Performance Analysis

There are a number of potential limitations of the performance of our system. First and foremost, the system we implemented in this section is based on the design with an ideal free-field setting. However, the listening room is not fully anechoic. The performance of spatial soundfield reproduction techniques is generally degraded by the effects of ambient reverberation (i.e. reflection from walls, floor and ceiling). In [7], Chua

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Bright zone Quiet zone

Figure 6.8: The real measurements of the sound at the centers of the bright zone and the quiet zone are plotted.

measured the reflection coefficients for the four walls⁹ in the Victoria electroacoustic lab. Given the measured reflection coefficients and the geometry of the room, we used the image source method [72] to simulate room reverberation and implemented the multizone soundfield repro-duction in Fig. 6.4 with the same set of loudspeaker filter gains. 100 image sources were included for each of the loudspeaker positions. The simulation of the acoustic energy of the reproduced soundfield over the desired reproduction region is plotted in Fig. 6.9. The solid line encircles the desired reproduction region. The dashed lines encircle the bright zone and the quiet zone with the marker “B” and “Q”, respectively. From Fig.

6.9, we can see that the performance of the desired multizone soundfield reproduction is significantly undermined comparing with the case of the ideal free-field settings of Fig. 6.5. In Fig. 6.9, the average acoustic contrast between the bright zone and the quiet zone is 12.43 dB, which

9The reflections from the floor and ceiling were not considered for simplicity in the 2-D case.

6.3. IMPLEMENTATION OF MULTIZONE SOUNDFIELD REPRODUCTION WITH FREE-FIELD ASSUMPTION

Figure 6.9: Soundfield reproduction of 1000 Hz signal at desired region by 24 loudspeakers given the measured the reflection coefficients and the geometry of the VUW electroacoustic lab. From [7], with permission.

is very close to the results obtained from the practical implementation in Sec. 6.3.3. Therefore, we can conclude that the performance of the free-field multizone soundfield reproduction system is mostly limited by the room reverberation. The effects of reverberation should be taken into account to improve our system performance, which will be introduced in the following section.

The 3-D directivity pattern of the employed loudspeakers was also studied and measured in [7]. It was shown that the employed loudspeaker features a directivity pattern between the spherical cap model [53] and the point source omnidirectional model within the frequency range of interest from 100 Hz to 5000 Hz. Comparing with the ideal point source model that radiates sound equally towards all directions, the sound radiation at the front side of the spherical cap based source model is stronger than its sidelobe and this beam-like directional property gets more obvious as the working frequency goes up to 20 kHz. The results in [7] show

that even though the directivity pattern of the employed loudspeakers within the frequency range of interest (100 Hz-5000 Hz) is non-ideal, it does not adversely affect the performance of the multizone soundfield reproduction system. For more details of the loudspeaker directivity pattern, refer to [7].

The following measures can be considered to further improve the perfor-mance for this task:

• To make the listening room less reverberant. As this part of work is based on the assumption that the ATF from loudspeakers follows the 2-D Green’s function in free-field, the reverberation in the listening room inevitably undermines the reproduction performance. A natural way to make the room less reverberant is to hang more absorptive curtains. Now only two sides of walls are covered by absorption panels.

• To make the loudspeaker placement more accurate. In our system, the loudspeakers are assumed to be placed uniformly along a circle of radius 0.75 m. Currently, the loudspeaker arrangement is only roughly placed.

• To replace the defective loudspeakers.

6.4 Implementation of Multizone Soundfield Re-production with Reverberation Equalization

In this section, we implement our proposed multizone soundfield repro-duction system with reverberation equalization in the Huawei media lab.

The goal was to reproduce the desired soundfield in a pre-defined acoustic bright zone and minimize the sound pressure in the specified quiet zone in a real listening environment.

6.4. IMPLEMENTATION OF MULTIZONE SOUNDFIELD REPRODUCTION WITH REVERBERATION EQUALIZATION

Figure 6.10: The media lab in Huawei European Research Center (Munich).

The multi-channel audio reproduction system in Fig. 6.10 consisted of 48 loudspeakers and two microphones. The loudspeakers were evenly placed on a circle of radius 2 m. The model of the loudspeaker was Canton CD 1020 and the microphone model was a DPA 4061 Omnidirectional miniature microphone. All loudspeakers were driven by two RME Fireface 800 soundcards,¹⁰four RME ADI 8DS AD converters, and six IMG STA-1508 8-channel amplifiers.

The testing environment was an office room with rectangular shape of dimensions 5.9 m length by 4.6 m width by 3 m height. The ceiling was treated with acoustic absorbers to reduce vertical reflections and the floor was covered by carpet. One wall of the testing room was completely covered by a sound-absorptive curtain and one wall featured large glass windows. No acoustic treatment was applied to the remaining two walls (as shown in Fig. 6.10). The testing room had features that created some diffusion in the lateral dimension: a doorway, a window, a PC, a wheel

10Each of the soundcard drives 24 loudspeakers and the two channels were perfectly synchronized in advance.

chair, two shelves that host soundcards/converters, etc. Overall, the reverberation time T60 was approximately 0.4 s at 1 kHz. Considering the circular setup of the loudspeaker array, we set the desired reproduction region D on the 2-D plane inside the loudspeaker array. The origin of the coordinates was the center point of the circular loudspeaker array.

In this section, we start with the real-world implementation work of the proposed Green’s function modeling approach in Chapter 4. Then, the narrow-band multizone soundfield reproduction using microphone feed-backs at the frequency of 1 kHz is presented. The setup and implemen-tation instructions of the wide-band multizone soundfield reproduction with adaptive reverberation cancellation system are also introduced.