The advent of electromyography (EMG) in the last several decades has allowed researchers to study the cellular and physiological processes responsible for purring in a range of mammals, including the Ursidea family (bears), Canidae family (foxes, wolves, dogs, etc.) and Felidae family . Electromyographs, used in EMG studies, record the electrical activity of skeletal muscles by measuring the electric potentials that arise when skeletal muscle cells are activated. Other researchers have studied purring by simply recording cat purrs and analyzing their wave forms. See the videos below for an example of the recording process (Figure 1)and the resulting waveform and spectrogram (Figure 2).
Figure 1. Dr. Robert Eklund recording a purring cheetah, at the Dell Cheetah Centre in South Africa.Source (accessed 11/21/2011)
Figure 2. Waveform (top) and spectrogram (bottom) for a typical Cheetah purr. The phases shown are egressive, ingressive, egressive, ingressive. Source (accessed 11/21/2011)
How Purring Occurs
The majority of mammalian vocalization, including human speech, occurs on a pulmonic egressive airstream; in other words, vocalizations are produced by passing air out through the larynx (Figure 3) . The vocal folds (Figure 4), which stretch across the larynx, are activated by the movement of air out of the lungs . Purring, however, occurs during both inspiration (breathing in) and expiration (breathing out) , and involves muscular activation of the vocal folds . Another somewhat unique characteristic of purring is that the sound is generated continuously,for several seconds and sometimes for as long as several minutes, during both inspiration and expiration phases with very short transition interruptions between the phases that range between 30-50 ms in the domestic cat and 50-200 ms in the cheetah .
Sissom, et al. (1991) demonstrated that the primary
mechanism behind the sound and the vibration produced during purring is a centrally driven laryngeal modulation of respiratory flow . The sound is generated by a sudden build up and release of pressure as the glottis is closed and then opened, resulting in a sudden separation of the vocal folds, which generate the sound . The laryngeal muscles, which move the glottis, are driven by a free-running neural oscillator, generating a cycle of contraction and release every 30-40 ms . Fundamentally, purring arises from the gating of respiratory flow by the larynx. Gibbs (1936) was able to elicit purring when he stimulated the infundibular region of the cat brain, suggesting that the laryngeal muscles are controlled by the central nervous system .
Characteristics of Purring
The vibration and sound of purring continues solidly during inspiration and expiration. The mean duration of each phase has been found to vary considerably between individuals of the same species , as well as between species . Eklund et al. (2010) found that phase durations were significantly longer in cheetahs than in the domestic cat. However, values of mean durations of each phase are debated in the literature: some results report ingressive phases (inspiration) to be considerably longer than egressive ones (expiration) , whereas others report expiration phases to be significantly longer than inspiration phases .
The fundamental frequency of purring in felids (measured in Hz), although different across Felidae species, has a range of 16-28 Hz . (see Table 1, reproduced from Peters 2002, for fundamental frequencies for several purring species). The fundamental frequency of purring is calculated from the vibrations per second of purring averaged over inspiration and expiration, in combination with mechanical activation of laryngeal muscles at the measured frequency. The frequency at mid-expiration exceeds that at mid-inspiration by 2.4 Hz. While purring can occur simultaneously with other vocalization, purring frequency is significantly lower than the voice frequency (meowing).
In a comparative acoustic analysis study of purring between cheetahs and domestic cats it was found that purring frequency is not correlated to body weight or size in mammals . This finding has also been reported by Sissom et al for pumas, cheetahs and domestic cats .
Table 1. Fundamental frequencies for several purring species. Source: Ref. 5
Figure 3. Antero-lateral view of the structure of the human larynx, which is similar to the larynx in Felidae species. Source
Figure 4. Laryngoscopic (dorsal) view of the vocal folds
and interior of the human larynx.Source