Acoustic Phonetics Workshop
Phonetics is the study of the physical properties of speech. This may mean the articulatory properties, that is, how your vocal tract is shaped during an utterance or the placement of your tongue. Or, the physical properties of speech may be an utterance’s acoustic properties, which are best represented visually as waveforms or spectrograms. In this workshop, we’ll use Praat to look at waveforms and spectrograms to identify the phonemes—the consonant and vowel sounds—in an utterance.
Praat is a free software program for phonetic analysis built by Paul Boersma and David Weenink from the University of Amsterdam. It is available free for download for Mac, Windows, Linux, and Unix from www.praat.org. This site also contains links to online manuals and users’ groups.
In this workshop we will work on phonemes in the context of a sentence spoken by a woman, The man in the cab wore a cap. The analysis we will do will demonstrate how to find and then define phonemes using waveforms and spectrograms. You will be comparing a minimal pair, /kæp/ and /kæb/
The purpose of the lab is to help you learn to use Praat to recognize the physical structure of stop consonants and vowels to examine and define the boundaries of phonemes, something we can do in one hour.
The Objects Window
Open Praat by going to the Start menu, selecting ProgramsPraat. Two windows will open: the left is the Objects window, and the right is the Picture window. You may close the picture window if you like, because we won’t use it immediately.
Our objects window is empty because we haven’t yet imported any sound files. It’s important to make a distinction between objects and files because nothing in the objects window is saved until you save it yourself. Thus, if you extract selections of the longer sound or make a text-grid (we’ll come back to this later) you must save it before you close the program, because it won’t be there the next time you want to work on it.
To get some objects in your window, select Read from file… from the Read menu. Browse for your student folder on Masu, where you copied the files from the Evolving_Comm folder. (If you need help finding your folder, please ask). Select the .wav file: the man in the cab. This file should now appear as an Object in the window, and suddenly, the buttons along the bottom of the window become available and options like Edit, Play, and Annotate appear to the right of the Object.
The Edit Window
While the Object is selected, click Edit and a new window will open. You should be able to see a waveform and a spectrogram. A waveform measures amplitude (in decibels) over time; it appears here as a black squiggly line on a white background. A spectrogram measures frequency (in Hertz) and appears mostly gray with darker patches, which show the intensity of the sound. The spectrogram probably looks familiar because of the workshop you did on animal calls using Raven Lite. If you don’t see the spectrogram, select SpectrumShow Spectrogram. If Pitch, Formants, or Intensity are displayed, turn these off using the drop down menus at the top of the window. Make your spectrogram easier to interpret by increasing the contrast between the lighter and darker parts. Select SpectrumSpectrogram settings… A dialog box will open. Click the button called Standards then click OK. Practice selecting segments of the waveform by clicking and dragging the cursor through the waveform. You can listen to any part of the wave by pressing the buttons below the spectrogram window. The bottom bar plays the whole sound, the middle bar plays the visible portion, and the top bar plays the portion that you have selected.
Zoom in on your selection by pressing the sel button on the bottom left of the window. This allows you to see the structure of the waveforms and spectrograms more clearly. Zoom out to the whole sound by pressing all. In and Out are nice, too, for looking at the sound at various levels of detail.
Let’s look at the sounds in cab and cap. First we’ll extract these sounds from the long file, and make them objects of their own. To do this, find the boundaries of the word cab and select it. You probably want to zoom in on the area using the In button so that you can more easily see the edges. Once you’ve isolated the word and selected it, choose FileExtract selected sound (preserve times). Check your Objects window. We now have an object called “Sound untitled.” Select it and rename it cab. (The Rename button is below the white space in the Objects window). Repeat all this for cap. We should probably save these files to your student folder, so that you can work on them later. Select the Object, then choose WriteWrite to WAV file.
Now select the object Sound cab and click Edit. Let’s make some observations:
1. We know that certain voiceless stops (e.g. /p, t, k/) in English are aspirated at the beginning of a syllable. That is, a little puff of air is released. Can you see it in the waveform? How about the spectrogram? Describe what you see. Remember that stop means that the vocal tract is closed at a certain point so that no sound comes through
2. Compare the waveform of the sound [kh] with wave of the vowel sound [æ]. (Zoom in a lot so that the wave is stretched out rather than all squiggly on top of itself). What do you notice about the vowel sound’s waveform? How about the spectrogram? Record, too, the duration of the sound. (Time is on the x-axis, and when the vowel is selected, the total duration in seconds will appear on one of the bars below the spectrogram). Open an Edit window for cap and compare the duration of the vowel sounds.
3. How about the /b/ sound at the end of the word? This is a stop, too, but this time, it’s voiced. How does the waveform change when the speaker says /b/ as compared to when she says /p/ at the end of cap? Does the voiceless /p/ in cap also show voicing? Why might it be voiced? Why not? Is this /p/ aspirated?
The Annotation Window
Close all your Edit Windows because it’s probably a big mess on your screen. Select one of your objects to Annotate. If you’re feeling ambitious and like you have all the time in the world, pick the man in the cab. You know what certain consonants and vowel sounds look like now, so it shouldn’t be too difficult to pick out those sounds in other forms. (Except wore. That sound looks nasty and it’s really hard to see any boundaries at all because of the kind of consonants and vowels in that word). It’s ok, though, to select cab or cap. Click AnnotateTo Text Grid.
A dialogue box will open. The default values are “Mary John bell.” These are totally meaningless and we want to get rid of them. We want two tiers, phonemes and words, so that we can describe the sound and the word boundaries. Type those into the All tier names box, separated by spaces. Delete “bell” from the box below it, and leave it blank. (Point tiers are for things like pulses, that is, things that have no duration. We don’t care about that in this workshop). When you click Ok a new Object called Text Grid whatever appears in the Objects Window.
When we open it to annotate, we want to see the sound along with it, so select both the Text Grid and it’s corresponding Sound Object by holding Ctrl while you click. With both Objects selected, click Edit.
Now you should see the waveform, the spectrogram, and two tiers, one labeled phonemes and one labeled words. Zoom in to more clearly see the sound boundaries in the waveform and the spectrogram and click on a boundary in either of those windows. A red line is drawn vertically through the whole window, and circles appear at the top of both the Phoneme Tier and the Word Tier. Click the circle in the Phoneme Tier and you’ve created a boundary for that phoneme. Repeat for all the phonemes in the word or phrase. In the Word Tier, you’ll want to create word boundaries so if you are working with a single word, the boundaries will just be at the beginning and the end. For the longer phrase, you’ll have more boundaries.
Let’s label the phonemes. We’ll be using the IPA, the font for which is installed on all the Language Lab computers. If you use Praat at home, you probably need to download and install the free fonts. Instructions for doing so are at the Praat website, www.praat.org. A weird thing about Praat is that it doesn’t allow you to change the font as you would in a word processing program, so the creators have made keyboard shortcuts to all the sounds. The full list is in the Praat Help menus, but I’ve included them here so you don’t have to find them. For cab or cap, you’ll only need three sounds, /k, æ/ and /b/ or /p/. To type æ, use this shortcut: \ae. For the man in the cab, you’ll need more, so ask me if you have decided to try it.
To type in your phonemes, click between the boundaries you’ve created and you’ll see a cursor at the top of the window. Type the phoneme and it will appear in the Phoneme Tier. Do this for the Word Tier as well. In the Word Tier, you can use the IPA or standard English spelling. When you’re done with your Text Grid, save it to your student folder by selecting it in the Objects Window and clicking WriteWrite to text file.
The Picture Window
Now we’ll make a drawing of our annotated Object. Select both the Text Grid and the Sound in the Objects Window and click Draw…. A dialogue box will open, and this time, the defaults are fine, so click OK. You now have a drawing in the Picture Window. This drawing can be manipulated in all sorts of ways; e.g. now you can change the font. Don’t try to do anything to it, though, because your drawing is fine as it is. I mention it here so that you know what is possible. You can also create a space for another drawing by clicking elsewhere in the Picture Window and stretching the pink box to the size that you want. That way, you can compare to waveforms side by side. All we’ll do today though is save the drawing so that it can be included in a Word document.
There are two options for saving:
1. In the Picture Window, select FileCopy to Clipboard. Paste it into a Word document and save from there.
2. Select FileWrite to Windows metafile. It will save to your student folder, and you can insert it into a Word document later. This option didn’t work on my home computer, but I’ve had success with it on Evergreen systems.
Why does this work matter?
The copy and paste method is easiest, and proven to work everywhere, so let’s do that. Type up the observations you made earlier in the same Word document and turn the whole thing in Monday Feb. 25. Make sure to put your name(s) on it.
While this is a tedious process, it’s actually very useful, and we’ve barely scratched the surface of what Praat can do. This kind of acoustic phonetic analysis is important because we can use the pictures to tell a story about English or any other language. For example, we compared vowel length in the context of an utterance, which can be useful to the description of a language. Praat has statistical functions that allow us to take the mean values of a certain sound as pronounced by a single speaker or many speakers, and using these functions, we may be able to say with more certainty, “the vowel sound /æ/ in American English looks like this:______.” That’s how textbooks are written and how there came to be a model for phonetic vowel charts that describe quality. We don’t know everything there is to know about English, and we certainly don’t all know Russian, or Japanese, or Tok Pisin, so if we were doing field work on any of these languages, we could use Praat to check out intonation patterns, vowel sounds, allophonic variations, and much more to more accurately describe the language.
Not only can we use Praat for phonetic description, we can also apply that description in uncovering the differences between the cognitive and physical realities of speech. We perceive speech through the sounds of a language, but we also bring a wealth of contextual knowledge of spelling, meaning, and the social world into our interpretation. The consonant in the middle of the words rider and writer, for example, are pronounced the same way in American English. The correct interpretation of this word, then, depends on context, as well as—in some varieties of English—vowel length and quality. Yet some people might claim they are pronouncing a [t] in writer. By using Praat, we can see that the sounds are the same and make inferences about the cognitive effects of communication from the physical ones.
This kind of work also has applications in sociolinguistics and discourse analysis. Through analysis of sound variations, we can track language change and accents, which have major social implications at a global level. We can also use Praat to discover, for example, the precise onset of overlaps in discourse, intonation contours, and duration of pauses, to assess and analyze what is going on in a conversation at the local level, and in this way, maybe figure out exactly how it is that we understand one another at all.