Beats of a Sinusoidally Amplitude-Modulated Signal with a Tone near the Modulation Frequency

Abstract

The “residue pitch” produced by a train of dc pulses will not beat with a search tone of similar frequency (Schouten, 1938). These results have been interpreted to mean that combination tones do not augment the residue pitch generated by low-amplitude periodic waveforms. This assumption was reexamined for the case of a sinusoidally amplitude-modulated (AM) signal synthesized from three inharmonic components—2050, 2400, and 2750 Hz. Acoustic energy at the modulation frequency (350 Hz) was 60 dB lower than the 2400-Hz component. The modulated signal was presented at 35, 45, 55, or 65 dB SPL, together with a 352-Hz search tone. Subjects adjusted the search tone amplitude for “best beats.” Two of the five subjects could detect beating at 350 Hz when the AM signal was less than 35 dB above threshold. When the search tone was set to 35 dB SL and above, all subjects heard a periodic sweeping pitch, located in the same approximate spectral region as the AM primary tones. This phenomenon, not previously described, will be discussed in relation to residue and place theories of pitch. [Experiments performed at Submarine Medical Research Laboratories, Groton, Connecticut, under ONR Contract with Connecticut College.]