The study of laughter provides a novel approach to the mechanisms and evolution of vocal production, perception and social behavior
One of my first goals was to describe the sonic structure of human laughter. This proved to be more difficult than I expected. Like other spontaneous acts, laughter often disappears when one attempts to observe it, especially in the laboratory. Some unconventional approaches were called for. Although I could occasionally elicit laughter from friends and colleagues during playful conversations, I was often forced to engage in shameless hamming (something that graduate school did not prepare me for). One of the most productive approaches was to encounter people in public places and simply ask them to laugh. The request was usually answered with a burst of laughter. About half of the laughing subjects reported that they could not laugh on command. Indeed, we have much less conscious control over laughter than over speech. It is easy to say "ha-ha-ha," but difficult to laugh on cue. We do not "speak" laughter.
In collaboration with an undergraduate assistant, Yvonne Yong, I took the recordings to the Sound Laboratory of the National Zoo in Washington, D.C. Here the laughs were analyzed with a sound spectrograph, a device that translates a sound into an image that reveals the changes in frequency and intensity of the sound over time. Giggles, shrieks and belly laughs replaced the laboratory's usual sonic fare of indigo bunting songs and the calls of golden lion tamarins. Laboratory workers gave us quizzical looks but politely refrained from asking about the origins of the sounds.
The sound spectra revealed the distinct signature of laughter. A laugh is characterized by a series of short vowel-like notes (syllables), each about 75 milliseconds long, that are repeated at regular intervals about 210 milliseconds apart. A specific vowel sound does not define laughter, but similar vowel sounds are typically used for the notes of a given laugh. For example, laughs have the structure of "ha-ha-ha" or "ho-ho-ho," but not "ha-ho-ha-ho." There are intrinsic constraints against producing such laughs. Try to simulate a "ha-ho-ha-ho" laugh—it should feel quite unnatural. When there are variations in the notes, they most often involve the first or last note in a sequence. Thus, "cha-ha-ha" or "ha-ha-ho" laughs are possible variants.
The explosively voiced blasts of a laugh have a strong harmonic structure, with each harmonic being a multiple of a low (fundamental) frequency. The harmonic structure is revealed in a sound spectrogram by the evenly spaced stacks of short horizontal lines in the spectrum, the lowest of which is the fundamental frequency. Given their higher-pitched voices, it is not surprising that the laughter of females has a higher fundamental frequency (about 502 hertz) than male laughter (about 276 hertz). Whether it is a deep belly laugh or a high-pitched titter, however, all human laughter is a variation of this basic form. It is this structure that allows us to recognize laughter in spite of individual differences.
The notes and internote intervals carry most of the information that allows us to identify a sound as laughter. If the sounds between laugh notes are edited out of a tape recording—leaving the notes separated by intervals of silence—a laugh still sounds normal. The internote time interval carries information, but the internote expiratory sounds do not. If the notes are removed from a recording and the gaps between intervals are closed, all that remains of laughter is a long, breathy sigh.
The stereotypic structure of a laugh is, at least in part, a result of the limitations of our vocal apparatus. It is difficult to laugh with abnormally long note durations, such as "haaa-haaa-haaa," or abnormally short durations (much less than 75 milliseconds in length). Likewise, normal note durations with abnormally long or short internote intervals do not occur. Try to produce a natural laugh with a long internote interval, such as "ha——————ha——————ha." As with the natural rhythms of walking or running, there are only so many ways to laugh.
The structural simplicity of a laugh is also suggested by its reversibility. A short segment of laughter—"ha-ha-ha"—played backward on a tape recorder still sounds rather like "ha-ha-ha." Indeed the sound spectrum of a laugh is similar whether scanned from left to right or from right to left—a laugh note has a high degree of temporal symmetry. Yet one aspect of a laugh that is not symmetrical is its loudness. Laughter is characterized by a decrescendo in which the laugh notes that are late in a sequence are usually lower in amplitude than earlier notes (presumably because we run out of air). Recordings of laughter played backward produce a bizarre-sounding crescendo.