At normal conversational speech levels, the speech sounds are distributed unequally as the distance between the speaker and listener increases. The low tone sounds from the vocal cords (voiced), vowels for example, travel at great distances, while the high tones of the teeth, tongue, and lip sounds (unvoiced), like the /s/ for example, don’t travel far. In fact, as distance increases, word clarity decreases because the unvoiced consonants lose power significantly after 6 feet. By 12 feet, some sounds can no longer be heard and at 24 feet all sounds that would help speech understanding are muted. The “critical distance” is a calculation of the best speech reception distance between speakers in a room (35’x40’x10’). This is why communicating at a distance can be challenging, especially with doorways, hallways, and objects between the speaker and listener.
Interference from Facing or Looking Away
In addition to reduced speech clarity at a distance, the direction of the speaker plays another important role. If the speaker is facing the listener, the speech is clearer. If the speaker is turned away from the listener, the speech is degraded. This is because the speech tones are unevenly distributed and higher tones, like the /s, f, th/, become lost just as they do at a distance, but even at shorter distances from behind. This is why communication is best at less than 6 feet when the parties are facing each other, especially in noise. Similar differences exist for both normal hearing individuals and for those that are hearing impaired using hearing aids. For those with hearing loss and not using hearing aids, communication is lost.
