Studies and Projects

Studies and Projects

Although this book is not an academic thesis, and is not intended to be one, I cannot help myself from sharing some of the results of my earlier research on cat sounds. You will see that it really is not so hard to conduct a small study. A phonetician’s best tools are their own ears, followed by an audio recorder (a smartphone or a video camera which allows you to record sound, for example), which make it possible to repeatedly and carefully listen to the same sound. That alone is enough to set you on the trails of some of the mysteries of the language of cats. You can learn a lot about some of the phonetic characteristics of the various sounds and their possible meanings this way.

In this chapter, I will present my previous phonetic studies to you. I conducted them in my free time, in the evenings and on the weekends, as during the day I am busy with my job as a teacher and researcher in phonetics at Lund University. My primary goals were to satisfy my own curiosity and to learn more about these cat sounds, which were so mystical to me back when I started.

With my descriptions of the studies, I would like to inspire you to try doing some research yourself—everyone can investigate cat sounds using simple methods and a little interest, time and patience. If it is too academic or too boring for you, no problem. You can easily skip this chapter and still understand the rest of the book.

My First Study: Phonetic Characteristics of Purring

I already mentioned the conference in 2010 where Dr. Robert Eklund held a lecture on his studies comparing the purring of a house cat to the purring of a cheetah, and discovered a great number of phonetic similarities. His lecture made me realize for the first time that maybe I could also contribute something to the research on cat sounds.

Once I got home again, I got out my old video camera and recorded the purring of our cat Vincent. It is not entirely straightforward to record a purring cat. I had my video camera on constant standby, and when Vincent was lying in his favorite spot resting, I tiptoed up to him with my camera, pressed the record button and petted him gently and carefully until he began to purr. Then I carefully put my hand on his body so that I could feel the movement of his breath.

Whenever his body lifted with an inhalation, I said “up” and “in” loudly into the microphone, and when it sank again as he exhaled I said “down” and “out.” I did that so as to distinguish the exhalation phases from the inhalation phases. Then I filmed for approximately another minute without commentary, so that I could record a sufficient number of inhalation and exhalation phases as materials for my research.

When we had to put Vincent to sleep and the mischievous triplets moved in with us just a few months later, I recorded their purring with the same methods. After that, I analyzed the sound recordings together with Dr. Robert Eklund using acoustic-phonetic methods. Using a computer program for speech analysis called Praat (it can be downloaded free of charge at we measured the length (duration), volume (acoustic sound level pressure or intensity) and pitch (melody) during both inhalations and exhalations and compared the results of all four cats.

The following figure shows our analysis in Praat. In this example, we can see that the intensity (loudness or volume) and fundamental frequency (pitch contour or melody; the curve in the third pane) are higher during exhalations than while inhaling (which can be recognized by the greater amplitude in the upper two panes and the higher contour in the bottom pane). Inhalations (or the ingressive phases) are marked with an I, and exhalations (or the egressive phases) are marked with an E.

The two upper panes show the waveform of the microphone signal (the top one is filtered to show only the lower frequencies, and the second is the original signal), the third is a depiction of the frequency analysis (a spectrogram) with a fundamental frequency contour (bottom curve), while the bottom pane shows the division of the audio recording into ingressive phases (inhalations) and egressive phases (exhalations).


Acoustic analysis of purring with the speech analysis program Praat.

The results of the study showed that two of the cats purred significantly louder during the egressive phase (exhalation) than during the ingressive phase (inhalation), while there were no differences in volume between inhalations and exhalations in the other two cats. The length of phases varied significantly between cats, but all cats had significantly longer inhalations than exhalations.

The fundamental frequency of vibrations is very deep in purring (roughly between 21 and 27 Hz), and all four cats were in the same frequency range. This is in line with other studies of feline purring. Two of the animals had a significantly higher fundamental frequency during the egressive phase, while it was the other way around with one cat, and there was no real difference between inhalations and exhalations with the fourth.

As far as I am aware, our study is the very first comparative and quantitative (it uses acoustic measurements and results) investigation into the purring of house cats. Our results were partially new, and they partially confirmed the results of earlier (nonacoustic) studies. The study provides an acoustic reference point for the peaceful, satisfied purring of cats, which we can use as a basis for comparison in future investigations into other forms of purring. After all, there are a few other situations in which cats purr.

A few years ago, a team of researchers in England discovered a “cry” embedded in purring (see page 127). They traced it back to the fact that cats purr much more loudly, or even scream, when they really want something (to get their human out of bed so they can have breakfast, for example). Further studies could therefore compare other types of purring in order to determine whether the pitch, volume or length of phases is different in other kinds of purring than in the results of the investigation above.

Tip: How does your cat purr? When does it purr? Is it possible that you can even distinguish between the purring your cat does while it is relaxing and the purr that it uses as a greeting when you get home from work? Maybe you can make a contribution to the research into cat sounds by recording different variations of purring and investigating them using phonetic methods, among which the first is careful listening.

My Second Study: Friendly Cat Sounds Directed at Humans as well as Other Cats

After completing my study on purring, I simply could not stop listening to my cats with my “phonetic ears.” My curiosity had been awoken. I wanted to collect detailed information about other cat sounds so as to be able to systematize them. Thus, I started following our triplets with my video camera, recording many different situations in which they made every possible sound. After a month, I had recorded 538 sounds, and I was quite proud of this myself, as it is not easy to record a cat at the exact second when they happen to say something. You first have to observe the situations where most sounds occur and be ready with a camera and a microphone in these moments. In our house, it was mostly when the cats were being fed or getting a treat, when one of the cats wanted to play with us humans or with its siblings, when one of the cats came up to us and we greeted it with affection, or when a bird or insect caught the attention of one of the cats.

I investigated these 538 sounds carefully and tried to subdivide each sound into one of five relatively rough categories: meowing, trilling, trill-meowing, chirping and other sounds. The following table shows the number of sounds per category that I had recorded from each of the cats as well as the total number of recorded sounds. The study focused only on friendly cat sounds, so I was investigating only some of the categories that I have described in the book until now. It was one of my first studies, and I did not yet know exactly how many and which categories there might be. In the category “other sounds” I put all the sounds that did not occur often, in this case purring and longer phrases with several different sounds. I did not continue investigating this category in this study, as it contained only eleven sounds—too few to be able to draw any general conclusions about them.

Number of sounds per cat recorded over a month, listed by category.

One can learn a lot from a table like that. For example, I saw immediately that Turbo was the big chatterbox and that Donna had not said much while I was recording. So Donna had produced the fewest sounds (73), Rocky somewhat more (152) and Turbo the most (313). The most common sound was the friendly and inviting trill-meow (246), followed by the greeting sound of trilling (184), then meowing (79) and chirping (18).

When I organized the sounds into categories, I did so according to the following criteria: length in seconds and the fundamental frequency, especially the lowest (minimum), the highest (maximum) and the average (mean). What all the sounds had in common was a very large variation in melody, often between 100 and 1000 Hz. It is a much wider range than we humans normally have in our voices. Meowing had the highest average fundamental frequency at 698 Hz, while trilling had the lowest at 358 Hz. All sounds aside from the trill-meow were also frequently of similar length. Length was measured at approximately a half second. Trill-meowing was significantly longer (approximately 0.8 seconds), which can be attributed to the fact that it is a complex or combination sound.

These results showed that there was a wide frequency range in the melody within each sound category, much larger than I had imagined. They also demonstrated that our female, Donna, had a much more high-pitched voice than her brothers, which can be attributed to the fact that Donna, as a female, is smaller than her brothers, Rocky and Turbo, so she has a smaller larynx (voice box) and a smaller mouth. With us humans, too, women and children generally have more high-pitched and brighter (with acoustically high resonances) voices than men.

As my study included only some 500 sounds from three cats, I could not draw any far-reaching conclusions from my results. In order to make general statements about cat sounds, one naturally needs a great number of additional recordings of cat sounds. So that was my next goal.

This study provided an additional insight. Perhaps there was something wrong with Donna, who had produced so few sounds. I continued to observe my cats and noticed that the boys often pushed themselves to the front and claimed a large part of my attention, while Donna stayed in the background. Had I been neglecting her? Did she need more time with me, so that she would show herself and become more talkative? So I started spending more time with her. I played with her, talked to her and paid attention to her, especially when Rocky and Turbo were nearby. After a few weeks, I noticed a big difference. Donna did become more outgoing and produced significantly more sounds.

Tip: You, too, can produce a similar study about the sounds of your cat. Record them for an hour in situations where they normally communicate with sounds (when they are hungry, want to be let out, or want to say hello to other family members, human or animals). Do this over a set period of time—a week, a month, every Saturday until Christmas, or whatever works for you. You certainly know when and under what circumstances your cat “speaks.” Then listen to the recordings repeatedly and carefully. To which category does each sound belong? Count the sounds in each category and create a table, which will make the comparison easier and tell you which sounds are the most common and which sounds are the rarest in your home.

My Third Study: Chirping and Chattering

I had not yet found anything about the chirping sounds that I had recorded in my second study in any academic journal article or book. I could find only descriptions of such and similar sounds online, on websites devoted to cats and their behavior. I grew curious and wanted to learn more about these mystical sounds.

So the next winter I arranged for a veritable bird buffet just outside our kitchen window in our garden, with bird balls, sunflower seeds, apples and peanuts. The birds quickly discovered the feast and the cats naturally noticed the birds on the other side of the window and made themselves comfortable on the windowsill, so as to be able to observe better. I put my old video camera in position and sat on the couch in the adjacent living room, remote control at the ready. In this way I could make video recordings of my cats without disturbing them. Every time I saw that one of my furry roommates was sitting on the windowsill watching the birds, I recorded their chirping and chattering sounds with the help of the remote control. After three months I had collected 255 sounds and began to categorize them.

It was not as easy as I had thought it would be to categorize these truly strange sounds. Because there were relatively few descriptions of these sounds in the research literature, it was difficult to find the right name for each sound. “Chattering” seemed to be the general term for all of these sounds, but because there were a great number of different variations, it seemed suitable to organize them in subcategories with appropriate names.

I did so by looking in the dictionary for names for bird sounds. Then I chose the name for each category that best corresponded to the cat sound. “Chattering” was often described as sound produced with the lower jaw and/or teeth chattering. That is why I use the term “chattering” only for the subcategory of voiceless chattering of the teeth. Next, I selected suitable names for the other subcategories. For example, Webster’s New World Dictionary describes a “chirp” as “a short, high-pitched sound, such as the one a bird or small insect makes,” which seemed to correspond best to the typical aehh, ehh, ehh cat sound, as my cats often produced this sound with very high-pitched and bright voices. The softer variant of aehh, ehh, ehh sounds more like uyh or hew and was given the name of “tweeting.” Longer tweeting sounds with more varying melody were given the name “tweedling” or “warbling.” The following table shows the distribution of the sounds by category for each of my cats. Again, Donna was the quietest of the bird watchers, while her brothers “spoke” more to the birds. Chirping was the most common sound, with 169 examples, and the voiceless chatter was the least common, with 22 recorded sounds.

Number of recorded sounds of three cats over a three-month period in different chirping and chattering categories.

After I had categorized all 255 sounds, I could start my measurements of length and pitch. The individual chattering sounds were the shortest (roughly 0.03 seconds), chirping and tweeting were also very short (roughly between 0.15-0.20 seconds), while tweedling was the longest (approximately 0.5 seconds). The chattering sounds were all voiceless, but chirping, tweeting and tweedling all averaged around 600 Hz. Despite the short length I discovered a wide pitch range in these sounds (between 230 and 1200 Hz for chirping). These results demonstrated to me that cats can vary their vocalizations by altering the pitch and melody. I did not yet know whether this was instinct or learned behavior. Maybe further studies would help me to discover more about these variations in melody.

Tip: What sounds does your cat make when it sees a bird or an insect? Some cats only chatter, while others tend to chirp and some “speak” to the small prey animals on the other side of the glass with tweeting tones. There are also cats who produce a wide variety of chirping and chattering sounds. If you are not exactly sure, put some bird food outside the window and listen carefully to how your cat “speaks” to the birds while watching them. Or make a video recording and listen to the sounds carefully and repeatedly. Even if you do not have any desire to turn your listening into a little academic research project, you will have a lot of fun listening, as they are often strange and funny sounds.

My Fourth Study: The Phonetic Differences
between Happy and Sad Meow Sounds

Early on it became clear to me that the meow sounds my cats made differed a lot depending on the situation or context. When I took them to the vet’s and they sat in their carriers in the waiting room looking at the other humans and animals anxiously, the sounds they produced were entirely different than when they were at home hungry in the kitchen and saw that I was preparing their food or a treat. Why was that? And did only I hear this difference, because I know my cats so well (or perceive sounds more exactly with my phonetic ears), or do others also hear the same differences?

In order to investigate, I worked with Dr. Joost van de Weijer, also a linguist and a cat fancier, to conduct an experiment in which thirty human listeners were asked to judge a number of meow sounds: six sounds that were recorded in a feeding context, and six meow sounds that were recorded in a veterinarian context, arranged randomly. After each sound, we asked the listener if the sound was from a feeding situation or from a veterinarian situation. It actually could be determined that there was a different melody in different situations. The feeding-related meow sounds varied more in the melody and more often ended with a rising tone (intonation, melody) than the veterinarian-related meow sounds, which were more likely to end in a falling tone and displayed much less variation in the melody. The following diagram shows the melody of the feeding meows (above) and the veterinarian meows (below).

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