4.3. Acoustic properties of a room
The acoustic properties of a room affect how sound interacts with the room and how well a listener can detect and comprehend the sounds that they hear. There is significant evidence from research studies that younger children and deaf children need a shorter reverberation time and greater SNR. Children with other speech, language, and communication needs, attention deficit hyperactivity disorder (ADHD), autistic spectrum disorder, visual impairment, and English as an additional language will also benefit from enhanced acoustic conditions.
Reverberation time is the time it takes for a sound to decay by 60 dB from when the source of the sound has stopped and is measured in seconds. Reverberation time usually changes at different frequencies.
Reverberation time will be affected by the size and shape of the room, construction materials, and the absorption properties of the materials within the room. Materials vary in how they absorb or reflect sounds at different frequencies. Hard surfaces such as wood, glass, and plasterboard reflect sound. A room with lots of hard surfaces will have a longer reverberation time. Carpet, soft furnishings, curtains, fabric blinds, and acoustic wall and ceiling tiles absorb sound, reducing reverberation times.
Longer reverberation times blur the speech signal and add to the background noise in the room.
Listeners hear speech directly from the speaker and also speech that has been reflected from hard surfaces around the room. The critical distance in a room is the point at which the sound pressure level of sound directly from the speaker equals the sound pressure level of reverberated sound.
A reverberant room has a shorter critical distance, so, the listener will need to be closer to the speaker to hear speech more clearly. A room with more sound absorbent surfaces and low reverberation will have a critical distance that is further away from the sound source. Beyond the critical distance, direct speech will arrive at the listener’s ears followed later by reverberated sounds, making speech discrimination more difficult.
Speech sounds vary in amplitude. Vowel sounds have a greater amplitude. Weaker consonant sounds may be masked if the reverberation time is long and noise level is high and the listener is beyond the critical distance.
Signal to noise ratio (SNR)
SNR is the difference between the signal that the listener needs or wants to listen to, eg speech and competing background noise. SNR is influenced by both direct and reverberated sounds. Larger differences give a clearer signal and greater speech clarity.
For deaf children, BATOD recommends an SNR of +20 dB across the frequency range 125 Hz to 750 Hz and +15 dB across the frequency range 750 Hz to 4 kHz.
Crandell, C.C. and Smaldino, J.J. (2000) ‘Classroom acoustics for children with normal hearing and with hearing impairment’, Language, Speech and Hearing Services in Schools, 31:4, pp362-370. Available from https://doi.org/10.1044/0161-1461.3104.362
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