When evaluating a space that has too much echo or the sound level is too high, reverberation time is generally used. Reverberation time is a measure of how long sound stays present within a space after it is made. More specifically, reverberation time is defined as the time required for the level of sound in a room to drop 60 dB after the signal is turned off.
The preferred reverberation time for a space is dependent upon its physical volume, as well as its intended use. For instance, for speech, we normally want a relatively short reverberation time within a space. If the reverberation time is too long and if the speaker does not speak slowly, a listener will actually hear sound from more than one word simultaneously. The result is a garbled sound that is not easily understood. On the other hand, if music is played within a space with a long reverberation time, the musical notes tend to blend together which is more pleasing than a dry dead sound. So the use of a space has a lot of bearing on what reverberation time is most desirable.
Mid-frequency sound (500 to 1,000 Hz) is generally the most critical because it is where the human ear is most sensitive and in the range where speech is produced. The reverberation time within a space can be controlled by the ratio of sound-absorptive surface area to sound-reflective surface area.
Reverberation time can be measured and can be predicted. It is a good tool for evaluating spaces before they are built. There are tables that show recommended reverberation times given the use and volume of the space. Here are a few examples:
- Conference Room, 30k ft3, recommended mid-frequency reverberation time 0.8 seconds
- Classroom, 40k ft3, recommended reverberation time 0.8 seconds
- Theater, 100k ft3, recommended reverberation time 1.0 seconds
- Multipurpose Auditorium, 200k ft3, recommended reverberation time 1.7 seconds
- Catholic Church (organ music), 300k ft3, recommended reverberation time 2.0 seconds
The design of music spaces requires attention to specific qualities such as reverberation, diffusion, absorption, shape, reflections, and volume to achieve optimum performance acoustically. It is desirable to have a space with a moderate reverberation time, good positive reflections relatively close to the source and diffusion. Sound diffusion is the random scattering of sound waves from a surface and is a beneficial characteristic of a music room, as it will give the musician and director the sensation that sound is coming from all directions.
The change in reverberation time is used to calculate the reverberant noise reduction. Table 1 shows an approximation of human sensitivity to changes in reverberation time and the corresponding reverberant sound level. Note that a 75% reduction in reverberation time results in a 6 dBA change which is required for the sound level change to be clearly noticeable.
Human Sensitivity to Reverberation Time
and the Corresponding Sound Level Reduction
|% Reduction in Reverberation Time||Reduction in Sound Level (dBA)||Change in Apparent Loudness|
|50||3||Just barely perceptible|
|75||6||Clearly noticeable reduction|
|90||10||About half as loud|
|99||20||About quarter as loud|
If you are working with a space where the primary concern is understanding someone talk (speech intelligibility), the more acoustical absorption the better. The same is true if your goal is just to reduce noise build up, such as in a gymnasium or pool. The benefit of doing an analysis is so you know what to expect and so that you do not spend more than needed to treat the space. You will not go wrong by adding as much absorption as possible but there do become diminishing returns.
For example, say that 2k ft2 of acoustical absorption reduces the reflected noise level by 3 dB. It would take another 4k ft2 of acoustical absorption to get 3 dBA more reduction and another 8k ft2 to get an additional 3 dBA reduction.
The NRC rating is an average of the absorption coefficient at 250, 500, 1000, and 2000 Hz. In general terms, the NRC represents the average percentage of sound that is absorbed by the material. An NRC of 0.85 generally states that the material absorbs, on average 85% of the sound for mid and high-frequency sound.
When predicting the reverberation time in a space you need to know the area and NRC of each surface type. There are tables of NRC ratings for a large variety of materials. There is information for each 1/3 octave band. Here are a few examples:
- Gypsum Board, NRC 0.05
- Empty Wood Pew (3.5 ft2/seat), NRC 0.15
- Carpeted Floor, NRC 0.35
- Occupied Upholstered Seat (5 ft2/seat), NRC 0.80
- Acoustic Ceiling Tiles, NRC 0.50 to 0.90
- Fiberglass Wall Panels, NRC 0.90
Here are some common acoustical treatments:
Fiberglass Acoustic Panels
Fiberglass acoustical panels are generally made with a 6 pound per cubic foot density fiberglass core wrapped with a fire retardant fabric or perforated vinyl. Panels are typically 1” or 2” thick and come in 2-foot increment sizes. Custom size and shaped panels would increase the cost. Panels should be 1” thick and can be mounted directly to the walls and ceiling.
Acoustic ceiling tiles have a variety of NRC ratings. You can hang them, mount them directly to a wall or ceiling or leave a air space. The air space provides better low frequency absorption.
The spray-on treatment is available typically as a relatively soft cellulose fiber-material or as a cement-based plaster. The material is not very resistant to abuse. Sprayed acoustic material is applied by many local insulation contracting companies.
1” thick fiberglass duct liner (black facing)
This can be applied to the ceiling and walls. A perforated metal covering can be installed to protect the panels. If a mylar facing is used, the seams can be taped to allow them to be washed.
Cotton Acoustic Panels
Sold as 1″ thick 6 pound/ft3 cotton insulation. They are made of recycled cotton. They are more expensive than fiberglass but they do not need to be covered and is easier to work with.
Acoustic Fabric Mounting System
They install a track in any shape, put in fiberglass, then stretch material between the tracks to cover the insulation.
Acoustic Baffles or Lapendaries
Acoustic baffles typically come in 4′ by 8′ panels which are 1 or 2 inches thick. Acoustic baffles consist of fiberglass wrapped in sail cloth or other fabrics. Lapendaries are long baffles that can stretch across a room.
Metal Acoustic Panels
Metal acoustic ceiling systems consist of a sheet of perforated metal with fiberglass resting on the metal. The fiberglass can be bagged in mylar. Wood can also cover the absorption with perforations or an open wood grid can be used.
Thank you for your interest in the Noise Engineers podcast.
Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.