Engineering Acoustics/Basic Room Acoustic Treatments

Introduction
Many people use one or two rooms in their living space as "theatrical" rooms where theater or music room activities commence. It is a common misconseption that adding speakers to the room will enhance the quality of the room acoustics. There are other simple things that can be done to increase the acoustics of the room to produce sound that is similar to "theater" sound. This site will take you through some simple background knowledge on acoustics and then explain some solutions that will help improve sound quality in a room.

Room Sound Combinations
The sound you hear in a room is a combination of direct sound and indirect sound. Direct sound will come directly from your speakers while the other sound you hear is reflected off of various objects in the room.



The Direct sound is coming right out of the TV to the listener, as you can see with the heavy black arrow. All of the other sound is reflected off surfaces before they reach the listener.

sound travels in the ground faster than in the air because the particles in the solid are closer so they vibrate faster and have a quicker reaction.

Good and Bad Reflected Sound
Have you ever listened to speakers outside? You might have noticed that the sound is thin and dull. This occurs because when sound is reflected, it is fuller and louder than it would if it were in an open space. So when sound is reflected, it can add a fullness, or spaciousness. The bad part of reflected sound occurs when the reflections amplify some notes, while cancelling out others, changing the character of the sound. It can also affect tonal quality and create an echo-like effect. There are three types of reflected sound, pure reflection, absorption, and diffusion. Each reflection type is important in creating a "theater" type acoustic room.



Reflected Sound
Reflected sound waves, good and bad, affect the sound you hear, where it comes from, and the quality of the sound when it gets to you. The bad news when it comes to reflected sound is standing waves. These waves are created when sound is reflected back and forth between any two parallel surfaces in your room, ceiling and floor or wall to wall. Standing waves can distort noises 300 Hz and down. These noises include the lower mid frequency and bass ranges. Standing waves tend to collect near the walls and in corners of a room, these collecting standing waves are called room resonance modes.

Finding your room resonance modes
First, specify room dimensions (length, width, and height). Then follow this example:



1. There are some room dimensions that produce the largest amount of standing waves.
a. Cube

b. Room with 2 out of the three dimensions equal

c. Rooms with dimensions that are multiples of each other

Absorbed
The sound that humans hear is actually a form of acoustic energy. Different materials absorb different amounts of this energy at different frequencies. When considering room acoustics, there should be a good mix of high frequency absorbing materials and low frequency absorbing materials. A table including information on how different common household surfaces absorb sound can be found on the website http://www.crutchfieldadvisor.com/learningcenter/home/speakers_roomacoustics.html?page=2#materials_table

Diffused Sound
Using devices that diffuse sound is a fairly new way of increasing acoustic performance in a room. It is a means to create sound that appears to be "live". They can replace echo-like reflections without absorbing too much sound.

Some ways of determining where diffusive items should be placed were found on http://www.crutchfieldadvisor.com/S-hpU9sw2hgbG/learningcenter/home/speakers_roomacoustics.html?page=4:

1.) If you have carpet or drapes already in your room, use diffusion to control side wall reflections.

2.) A bookcase filled with odd-sized books makes an effective diffusor.

3.) Use absorptive material on room surfaces between your listening position and your front speakers, and treat the back wall with diffusive material to re-distribute the reflections.

How to Find Overall Trouble Spots In a Room
Every surface in a room does not have to be treated in order to have good room acoustics. Here is a simple method of finding trouble spots in a room.

1.) Grab a friend to hold a mirror along the wall near a certain speaker at speaker height.

2.) The listener sits in a spot of normal viewing.

3.) The friend then moves slowly toward the listening position (stay along the wall).

4.) Mark each spot on the wall where the listener can see any of the room speakers in the mirror.

5.) Congratulations! These are the trouble spots in the room that need an absorptive material in place.  Don't forget that diffusive material can also be placed in those positions.

References Sound
http://www.ecoustics.com/Home/Accessories/Acoustic_Room_Treatments/Acoustic_Room_Treatment_Articles/

http://www.audioholics.com/techtips/roomacoustics/roomacoustictreatments.php

http://www.diynetwork.com/diy/hi_family_room/article/0,2037,DIY_13912_3471072,00.html

http://www.crutchfieldadvisor.com/S-hpU9sw2hgbG/learningcenter/home/speakers_roomacoustics.html?page=1

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