An amplitude-modulated tone produces a three-line spectrum

Evidence

By way of contrast and to complete the story, sounds that have no definite musical pitch (but a pitch nevertheless—see what follows) associated with them, such as the “ss” in sea (Figure 3.9 later in the chapter), have an acoustic pressure waveform that does not repeat regularly, and is often random in its variation with time and is therefore not periodic. Such a waveform is referred to as being “aperiodic” (see Chapter 1). The spectrum of such sounds contains frequency components that are not related as integer multiples of some frequency, and there are no harmonic components. The spectrum will often contain all frequencies, in which case it is known as a “continuous” spectrum. An example of an acoustic pressure waveform and spectrum for a nonperiodic sound is illustrated in Figure 3.6 for a snare drum being brushed. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 822 (#exemplifies, via linked claim)

If the level of sound fluctuates relatively slowly over a certain range of rates, between about 1 Hz and 16 Hz, a significant fluctuation strength may be perceived. When the modulation rate far exceeds 16 Hz, our hearing is unable to follow the level of sound, and the sound is perceived as rough, associated with the psychoacoustic quantity roughness. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217 (#expands, via linked claim)

Categorization

• XREF: connects to: Rapid changes in a sound’s amplitude envelope force broader spectral bandwidth — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 489; Pitch strength of amplitude-modulated tones depends critically on the carrier-to-modulation frequency ratio; Modulation frequency determines whether sound is heard as fluctuation, roughness, or multiple tones — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217 (+4 more)

Broader Connections

• Periodic sounds give line spectra while aperiodic sounds give continuous spectra #exemplifies (0.90) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 822
• Pitch strength of amplitude-modulated tones depends critically on the carrier-to-modulation frequency ratio #expands (0.80)
• Modulation frequency determines whether sound is heard as fluctuation, roughness, or multiple tones #expands (0.80) — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217
• Rapid changes in a sound’s amplitude envelope force broader spectral bandwidth #expands (0.75) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 489
• Amplitude-modulated tones gain an extra detection cue that noise and FM tones lack #expands (0.75)