While analyzing an area containing an excessive amount echo or the noise buildup that is too high, reverberation time analysis is often used. Reverberation time is a measure of the time noise remains in a space soon after it is produced. Particularly, reverberation time pertains to the time needed for noise in a space to fall 60 dB following it being turned off.
The preferred reverberation time in a space depends upon its size, together with its use. For example, with talking, a short reverberation period is desired. Should the reverberation period be too long, a person listening will hear noise from more than one word at the same time and the speech will be garbled and not easily understood. Alternatively, for music in a space with a prolonged reverberation period, the musical notes blend collectively which is much more pleasing than dried lifeless noise. So how the area is used has a great deal to do with what reverberation period is most attractive.
Middle frequency noise (500 to 1, 000 Hz) is the most crucial because it is where people are most sensitive as well as the primary frequencies where conversation is generated. The reverberation period in a space may be controlled by the percentage of noise-absorptive material versus the amount of noise-reflective material.
Reverberation period may be tested and predicted. It is just a beneficial instrument in analyzing places prior to their construction. There are charts that indicate advised reverberation times based on the size and use of the room.
Design of music areas demands awareness of particular qualities in addition to reverberation, such as: diffusion, shape, and size to achieve the best acoustical environment. It really is helpful to control the reverberation time along with the reflections, shape, size and diffusion.
Noise decline can be calculated using the reverberation period. Here are some relationships between reduction in reverberation time (T60), noise level and perceived loudness.
- 20% reduction in T60, 1 dBA sound reduction, imperceptible
- 50% reduction in T60, 3 dBA sound reduction, just barely noticeable
- 75% reduction in T60, 6 dBA sound reduction, clearly noticeable
- 90% reduction in T60, 10 dBA sound reduction, 1/2 as loud
- 99% reduction in T60, 20 dBA sound reduction, 1/4 as loud
In case you are working in a room where speech is main issue (speech intelligibility), the lower the reverberation time (more absorption) the better. The identical is true to reduce noise in a space, as in a factory or gym. Predictions will help show the expected reduction and prevent spending too much on acoustical treatments. The effectiveness of adding absorption decreases when there is more and more absorption.
For example, if 2000 ft2 of absorption decreases the noise by 3 dB, it may require 4000 ft2 more for another 3 dBA decrease and 8000 ft2 more for another 3 dBA.
NRC is an average absorption at two hundred and fifty, five hundred, one thousand, and two thousand Hz. NRC is the average amount of noise that is absorbed by something. An NRC of 0.70 means that about 70% of the mid-frequency noise is absorbed.
There are tables of the NRC for different surfaces along with the octave band absorption. This is needed in conducting predictions.
Below are a few acoustical absorption materials:
- Ceiling Tiles
- Spray-on Absorption
- Fiberglass Acoustic Panels
- Acoustic Stretch Fabric Systems
- 1” thick fiberglass duct liner (black facing)
- Cotton Acoustic Material
- Acoustic Baffles
- Metallic Acoustic Panels
- Wood Acoustic Panels
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