AIR-MODULATED SOUND SOURCE AND EXPERIMENTAL INVESTIGATION OF THE SPEECH INTELLIGIBILITY IN LONG DISTANCE BROADCASTING

Air-modulated sound source has been used widely as a sound source analogous to high intensity noise. Investigations show that suitably raising the upper limit frequency of the air-modulated loudspeaker it can be used as a broadcasting sound source to reproduce speech and music. The power output of the sound source developed is 2000 W, the power output at 1000 Hz deviates less than 4 dB from the highest power output in its frequency range. The power output and sensitivity vs frequency curves and directivity patterns are given. The sentence intelligibility when recorded word by word at 5.8 km distance from single source is 56%. And for an array of four loudspeakers, the intelligibility when recorded word by word is 58% at 8.7 km from the sound source, and 32% at 10.2 km from the sound source. The subjective rating for the short passages is nearly intelligible at 8.7 km from the sound source, and less intelligible at 10.2 km from the sound source. The experimental result shows that the obtained values are dependent on the education level and the attention of the listeners. Since experimental condition is similar to practical condition （average weather, listening in the field and untrained listeners） and obtained conclusion are of some practical value.