-dimensional audio technology. A nonparametric method is proposed for modifying the reverberation time, maintaining the frequency response trend (frequency locations of the spectral peeks and troughs) so that entire tonal characteristics might be preserved. The method can be interpreted to move the poles and zeros of the transfer function close to (for making reverberations longer) or far from (for shorter reverberations) the unit circle in the z plane. Exponential time windowing moves the minimum-phase zeros along the radial direction, while the pole and zero symmetrical locations of the all-pass part can be controlled after causal and noncausal cepstral decomposition. The locations of the all-pass poles can be moved by exponential windowing the time response reconstructed from the causal cepstrum component. The causal cepstrum obtained again after moving the all-pass pole locations is utilized in creating the noncausal part of the modified all-pass phase cepstrum. Rendering of the reverberation effects on the frequency response in which the frequency response trend is confirmed. Examples of frequency-dependent reverberation control applications are also given.

Subjective Testing of Compression Drivers

Authors: Geddes, Earl R.; Lee, Lidia W.; Magalotti, Roberto

[Engineering Report] A subjective test was devised and performed in order to assess the factors that influence the perception of sound emitted by compression drivers. A musical passage was high-pass filtered and played through three compression drivers of similar characteristics, loaded by a plane-wave tube, and recorded. To obtain different levels of nonlinear distortion, the passage was played at three different voltage levels on each driver. The resulting sound files were recombined with the low-pass-filtered portion, yielding nine complete sound pieces whose only differences from the original passage were caused by the drivers’ behavior. The nine stimuli were then presented, in a double-blind test, to 27 subjects, who were asked to rate audible differences when compared to the original passage. Analysis of the results shows that the differences in frequency response between drivers are statistically significant, whereas differences in playing level, and therefore nonlinear distortion, were not significant. This unexpected result implies that nonlinear distortion is not audible under these test conditions, and it leads to important conclusions regarding the design objectives of compression drivers.

[Feature Article] Recent research in digital signal processing, psychoacoustics, and microphone technology is transforming the resources available to those with hearing impairments. While highly directional microphone arrays bring with them the promise of more selective listening, binaural processing is enabling hearing aids to take advantage of the spatial properties of sound signals and human perception. Advanced means of tuning hearing aids make them more specific to the wearer, and assistive technology can be used to integrate modern communications devices.

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27th Conference Report, Hillerød, Denmark

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Audio Processing for the Hearing Impaired

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