High Intelligibility Classrooms
A technical brief by Art Noxon, Acoustic Engineer
There are classrooms and there are high intelligibility classrooms. Who gets high intelligibility classrooms?
- Hearing and learning disabled
- Language Arts
- Vocabulary oriented classroom
- Music and practice rooms
- TV learning centers
- Multilingual population classrooms
- Larger classrooms and lecture halls
Normal classrooms are quite reverberant, due to the hygiene tradition of public spaces. Unfamiliar speech is much harder to understand in a reverberant space. Familiar speech can be more easily understood in a semi-reverberant space than is unfamiliar speech. For example, instructions in a gym class will contain almost no new words. The familiar context and expectation of word sequences will help the listener hear, actually guess/expect what is being said.
Learning a foreign language in a gym would be almost impossible because there is no backlog of experience that can lead to expected word sequences. This applies also to classes steeped in new vocabulary such as biology.
Children who are hearing or learning disabled are much more easily distracted by the reverberation of sound. They seem to fall short in their ability to discriminate between the intentional, direct sounds from the teacher and the time delayed cacophony of these same sounds after they have undergone multiple reflections within the room.
For bilingual students, the classroom language is their adopted language. They are not used to the context of this second language and therefore have reduced confidence in expectation of word sequences. They cannot hear through classroom noise as easily as primary language based students. These second language students operate with a learning disadvantage due to poor room acoustics.
Larger classrooms and lecture halls lose yet another cue factor that does exist and helps students to “see through” noisy, reverberant classrooms. This is the visual cue. In small classrooms, the student can focus on the mouthing of words to help understanding. Over long distances, this lip reading factor disappears leaving only body language as support for sentence detail.
Even normal sized classrooms suffer from the loss of visual cues. In the back of the room, reverberant levels are greatest and the direct signal is weakest. The distance from student to teacher is greatest and visual cues are weakest. This is complicated as the heads of other students are often in the way. We can next add or factor in the general attitude of the students who choose to sit in the back of the room. Accompanying all this is the shuffle and murmur or “self noise” noise levels. There is little wonder these far field student positions will be accompanied by lower levels of attentiveness. The conglomeration of these effects serves to suggest that wide shallow classrooms ought to be preferred over long narrow classrooms.
Music practice rooms are like language classrooms except students anticipate not their own voice but the voice of their instrument. Still their ears must synchronize their own sounds to those of the ensemble and the conductor’s expectations. Excessively reverberant spaces isolate the student. Rather than perceiving themselves set into a teamwork relationship with other members, they find themselves playing against a wall of sound, the reverberant sound field. They cannot practice and develop the skill of hearing themselves within the context of other individual instruments if they cannot separate out to render distinct the other instruments from the reverberant sound field.
Another area for enhanced intelligibility is the rapidly growing TV monitor education format. In this setting there often is no visual available for the student, no lip movement and no body language need be present. Additionally, the fixed sound source associated with TV sets is very different from the ever moving source of a live teacher. Audio speakers have sound dispersion characteristics, high frequencies radiate forward while the lower tones radiate equally in all directions. The human voice is the same in this regard. The difference is that a person continuously moves head and body. The higher frequency beamy part of the soundfield is always being cast about, not so with the fixed position TV monitor. With the lower registers, a fixed TV or audio monitor will acoustically couple very easily to room resonances which establish those monotonous drone tones and room boom that accompanies the presentation. Sound is quite different with an animated source, where every movement changes the coupling to room resonance and so the coloration is continuously changing and much less dreary. Acoustic control of the walls and ceiling near the TV monitor is very important in developing a comfortable fit for student audition.
I have reviewed the more important consequence of classroom cacophony. I am confident you are fundamentally aware of these factors but I must say that architects in general are one to two decades behind most current practices in acoustics. It is imperative that any school plan for the 90’s include considerations for the intelligibility factor. The architect is presently familiar with absorption coefficients and decay rate criteria for sound in various types of spaces. The sound system and acoustic engineers have testing equipment such as sound level meters, octave band and RT-60 analyzers to support this level of understanding.
As with all science and practice, sophistication and experience produce new levels of understanding and competency in manipulating the variables. So it is with psychoacoustics. And the emergent topic these days is intelligibility. There already exists intelligibility standards and intelligibility testing equipment. Crown International (U.S) makes one piece of gear and B & K (Denmark) makes another. Most of the top acoustic engineers and sound system designers already have this equipment and use it regularly in setting up churches, halls, or any area of amplified sound. The technology can easily be extended into the acoustics of the classroom.
The important point to remember is that different intelligibility standards will go with different applications. Some of this criteria for the classroom no doubt has yet to be developed.