- What is it?
- Reverberation time is basically how "loud" a room or space is. Specifically, the time (measured in seconds) that it takes for a certain noise level to drop 60 decibels (dB). The 60 decibel drop is an arbitrary decision to standardize the measurement, but it is a relatively useful choice, given it is about the difference between an orchestra at full fortissimo and the background noise in a room.
- Why is it important?
- Reverberation time contributes (or detracts) from the quality of a space in several ways, in different environments. Here's a chart showing optimum reverberation times for various rooms.
- The biggest difference (or at least the most recognized) is made in performing arts environments, one of our specialties.
- In choral or lyrical instrumental performance environments, a long reverberation time (like a cathedral) is desirable, stretching and blending the various sounds into one another, adding a great resonance to people's voices, making them sound better. It's probably no coincidence that people tend to sing in the shower, usually the room with usually the longest reverberation time, due to usually having all hard finishes.
- In lecture halls or spoken-word environments, a short reverberation time is desirable, as long reverberation times will blend the consonants together, making certain words hard to distinguish.
- In practice rooms and rehearsal halls, many musicians prefer to have short-resonance spaces, so as to be able to hear themselves accurately, without the embellishment and "sugar-coating" of a well-designed hall. (Though, some of us enjoy practicing in reflective stairwells, using the reverberation to fool ourselves into thinking we sound much better than we actually are.)
- Outside of performance buildings, reverberation times still make a difference:
- Often, the desire for low-maintenance finishes in restaurants and kitchens makes people select too many hard finishes. This results in a long-resonance space, and therefore makes for very loud, even unpleasant dining experiences, driving some customers away.
- In classrooms, studies have linked poor acoustical design with several metrics. Students who cannot make out what the teacher is saying tend to have lower test scores. Also, teachers who must continually shout over background noise tent to have to take more sick days.
- To hear the difference, check out an audio comparison at Armstrong Ceiling's website, here.
- How do we do it?
- Reverberation time is usually measured in the field by playing a sound, stopping it, then recording how long it takes for the sound level to drop 60 decibels. It can, however, be calculated ahead of time by taking the volume of the room (multiplied by a constant), and dividing it by the sum of the total Sabins of absorption in the room. See here for more detailed information.
- A sabin, named for the Harvard acoustic researcher who first researched reverberation times, is a measurement of absorption, whose units are curiously measured in area (m^2, or sqft.) However, to be a useful measurement in sabins, the area is multiplied by a coefficient of how much sound it absorbs. For example, a square meter (or square foot for imperial units) of open window reflects no sound, and so have a perfect coefficient of 1, and would be measured as one sabin. Something of the same size that reflects a quarter of the sound back would then be 0.75 sabins.
- Notably, since materials will absorb sound differently at different frequencies, a standardized metric, called the Noise Reduction Coefficient (NRC) exists to test a material at four common octave bands; at 250, 500, 1000, and 2000 Hz; (Approx Middle B4, B5, B6, and B7) and average performance between the four. Typical NRCs vary from 0.70-0.90 for high-absorption acoustic ceilings, to 0.05 for glass, brick, or gypsum board.
- These calculations can also be done in reverse, where we take a room of a desired size and desired reverberation time, and calculate out the required absorption needed, which we can then incorporate into the design of the room. Most projects we do this ourselves, but in highly sensitive cases like music schools and performance halls, we will partner with an acoustical consultant to get it just right.
- In many cases, the equations can be simplified, and a good reverberation time can be obtained by simply selecting a good ceiling and floor finish.
- However, in more specialized cases, we often need to add acoustical panels around or in the space, both to absorb sound, and to break up any strong echoes We also sometimes add movable curtains that can be opened to allow the acoustical qualities of the room to be changed as desired. All of these elements need to be carefully placed, both to work with the acoustic geometry of the rooms, as well as to fit within a coherent room design.
- When in the process do we do it?
- Early in the design process, we will take into account the desired volumes of spaces, and select a ceiling and a ceiling height that works for most of the simple spaces
- Later in the design process, for any particularly complicated or demanding spaces, we will look into any necessary additional acoustic panels or elements, and incorporate them into the design of the spaces.
- Where are some good examples that we've worked on?
- Casper College
- At Casper College, we worked closely with our acoustical consultants to properly balance the acoustics of all the major spaces. The required acoustic panels were treated as a design opportunity, helping to give character to the various spaces.
- The main concert hall was made up of glass-fiber-reinforced gypsum panels on the ceiling and walls to evoke the windswept plains of the area, while diffusing the sounds throughout the hall, with absorbing panels at the rear. There are also absorptive curtains that can be deployed in front of these diffusive panels that will allow the users to controls the acoustics of the space.
- In the instrumental and choral rehearsal halls, the adjustable acoustic curtains were hidden by pilasters that, along with the curved acoustic diffuser wall panels, also give detail and a human scale to the large space.
- At the percussion rehearsal hall, absorptive wall panels were placed both low and high, to avoid reflective standing waves and flutter echoes, while the mid-height diffusive acoustic wall panels were woven together with both absorptive and diffusive concrete blocks, to accomodate the deeper tones of a percussion suite. They were carefully placed on the walls as an expression of the common basic rhythms that percussionists play, while additional glass-fiber-reinforced panels from the main auditorium were hung in the space to diffuse the sound from below.
- Mimosa Park Elementary
- At Mimosa Park Elementary we preemptively addressed acoustical concerns about an overly-loud cafeteria space by spraying a acoustical treatment onto the ceiling, adding acoustical panels on all walls, and adding hanging acoustic panels in the center of the space. This was done early enough in the design and construction process that all of these elements were folded into the coherent design of the cafeteria, instead of tacked on afterwards.
- JB Martin Gymnasium
- At JB Martin Gym, we were specifically tasked with fixing the acoustics of an existing gymnasium, that was far too "loud" a space to be useful. Nearly all speech was unintelligible, even through the loudspeakers, and being in the space during a loud athletic event was almost unbearable. HMS remedied the situation by applying a sprayed acoustic ceiling to the entirety of the gym ceiling, as well as placing acoustic wall panels at the perimeter, solving the school's problem, even with an increased capacity of the gym.
Overall, reverberation time is one of the many factors that affect a user's perception of a space, and is therefore only one of the many factors we take into account for the design spaces and buildings.
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