21 Jan Intelligible Design: Reverberation time and hearing
Reverberation time (RT) is a measure of how long it takes for sound to decay in a space. It can affect how well you understand speech and even change the way music sounds. Acousticians use RT to describe the acoustics of rooms, and can establish and achieve reverberation time criteria for rooms where it is important to understand speech, or where musical performances take place. The reverberation time criterion can vary with frequency; it may be desirable, for instance, to have a longer RT at lower frequencies than at higher frequencies.
Essentially, RT refers to the amount of time it takes for sound energy to bounce around a room before being absorbed by the surface materials and air. Large-volume spaces with lots of hard finishes (e.g., concrete, stone, plasterboard) will have longer RTs, while smaller spaces with soft finishes (e.g., carpeted floors, acoustically absorbing wall panels, curtains, acoustic ceiling tiles, furniture with open-weave fabric covers) will have shorter RTs.
Surfaces and speech
Listen to the samples below. You’ll notice the difference between speech recorded in a typical classroom and speech recorded in a classroom of the same dimensions with hard, reflecting surfaces.
It is usually easier to understand words that are spoken in a less reverberant space; this is particularly true for younger children and people with hearing impairments who may not be able to pick up on the context or hear all parts of each word clearly. In spaces where speech intelligibility is critical (e.g., for public address and voice alarm systems at train stations, stadiums and other places where large crowds gather), acousticians conduct extensive modelling and testing to ensure the reverberant noise level is suitably controlled.
Reverberation time and performance venues
Music, on the other hand, should sound rich and envelope the listener. Not only should the reverberation time be longer in a performance space, but low, medium and high frequencies need to be balanced. Here are a few examples simulated using a computer model of the Royal Festival Hall in London:
The first sample represents balanced reverberation times.
This next sample features too much low-frequency absorption, so the low frequencies lack sufficient reverberation.
This sample features too much high-frequency absorption.
There is an optimum RT for all spaces, depending on their use. The design becomes more challenging for multifunctional spaces, where a good acoustic environment is required for both speech and music (i.e., both long and short reverberation times are required). In these instances, acousticians can incorporate variable absorption into the design. This could be curtains that bunch or extend, or flipping panels that absorb sound on one side and reflect it on the other. Many concert halls use variable absorption, but these features are usually installed discretely.
Reverberation time also depends on the volume of a space. Some theatres have connecting chambers where doors can be opened and closed, thereby increasing the volume and extending the RT. This can create additional acoustical challenges and lead to other unwanted effects (e.g., double decay due to the coupling of spaces).
Speech and music are not the only sounds that persist in a reverberant room. All sound sources—whether it’s a person sneezing, a puck hitting the boards or a shouting audience—will keep rattling around through a highly reverberant room. As a result, excessive reverberation times can have an adverse effect on noise levels.
Absorption and reflection: Examples of extremes
There are places throughout the world where the reverberant sound field results in interesting acoustics. Examples include:
In an anechoic room, all surfaces (walls, floor, ceiling) are completely covered in acoustic absorption, so the reverberation time is essentially zero. A similar environment can be found outside, for example, in a large open field. Check out this video made in the anechoic chamber at Brigham Young University in Utah.
Designed with hard, reflecting surfaces, reverberation chambers are used today for acoustic measurements and testing. These spaces, which allow sound to bounce around for a longer period of time, have been around for millennia. Take for example the Hypogeum in Paola, Malta, which was built nearly 4,000 years ago.
The reverberation time in spaces we encounter is typically between 0.5 – 2 seconds, yet a few years ago, the record was broken for the world’s longest reverberation time measurement. In an oil storage tank in Scotland, acousticians measured a 75 second reverberation time!
Finding the right reverberation time
Good acoustic design optimizes reverberation time for every space, a process that considers a range of factors including the room’s function and size, as well as the characteristics of its surfaces. At BKL, we have designed the acoustics for rooms of every size and function including studios, sports arenas, libraries, classrooms, theatres and more. Contact us to learn more about our acoustical design services.