If in 25 years, full of publications allowed to demystify the sound recording into multichannel and the various processes of reproduction, work concerning acoustics of the places of diffusion into multichannel are rare.

The room and loudspeakers are the master trumps of a good reproduction. In no case, the quality of the loudspeakers will not make it possible to correct defective acoustics of a room.

The passage of stereophony to the 5.1 is not inevitably compatible. Volumes associated with the buildings as well as the principles suitable for stereophony as the LEDE are characteristics which must be reconsidered.

Standardization ITU is currently the single reference concerning the audio multichannel diffusion. For obvious reasons of compatibility, it must be respected. The recommendations are not numerous but one realizes that certain rules result from this. Various working groups resulting from the NHK or the AES contribute to argue to the acoustic characteristics of the studios into 5.1.

There is no question of taking here into account the various means of acoustic and virtual recording for the creation of a musical work. The considerations presented here relate to only the sound reproduction and are thus common to all types of recording on the other hand, we will analyze only the studios dedicated to the production or the sound post-production. One will thus treat neither of the constraints of the sound related to the image nor of the special formats of reproduction other than the 5.1.

2 general Considerations of the 5.1

2.1 Recall concerning stereophony

An installation 5.1, must be able to diffuse stereophony as well as a room intended for this one. What wants to say that certain rules of bases must be preserved:

The buildings will have to be symmetrical compared to the median. A dissymmetry will involve a distortion of the frontal image.
The zone of listening in stereophony is a triangle whose base widens as one moves away from the enclosures. This zone is limited by the back wall which must be located enough far from the listener. One generally recommends for a rectangular room that the median of the axis of listening corresponds to the length of the room.
The LEDE, is a tendency of treatment of the walls consisting in dry front wall studio to avoid first harmful reflections and clearing up the back of the room, generally with diffusing walls. This causes to increase early energy and the sound force. This method is prone to controversy for the 5.1 bus it imposes the placement of the back loudspeakers on a reflective or diffusing zone.
The recording studios have a reverberation time which fluctuates between 0.2s and 0.4s according to the volume of the room. One generally notes that TR60 holding in a gauge of ± 0.03s between frequency 100Hz and 10kHz. These measured to Radio France values are similar to those met for the private studios.
Dimensions of a rectangular room will respect the following formulation in order to limit the standing waves.

1,1 W/H ≤ L/H ≤ 4.5 W/H - 4

with L/H < 3 and W/H < 3

Where W is the depth, H the height and L is the length of the room.

One can take for example the reports/ratios suggested by the NHK which answers the preceding equations:

H = 1.59 W ±0.7 = 2.52 L ±0.28

2.2 Requirements of the 5.1

Configuration ITU by a preoccupation of downward compatibility, imposes the physical angles of reproduction of Figure 1.

Figure 1. Angle of reproduction (specification ITU)

It specifies a height of frontal listening with 1.2m and 1.2m or more for the back wall. It will be understood here that it is the position of the tweeter which is taken into account. If the Loudspeakers are positioned in height, it will have to be downwards tilted to avoid filtering combs from there at the frequencies of connection of the various transducers. These defects come from the difference of the distance between the ear of the of sound engineers and the various transducers.

In impossibility of placing the sources on the same ray, a delay will have to be applied to the signal nourishing the back loudspeakers. This possibility is interesting in the case of small volume but it is an artifice which is not completely transparent for total listening. Indeed the report ratio direct field/reverberated field will be different between before and the back.

The international recommendation specifies that the localization of the sound image is not necessary on the side and on the back. It conceives the use of the back loudspeakers as an improvement of spatialization what makes it possible, according to it, to increase the listening zone of stereophony.

It is seen well here that these requirements miss precisions. Because the addition of sound sources in a room contributes to destabilize the frontal image and to create of more restricted stereophonic listening zone.

Thus the 5.1 must be interpreted like a system allowing a better immersion in the sound environment. It is obvious that this vision minimalist of the function of the back sources can be extended to the wish of information of complete azimuth localization. In this case, the zone of listening is very reduced because it depends on the robustness of the localization. Moreover configuration 5.1 allows only a bad side localization. [5]

A technical document of the AES written recently [7] suggests values and parameters concerning a listening of quality into 5.1.

It mentions mainly:

· A surface of listening higher than 40m ². Consequently, the studios dedicated to the 5.1 will have to be larger than those employed in stereophony. Japanese proposes to bring back this value to 30m ² because they are accustomed to working in buildings comprising of more exiguous surfaces (work and residence). This value seems more reasonable but it will not be inevitably compatible with the suggestions which follow. Large many studios dedicated to multichannel will have surfaces even weaker considering the costs per m ². One can fear that buildings of low dimensions make it possible only to one single person to be located in the zone of correct listening.

· A distance from listening ranging between 2 and 6.8m of the enclosures as well as a zone of minimal listening whose surface is a disc of 0.8m of ray compared to the central point. This value depends primarily on the effect of precedence. As the energy of the direct wave decrease of 6dB to each doubling of the distance, an important ray for the sources will thus support a zone of extended listening less sensitive to the effects of precedence.

· The early reflections must have a level lower than 10 dB compared to the direct field. This recommendation does not authorize almost any more the installation of hard reflective materials. These materials will have to be diffusing to attenuate the energy of the reflections of the first orders.

· Tr of about 0.25 (V/Vo) ^1/3 where Vo is a volume of reference of 100m3. Thus, a studio of low volume of 80m3 will have a time of reverberation of 0.23s while a volume of 160m3 will require 0.3s. This is almost dry compared to the average values measured in the studios of recording generally between 0.3 and 0.4s. So in general, the time of reverberation of the living rooms of listening at the private individuals approaches these values for stereophony, it requires that the installations home theatre 5.1 of general public is much better treated in order to preserve the conditions of listening of the mixing.

· A frequency response (50Hz to 2 kHz) to the point of listening in a range of ±3 dB in the zone of listening (be authorized 3dB additional attenuation for the higher frequencies). These measurements could be reached only with one total control of standing waves. All the professionals speakers have a 1 meter frequency response included in the range, this information relates to only acoustics of the room. To respect this recommnadation for frequencies lower than 100 Hz seems illusory.

· A background noise lower than curve NR10 and in all the case never higher than the NR15. The specified levels of noise are really low. If air-conditioning is old, these levels could not be reached. A value of NR18 seems already sufficient.

· A level of listening of reference fixed at 78 dB (A) or 85 dB (C).

· The loudspeakers will be placed at a minimal distance from the walls of 1m. A source placed at a distance D makes that the response curve will be dug for the frequency of which the wavelength L =4*d is F = C (4*d). Thus with 1m a hole will occur to 85 Hz. More the loudspeakers would be positioned close to the walls more the cancellation frequency will be located in the audible spectrum. This disadvantage will not exist any more if the loudspeakers are baffled.

· The frontal sources will be matched to 0.5 dB what requires the use of transducers resulting from the same series.

· Frequency response of the enclosure to 1m must hold in a range of 4 dB between 40Hz and 16kHz. These values are in agreement with those of the characteristics of the professional monitors.

· An index of directivity ranging between 6 and 12 dB. These specified values a well controlled directivity and exclude the use of broad band loudspeakers. The quality of a loudspeaker is related to a directivity according to the frequency. The goal is to obtain a directivity according to the frequency which does not present an accident to the transducers connection frequencies. They are the most significant improvements made by the various loudspeakers manufacturers these last years. It is mainly by studies of profile of horn and throat that we can obtain a linear radiation according to the frequency. Nonlinearities will have like consequence to colour the room and it is mainly this parameter which makes that a loudspeaker does not sound like another.

· A frequency response to 30° will not have to be attenuated of more than 4 dB compared to the axis. These values will make it possible to minimize the effects of precedence in the zone over listening. The baffle mount of the loudspeakers will tend to improve considerably the data measured in free field. It will be possible to use more directing speakers if they are mount in the wall.

The given values are important, it does not exist almost any stereophonic studio answering to these values at Radio France. Let us note that the Japanese of the NHK particularly propose specifications even more constraining for the time of reverberation. Radio France will be equipped soon with a multichannel studio of quality which will not respect either all these recommendations.

In short, according to these recommendations the diffusion in 5.1 requires greater volumes and must be better treated. Times of reverberation recommended require use of absorbing materials and other at the same time diffusing and reflective.

2.3 Point 1 of the 5

The conditions of use of a subwoofer are relatively simple but are so rarely observed:

Knowing that musical instruments or noises including low frequencies contents are few and that the subwoofer wave cannot be propagated linearly in small buildings, we can say that the reproduction of the frequencies towards 50 Hz is not inevitably necessary. Moreover professional studios have loudspeakers whose characteristics at the low frequencies are more than honourable.

The sub is recommended for studios with weak budget whose principal sources are free stranding small satellites.

Let us add finally that the adjustment of a sub with the other transducers is very difficult to realize (phase, filtering…). The installation of a sub without adjustment will have like direct consequence to generate a disastrous frequency response…

The audio professionals will choose the 5.0 and will leave the 5.1 to the field of the cinema and the home theatre.

3 Acoustics considerations

The increase the number of loudspeakers for a configuration 5.1 involves a greater excitation of the room than in stereophony. We will take more precaution to minimize the harmful effects of the room.

3.1 Standing waves

Less the room bulky less standing waves appears, it is thus absolutely necessary to respect dimensions length, width and height whose dimensions do not have a multiple report/ratio. This is unfortunately only one precaution necessary but no sufficient. A current tendency which requires a consequent architectural study is to create studios without any parallel surface. If one adds an effective treatment absorbing in the low register, the standing waves of the room will be very strongly reduced.

3.2 In wall loudspeaker

It consists to integrate it in the frame of the studio so that only the front face of the loudspeaker is visible. The advantages of it are undeniable, the baffled installation have a cost but as the sound quality of a studio is the principal objective, the baffles is necessary. This mode of installation to another advantage because it requires to think the design or the restoration of the studio from an acoustic point of view what is often forgotten…

The majority of the professional loudspeakers are rectangular, and the sound diffracted by the edges of the enclosure reacts as a sound source which is in opposition of phase to certain frequencies.

A loudspeaker radiate omni directional at low frequency. The principal consequence of these physical laws is the creation of a wave returned by the back wall which will reinforce or attenuate the principal wave according to the wavelength of the signal. The second consequence is the addition of diffuse reflections created by the back hemisphere of the enclosure which will come to corrupt the fidelity of the home record. Figure 2 is a simulation of the contribution of reflections [6]. One cut off with the echogram from a free standing mount with 1m from a wall treated with an absorbing material, that of a baffled enclosure. Positive emergences are thus the reflections induced by the back of the enclosure. Some of between they are not negligible and contribute to reinforce the acoustic signature of the listening room.

Figure 2. Additional reflections created by the back hemisphere

The baffling in a wall has like direct consequence to increase the very low level frequencies. The measurement of Figure 3 indicates an increase of 4 dB for an assembly with a plate baffle. Loudspeaker intern correctors are necessary to avoid the addition of an extra equalization for the monitoring. Let us note on this subject which if acoustics of the cabin is adequate, the presence of an equalizer is not necessary and that an electric corrector does not allow an acoustic linearization of the cabin. The totality of the large studios of sound recording installed by the Genelec company do not have added modules EQ and the curves response obtained to the place of the sound engineer have values which hold in a suitable range. An electric correction must be only considered with several points measurement in the listening zone.

Figure 3. Measure effect of embedding. Increase in the level of 4 dB in lower parts of 250 Hz.

Figure 4. Measure directivity

The directivity of the enclosure is also strongly modified. The following diagrams of directivity show a loss of directivity for the front hemisphere. The consequence is a greater homogeneity of the coverage of the listening zone because of a broader aperture unfortunately that also contributes to make react more the room in no desired zones. One will thus retain by this handling that the values of aperture given by the manufacturers are to underestimate for a baffled mode and that greater directivity can be used without harming to the homogeneity and the robustness of the localization of the room.

Figure 5. Directivity of an enclosure on foot

Figure 6. Directivity of an embedded enclosure

3.3 Early reflections

It is necessary for an acoustic recording, to listen the room where is the sound produced and not the room of listening. The studio of recording is of lower volume than the room of spectacle. The intensity of the early reflections is the acoustic signature of a room (volume, colour, distinctness). If those of the studio of recording are not attenuated enough, it is its signature which will be intended with the reverberation of the other room Figure 7. The ITD (Initial Time Delay, is the time of the first reflection after the direct wave (this value is an index codifying the size of the room). The firsts reflections are most disturbing and will have to be attenuated in priority. The ITD is the acoustic parameter characterizing the size of the room. It is a reason for which cabins LEDE require a time of ITD ranging between 8 and 20ms to do not corrupt the perception of the recording sound space.

On Figure 8, Angus [4] proposes a curve showing the maximum level of the reflections according to time in order not to disturb the localization of the phantom image. It concludes from it that the participating walls with the first reflections must have an absorption coefficient higher than 0.9.

Figure 7. The impulse response of the space of recording is added with the listening place.

Figure 8. Gauge concerning the amplitude limits not to exceed according to time

3.4 Shape of room

The points exposed higher show all the importance of the room geometry. One realizes well that the reflections of strong intensity are responsible for poor acoustics. The solution to avoid them is either to weaken them by effectively treating the zones of impact of the first order reflections or to modify the geometry of the room so that the harmful reflections do not return in the field of the listener [3].

A mixed solution is surely the best compromise because the zone of listening is sometimes shared by several operators.

They are obviously the most disturbing reflections of order. The reflections of a higher order are weakened by their longer ways (decrease in 1/d) and by the fact that they lose energy with each impact on the walls (by diffusion and absorption).

It is very difficult to envisage on plan the comportment of these reflections. The walls like the ceiling or the ground are often forgotten, of the short reflections of second order on reflective walls not belonging to the same plan will not be displayable, the use of an acoustic simulation per computer is completely relevant to obtain various echograms in the zone of listening. Figure 9 shows to the results obtained by Mr. Gall for a simulation of a studio of Radio France [6]. The numbered reflections are those first order and the small ones between 3 and 18ms simulate the first order reflections of diffraction by the walls and the edges. In this emergent example two peaks with 10dB in lower part of the direct wave and will have to be treated firstly.

Figure 9. Echogram simulated in the place of the sound engineer

One should not forget to add the echograms with the five loudspeakers. The position of the central source makes that a particular attention will be carried out for the treatment of the back wall. Indeed the exiguity of certain studios brings the back wall very near to the listener and even a diffusing treatment will not make it possible to completely remove the direct reflex ions. Others solutions implementing tilted sides absorbents allowing to reject the waves reflected out of the field of listening. Figure 10 and Figure 11 give an example of geometry illustrating this principle for the position of the sound engineer.

Figure 10. Solution NHK

Figure 11. Top view

4 Conclusion

A studio 5.1 of small volume is not advised. More surface is reduced more the listening zone too.

A studio 5.1 imposes a higher design more are the loudspeakers more the room is requested and influences listening. It is obvious that the architectural study of a new studio starts beforehand with the study of acoustics of the place. But this was already true for stereophony even if one with tendency to forget it…

The more we excite a room, the more we must treat the walls in absorbing. This consideration requires the use of dry acoustic studios for the multichannel. The music's, the fictions, the operas mixed in studios designed goods, will answer the aesthetics wished by the producer or the sound engineer. But the vocation of sound works is to be diffused massively at the private individuals in parallelepipedic and sometimes poor acoustic buildings. Does the private individual have to treat his living room so that it approaches the conditions of professional listening or is the sound engineer must work the sound so that it is acceptable in rooms equipped with poorer acoustics?

5 Literature

[1] Multichannel surround sound systems and operations technical document AES TD1001.0.01- May 5, 2001

[2] Listening conditions for the assessment of sound technical program material Document EBU 3276-E, Pre-Press version-February 1999

[3] R. WALKER, Multichannel control room acoustics and the development of early reflection control, extract of the conference Multichannel sound recording and reproduction organized by the AES and the SFA in Paris, le26 & October 27, 2000

[4] J. ANGUS, The effects of specular versus diffuses reflections one the frequency response At the listener Journal of the Audio Engineering Society Volume 49 Number March 3, 2001

[5] L. GIVERNAUD, Study of the perception of the localization for a acoustics sound recording of difference in intensity in 5.1. Memory Cnam 12/2000

[6] T. GALL, Study of a control of mixing in multichannel configuration with embedded enclosures. Trainee DEUST 07/2001

[7] Technical document AESTD1001.0.01-05 Multichannel surround and operations. AES TC-MBAT 2001

[8] R. WALKER, BBC resources. Controlled Image design 1995. Research and development carry forward

[9] F. ERNOULD Placement of the enclosures 1998

[10] Multichannel stereophonic sound system with and without accompanying picture. ITU-R BS.775-1

[11] David BELL, Whitemark STUDIO SOUND Changing Rooms. 02/2001