Random Amplitude Modulation Research Articles

Sound Source for ASD Sonic Test Facility

The sound source for the ASD Sonic Test Facility now under construction consists of two banks of independently controlled sirens designed to give sinusoidal output over a range of operating pressures. The main fixed bank of sirens (frequency range 50 to 2000 cps) employs 25 sirens, each using a 24-in. rotor; a second movable bank (frequency 500 to 10 000 cps) employs nine sirens each using a 12-in. rotor. Each siren employs a modulator which permits random or coherent amplitude modulation of the acoustic output at modulation frequencies from 0 to 25 cps for the large sirens and from 0 to 50 cps for the smaller sirens. Maximum acoustic power output of the fixed bank of sirens is expected to exceed 106 w in the midfrequency region from 200 to 800 cps. Finite amplitude limitations will be discussed. (Sponsored by Aeronautical Systems Division, Air Force Systems Command, U. S. Air Force.)

Articulation Reduction by Combined Distortions of Speech Waves

Previous investigators have shown that speech waves can undergo any one of a number of severe forms of distortion in low ambient noise levels without serious reduction of word articulation. There are well-known notable exceptions (e.g., center clipping). However, it is not enough to avoid these exceptional forms of distortion. In the study reported here it has been demonstrated that combinations of speech-wave distortions, which individually are quite innocuous with regard to word articulation, can be devastating in their combined effect, even in the absence of serious noise. Four types of speech-wave distortion were studied, individually and in combination, as follows: gross attenuation of high-frequency components, multiple echo, random amplitude modulation, and gross irregularity of response-frequency characteristic. Ambient noise was also a controlled environmental condition in some phases of the investigation.