Journal of the Audio Engineering Society

2017 March - Volume 65 Number 3


Sound-Source Localization On Flat-Panel Loudspeakers

Authors: Heilemann, Michael C.; Anderson, David; Bocko, Mark F.

The authors describe a method for creating a two-dimensional audio display by controlling the bending vibrations of a flat panel loudspeaker in spatial regions. This method can simultaneously produce multiple independent audio sources on the surface of the panel using an array of independent force actuators that then govern the size, shape, and location of each source. The minimum size of an audio source is determined by the spatial Nyquist frequency of the exciter array and by the spatial vibration profile of the panel, which can be dynamically controlled by signal processing. A prototype panel with an optimized array of eight force exciters shows that the vibration profile of the panel closely approximates the user-defined target shape at all frequencies within the bandwidth of the actuator array. In addition, multiple loudspeakers can be used to render a specific sound field for applications such as beam-forming or wavefield synthesis. This technology could be applied to a thin OLED display that was driven from the back by an array of force actuators to create a multimodal video-audio display with spatial audio capabilities. The video screen also functions as the loudspeaker with the potential for center, left, and right channels.

This research explores how expectations about room acoustics influence the perceived quality when a listener is forced into an acoustical environment that is different from the actual listening room. The difference is the so-called room divergence effect. In listening tests, the expectations of the listeners were altered in order to measure an effect on the perception of externalization. Two groups of 31 listeners were familiarized with two different rooms using a localization test with visual directional feedback. One group was familiarized with a reverberant room and the other group with an acoustically dry room. Audio signals were presented with a static and individualized binaural headphone reproduction system. The disparity between the group ratings represents the effect of training on externalization. The results show that training modifies the listeners’ expectations. The reduction of the room divergence effect after training is an indicator for the existence of an auditory room model that is based on prior knowledge and expectations.

In multichannel reproduction, listening at off-center positions involves differences in the arrival times of the signals from different loudspeakers, which influences localization. The difference between transient or nontransient signals was found to have an influence on the localization of an auditory source image with third-order Ambisonics when the listener is located off-center. The transients were found to have larger errors on the placement of the Ambisonic pointer because of stronger localization dominance of the earlier arriving loudspeakers. Subjects performed consistently over a number of repetitions, but there was a larger difference among different subjects. Results were compared to those obtained with several prediction models, including an extended version of the energy vector model that incorporates the precedence effect. Compared to two binaural models, the extended vector model is shown to provide the best predictions over all conditions. The results confirm that the type of signal must be taken into account in predictive modeling. Furthermore, the extended energy vector exhibits about 50% less error than the standard energy vector.


An experiment was performed to determine the attributes that contribute to listener preference for a range of spatial audio reproduction methods. Experienced and inexperienced listeners made preference ratings for combinations of seven program items replayed over eight reproduction systems, and reported the reasons for their judgments. Automatic text clustering reduced redundancy in the responses by approximately 90%, thereby facilitating subsequent group discussions that produced clear attribute labels, descriptions, and scale end-points. Twenty-seven and twenty-four attributes contributed to preference for the experienced and inexperienced listeners respectively. The two sets of attributes contain a degree of overlap (ten attributes from the two sets were closely related); the experienced listeners used more technical terms while the inexperienced listeners used broader descriptive categories.

Evaluation of Spatial Audio Reproduction Methods (Part 2): Analysis of Listener Preference

Authors: Francombe, Jon; Brookes, Tim; Mason, Russell; Woodcock, James


A paired-comparison preference rating experiment was performed in combination with a free-elicitation task for eight reproduction methods (consumer and professional systems with a wide range of expected quality) and seven program items (representative of potential broadcast material). The experiment was performed by groups of experienced and inexperienced listeners. Both groups preferred systems with increased spatial content; nine- and five-channel systems were most preferred. The use of elicited attributes was analyzed alongside the preference ratings, resulting in an approximate hierarchy of attribute importance. Three attributes (amount of distortion, output quality, and bandwidth) were found to be important for differentiating systems where there was a large preference difference; sixteen were always important (most notably enveloping and horizontal width); and seven were used alongside small preference differences. Although the presence of more spatial content increases preference, adding loudspeaker channels does not necessarily give a corresponding increase in preference.

Engineering reports

Design of a Loudspeaker Array for Personal Audio in a Car Cabin

Authors: Liao, Xiangning; Cheer, Jordan; Elliott, Stephen J.; Zheng, Sifa

The automobile cabin is an appropriate listening environment for a sound delivery system that allows multiple individuals to each hear a different sound program without using headphones. While an array of four standard loudspeakers can create acceptable contrast between front and rear zones, this configuration is limited to relatively low frequencies. But by mounting a loudspeaker array on the ceiling of the car cabin, a larger bandwidth for control is created. Two algorithms, acoustic contrast control and the least squares method, are considered for producing two independent listening zones: one zone for the front passengers and the other for the rear passengers. Free-field simulations were used to calculate the response of the source array and to investigate the performance of the two control algorithms. Since the performance of the least squares method is dependent on the chosen target sound pressures, a method for selecting the target sound pressures is also proposed. Finally, the proposed loudspeaker array is implemented in a real car and the measured results are found to be similar to those predicted from the simulations.


Spectral smoothing is a standard operation in many fields of audio. It reduces the often overwhelming detail of high-resolution spectra to the relevant information. A method is presented for fractional-octave smoothing that preserves symmetry after smoothing for spectra that were originally symmetric in log-frequency. While existing methods require interpolation of the FFT spectra to a log-frequency scale, the proposed method uses an analytically-derived smoothing window and operates directly in the FFT domain. This approach retains compatibility with the well-established spectral smoothing techniques such as complex smoothing. The proposed method is compared with two existing methods. The first uses a symmetric (on a linear scale) smoothing window, which exhibits the correct bandwidths but does not span the correct fractional-octave frequency ranges. The second interpolates the spectrum to logarithmically-spaced frequencies and then uses a symmetric fixed-width smoothing window. Results show that the proposed method achieves nearly identical smoothed spectra to the second method, but without the need for interpolation, and that the first method indeed skews the log-symmetry of the original spectra.


[Feature] Approaches to directivity evaluation based on measurements made near the loudspeaker may make it possible to avoid the need for highly sophisticated anechoic chambers and exceptionally low noise floors. It seems that far field measurements may be able to be inferred from holographic or radiation mode analysis. While some doubt is cast on the commercial value of distortion reduction in loudspeakers using signal processing, the prediction and control of distortion is still an important factor in loudspeaker design, particularly for micro-loudspeakers. Loudspeaker driver modeling is becoming ever more advanced, enabling the detailed evaluation of nonlinear effects arising from things like flux modulation that could not easily be studied before now.

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