Eklund, Robert

Schötz, Susanne

English

Lund University Publications

LUND UNIVERSITYPO Box 117221 00 Lund+46 46-222 00 00A comparative acoustic analysis of purring in four catsSchötz, Susanne; Eklund, RobertPublished in:Proceedings from Fonetik 2011, Speech, Music and Hearing, KTH, Stockholm, TMH-QPSR2011Link to publicationCitation for published version (APA):Schötz, S., & Eklund, R. (2011). A comparative acoustic analysis of purring in four cats. In Proceedings fromFonetik 2011, Speech, Music and Hearing, KTH, Stockholm, TMH-QPSR (Vol. 51). Department of Speech,Music and Hearing, KTH, Stockholm.Total number of authors:2General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portalRead more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.A comparative acoustic analysis of purring in four cats Susanne Schötz 1 & Robert Eklund 2,3,4 1 Humanities Lab, Centre for Languages and Literature, Lund, Sweden 2 Voice Provider, Stockholm, Sweden 3 Department of Cognitive Neuroscience, Karolinska Institute, Stockholm, Sweden 4 Department of Computer Science, Linköping University, Linköping, Sweden Abstract This paper reports results from a comparative analysis of purring in four domestic cats. An acoustic analysis describes sound pressure level, duration, number of cycles and fundamental frequency for egressive and ingressive phases. Significant individual differences are found between the four cats in several respects.  Introduction The domestic cat is one of the most popular pet animals in the world, and virtually everyone is familiar with its trademark “purring” sound. Contrary to what might be believed, it is not known exactly how purring is produced, and there is a surprising lack of studies of purring, even descriptive.  This paper compares a number of acoustic characteristics of purring in four domestic cats, with focus on sound pressure level, duration, number of cycles and fundamental frequency of ingressive and egressive phases. The domestic cat There are 35 to 40 felid species in the world today (Sunquist & Sunquist, 2002), and the domestic cat (Felis catus, Linneaus 1758) is by far the most well-known and common cat with an estimated number of 600 million individuals (Driscoll et al., 2009). It was long suggested that the cat was first domesticated in ancient Egypt around 3600 years ago, but it is now believed that domestication took place 10,000 years ago in the Fertile Crescent. The closest relative of the domestic cat is considered to be the African wildcat (F. silvestris lybica) (Driscoll et al., 2007; Driscoll et al., 2009). Today around 60 breeds of domestic cats are recognized (Menotti-Raymond et al., 2008). Although varying considerably in size and weight, a domestic cat normally weighs between 4 and 5 kilos, and is around 25 centimeters high and 45 centimeters long. Males are significantly bigger than females, and are on average 20% heavier than are females (Pontier, Rioux & Heizmann, 1995). Purring As mentioned above, it is not known exactly how purring is produced, and the term as such has been used quite liberally in the literature. In a major review paper Peters (2002) employed a strict definition of purring as a continuous sound produced on alternating (pulmonic) egressive and ingressive airstream. Given this definition, purring is only found in the “purring cats” (i.e. all felids but the non-purring/“roaring cats” lion, tiger, jaguar, leopard; whether or not the non-roaring snow leopard can purr remains unsettled) and in the Genet. A number of different purring theories are found in the literature. McCuiston (1966) suggested that purring was hemodynamic and that the sound consequently emanated from the bloodstream running through the thorax. This theory was proven wrong by Stogdale & Delack (1985). Moreover, both Frazer Sissom, Rice & Peters (1991) and Eklund, Peters & Duthie (2010) reported that purring maximum amplitude occurs near the mouth and nose. It has recently been suggested that purring “is caused by rapid twitching of the vocalis muscle, whereas the large pads within the vocal folds of Pantherinæ might impede rapid contractions of this muscle and thus make it difficult to purr” (Weissengruber et al., 2008:16; see also Weissengruber et al., 2002). Contrary to what is often believed, cats do not exclusively purr when they are content, but also when they are hungry, stressed, in pain or close to dying, and behaviourists have suggested that the function purring serves is to signal that the cat does not pose a threat (Eldredge, Carlson & Carlson, 2008:297). Previous research There is a surprisingly small number or papers devoted to felid purring, and several of these papers are also impressionistic in character. One of the first papers exclusively devoted to purring the domestic cat was Moelk (1944), but the focus of her paper is a classification of different kinds of purr and how they are used, and no acoustic analysis is presented. Frazer Sissom, Rice & Peters (1991) reported that domestic cats purr at a frequency of 26.5 Hz, while Eklund, Peters & Duthie (2010) reported the figure 22.6 Hz.  Remmers & Gautier (1972:359) reported that egressive phases in purring cats had a duration of 730 ms, while ingressive phases had a duration of 690 ms. Data collection Continuous calm purring was collected from the four domestic cats Donna (D; female, age 6 months, 3.0 kilos), Rocky (R; male, 6 months, 3.6 kilos), Turbo (T; male, 6 months, 3.6 kilos), and Vincent (V; male, 16 years, 5.2 kilos). All cats were recorded in a quiet home environment using a Sony DCR-PC100E digital video camera recorder with an external Sony ECM-DS70P electret condenser stereo microphone. This microphone is small in size, and could easily be held close to the muzzle without scaring or disturbing the cat.  Figure 1 shows the microphone positions during the recording sessions with the four cats.  Videos are available at http://purring.org   Figure 1. The microphone positions of all four cats during data collection.   To be able to identify egressive and ingressive phases in the recorded audio files, the first author kept her hand on the side of the cats’ chests during the recording session while saying the words “in” and “out” according to the expanding (in-breath) or collapsing (out-breath) rib cage several times during the recording sessions.  Method Data post-processing All videos were transferred to iMovie, and audio files (wav, 44.1 kHz, 16 bit, mono) of about 70 seconds for each cat were extracted with Extract Movie Soundtrack. The waveforms were normalised for amplitude with Audacity, and low-pass filtered copies were created with Praat (10–40 Hz, smoothing at 10 Hz). These copies were used together with the original normalised waveform, spectrogram and Praat’s pitch analysis to facilitate manual segmentation and counting of respiratory cycles per phase.  Figure 2 shows an example of the manual segmentation in Praat.   Figure 2. Manual segmentation of ingressive (I) and egressive (E) phases in Praat using the low pass filtered (top pane) and original (mid pane) waveforms as well as the original spectrogram and pitch contour (bottom pane).  The respiratory cycles per phase were labeled manually from the waveforms and counted with a Praat script. Figure 3 shows an example of the procedure.   Figure 3. Manual labelling of cycles (pulses) per ingressive (I) and egressive (E) phases in Praat using the low pass filtered (top pane) and original (mid pane) waveform.  Egressive–ingressive identification In order to ascertain that the egressive and ingressive phases were correctly identified, the parts of the recordings where the first author said “in” and “out” were located. Phases were then easily identified based on their distinct sound and waveform characteristics.  Analyses Analyses were carried out with Praat. Statistics were calculated with SPSS 12.0.1.  Table 1. Summary Table. For all four cats results are given for sound pressure level (SPL), durations, cycles per phase, and fundamental frequency. Results are presented independently for egressive and ingressive phases, and statistical tests are performed on differences between egressive and ingressive phonation.   Donna (D) Rocky (R) Turbo (T) Vincent (V) Phonation type Ingressive Egressive Ingressive Egressive Ingressive Egressive Ingressive Egressive No. phases analysed 39 39 40 40 61 61 61 61 Mean SPL (dB) 72.4 74.6 72.14 71.93 70.66 76.43 71.85 71.72 Mean SPL (dB) egr+ingr 73.48 72.03 73.52 71.78 Standard deviation 0.8209 1.2974 0.9614 1.7693 1.96 3.20 1.0661 1.6260 ∆ t test (paired-samples, two-tailed) p < 0.001 p = 0.427 p < 0.001 p = 0.426 ∆ Wilcoxon (two related samples) p < 0.001 p = 0.249 p < 0.001 p = 0.224 Mean duration (ms) 673 587 819 756 604 511 511 484 Mean duration egr+ingr 632 788 558 498 Standard deviation 120.80 82.70 169.23 130.05 58.90 45.09 85.10 69.72 Maximal duration 921 838 1038 997 773 634 719 614 Minimal duration 413 443 432 365 480 419 319 266 ∆ t test (paired-samples, two-tailed) p < 0.001 p = 0.011 p < 0.001 p = 0.010 ∆ Wilcoxon (two related samples) p < 0.001 p = 0.013 p < 0.001 p = 0.004 Mean no. cycles/phase 16.58 15.95 21.28 20.15 13.92 12.46 13.41 13.16 Mean no. cycles/phase egr+ingr 16.31 20.72 13.19 13.3 Standard deviation 1.41 2.25 4.33 3.56 1.99 1.20 2.52 1.93 Maximal no. phases/cycle 22 22 29 28 21 15 18 17 Minimal no. cycle/phase 10 12 11 10 10 10 9 7 ∆ t test (paired-samples, two-tailed) p = 0.178 p = 0.090 p < 0.001 p = 0.437 ∆ Wilcoxon (two related samples) p = 0.132 p = 0.073 p < 0.001 p = 0.456 Mean fundamental frequency (Hz) 24.63 27.21 26.09 26.64 23.00 24.43 23.45 20.94 Mean frequency egr+ingr (Hz) 25.94 26.36 23.72 22.2 Standard deviation 1.14 1.82 2.08 1.24 1.85 1.45 3.62 2.14 Highest fundamental frequency 27.5 33.2 33 29 27 28 28.8 24 Lowest fundamental frequency  21.6 24.2 23 24 19 20 18.2 17.1 ∆ t test (paired-samples, two-tailed) p < 0.001 p = 0.174 p < 0.001 p < 0.001 ∆ Wilcoxon (two related samples) p < 0.001 p = 0.067 p < 0.001 p = 0.002  Results Summary results are presented in Table 1 above. I. Intracat analyses We first analysed within-cat variation. Amplitude The normalised waveforms were used to extract the mean relative amplitude (SPL) in each ingressive and egressive phase for comparisons within each cat. Mean relative SPL as derived from the normalised waveforms varied between 70.66 dB (T) and 72.4 (D) in the ingressive phase and between 71.72 (V) and 76.43 (T) in the egressive phase. For two of the cats (D/T), mean SPL was significantly higher in the egressive phases than in the ingressive ones, in contrast with Moelk (1944) and Peters (1981). However, no difference in mean SPL was observed for the other two cats (R/V).  Duration Mean durations of the phases varied considerably between the four cats, ranging from 511 ms (V) to 819 ms (R) in the ingressive phase, and from 484 ms (V) to 756 ms (R) in the egressive phase. Ingressive phases were significantly longer than egressive ones in all four cats, contrary to the results reported in Remmers & Gautier (1972:359). Cycles per phase The mean number of cycles per phase varied between 13.41 (V) and 21.28 (R) for ingressive phases and between 12.46 (T) and 20.15 (R) for egressive phases.  For all cats, the mean number of cycles per ingressive phase were higher than it was per egressive phase, thus replicating the results reported in Eklund, Peters & Duthie (2010) .   Fundamental frequency All four cats showed fundamental frequencies that compare well to previous studies (Frazer Sissom, Rice & Peters, 1991; Eklund, Peters & Duthie, 2010). For the ingressive phase, mean F0 ranged from 23.00 Hz (T) to 26.09 Hz (R), while the values for the egressive phase ranged from 20.94 Hz (V) to 27.21 Hz (D). Two of the cats (D/T) had significantly higher F0 for the egressive phase as compared to the ingressive phase. One cat (V) showed the opposite pattern with significantly higher F0 in the ingressive phase, while no significant difference was found for one cat (R). II. Intercat analyses Having performed within-cat analyses, we then turned to between-cat analyses. No intercat analyses of sound pressure level were performed since these were seriously affected by individual microphone positioning. All significance tests referred to are t tests (two independent samples, equal variances assumed, two-tailed). Duration All pair-wise comparisons revealed significant differences (p < 0.001) with the exception of T/V egressive duration (p = 0.012). Cycles per phase All pair-wise comparisons revealed significant differences (p < 0.001) with the exception of T/V number of ingressive cycles (p = 0.305) and number of egressive cycles (p = 0.017). Fundamental frequency All pair-wise comparisons revealed significant differences (p < 0.001) with the exception of D/V ingressive frequency (p = 0.052), T/V ingressive frequency (p = 0.393) and D/R egressive frequency (p = 0.111). With regard to combined fundamental frequency, all pairwise comparisons were significantly different with the exception or D/R (p = 0.127). Discussion To the best of our knowledge, this paper constitutes the first comparative and quantitative report of purring in domestic cats. As was the case in Eklund, Peters & Duthie (2010), previous research was both confirmed and contradicted. The lack of quantified reports in the literature makes far-reaching conclusions difficult, but our results hint at a certain degree of variation between individual cats in how purring is manifested, even if overall figures lie within the same general range. Acknowledgements Thanks to Gustav Peters for insightful comments. References  Driscoll, C. A., J. Clutton-Brock, A. C. Kitchen & S. J. O’Brien (2009). The taming of the domestic cat. Scientific American, June 2009, 68–75. Driscoll, C. A., M. Menotti-Raymond, A. L. Roca, K. Hupe, W. E. Johnson, E. Geffen, E. H. Harley, M. Delibes, D. Pontier, A. C. Kitchener, N. Yamaguchi, S. J. O’Brien & D. W. Macdonald (2007). The Near Eastern Origin of Cat Domestication. Science 317:519–523. Eklund, R., G. Peters & E. D. Duthie (2010). An acoustic analysis of purring in the cheetah (Acinonyx jubatus) and in the domestic cat (Felis catus). In: Proceedings of Fonetik 2010, Lund University, 2–4 June 2010, Lund, Sweden, 17–22. Eldredge, D. M., Delbert G. Carlson & L. D. Carlson (2008). Cat Owner’s Home Veterinary Handbook. Third edition. Hoboken, New Jersey: Wiley Publishing. Frazer Sissom, D. E., D. A. Rice & G. Peters (1991). How cats purr. Journal of Zoology 223:67–78. McCuiston, W. R. (1966). Feline purring and its dynamics. Veterinary Medicine/Small Animal Clinician 61:562–566. Menotti-Raymond, M., Victor A. David, S. M. Pflueger, K. Lindblad-Toh, C. M. Wade, S. J. O’Brien & W. E. Johnson (2008). Patterns of molecular variation among cat breeds. Genomics 91:1–11. Moelk, M. (1944). Vocalizing In The House-Cat; A Phonetic And Functional Study. 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A comparative acoustic analysis of purring in four cats

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A comparative acoustic analysis of purring in four cats

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