Can You Repeat That? Optimizing Acoustics with Speech Intelligibility
How the Science Developed for Speech Intelligibility in Performance Theatres Can Be Applied to Everyday Buildings
Humans have used speech to communicate for thousands of years. Our brains and ears have evolved in tandem to optimize our ability to communicate via speech. When it comes to making different sounds, our vocal reproduction system is quite a marvel of versatility. For example, the Taa language spoken in parts of Botswana and Namibia is believed by most linguists to have the largest sound inventory with more than 100 consonants.
The vocal cords are a marvel of engineering as shown through this video demonstrating how vocal cords work and this one showing that vibration is a key component.
As our ears and brains adapt, it means we can take the complex acoustical field of our surroundings and extract useful sound information from it. The environment can introduce sounds that could help or hinder the extraction of this useful information. For example, noisy environments may mask the incoming acoustic signal, or a site generating an echo could mix up the signal. When sound is at a premium, the pursuit of design for spaces that enhance the speech intelligibility has led us to understand a few aspects of the more ideal acoustic environment.
The Impact of Background Noise on Speech Intelligibility
Even as they have developed, our ears do best when they are focused on sound. Ambient noise levels can severely impact speech intelligibility. Speech intelligibility is a measure of how comprehensible speech is in given conditions. If you are listening to a speech with construction in the next room, the ambient noise will mask the speech sounds completely and render it inaudible. It means the relative loudness of the speech to the background noise becomes critical. In the end, the acoustics of a space will not matter if the background noise level is too high.
The Shortest Distance Between Two Points
We often think of acoustics and speech intelligibility when discussing the design of spaces used for musical or theatrical presentations. In our previous blog post about Performance Arts Venue Acoustics, we discussed how the arrival pattern of how the sound gets to the listener also affects speech intelligibility. If we want to optimize for speech, the greater the early energy, the better the intelligibility. Much like our vision, our ears have a flicker fusion threshold of about 50 milliseconds. This means those sound reflections that arrive within 50 milliseconds of the direct sound get used by our brain to enhance the speech intelligibility. Acoustic reflections that arrive later than 50 milliseconds after the sound can no longer be used by the brain to enhance speech intelligibility, and detract from it.
This is a critical component of performance theatre design. Most performance theatres rely on the two above metrics (in combination with some others) to ensure adequate speech intelligibility and acoustic strength in a performance venue.
Improving Speech Intelligibility Beyond the Performance Theatre
Speech intelligibility is critical in other types of venues as well and the same approach to acoustical analysis can be just as valid. Be it in universities, research institutes, courthouses, schools, or meeting rooms, using the acoustical science of performing arts venues can enhance the experience of a space to a more positive one. Sound quality and speech intelligibility are needed everywhere. Good speech intelligibility in a courthouse will ensure everyone can hear the proceedings. In a university classroom or lecture hall, it impacts whether the students can hear the instructor and whether the instructor can hear questions from the class.
If you are designing a space that needs an audience to hear clearly, there are three main considerations to ensure adequate speech intelligibility:
1) Make sure the space is quiet enough.
Even the best acoustics cannot overcome base level background noise that drowns out the person speaking. The operative word is quiet enough. Once the background noise is under control, further reduction should be done with caution. If a space is too quiet, it runs the risk of being disrupted by individual sounds that would otherwise be masked by the background noise.
2) Control reverberation.
Managing the reverberation time in a space will almost always reduce the late-arriving reflections at a listener’s ear and will enhance speech intelligibility. This can be done through conventional means such as providing acoustically absorptive finishes in the space, and by limiting the total volume of the space.
3) Strategic reflections.
Reflections arriving at the listener within 50 milliseconds of the direct sound will increase speech intelligibility. This timing will be affected by the shape of the room, but for most rectangular lecture spaces, this means the ceiling ought to stay reflective, and acoustic absorption should be placed near the back of a room first, and above the ear line of listeners.
Creating Better Acoustics for All Venues
The human ear is a marvel of evolution and does an excellent job of delivering information to the brain. Designing a room should enhance the capabilities of the ear rather than introduce elements that compete with its role. Controlling noise and reverberation should deliver the right level of speech intelligibility.
While the importance of speech intelligibility is most commonly thought of for performance scenarios, achieving good acoustics is important to many venues. Whether the space is for meetings, education or legal proceedings, ensuring the comfort of those using the facility is integral to a successful design. With so many well-designed performing arts venues, we can take many key learnings from these facilities to apply to everyday facilities. If you have any questions or need expert advice on how to acoustically design your space, please contact us at [email protected].
