# 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.

![](sound_lady.jpg "sound_lady.jpg")

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.

![](sound.jpg "sound.jpg")

### 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:**

![](equationandexample.jpg "equationandexample.jpg")![](Resmodepic.jpg "Resmodepic.jpg")![](exampletable.jpg "exampletable.jpg")

#### Working with room resonance modes to increase sound quality

##### 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

##### 2. Move the chair or sofa away from the walls or corners to reduce standing wave effects

### 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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