Designing a cinema involves multiple disciplines that often compete with one another. At the heart of any  list of technical requirements for a cinema auditorium are really two essential elements: sound and picture. 

Assuming both sound and picture are of equal importance, the latest visual technology, with its high dynamic range and crisp resolution, is often prioritized by theatre operators, their staff and even those helping design and build cinemas. Thus, this post will focus on sound, specifically, the acoustics of the auditorium, and will outline some fundamental conditions for creating a successful listening environment.

Why Good Acoustics are Important in Cinema
Most audio professionals would agree: good sound starts with good acoustics. Or as the “Star Trek” engineer Scotty famously said, “I can’t change the laws of physics, Cap’n!”

So, what do we mean by good acoustics? In simple terms, good acoustics means the room doesn’t interfere with the sound coming from the speakers.

It is important to note at this point that an auditorium intended specifically for amplified pre-mixed sound – such as a cinema – requires different specifications than, for example, a concert hall where the acoustics of the room itself become a part of the actual sound being generated de novo. Movie soundtracks have acoustic environments and elements added on a mixing stage, requiring playback in the cinema to be as neutral as possible – not adding anything extra or detracting from what’s on the track. Common issues caused by poor room auditorium acoustics include echoes (slap back or flutter echoes), resonant frequencies that emphasise certain parts of the frequency range and an uneven frequency response. These can result in muddied audio, indistinct dialogue, and poor imaging, some of which can also be a result of the audio system itself.

Fixing these problems starts with analysing the room; listening critically and taking measurements to see if what hear matches. This will help diagnose any problem so that a suitable acoustic treatment can be applied to improve the listening experience.

What is Acoustic Treatment?
As the name suggests, acoustic treatment involves installing materials that alter how sound behaves in a room, with the goal of enhancing the acoustic response of the room. After investigating the auditorium to determine any acoustic issues and if we take a broad overview, there are three options that can be addressed following an investigation of the room and the conclusions drawn:

1. Do Nothing – if the room already sounds good and acoustic response proves satisfactory then no further action is needed.
2. Absorb – use a solution to absorb the frequencies that are too dominant.
3. Diffuse – to reduce echoes and flutter.

A Simple Guide to Cinema Sound Checks
Without going into forensic detail that requires bringing in expensive gear or consultants, there is one test kit that is always available: your ears! 

Here are some easy-to-perform tests that can be carried out in the auditorium:

• Clap Test – stand in the center of the auditorium, clap your hands, and follow up with a loud “heeeeey” that sweeps through different pitches. Listen for resonances or discrete echoes. If you hear them, the room needs work.
• Location Check – stand in different areas in the auditorium and repeat the clap test. Pay attention to any anomalies and the reverberation time — ideally, it should be short (1.5–3 seconds), depending on the size of the room.
• Speech Test – have a normal conversation with a colleague while moving around the auditorium. If speech doesn’t sound natural, or is hard to understand, that’s a red flag. Resonances, too much reverberation, echoes, etc. , can make speech indistinct and dialogue hard for an audience to hear.

Two practical notes: 

1. If your cinema has curtains in front of the screen, they should be left open. Curtains tend to be absorbent and could damp down reflections when closed.
2. Seating under balconies often creates acoustic issues and needs careful checking.

A Common Cause of Acoustic Issues
In most cases, acoustic issues in cinemas boil down to one culprit – hard surfaces. Concrete walls, wooden floors, hand railings and even the projector porthole glass, can bounce sound around the room and cause “coloration,” or distortion, making dialogue unintelligible. If, during investigations, any of the symptoms noted above are present, check for hard surfaces, particularly those in the line of sight from the screen loudspeakers. A major fault that is still common today is the surface of the rear wall – this is often hard and reflective, bouncing sound from the screen speakers back into the room. Depending on the acoustic transparency of the screen, the sound may be reflected back into the auditorium again, thus compounding the problem. Any hard, reflective surfaces causing acoustic problems need to be treated with absorbent and/or diffusive elements.

How to Solve the Problem
If acoustical problems are identified, it is wise to consult an acoustician to determine what treatment is needed and where. An acoustician can identify the frequency range in which treatment needs to be applied; the low, mid or high frequencies. 

The mid-frequency range is where the human ear is the most sensitive. High frequency problems are fairly easy to address. Low frequency problems require larger,  more specialized treatment.

Many problems can be solved by absorptive materials and/or elements but dampening down the room too much can make the atmosphere a little oppressive. Diffusion, on the other hand,  uses irregular surfaces to break up the sound waves and scatter them, thus diminishing echoes and resonances and opening up the room. A good mix of absorption and diffusion is the best of both worlds, making for a more pleasant, natural experience.

And though a frequent concern is whether acoustic treatment will spoil design aesthetics, good acoustics can look good, too. Many companies (like EOMAC and Unidus Acoustics) make attractive, decorative panels that also do the job acoustically.

Balancing Materials, Design and Budget
It may sound obvious, but only do what is necessary. An acoustician can prioritize issues and avoid wasting money on over-treatment.

If budget is an issue, make a start by addressing any serious problems first and fine-tuning later. However, make a complete schedule, or checklist, of what needs to be done and go through it in steps.

Flutter echoes are often the first thing to tackle. One of the easiest solutions: install heavy velvet drapes with large pleats (10–15 cm), ideally weighted and spaced 20–30 cm from the wall. This can also enhance the look of an auditorium. If the side walls are creating problems, the same treatment can apply.

And don’t forget that certain absorbers are available free of charge – the audience. Human bodies (and even seats) are pretty good at absorbing sound waves. To maximise the efficiency, aim any high-frequency speaker horns so that the on-axis output (90° from the horn) hits the seats about two-thirds of the way back. This reduces rear-wall reflections and limits sidewall bounce.

Why Cinema Professionals Should Care
As an esteemed colleague, Dr. Floyd Toole of JBL, once said in a presentation on acoustics: “It is not rocket science – but it is science!”

The aim for those of us who specialise in the audio field is to remove the mystery surrounding the subject and bring acoustics from the realm of the ‘black arts’ into the real world. Once there’s a basic understanding of how sound behaves, that knowledge can be used to create a better film presentation. It can be addressed and utilised effectively to make a cinema’s performance the best it can be. Cinema is a holistic art. When sound, image, and space work together, the result is greater than the sum of its parts.