AM and FM Difference Limens and their Reference to Amplitude-Frequency Changes of a Sound in a Room

The paper discusses determination of difference limens for AM and FM signals and their reference to changes in the amplitude modulation depth of an AM signal and changes in the frequency deviation of a FM signal propagating in a room. Difference limens for AM and FM were determined for periodic and random modulation signals. Additionally, difference limens of amplitude modulation were also determined for octave noise bands, which helped refer them to an objective method for the evaluation of speech intelligibility in a room (the RASTI method).

Results of experimental investigations showed that difference limens of AM and FM signals are approximately independent of modulation frequency. They are linearly rising functions of the AM index (AM case) or the frequency deviation (FM case) of respective reference signals. It was found out that in a number of cases the values of the difference limens are lower than changes of the AM index of the amplitude modulated signals, or changes in the frequency deviation of FM signals, propagating in a room. This means that changes in the amplitude modulation depth for AM signals or changes in the frequency deviation of FM signals introduced by a room can be perceived by subjects. This fact can have an important practical significance for the evaluation of speech intelligibility or evaluation of the quality of music in a room.