Purpose of Spatial Audio Systems

The extension of the stereo system to 5.1 includes the extra centre and surround channels which provides a number of benefits and were explained in Section 3.2.5. In summary these were that the centre channel allowed anchoring of the centre stereo soundfield for off centre listeners, including better tonal balance. The extra rear channels provide two uses, one for the creation of rear sound effects and two for lateral energy, with lateral energy being improtant for envelopment. Finally, it was also stated in Section 3.2.5 that the 5.1 system is the optimum layout and number of speakers for reproducing the spatial impression of a diffuse soundfield in the horizontal plane and is closely related to listeners envelopment (Hiyama et al., 2002).

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The term envelopment or more specifically listener envelopment was first coined in relation to concert hall acoustics, where the perception of envelopment is judged highest when late reverberant sound is arriving equally from all directions (Beranek et al., 1996).

In multichannel systems envelopment can be recreated in two different ways, the first can be by reproducing reverberant sound which has been recorded with appropriate techniques in a real space, or produced using digital signal processing or by using dry direct sources placed around a listener which exhibit special qualities like, soundfield density (number of mono sources), interchannel correlation, location or position of the sources and frequency content (Conetta, 2011). Examples of dry direct sources might be random noise signals which exhibit low correlations like that used by Hiyama et al. (2002) to approximate a diffuse field.

The distinction between the direct and indirect envelopment is made clear by Rumsey (2002) where he explains that in reproduced sound subjects described being enveloped when surrounded by dry direct sources, however in terms of concert hall acoustics this is never the case since envelopment is a property of late reflected sound, however if the listener were placed in the middle of the orchestra they would claim to be enveloped or inside the music.

The perception of envelopment in multichannel reproductions has been explained in that two different types of envelopment can be expereinced one of which has a different context to that experienced in concert hall acoustics.

The clear benefits of the extra channels in a 5.1 system are established, in the fact that the rear channels can provide the sensation of envelopment such as that experienced in a concert hall environment and is a highly rated attribute (George et al., 2010).

3.10.1 Objective Measures

A measure which has been shown to be closely related to ASW and envelopment is called IACC (Inter-aural Cross Correlation) (Nakayama et al., 1971), (Hiyama et al., 2002). This measurement has been used extensively in concert hall acoustics and in reproduced sound (Mason, 2002), (Hirst et al., 2006).

IACC is defined as the maximum value of the normalised cross correlation function for the time interval of ± 1ms. See Equation 3.15 and 3.16 where Pl and Pr are the impulse responses at the left and right ears, 𝜏 is the offset between ±1ms and is set to account for the maximum

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inter-aural time difference. t1 and t2 are the time limits of the integration, different variants of IACC exist depending on the time limits. The version used in this work is IACC for which t1 would be 0 and t2 = inf. This IACC is based on the degree of correlation between the impulse responses at the left and right entrance of the ear. A well-defined source has a sharp maximum of 1, whilst subjective diffuseness has a low value ~<0.15 (Ando, 1998)

Equation 3.15

Equation 3.16

A large amount of research has been carried out into IACC and the measurement is not a perfect solution since it does not take into account the effect of low frequencies due to the wavelength of the signal and the distance between the ears (Tohyama and Suzuki, 1989). As a consequence, it has been explained by Blauert and Lindemann (1986) that in terms of concert hall acoustics early lateral reflections which contain mainly frequencies up to 3 kHz contribute solely to the expansion in the front back direction, referring to the attribute envelopment.

3.10.2 Importance of the Direction of Reflections in Concert Hall Acoustics in Terms of

Subjective Preference

It has been explained in Section 3.10 that lateral reflections are the most important, therefore what part do ceiling reflections have? It has been reported by Marshall (1967) that diffusion of ceiling reflections are advantageous since this lowers the masking level which provides audibilty to lateral reflections which are important for what he terms spatial responsivness, which could be assumed to be reffering to spatial impression.

In the same way it is also reported by Furuya et al. (2001) that if reflections arriving from directions other than lateral are excluded in terms of envelopment, this could be detrimental,

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in that late sound from other directions like overhead are also effective for experiencing the three dimensional fullness of sound, this was also confirmed by Wakuda et al. (2003).

However,it is stated by Barron (2009) that ceiling reflections will not add to spaciousness, this sentiment is also echoed by Griesinger (1996) who states that it is the lateral or diffuse components which matter, reveberation from the front or overhead is not perceived as spacious.

The previous statements underline the fact that lateral reflections in the horizontal plane are important in concert hall acoustics and there is evidence that reflections from the vertical plane in concert halls do not add to envelopment/spaciousness, however the direction of arrival of reflections from the vertical plane are still important. Since it has been explained that reflections from the vertical plane ariving in the median plane are not useful in lowering the IACC, because reflections arriving at the listener from the median plane will lead to identical signals at each ear increasing the IACC (Schroeder, 1979), therefore it would seem that in order to reduce the IACC the reflections from the vertical plane should arrive at the listener from non central directions similar to the horizontal case.

3.10.3 Clarity

There is some uncertainty regarding the importance of ceiling reflections in concert hall acoustics, although there is some evidence that ceiling reflections add to the clarity of sound for listeners. This is stated by Barron (2009), however care has to be taken with reflections from overhead since it has been established that these reflections can cause tone colouration effects. In the same way, it has been reported by Silzle et al. (2004) for elevated speakers that the comb filtering effects of higher frequencies are much more audible in the vertical plane than the horizontal plane, this due to the binaural processing of the two ear signals in the horizontal plane which makes it much less audible compared to the vertical plane. The theory being that sources arriving from the vertical plane are subject to unmasking making comb filtering more audible.

It is also stated by Lokki and Pätynen (2011) that early lateral reflections should reach the listener from slightly elevated angles as this helps to optimize high frequency content and harmonics. This theory is supported by Hartmann (1983) who found that localisation accuracy was affected by the direction of which early reflections appeared and that fewer localisation errors were noted when early reflections came from above and below instead of

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lateral directions. Hartmann also makes the distinction between lateral reflections for a sense of surround, but not for good localisation of sources in the horizontal plane.