## Phonetics R, Praat code in GPL3, Paper and Data to Download

This posts brings R, Praat and Python code I used to write my Phonetics MA paper, as well as the paper itself to download, plus the acquired data. I won’t go into too many details about the downloads, but I will note that I hope they will be of some use to people searching for similar things, approaches – or simply, to see how useful free and open source software is to researchers.

## R, Python, Praat Code

The R, Python, and Praat code is hosted on Github under the label r-diphthongs-sr-en (here is the zipped version, which may not be up to date, but again not too different). The software tool that that took me the most time to write was a set of scripts in R language. It was designed to load the data I acquired with Praat and to list tables and create plots (the R plots and diphthongs you can see here). The code takes length, pitch, formants and intensity of diphthongs as the input.

## Data: Diphthong Measurements, RP Speaker versus ESL Speakers

In my research I compared the lengths, formants, intensity and pitch of the selected diphthongs, as pronounced by of a group of female ESL speakers (native language Serbian), with a referent RP speaker. The data (see it here or at the above links) was extracted by using Praat TextGrids (this is how I checked them), and if you’re interested to see which methods and techniques I used to segment the files, you can see this chapter (the link to the integral paper is below). The data linked contains 8 diphthongs in 2 contexts (short/long), as recorded and pronounced by 15 ESL speakers and 1 RP speaker. The diphthongs were pronounced within 32 words (I wrote this script to select the corpus).

## MA Paper: “Pronunciation of English Diphthongs by Speakers of Serbian: Acoustic Characteristics”

The paper is titled “Pronunciation of English Diphthongs by Speakers of Serbian: Acoustic Characteristics” and the most current (but not error free) version you will find here: http://www.languagebits.com/files/ma-paper/

## So, Why Putting All This Online?

The most of the code here is tailor-made for my research, and I am aware that it cannot as-is be used in some other project. However, I believe it is a very useful heap of ideas. For example, Praat scripts and TextGrigs show some advanced tips for data extraction and control, which are backed up by a phonetic discussion about segmentation (itself a demanding task). The Python is used for corpus search and integrates a script from NTLK Toolkit to verify the sound signal annotations (as well as for the control of recording, but about that some other time). Finally, R scripts show how custom-made project is limited only by imagination, and how simple operations and filtering can significantly contribute to the final result (what I’m saying here is: don’t use Excel, learn R).

I also firmly believe that data, especially scientific (even in a such humble work, as an MA paper is), should be free, and that ideas should be free. Moreover, I have in mind Ladefoged’s words from his Phonetic Data Analysis:

After you have written everything, I hope you will publish a complete account of the work, even of it is only on your web site. Private knowledge does the world no good. … In addition, make sure that your data is stored in such way that it can be found and used by others. (p 192)

Cheers!

## Resonant frequencies and the vocal tract length

This post is about resonant frequencies of a tube, in the context of speech and the neutral vocal configuration. Two formulas are given: the first to calculate the resonant frequencies when the length is known, and second, to calculate the length when the frequency of a formant is known. Finally, there is a real-life example: a calculation of a speaker’s vocal tract length after measuring the formants in schwa.

The speech mechanism in vowels is described by a model that uses the physical properties of tubes. A tube is a simple apparatus that, if attached to a source of sound, can emit harmonic frequencies. When attached to a sound speaker at the end, the tube acts as a resonator that “has an infinite number of resonances, located at frequencies given by odd-quarter wavelength” (Kent and Read 14). The resonant frequencies of a tube closed at one end are calculated by using this formula (Johnson 96):

$Fn=\frac{(2n-1)c}{4L}$

Where n is an integer, L is the length of the tube and c is the speed of sound (about 35,000 cm/sec).

This was very interesting to me, so I decided to experiment with the formula in R language. The purpose was to calculate average frequencies of a vocal tract in the neutral configuration (a position of vocal organs where a tube without obstacles is created from the larynx to the lips). So, the formula written above in R looks like this:

freq <- ((2*i-1)*35000)/(4*tract.len)

For a given speed of sound c=35000, the formant number i and the tract length, we can calculate estimated formant values. As an example, we can insert L = 17.5 cm in the formula, the average length of human tract16 from glottis to lips (15). In this case the first formant, or the first resonance frequency, occurs at 500 Hz, the second at 1500 Hz, the third at 2500 Hz, and so on. Here is the output form R code located here:

> Resonance(17.5)
Tract length is 17.5 cm.
formant 1: 500 Hz
formant 2: 1500 Hz
formant 3: 2500 Hz
formant 4: 3500 Hz
formant 5: 4500 Hz

Of course, we can reverse the calculation; by entering formant frequency and the order of the formant we can calculate an average length:

prep <- 35000*((formant/2)-0.25)
length <- (prep/freq)

This is the result of  Length function of the code:

> Length(1000, 1)
Estimated tract length is 8.75 cm, where formant number 1 has value of 1000 Hz.

This length corresponds to vocal tract lengths measured in infants.

To make the calculations even more interesting, we can measure the frequency of the first formant of speakers, and then “calculate” the length lengths of the vocal tracts. Here is an example: we recorded a speaker and examined the sound data. Since schwa sound is pronounces in (approximately) the neutral configuration, we measured the formants where this sound (IPA: ə) was articulated. In this case, that was near the end of the word  abjured /əbˈdʒʊəd/. The first three formant values in the sample female speaker were:

Time_s   F1_Hz   F2_Hz   F3_Hz
4.633178   549.304326   1750.098455   2915.885791

If we enter 549.3 Hz in the second formula, we get:

> Length(549.304326,1)
Estimated tract length is 15.92 cm, where formant number 1 has value of 549.3043 Hz.

This is, it seems, an acceptable value for this speaker.

The measurements and image was obtained by using Praat, free phonetic software. Calculation and the code example were written in R programming language.

## Checking Praat’s TextGrids in Python

A TextGrid file contains data about intervals, segments, times etc. of the corresponding signal file (audio in wav, mp3, aif…). Because grids are in plain-text  – they can be analysed / checked / extracted  automatically, or parsed.

In case you are a linguist/phonetician you might be using Praat, a small, but very powerful, programme for phonetic analysis. Chances are have a lot of speakers and recordings. You will probably segment signals in Praat, and save the segmentation in TextGrids.

Thanks to Margaret Mitchell and Steven Bird, who contributed the parser for Praat TextGrid to Natural Language Toolkit, automated analysis is now much easier.

TextGrid parser is a part of NLTK and it is located here.

I am grateful to the authors, because they saved me a lot of time during segmentation checks. All that was needed was a Python script that uses the above code to load TextGrid content, and then write a set of checks for each file/speaker.

Checking file  03-speaker-im.TextGrid
Checking proper tier names...
Checking if tiers contain 32 items...
Checking if all tiers have valid text...
Checking if the diphthongs have pairs...
Checking if all words are present...
Checking if the words and diphthongs match...
Mismatch: "ay_l" not allowed in "dice", at position 24.
It should say "ay_s".

Here, for example, my script warned me that I have a wrong label for a diphthong in the file number 3. To spot that “manually” it would require a lot of time and attention.

I hope this post might help other researchers, and here is the Python script I wrote for my phonetic research.