Cinema Room Acoustics

Why should you care?

Movie soundtracks are getting better, more dynamic and more complex, directors are also putting far more budget and focus on producing a great audio experience. Danny Boyle stated on the Chris Evans breakfast show on Radio 2 (22-3-13) that he ring fences a huge percentage of the budget to the soundtrack of his films.  He feels that over 50% of the film experience is down to the audio, a view which is shared by George Lucas. It’s clear that the sound design and equipment should have at least the same proportion of the budget as the projector and screen; however this is rarely the case.

Everyone while watching a film has struggled to hear the dialogue, or become overwhelmed by the explosion effects. These issues are well understood and are avoidable in a well designed cinema.

How can it be fixed?

Three main areas need to be considered, locations of the speakers, location of the audience and the properties of the room.

Locations

The location of the audience and the loudspeakers are the easiest to get right in dedicated rooms, and we really have no excuses for getting it wrong. THX and Dolby both give detailed specifications for this, it’s the designers job to get as close to this requirement as possible within the restrictions of the room and considering the room properties.

Room Properties

Sound reaches your ears either directly from the speakers or via a reflected path off walls, ceiling, furniture and the floor, see figure 1.

In the example in figure 1 the reflected sound reaches your ears very soon after the direct sound, this will smear the sound making dialogue hard to understand.

By delaying the reflected sound as shown in figure 2 using acoustic treatments or using the room layout as shown in figure 3 the situation can be much improved. This leads to less fatigue when trying to understand dialogue and a better movie experience.

In addition to the reflected and direct sound we must also consider the frequency response of the room. One of the main reasons explosions can be overwhelming and much too loud compared to other effects is that the room can go into resonance due to its dimensions.

Careful selection of room dimensions, seating and loudspeaker positions can reduce this issue so that only minimal equalization is needed. No more having to change the volume throughout the film.

Loft conversions as cinemas are popular here in the UK and its very common that due to the slopping ceilings the issue described above can be more prominent, figure 4 shows an example.

In the room shown in figure 4 a huge peak in the frequency response will be present. This will cause a significant issue unless treated to minimize it, certain effects and instruments will be too loud and others will be far too quiet.

An example

The room shown in figure 5 and 6 below is a cinema which was completed by AV Candy, at first glance there are no obvious acoustic treatments or design features, the interior design meant that treatments were not desirable.

It was clear from the outset that the room was not going to work well if we maintained the current traditional orientation (screen at the end of the long dimension), after an explanation to the client with the necessary drawings we got approval to rotate it. This has many advantages:

  1. Room symmetry improves – improving frequency response
  2. The loudspeakers and audience can more closely meet the location specifications – improving locatisation and uniformity within the sound field
  3. The direct to indirect sound is improved due to the side walls been further away from the seating and loudspeakers – improving the time response of the room

The room dimensions (although we had no influence here) are also close to the ‘golden ratio’ meaning the modes are evenly distributed at low frequencies, this combined with the subwoofer positioning work has led to even coverage. Several subwoofer locations have been wired for incase additional units are added to further smooth out the response.

A good thick and fibrous carpet and rug with heavy curtains complete the acoustic design.

In summary

Split the audio and vision budgets equally.

Although the room properties room plays a huge part in the design, we can minimize the issues discussed above leading to a better cinema experience

Simon Redfearn

Director

AV Candy

Home cinema and multi-room specialists in the Midlands.

www.av-candy.co.uk

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