Speech Intelligibility in the Classroom

Speech Intelligibility in the ClassroomThere are some serious misunderstandings that reach me about the quality of sound systems in the classroom.

Unlike Hi-Fi systems where listeners are used to, and some obviously enjoy, booming bass sounds and higher pitched screaming guitars or the full range of an orchestra, the need to make the voice of a teacher intelligible to a classroom of young pupils requires very different sound qualities.

The human voice generates most of it's power between 100 Hz and 2 kHz, this is a tiny proportion of the full Hi-Fi range but unless consonants can be heard in this frequency range, speech becomes almost unintelligible. In Sound System Engineering by Davis and Patronis they state that ā€œIt can be seen why the telephone with it's limited response works so well as do small radios with well-designed 4ā€- 8ā€ loudspeakers covering the range of 125Hz-5000Hzā€

Comparing music on a Hi-Fi set-up with that played into a properly engineered classroom audio system, the classroom system will sound flat and the big mistake is to assume that the quality of the system is not good. This could not be further from the truth.

Speech intelligibility calculations and tests show that the articulation loss of consonants in speech (%ALCONS) which is normally measured at 2kHz should not exceed 10% to 15%. The most important factor of the audio system for classrooms, is to have the correct frequency spectrum shape at 2 kHz. Many of the available sound systems ignore this in their design and would need some equalisation to correct these defects at 2 kHz.

So the simple answer is that a normal music amplifier system is unlikely to perform well for intelligibility in the classroom despite an untrained listener thinking that the boomy sounding base relates to a higher quality system

Our classroom audio systems are designed specifically for maximum speech intelligibility at 2 kHz in terms of frequency response, articulation loss and sound pressure levels and perform exceptional well at improving listening and as a result improved learning outcomes. Don't be fooled by those who offer to sell you under powered and poorly designed systems with booming bass !

David Edis-Bates C.Eng MIET


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