Acoustic design maximizes the cinema experience

What makes movies magic? The smell of theatre popcorn, the plush seats, the huge screen and booming sound elevates the experience from the ordinary to the otherworldly. To give true immersion in a movie, however, the sound needs to be also believable and give a sense of space and direction; to bring the world all around the moviegoer.

With the planning of acoustic improvements, Bio Rex's renewed primary hall was finished with a superb sound

Traditionally movie sound has been realized as front facing sound (the Left – Center – Right channels). With the advent of more immersive sound, more speakers were added into halls to give a possibility for the sound designer to have sound effects surround the moviegoer. In the popular 7.1 sound, for instance, these so-called surround speakers are divided into four blocks, two on each side of the theatre. These are used in a rather coarse way, enabling the sound designer to only call on the channel to add surround effects to the movie.

With the new Dolby Atmos system, the paradigm of movie mixing and sound design has been changed: instead of mixing the movie as channels, the sound designer can move sound objects inside a virtual space that translates to sound moving between the individual surround speakers.

One of the first movie theatres to employ the Dolby Atmos in Finland was Bio Rex Prime Hall in Hämeenlinna City. The hall’s acoustic designer, Akukon's Janne Riionheimo, started his project with a series of reference measurements in several theaters. Measurements were carried out in different halls to provide information in support of sound system selection and acoustical design.

- I first went to measure Bio Rex's halls that were user-friendly or were somewhat problematic. This primary hall in Hämeenlinna city was problematic because the sound kind of blurred and the speech was not as clear as in owner's other theaters, says Riionheimo.

Based on the measurements, the problems of the movie theater were traced to the hall's acoustics and sound system. A too long reverberation time was found out on bass frequencies. Also, Riionheimo found issues in the original sound system, particularly in a too wideband LFE channel. The speaker did not have a low-pass filter at normal 125 Hz but repeated the material up to 500 Hz, which in some cases made the sound "muddy".

The purpose of the acoustics improvement was to increase absorption at frequencies below 200 Hz.

- A thicker acoustic damping layer was built on the rear wall, additional damping was installed in the roof, side walls were slightly thickened. As the theatre moved to a curved screen, this generated room for bass dampening. This additional space was also used to turn the speakers towards the listening area. It is in the interest of the owner of the house that all seating positions are equally good.

A measured hall reborn in the listening room

In Bio Rex cinemas Riionheimo measured the loudspeaker impulse response from each speaker using six microphones.

- With the impulse response, I can calculate objective parameters such as reverberation time, clarity, frequency response, and sound direction in the measured theatre. With the help of six microphone measurements, the sound field can also be auralized to headphones or a listening room. So any hall can also be listened to afterwards. It is also possible to compare different hall auralizations with a flick of a switch!

From better acoustics to applied speaker settings

In addition to acoustic improvements, Akukon's tasks included designing the Dolby Atmos speaker layout.

- There are some good tools and documentation available from the manufacturer, but practical planning requires work to adapt it to a specific theatre. Together with the architect we designed the positioning and orientation of the loudspeakers.

- The problem lies in aligning the front speakers to the Central Listening Area (CLA), where the left and right loudspeakers must be turned at about 20 degrees to the CLA. In the movie theater, the loudspeakers are flush mounted in the front wall according to specs, but almost always, as it is now, the front flush mount wall was flat. The loudspeakers have holes on the front wall to which they are mounted to and it is important that the front edge of the loudspeakers is flush in the wall.

In Bio Rex's case, how to turn the speakers towards the CLA and still maintain flush mounting?

- We talked with JBL and Dolby, and they came up with good practical tips and advice. Interestingly, there is no official guidance, so I decided to apply some design sense. The loudspeakers were flush mounted in acoustic dampening structures of the side walls, which also served to dampen the hall’s bass reverbation, the backward radiated sound of the speaker, and the loudspeaker side reflections. Based on current measurements, the development work done here work perfectly, says Riionheimo.

A little something to learn for Dolby

Five subwoofers came in and according to Studiotec Ltd's Matthias Möttö's proposal they all were lined up in a row. Dolby Atmos was installed by Studiotec.

- This caused a bit of rumbling, but because there was enough headroom, it was accepted by Dolby.

In special cases where the hall or speaker layout does not fully comply with the Atmos-Specs, the plan goes to so-called Concession Team handling. They try to harmonize all the special cases and use earlier cases to help to solve future problems.

When obtaining approval Dolby was very accurate about the layout.

- Ceiling loudspeakers were moved 10 cm in the center and front speakers up 10 cm. Also, the sidewall lines were slightly adjusted. The reason for this was the falling ceiling and the low back of the hall so that there is not enough angle between side and ceiling speakers.

The margin of acceptance may be as small as 10 cm

Measuring and calibration of Bio Rex's 1st Hall were made together with Studiotec Ltd's Möttö, who is a certified implementator.

- This project inspired me, and just recently I took part in Dolby's training in England, so I can now do complete design and implementation of Atmos Theaters.

About Dolby Atmos

Audio objects move in between speakers

Dolby Atmos is an audio format for immersive audio playback. In addition to the 5.1 or 7.1 speakers, the audio is also played with ceiling-mounted type-approved loudspeakers, resulting in the more profound three-dimensional sound image.

Furthermore, unlike older Dolby systems, in Dolby Atmos each speaker operates as a discrete speaker, where the surround channels are typically played from all the cinema's speakers on the side or behind. Therefore, the movie soundtrack can contain individual audio objects, that move from one speaker to another, improving the spatial accuracy of the sound.

According to Dolby, the format also enhances the sound quality, because the loudspeakers have to meet even tighter specifications. For cinema this means adding additional speakers, possibly new amplifiers and a new processor.

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