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Chalkboard Presentation - Acoustics 101

A basic introduction to room acoustics.

Cathy Hutchison

on 29 December 2014

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Transcript of Chalkboard Presentation - Acoustics 101

The room shape affects the acoustical characteristics of the room.
changing the finishes
The room finishes are the types of materials used in both construction and the furnishing of the room.
The remedy for a low signal to noise ratio is to either increase the signal level or lower the noise (whatever that might be). Some common methods for making improvements are to:

Reduce reverberation by adding sound absorption
Use high directivity speakers or use a distributed system
Absorb or redirect reflections using materials/shaping
Eliminate amplifiers on stage or use in-ear monitors
Isolate acoustic drums or use electronic drums
Welcome to Acoustics 101
One of the best ways to describe anything related to acoustics and how we perceive sound is to look at the signal-to-noise ratio.
In any given situation, the “signal” is the sound that we want to hear while the “noise” is the sound that we don’t necessarily want to hear.
One obvious example would be for lectures, the signal would be the speech of the lecturer while the noise could be an air conditioner, traffic noise, a crying baby or anything else that is unwanted and adds to the background noise level.
The following chart illustrates the difference in the signal and noise levels and how, in this example, there is sufficient Signal to Noise Ratio (S/N) to provide acceptable intelligibility during speaking.
Under different circumstances an acoustic environment with excessive reverberation where low directivity speakers are used might result in a low signal to noise ratio or the noise actually being louder than the signal. For lecturing, the result would be poor intelligibility as illustrated by the example here.
A less obvious example might be what we hear during a concert. If a band is playing and you want to hear the singer (signal) the rest of the band could actually be considered noise. If the singer is not mixed above the rest of the band, the resulting poor signal-to-noise ratio equates to poor clarity and a lack of understanding of the words. With this concept in mind, it is easy to see how clear sound quality in a room can be difficult to achieve.
When you are trying to discern the words being sung by a singer, anything that competes or adds sound energy to the room is considered noise. Some of the things that can be considered noise can include:
Instrument amplifiers on stage
Acoustic drums
Excessive reverberation in the room (not enough sound absorption)
Poor directivity (sound control) in the main speaker system (sound energy into the room rather than into the seats)
Late arriving reflections from the room
In a good acoustic environment with low background noise levels and a well-designed speaker system, a good signal to noise ratio can provide excellent sound quality and a good listening experience.
The ratio of the direct sound to reverberant sound (D/R) is one specific application of signal-to-noise. For example, two people standing at either end of a racquetball court trying to talk would have great difficulty understanding each other. They need to speak loudly but will still have trouble understanding each other because so much sound energy is bouncing around the highly reverberant room.
However, in the same room, when the two people stand right next to each other, they don’t have to talk as loud and can be easily understood. The listener still hears the direct sound at the same level, but there is much less sound bouncing around the room.
The acoustical characteristics of a room are determined predominantly by:
Size - which is largely driven by square footage or seat count plus the ceiling height - has the greatest impact on the acoustical quality.
In existing rooms, it is difficult to change the shape or the size of the room. This means is the only practical means for affecting the natural acoustics of the room.
The two acoustical attributes that we investigate and analyze are the characteristics of reverberation and reflected sound energy.

Reverberation is continuation of sound in an enclosed space after the initial source has been terminated.
Mehta, Johnson, Rocafort; Architectural Acoustics: Principles and Design; 1999
Reverberation time (RT60) is the time it takes for sound to decay 60 decibles (dB) once the source has stopped.
Multi-purpose auditoriums generally need to have reverberation characteristics somewhere close to the middle of their target range in order to accommodate a variety of acoustical needs ranging from speech to various styles of music.
Relfection Characteristics
Most of the sound energy is reflected without losing much energy.
Depending on the absorbing power of the material, the energy of the reflection is diminished.
This scatters the sound energy, reducing the power
sent to any one location.
Direct Sound
Reflected Sound
In general, the later the sound arrives (following the direct sound) and the higher in level at which it arrives, the more noticeable the reflection will be. The design goal is to attenuate or eliminate any late arriving reflections that will degrade clarity or speech intelligibility.
Reflected sound energy late in time can usually be perceived and identified by a listener as a distinct echo.
Acoustical Balance -
Worship Acoustics Example

It is highly preferable to provide reflecting surfaces close to listeners (and participants) while most of the sound absorbing surfaces are furthest from the listener. This will result in an increase in early energy and greater clarity and intelligibility.
Reflected Sound
Direct Sound
Early Reflector
In the end, it's important to weigh all the pros and cons when deciding the acoustic personality of the intended space. Having a good understanding of the purpose of the space will help you make the right decisions. What is good for some programs is bad for others.
Egan, David; Architectural Acoustics; 1988
Poor congregational singing and orchestral music
Good clarity for contemporary music
Worship space with a short reverberation time.
Worship space with a long reverberation time.
Poor clarity for contemporary worship and speech
Good congregational singing and orchestral music
Thanks for joining us.
The Top Three Acoustical Issues:
Late Arriving Reflections
High Background Noise
Excessive Reverberation
Size = Architecture
Shape = Architecture
Finishes = Architecture
So...how do you get the room that you want?
DESIGN based on the program
MAXIMIZE the listening experience
HOLISTIC facility design
DESIGN EARLY with integrated team

In a good acoustic environment with low background noise levels and a well-designed speaker system, a good signal to noise ratio can provide excellent sound quality and a good listening experience.
This is an example of an improvement in the direct to reverberant ratio.
High Level
Low Level
Solutions in a challenging amplified music or speech environment:
Acoustical Attributes
Depending on the type of program that takes place in a room, there is an associated reverberation time range that is considered appropriate.
Under certain conditions, acoustic drums and/or instrument amplifiers could be considered noise when they prohibit the listener from hearing a vocalist or speech environment.
a lighthearted presentation
by Idibri
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