06.06.14

How to: Acoustically Treat a Home Theatre

Peter-Janis

By Peter Janis, Primacoustic. In a movie theatre, the room acoustics play a critical role in making sure that everyone can clearly hea...

Peter JanisBy Peter Janis, Primacoustic. In a movie theatre, the room acoustics play a critical role in making sure that everyone can clearly hear the dialogue, no matter where they are seated. By eliminating near-wall reflections, those seated at the periphery are not fighting to discern the sound from the speakers versus the sound reflecting off the walls. The padded seats have been carefully designed to be comfortable, and when not in use, they help control bass. When a theatre is properly treated, it improves our ability to comprehend what is being said. This is known as 'intelligibility'. The image below shows a vector as the direct sound from the loudspeaker arrives at the listening position. A few milliseconds later, the reflected sound arrives. Depending on distance and frequency, if left untreated these sounds interact and either amplify or cancel each other out, reducing intelligibility. [caption id="attachment_5707" align="aligncenter" width="375"]Example of how reflected sound can interfere with direct sound. Example of how reflected sound can interfere with direct sound.[/caption] Your home theatre is no different, only smaller. This means that you do not need as much treatment, and with a little forethought on panel positioning, you can dramatically improve the sound of your room, add excitement to every event and make it look terrific too! Understanding the Problem In a typical 5.1 system, the centre channel is the most important, as this is where the dialogue is primarily generated. Our main goal is to ensure that the message or dialogue is clearly understood by all who are in the room. [caption id="attachment_5720" align="aligncenter" width="400"]Effect on listener of sound being reflected off untreated walls. Effect on listener of sound being reflected off untreated walls.[/caption] The direct sound (green) is the sound we want to hear. But just as in the movie theatre, if the walls are left untreated, sound will echo off the nearby surfaces and will interfere with the direct sound. Primary or first order reflections (red) are the most powerful and are generally the first we will want to control. Then, the trailing reverberant field (blue), or flutter echo, should be addressed. Applying the Solution The room can be treated by mounting acoustic panels to the wall surfaces, but before this is done, we must understand where to place them. Sound in the voice range tends to be directional. As such, we can draw vectors on a floor plan to strategically position the acoustic panels where they will be most effective. The more panels we use, the 'darker sounding' the room will be. Most find that 25% to 30% wall coverage provides plenty of absorption without over deadening the room. [caption id="attachment_5721" align="aligncenter" width="400"]Reflections and reverberation can be reduced by placing acoustic panels strategically around the room. Reflections and reverberation can be reduced by placing acoustic panels strategically around the room.[/caption] Managing the Acoustics In Different Rooms Although dedicated home theatres tend to be large rectangular spaces, most home theatres are in fact multi-purpose rooms that combine a traditional living room with a home theatre. Generally, we want to control primary reflections and reduce flutter echo while creating a balanced audio environment. The following images are some examples of acoustic treatment using a room kit as it would be applied to various room layouts. [caption id="attachment_5712" align="aligncenter" width="265"]Example A Example A.[/caption] In Example A above, the direct sound (1) from the centre speaker carries the dialogue. We can predict a powerful first order reflection off the wall at the top (2). Because the distance to the other wall at the bottom is greater, we can leave this wall uncovered. Flutter echo (3) will be taken care of by the absorption on the wall at the top and behind the primary listening position. [caption id="attachment_5711" align="aligncenter" width="265"]Example B Example B.[/caption] In Example B above, dialogue (1) will typically come from the centre channel. Ideally, we will treat the side walls to reduce first order reflections (2) and then mount a series of acoustic panels behind the listening position (3) to absorb front to back echo and flutter. [caption id="attachment_5710" align="aligncenter" width="265"]Example C Example C.[/caption] In the type of room in Example C above, the centre channel (1) will compete with powerful first order reflections (2) off nearby walls. Treatment usually begins with side walls and then behind the listening position. The side wall treatment also helps reduce flutter echo (3). Determining Coverage The most common question we get is, 'How much treatment should I use in my room?' Although there are no hard rules when it comes to acoustic treatment, most dedicated home theatres tend to have more acoustic coverage, while multi-purpose family rooms have less. This has more to do with available wall space than anything else. To determine the coverage, we have created a series of tables that allow you to choose between various degrees of treatment, depending on your budget and the desired outcome. Often, a 'light' level of treatment provides sufficient sound abatement while keeping the budget in check. If budgets are tight, start with minimal treatment and then add more panels as funds become available. To use the tables, estimate the floor size of your home theatre or living room and the average height of your ceiling. [caption id="attachment_5706" align="aligncenter" width="599"]Room calculation tables in feet. Room calculation tables in feet.[/caption] [caption id="attachment_5705" align="aligncenter" width="600"]Room calculation tables in metres. Room calculation tables in metres.[/caption] So for instance, say you have a home theatre that is 14 feet wide and 16 feet deep with an 8ft ceiling. Multiply 14 x 16 to get the square footage which, in this case would be 224 sq-ft. You would go to the table for 8 foot ceilings and then consider your options. If the home theatre also serves as a family room, then you may be limited as far as available wall space is concerned, and opt for 48 sq-ft of absorption. This could be done using the Primacoustic London 10 room kit for example, or maybe 12 of our Control Columns (4 sq-ft each), or even a mix of panels including Cumulus tri-corner traps, or some great looking Primacoustic Accent panels. These will help reduce powerful primary reflections and make movie watching better. If on the other hand you really like to crank up the volume and want the same effect as a full-on movie theatre, you can use a greater amount of absorption by heavily treating the side and rear walls. If you are not sure, always start with less. You can always add more treatment after you listen to your room. Conclusion Go to any movie theatre and you will notice acoustic treatment covering a large percentage of the walls. Chairs are soft and well padded, and the whole room has been carefully crafted to work as a single system. When properly put together, the combined experience of a powerful visual presentation, good acting and great sound becomes an emotional ride that can take you away, make you laugh or keep you riveted on the edge of your seat. With thoughtful positioning of acoustic panels in a room, there is no reason why a similar experience cannot be enjoyed at home. Peter Janis is President of Primacoustic, a division of Radial Engineering Ltd. Primacoustic is a leading manufacturer of high-performance acoustic materials. www.primacoustic.com You are welcome to comment on this article. See below.