|The Science of Domestic Concert Hall Design|
by Ralph Glasgal
Ambiophonic Principles for the Recording and Reproduction of Surround Sound for Music - Part 9
Angelo Farina, Ralph Glasgal, Enrico Armelloni, Anders Torger
6. SUBJECTIVE COMPARATIVE TESTS
At the time of writing, only a very preliminary comparative test was performed among the three alternative systems: Stereo Dipole, Ambisonics and Ambiophonics.
The listening environment was a very damped listening room, equipped with a pair of Quested F-11 self-powered monitors employed for the Stereo Dipole and 8 General Music self-powered monitors for the 8-ways Ambisonics cubic rig. Fig. 30 shows a photograph of the listening room, which is actually capable of providing optimal listening conditions for just one listener standing in the center.
A panel of 9 students was employed for the comparative tests. Each subject was asked to rank, in order of preference, three repetitions of the same music sample, 40s long. One of them was presented through Stereo Dipole only, one through the Ambisonics rig only, and the third one employing simultaneously both systems, that is by Ambiophonics. It must be noted that all the signals were pre-computed off-line by means of the Aurora plugins , and CoolEditPro was employed for playing the 10-channels session through a multichannel sound board (Echo Layla). The first two types of signals (Stereo Dipole and Ambisonics) were obtained by muting the other channels, whilst the third (Ambiophonics) was played with all 10 channels unmuted.
The above methodology is somewhat misleading regarding Ambisonics, because in practice the impulse responses employed for convolution were deprived of the direct sound, and this caused the reproduction of the sound through the Ambisonics rig to be excessively diffuse, with poor localization of the sound coming from the frontal stage. On the other hand, a brief informal test conducted employing IRs containing also the direct sound shown that the difference was not very evident, and thus it was considered not be worth the increased complexity required.
Each subject had to rank 3 music pieces, each one processed with a set of IRs corresponding to a different theatre, coupled with a different set of crosstalk cancelling filters.
The following music samples were employed:
This, of course, did not explore all possible combinations; furthermore, the results are certainly dependent on the source material, and it was not checked if the cross-talk filters were really the optimal ones for each recording (the coupling was based only on "a priori" knowledge of the miking technique employed for each recording).
It is planned to take the occasion of the public demonstration of the three systems planned at the 19th AES Conference for collecting a large number of qualified subjective listening tests. Nevertheless, the results of these first low-quality subjective tests are encouraging. The following table reports the average scores obtained by the three methods, obtained assigning score 1, 2 and 3 respectively to the sound samples in their ranked order.
A multi-dimensional statistical test applied to the raw subjective responses demonstrated that the ranking is statistically significant (the 95% confidence interval resulted equal to 0.62, and thus both differences between average scores are significant).
Of course, the limited number of subjects and sound samples for each subject, and the fact that they were not trained, nor selected for their discriminative listening capabilities, means that the above results must be considered just a very initial confirmation of the validity of the Ambiophonics method.
This paper presented the theoretical background and the practical implementation of the Ambiophonics surround system. The system can be seen as the superposition of two already established surround techniques (Stereo Dipole and Ambisonics): each of them is employed for what it does best.
The superposition of the two cooperating systems produces significant advantages, which are clearly outlined by the theoretical analysis, and were confirmed by listening tests.
Although Ambiophonics until now has been a quite expensive system suited only for audiophiles in the high-end, its modern implementation in the form of freeware software for low-cost PCs opens its use to the vast majority of music lovers, provided that they take care of allocating a suitable, well treated listening environment.
David Johnston, author of CoolEditPro , is acknowledged for his excellent software, which was kindly made available free for this research.
This work was partially supported through a research convention between ASK Industries, Reggio Emilia, Italy and the University of Parma, co-funded by the Italian Ministry for University and Research (MURST) under the grant MURST-98 #9809323883- 007.
The substantial support and liberal contribution of the Ambiophonics Institute was fundamental for the research work described here.