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In an enclosed environment sound can continue to reflect for a period of time after a source has stopped emitting sound. This prolongation of sound is called reverberation. Reverberation time (RT60) is defined as the time required, in seconds, for the average sound in a room to decrease by 60 decibels after a source stops generating sound.

Reverberation time is the primary descriptor of an acoustic environment. A space with a long reverberation time is referred to as a "live" environment. When sound dies out quickly within a space it is referred to as being an acoustically "dead" environment. An optimum reverberation time depends highly on the use of the space. For example, speech is best understood within a "dead" environment. Music can be enhanced within a "live" environment as the notes blend together. Different styles of music will also require different reverberation times.

Reverberation Time
.8 - 1.3
1.4 - 2.0
2.1 - 3.0
Fair - Poor
0.8 - 1.1
Contemporary music
Fair - Good
1.2 - 1.4
Choral music
Poor - Fair
Fair - Good
Good - Fair
1.8 - 2.0+
* With an adequately designed and installed sound system, speech intelligibility concerns can be mitigated.
** The optimium reverberation time can be somewhat subjective and can shift based on numerous variables.

Reverberation time is affected by the size of the space and the amount of reflective or absorptive surfaces within the space. A space with highly absorptive surfaces will absorb the sound and stop it from reflecting back into the space. This would yield a space with a short reverberation time. Reflective surfaces will reflect sound and will increase the reverberation time within a space. In general, larger spaces have longer reverberation times than smaller spaces. Therefore, a large space will require more absorption to achieve the same reverberation time as a smaller space.

Reverberation time can be calculated in the preliminary design stage. This is very beneficial in determining how well a space will function for its intended use and if more or less absorption is needed within the space. There are several formulas for calculating reverberation time, the most common formula is the Sabine Formula, created by Wallace Clement Sabine. The formula is based on the volume of the space and the total amount of absorption within a space. The total amount of absorption within a space is referred to as sabins. It is important to note that the absorption and surface area must be considered for every material within a space in order to calculate sabins.

Reverberation time can also be adjusted within an existing space. Tests can be performed in a space to determine the existing reverberation time. Absorptive materials can then be added to or removed from a space to achieve the desired reverberation time. Whenever possible it is highly advisable to consider reverberation time and other aspects of acoustics in the design stage. Making revisions to a space after the fact can be more costly and compromise aesthetics.

Reverberation time is not the only descriptor of an acoustic environment. There are several other principles to consider. A few of the more important considerations include: reflections, loudness (strength), clarity, warmth and intimacy. Questions to consider in each of these areas:
Reflections: Does the reflection of sound within the space cause negative results such as an echo or a megaphone effect? Or are reflective surfaces helping to benefit sound distribution?

Loudness (strength): Is the volume of the sound loud enough? Is it too loud? Or does it seem louder than it would at the same distance outdoors?

Clarity: Can I hear each of the various instruments clearly? Can I understand what is being sung by a solo vocalist, or what is being said by a speaker?

Warmth: Is there a balance of sound throughout the various frequencies? Or is the sound overpowered with too much bass or too much treble?

Intimacy: Do you feel like you are a part of the performance? Or do you feel like the music or speech is taking place in a separate environment?
It is highly advisable to hire an acoustical consultant to assist with reverberation time issues. Determining an optimum reverberation time can be difficult, particularly in multi-use facilities. In many cases the reverberation time must be balanced in order to provide an environment suitable for both speech and music. NRC ratings can be misleading, and can vary depending on the specifics of a particular material being used and how that material is being installed. If proper NRC ratings are not used within the reverberation time formula, the calculation will not yield accurate results. Locations of materials within the space should also be considered. Qualified acoustical consultants have experience with these issues and can perform tests and calculations in order to determine the best possible solutions for your specific environment.

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