Home Theater Acoustics 101

Reverberation Time

Essential Acoustic Principles For Home Theaters

We are going to discuss elementary acoustics principles so you can gain some knowledge in order to comfortably interact with your vendors. Don't worry we won't get to technical.

Reverberation Time:

When we talk about acoustic panels for Home Theaters, we must have a clear understanding of the term Reverberation Time (known as RT). This refers to how long it takes for sound to decay in any given space.

The longer a sound lingers in a room, the more it will create an echo effect in that room. Sound continuously bounces off of the multiple hard surface in a room. If there are no materials that can absorb the sound, then the sound will keep bouncing off these surfaces until it eventually dissipates. Thus the sound will take a long time to decay. This results in a room that has a high RT (echoes and noise).

So we use acoustic treatments to absorb the sound waves. The Acoustic Panels increase the rate of decay of the sound waves. This results in a lower RT for the room. If the sound decays at a quick rate, we reduce the echo effect in the room and increase the clarity of the conversations and music in the room.

Each type of room has a different goal for it’s RT. A library would want a very low RT, while a restaurant may want a higher RT to keep a controllable “buzz” in the room.

NRC Ratings

NRC Explained:

NRC (Noise Reduction Coefficient) is an average rating of how much sound an acoustic panel can absorb. This gives us a quantitative measurement of a sound control product so we can compare it to other products. If the acoustic panel has an NRC rating of .80, that means that the panel will absorb 80% of the sound, while reflecting the remaining 20%.

As you can see from the ratings chart , the thicker panels will have a higher NRC, but we have had great success using the 1″ acoustic panels for our Home Theaters.

Some people swear by the thicker panels. However, the majority of our Acoustic Panels are 1″, which work very well as long as you have enough wall coverage (at least 30%).

Sound Absorption

Sound Absorption:

The primary goal of your Home Theater acoustic panels is to enhance the sound in your room. This is accomplished through the use of three types of acoustic properties: Absorption, Diffusion & Reflection.

Absorptive panels will be used the majority of the time. In your untreated theater, the sound will bounce off the many hard surfaces of the room and reach the listener at different time intervals. This results in an echo effect and “muddy” dialog. By installing absorptive panels, you keep the sound waves from bouncing off the walls and other hard surfaces in your Home Theater. As the name suggests, the absorptive panels actually absorb the sound waves. This prevents the sound waves from continuously bouncing from wall to wall and eventually to your ears.

In order to achieve a tangible level of Home Theater sound enhancement, we recommend that at least 30% of the wall space be covered with Acoustic Panels.

Diffusion

Diffusion:

In rooms where there is a potential for too much sound absorption, we recommend the inclusion of Diffusive acoustic panels for your Home Theater. Home Theaters that contain wall to wall acoustic panels will suffer from too much sound absorption. This results in a room that sounds “dead”. A room that contains wall to wall panels should allocate Diffusion to at least 20% of the panels. The back part of the theater usually contains Diffusion.

In the front part of the theater, you want Absorptive panels to absorb sound waves at the first reflection point from the front speakers. This basically represents the first line of defense against bouncing sound waves.

A true Diffusive panel’s surface will contain a variety of angles, shapes, and depths. This helps scatter the sound waves so there is still positive sound energy in the room, but the scattered sound waves will NOT continuously bounce off the walls. To a lesser degree, a Diffusion panel can also be created by converting several segments of an absorptive panel into a hard service. Basically, the panel will contain a checker board of alternating absorptive and reflective segments.

The majority of our projects do not require Diffusive panels. We treat each Home Theater Acoustic Panel as it’s own unique design platform (via art, acoustic art panels, custom movie posters or photos), therefore our panels tend to be quite large. However, we do leave enough exposed reflective wall space to keep a positive energy in the room.

Architectural elements such as chair rails, crown moldings, columns and even bookshelves can also assist in sound diffusion.

Panel Placement

Acoustic Wall Panel Placement:

Side Wall:

The most important location for acoustic panels in your Home Theater is on the front part of the side wall, called the First Reflection Point. This represents the exact location on the side walls where the sound waves from your front speakers will hit the wall. Having an acoustic panel on the first reflection point goes a long way in helping to control the reverberation in the room.

So how do we find the first reflection point? It’s actually quite easy. Place a temporary mirror on the side wall. When you are sitting in your favorite seats, move the mirror to a location where you can see the front speakers in the mirror. The location of the mirror will represent your first reflection point. This is where you should have an acoustic panel.

Front Wall:

We recommend panels on the front wall bracketing the screen or TV. These front panels will absorb the excessive sound waves that might bounce off the rear wall. If you have a perforated screen, you can also place panels behind the screen.

Rear Walls:

Many people recommend Diffusion for the rear wall. This keeps the room sounding “alive” and vibrant. Diffusion is recommended when you may have too much absorption in the room. If you have a series of smaller panels, then you should use absorptive panels on the back wall as well.

Ceilings:

It is not necessary to place panels on the ceiling if you have a carpeted room because the sound energy emitted from the center channel tends to not bounce from the floor to the ceiling and back to the floor.