basics and general formats: Surround Sound

The A-Chain is the part of the cinema reproduction system from the audio playback system (magnetic tape, optical film or any other) to the ‚Main Fader’ input. The main fader is the control device for the general playback level in the motion picture theatre.
All parts in the signal chain after the main fader is called the B-Chain. This are equalizers, power amplifiers, crossover networks and loudspeakers. The sound transmission through the projection screen and the auditorium acoustics are also parts of the B-Chain.

Optical soundtracks were the first method to put audio on film material. The optical soundtrack is still today the standard for audio transmission in movie theatres (analog and digital). The analog optical sound track is an audio stereo waveform printed parallel to the images in an opaque area on the side of the celluloid material. This can be Dolby A or Dolby SR, there the surround channels are matrixed into. Because light from the projector is directed onto the sound track, variations in the width of the tracks are detected by a solar cell, which converts the light into an electrical waveform. The soundtrack is printed photographically on the film with the images. This method is very durable and the sound track is as long lasting as the images itself.
In a mono print (mono cinema sound) there are still two tracks, both identical with the same content. A stereo track has the left and right track on the two sound tracks. With the matrixed surround formats the surround signal is modulated on the two tracks and will be extracted by a certain circuitry.

The front loudspeaker systems (two-way, three-way or four-way systems full range inclusive low bass) are normally installed in a sound baffle. This sound baffle is also a part of the THX recommendations. For larger cinemas a five channel setup is necessary to ensure a smooth coverage over the entire screen width.

L,C,R loudspeakers installed in sound baffle ('THX baffle')	perforated screen in front of loudspeakers	masking system around screen motorized for different movie formats

front loudspeakers L - C - R	front loudspeakers L - CL - C - CR - R

The front loudspeaker channels should be installed in a height of about 2/3 of the cinema screen and should be angled to the ear height of the seats in about 2/3 of the audience area. This slight angle also prevents reflections between the perforated screen and the speaker itself (a non-angled speaker could create standing waves between the speaker surface and the screen). The outside left and right speakers should be slightly angled to the centre of the room.

front speakers angled to 2/3 of the audience area (ear height)

Movie theatre screens are always perforated to enable the speaker positions behind the screen. Today a micro-perforation in accordance with high definition picture standards.

micro perforation of a movie theatre screen (here Gerriets 'Gammalux Micro') with 300,000 holes per square meter and about 6% open surface, color: plain white

The masking system around the screen (black material) must also be acoustically transparent because for smaller movie formats the material can cover the left and right loudspeakers.

In Home Cinema the 7.1 Surround format features four surround channels. Surround receivers create a 7.1 mix with four surround channels out of the original 5.1 movie mix supplied by a DVD to produce a more impressive surround sound experience.
In professional cinema the eight channels surround setup consist of five front channels and two surround channels. The most prominent professional format for this setup is Sony's SDDS Format (Sony Website).

Home Cinema 7.1 Surround	Eight-Channel Cinema Surround with Center-Left and Center-Right

The electroacoustic response of the B-Chain is measured as an average of a sufficient number of measurements throughout the audience area. The measurement is done as a comparison of the frequency response to a linear pink noise input signal at the main fader input (usual signal input of the B-Chain).
The used pink noise should be wide-band pink noise from 25 Hz to 20 kHz with a tolerance of +/- 0.5 dB.

This 'old-fashion' RTA measurement is definitely not sufficient for todays sound reproduction needs but because of the historical evolution of cinema sound and the large number of installed cinemas all over the world, it is very difficult to change setup procedures without creating big confusion. Sound reproduction in cinemas was already standardized in the 1930s and changed slowly since. RTA measurements cannot distinguish between the first arrival of direct sound and the built-up reverberation in the room. But listening to a sound system, our hearing-brain system psychoacoustically differentiates very well between these parts of the received sound. Reverberation and reflections are received in a very different way than the first wave front of direct sound from the loudspeakers.

In the 1970s the so-called "X-Curve" was developed by Ioan Allen of Dolby Laboratories. The only available measurement systems were RTA-type devices that show sound pressure level on individual frequency bands (usually 1/3 octave). With carefull equalization and control via the RTA measurement cinema loudspeaker channels were adjusted for a wide and flat frequency response (within the possible limits).
The X-Curve was standardized by ISO sand SMPTE and the standards today are ISO 2969 and SMPTE 202M-1998. This standardized frequency curve suggests a favorable average response curve measured at a number of positions in the room. The measurement shall be executed with wideband pink noise fed in at the main fader input of the cinema sound system and measured with the fully installed system including the perforated screen, individually for each channel. The curve shall be flat between about 60Hz and 2kHz with a roll off below 60 Hz and above 2 kHz with about 3 dB/octave. Above 10 kHz the roll off shall be 6 dB/octave. For different room sizes corrections in the high frequency slope are suggested (smaller rooms with less attenuation to the high frequency range).

Because pink noise and an RTA measurement are often very critical for speaker measurements in rooms with the involved room acoustic the X-Curve cannot ensure and cannot indicate a correct set up frequency response of the speaker channels and cannot ensure a 'flat' frequency response for any of the listeners in the cinema. Todays sophisticated measurement tools (TEF, FFT) are able to distinguish precisely between the different part of the sound response measured in the room and can guaranty an optimum in the frequency response perceived by the listener.

The upper roll-off in the X-Curve was intended to represent the typical high frequency roll off happening by installing speakers in large rooms. To set up a speaker system reproducing pink-noise and be measured in some distance a flat frequency curve would result in a much too high increase of the high frequency range and finally in a much too bright sound. The mis-proportion between low and high frequency content in the RTA measurement is caused by the energy-frequency relation of the built-up reverberation in the room. Low and mid frequency reverberation is always much higher than high frequency reverberation (air absorption within the late part of the reverberation field). The RTA measurement adds the reverberation field to the direct sound of the speaker resulting in a curve with a roll-off in the high frequency range.
The low frequency roll-off was taken from the average frequency response of good sounding theatres at the time. Because dubbing rooms etc. incorporate the same X-Curve, the missing energy is easily compensated in the film mix already.
By using the X-Curve in a majority of theatres a more or less satisfactory general response curve could be ensured.

X Curve

Each channel of the surround system (front, surround and LFE) should output a sound pressure level of 85 dB(C)-slow reading- at standard fader settings. This should represent a recorded reference level of -20 dBFS. The maximum sound pressure level that could be output of the sound system (each individual speaker) should be up to 105 dB without amplifiers overloading or clipping.
The LFE channel is set to be 10 dB higher than the master channels. Measuring with pink noise and a C-weighted SPL meter the measured dB(C) level should be about +4dB higher than the master channels. The measurement will not show +10dB because of the narrower bandwidth of the used pink noise.
The recorded content for the LFE channel is set 10 dB lower to -30 dBFS. Within an AC-3 signal stream (Dolby Digital) the LFE channel is carried with 10 dB less level. In the Dolby processor this will be compensated and the system output of the processor will produce the same level for the master channels and the LFE channel.

All movies should be presented at these above mentioned standard fader settings with a SPL of 85 dB(C). Sometimes loud trailers may need compensation downwards but normally standard level should be executed all time. BUT, often this is felt as too loud and in many theatres the standard fader level is set to 79dB(C), -6dB down from the recommendations to create a more comforting level.


The surround speaker channels are set individually to -3dB compared to the master channels because the acoustical sum of both surround channels adds up to 0dB if the content is equal. The coverage should be within +/-2dB uniformly over all seating area.

A separate low frequency channel was introduced to reproduce special low frequency content especially for movie production. This LFE channel (Low Frequency Effects) has normally a bandwidth up to 120 Hz and should be able to generate a 10dB higher gain in this frequency range. This is achieved by a 10dB gain in the amplification-speaker system, not in the original recording of the sound material.



Because the LFE material is by definition only the low frequency effects material (explosions etc.), the low frequency content of the regular stereo signals should be reproduced by the main stereo loudspeakers. This is a generally so in cinema systems but for home cinema a different approach is used. With a proper bass management system all low frequency content of all channels can be fed into the subwoofer channel. This subwoofer channel is not a LFE channel anymore but a reproduction channel for all low frequency content, it carries the low-frequency content of all five channels plus the LFE content. Because of psychoacoustic reasons the position of the low-frequency content is difficult to localize and the splitting and mixing into one channel will not be recognized. With bass management the low frequency range for the content of the regular five channels must be properly defined (bandwidth, level, crossover frequency).

LFE channel without bass management	bass management for all channels

Surround speakers must provide uniform coverage of all audience. Here the spacing between speakers, height above floor and aiming angle are important.
There are two basic designs for surround speakers: dipoles and direct radiators. Dipoles distribute sound in both directions forward and backward in parallel to the walls. This can create a very even sound distribution. Direct-radiating speakers project sound in the forward direction.
In todays movie theatres seats are often placed very close to the surround speakers to maximize the seating area. Because of the necessary dry room acoustics this can cause localization of the loudspeakers rather than to hear the proposed diffuse sound field.
These circumstances and the high output levels needed can only be compensated by a higher number of speakers and grouping surround speakers closer together.

surround speakers should be angled to the farthest outside seat (ear height) of the opposite side

In cinema installations no single surround speakers on each side wall are used but an entire set of speakers covering the side and real walls. This is necessary because unlike in music listening situations the entire audience in a cinema can not sit in the 'hot spot' (or only a few tickets would be sold ..). By using a set of surround speakers on the side and rear walls surround sound and ambience impression can be provided for the whole audience except a few uncomfortable positions in the very front and the extreme side seats where the signal from the other side cannot be heart.

Because of limitations in the analogue transmission path for cinema audio initially only one surround channel could be provided that was matrixed within the two-channel signal but with the upcoming digital signal distribution six discrete channels could be implemented: Dolby Digital 5.1.

In 1998 the Dolby Digital Surround EX was introduced, a joined venture between Dolby and Lucasfilm THX, to give movie producers the ability to differentiate between left, right and rear surround signals.

Dolby SR


Dolby Digital - 5.1


Dolby Digital Surround EX

Dolby SR (one surround channel)	Dolby Digital 5.1 (two surround channels)
Dolby Digital Surround EX (three surround channels)	Surround 7.1 (four surround channels)

Not jet used in movie theatres as surround loudspeakers are smaller line arrays (larger column speakers). The characteristic of these systems could create a smoother coverage from over the entire audience area without being localized...

Despite the ITU standard with 30 degrees listening angle for the left and right speakers, for cinema monitoring a different setup is used. The front left and right monitor speakers are angled by 22.5 degrees each to complement the listening situation in the best cinema seats with an viewing angle of the Cinemascope screen of 50 degrees (25 degrees each side).

typical monitoring situation for cinema sound mix

For the surround channels in cinema monitoring often a set of smaller surround speakers is used similar to the real setup in a movie theatre.

THX Ltd was established in 1983 by the George Lucas company with the purpose to create standards to correct inadequate movie playback in cinemas. " ..is the founder of quality assurance programs for superior cinema presentation.." (from the THX Website)

THX Website: "A THX Certified Cinema is built from the ground up, taking extra steps, often times installing additional materials and equipment, to create an environment that will totally immerse you in the story. Whether you’re on a first date, going for a blockbuster opening night, or catching a matinee, get ready to experience the magic and emotional impact created by the director for you to enjoy."
See THX.