# Engineering Acoustics/Room Acoustics and Concert Halls

# Room Acoustics and Concert Halls

## Introduction

From performing on many different rooms and stages all over the United
States, I thought it would be nice to have a better understanding and
source about the room acoustics. This Wikibook page is intended to help
the user with basic technical questions/answers about room acoustics.
Main topics that will be covered are: what really makes a room sound
*good* or *bad*, *alive* or *dead*. This will lead into absorption and
transmission coefficients, decay of sound in the room, and
reverberation. Different use of materials in rooms will be mentioned
also. There is no intention of taking work from another. This page is a
switchboard source to help the user find information about room
acoustics.

## Sound Fields

Two types of sound fields are involved in room acoustics: Direct Sound
and Reverberant Sound.

### Direct Sound

The component of the sound field in a room that involves only a direct
path between the source and the receiver, before any reflections off
walls and other surfaces.

### Reverberant Sound

The component of the sound field in a room that involves the direct path
and the path after it reflects off of walls or any other surfaces. How
the waves deflect off of the mediums all depends on the absorption and
transmission coefficients.

Good example pictures are shown at Crutchfield
Advisor,
a Physics Site from MTSU,
and Voiceteacher.com

## Room Coefficients

In a perfect world, if there is a sound shot right at a wall, the sound
should come right back. But because sounds hit different materials types
of walls, the sound does not have perfect reflection. From 1, these are
explained as follows:

### Absorption & Transmission Coefficients

The best way to explain how sound reacts to different mediums is through
acoustical energy. When sound impacts on a wall, acoustical energy will
be reflected, absorbed, or transmitted through the wall.

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

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Absorption Coefficient:
![](Absorption_Coefficient.jpg "Absorption_Coefficient.jpg") NB: this
chemical structure is unrelated to the acoustics being discussed.

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Transmission Coefficient:
![](Transmission_Coefficient.jpg "Transmission_Coefficient.jpg")

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If all of the acoustic energy hits the wall and none is reflected, the
alpha would equal 1. The energy had zero reflection and was absorbed or
transmitted. This would be an example of a *dead* or *soft* wall because
it takes in everything and doesn\'t reflect anything back. Rooms that
are like this are called Anechoic Rooms which looks like this from
Axiomaudio.

If all of the acoustic energy hits the wall and all reflects back, the
alpha would equal 0. This would be an example of a *live* or *hard* wall
because the sound bounces right back and does not go through the wall.
Rooms that are like this are called Reverberant Rooms like this
McIntosh room. Look how the
walls have nothing attached to them. More room for the sound waves to
bounce off the walls.

### Room Averaged Sound Absorption Coefficient

Not all rooms have the same walls on all sides. The room averaged sound
absorption coefficient can be used to have different types of materials
and areas of walls averaged together.

------------------------------------------------------------------------

RASAC:    ![](Ralpha.jpg "Ralpha.jpg")                
![](Sequals.jpg "Sequals.jpg")

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### Absorption Coefficients for Specific Materials

Basic sound absorption Coefficients are shown here at Acoustical
Surfaces.

Brick, unglazed, painted alpha \~ .01 - .03 -\> Sound reflects back

An open door alpha equals 1 -\> Sound goes through

Units are in
Sabins.

## Sound Decay and Reverberation Time

In a large reverberant room, a sound can still propagate after the sound
source has been turned off. This time when the sound intensity level has
decay 60 dB is called the reverberation time of the room.

------------------------------------------------------------------------

![](Revtime.jpg "Revtime.jpg")           ![](Reverberation_Time.jpg "Reverberation_Time.jpg")

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Great Reverberation
Source

## Great Halls in the World

Foellinger Great
Hall

Japan

Budapest

Carnegie Hall in New
York

Carnegie
Hall

Pick Staiger at Northwestern
U

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Concert Hall
Acoustics

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## References

\[1\] Lord, Gatley, Evensen; *Noise Control for Engineers*, Krieger
Publishing, 435 pgs

Back to Engineering
Acoustics

Created by Kevin Baldwin
