Montreal Forced Aligner


January 22, 2023



The Montreal Forced Aligner is a forced alignment system with acoustic models built using the Kaldi ASR toolkit. A major highlight of this system is the availability of pretrained acoustic models and grapheme-to-phoneme models for a wide variety of languages, as well as the ability to train acoustic and grapheme-to-phoneme models to any new dataset you might have. It also uses advanced techniques for training and aligning speech data (Kaldi) with a full suite of training and speaker adaptation algorithms. The basic acoustic model recipe uses the traditional GMM-HMM framework, starting with monophone models, then triphone models (which allow for context sensitivity, read: coarticulation), and some transformations and speaker adaptation along the way. You can check out more regarding the recipes in the MFA user guide and reference guide; for an overview to a standard training recipe, check out this Kaldi tutorial.

As a forced alignment system, the Montreal Forced Aligner will time-align a transcript to a corresponding audio file at the phone and word levels provided there exist a set of pretrained acoustic models and a pronunciation dictionary (a.k.a. lexicon) of the words in the transcript with their canonical phonetic pronunciation(s).

The current MFA download contains a suite of mfa commands that allow you to do everything from basic forced alignment to grapheme-to-phoneme conversion to automatic speech recognition. In this tutorial, we’ll be focusing mostly on those commands relating to forced alignment and a side of grapheme-to-phoneme conversion.

Very generally, the procedure for forced alignment is as follows:

  • Prep audio file(s)
  • Prep transcript(s) (Praat TextGrids or .lab/.txt files)
  • Obtain a pronunciation dictionary
  • Obtain an acoustic model
  • Create an input folder that contains the audio files and transcripts
  • Create an empty output folder
  • Run the mfa align command


You can find the streamlined installation instructions on the main MFA installation page. The majority of users will be following the “All platforms” instructions.

As listed there, you will need to download Miniconda or Anaconda. If you’re trying to decide between the two, I’d probably recommend Miniconda. Compared to Anaconda, Miniconda is smaller, and has a slightly higher success rate for installation. Conda is a package and environment management system. If you’re familiar with R, the package management system is similar in concept to the R CRAN server. Once installed, you can import sets of packages via the conda commands, similar to how you might run install.packages in R. The environment manager aspect of conda is fairly unique, and essentially creates a small environment on your computer for the collection of imported packages (here the MFA suite) to run. The primary advantage is that you don’t have to worry about conflicting versions of the same code package on your computer; the environment will be a bubble in your computer with the correct package versions for the MFA suite.

Once you’ve installed Miniconda/Anaconda, you’ll run the following line of code in the command line. For Mac users, the command line will be the terminal or a downloaded equivalent. For Windows users, I believe you will run this directly in the Miniconda console.

conda create -n aligner -c conda-forge montreal-forced-aligner

The -n flag here refers to the name of the environment. If you want to test a new version of the aligner, but aren’t ready to overwrite your old working version of the aligner, you can create a new environment using a different name after the -n flag. See @ref(Tips and tricks) for more on conda environments. If it’s worked, you should see something like:

# To activate this environment, use
#     $ conda activate aligner
# To deactivate an active environment, use
#     $ conda deactivate

And with that, the final step is activating the aligner. You will need to re-activate the aligner every time you open a new shell/command line window. You should be able to see which environment you have open in the parentheses before each shell prompt. For example, mine looks like:

(base) Eleanors-iPro:Documents eleanorchodroff$ conda activate aligner
(aligner) Eleanors-iPro:Documents eleanorchodroff$ 

You can run the aligner from any location on the computer as long as you’re in the aligner environment on the command line.

NB: The Montreal Forced Aligner is now located in the Documents/MFA folder. All acoustic models, dictionaries, temporary alignment or training files, etc. will be stored in this folder. If you are ever confused or curious about what is happening with the aligner, just poke around this folder.

Running the aligner

To run the aligner, we’ll need to follow this procedure:

  • Prep audio file(s)
  • Prep transcript(s) (Praat TextGrids or .lab/.txt files)
  • Obtain a pronunciation dictionary
  • Obtain an acoustic model
  • Create an input folder that contains the audio files and transcripts
  • Create an empty output folder
  • Run the align command

In this section, I will give a quick example of how this works using a hypothetical example of aligning American English. Through this, you’ll see the primary mfa commands that you’ll need for alignment. Then in the following sections, I’ll go into more detail about each of these steps.

For the first steps of prepping the audio files and prepping the transcripts, I’ll assume we’re working with a wav file and a Praat TextGrid that has a single interval tier with utterances transcribed in English. We can have multiple intervals per TextGrid. The wav file and TextGrid must have the same name.

After we have the audio files and transcript TextGrids, we’ll place them in an input folder. For the sake of this tutorial, this input folder will be called Documents/input_english/.

We will also need to create an empty output folder. I will call this output folder Documents/output_english.

We can then obtain the acoustic model from the internet (yes, you’ll have to be online the first time you do this) using the following command:

mfa model download acoustic english_us_arpa

(You can also use mfa models download acoustic english_us_arpa.)

And we can obtain the dictionary from the internet using this command:

mfa model download dictionary english_us_arpa

(You can also use mfa models download dictionary english_us_arpa.)

The dictionary and acoustic model can now be found in their respective Documents/MFA/pretrained_models/ folders.

Once we have the dictionary, acoustic model, wav files and TextGrids in their input folder, and an empty output folder, then we’re ready to run the aligner!

The alignment command is called align and takes 4 arguments:

  • path to the input folder
  • name of the dictionary (in the Documents/MFA/pretrained_models/dictionary/ folder) / path to the acoustic model if it’s elsewhere on the computer
  • name of the acoustic model (in the Documents/MFA/pretrained_models/acoustic/ folder) / path to the acoustic model if it’s elsewhere on the computer
  • path to the output folder

I always add the optional --clean flag as this “cleans” out any old temporary files created in a previous run. (If you’re anything like me, you might find yourself re-running the aligner a few times, and with the same input filename. The aligner won’t actually re-run properly unless you clear out the old files.)

Important: You will need to scroll across to see the whole line of code.

mfa align --clean /Users/Eleanor/Documents/input_english/ english_us_arpa english_us_arpa /Users/Eleanor/Documents/output_english/

And if everything worked appropriately, your aligned TextGrids should now be in the Documents/output_english/ folder!

File preparation

Audio files

The Montreal Forced Aligner is incredibly robust to audio files of differing formats, sampling rates and channels. You should not have to do much prep, but note that whatever you feed the system will be converted to be a wav file with a sampling rate of 16 kHz with a single (mono) channel unless otherwise specified (see Feature Configuration. For the record, I have not yet tried any other file format except for wav files, so I’m not yet aware of potential issues that might arise there.


The MFA can take as input either a Praat TextGrid or a .lab or .txt file. I have worked most extensively with the TextGrid input, so I’ll describe those details here. As for .lab and .txt input, I believe this method only works when the transcript is pasted in as a single line. In other words, I don’t think it can handle time stamps for utterance start and end times.


The filename of the wav file and its corresponding transcript must be identical except for the extension (.wav or .TextGrid). If you have multiple speakers in the alignment or training, you can actually implement some degree of speaker/channel adaptation. You can do this either by placing speaker-specific files in its own subfolder within the input folder, or by adding an optional argument to the mfa align command to use the first n characters of the filename.

If you go forward with this, it helps to have the speaker ID as the prefix to the filenames. For example:

spkr01_utt1.wav, spkr01_utt1.TextGrid  
spkr01_utt2.wav, spkr01_utt2.TextGrid  
spkr02_utt1.wav, spkr02_utt1.TextGrid  
spkr02_utt2.wav, spkr02_utt2.TextGrid
spkr03_utt1.wav, spkr03_utt1.TextGrid  
spkr03_utt2.wav, spkr03_utt2.TextGrid  
spkr04_utt1.wav, spkr04_utt1.TextGrid  
spkr04_utt2.wav, spkr04_utt2.TextGrid

In this case, we can then tell the aligner to use the first 6 characters as the speaker information. The -s flag stands for speaker characters.

mfa align -s 6 --clean /Users/Eleanor/input_english/ english_us_arpa english_us_arpa /Users/Eleanor/output_english/


mfa align --speaker_characters 6 --clean /Users/Eleanor/input_english/ english_us_arpa english_us_arpa /Users/Eleanor/output_english/

Input and output folders

I would recommend creating a special input folder that houses a copy of your audio files and TextGrid transcripts. In case something goes wrong, you won’t be messing up the raw data. You can place this folder basically anywhere on your computer, and you can call this whatever you want.

You will also need to create an empty output folder. I recommend making sure this is empty each time you run the aligner as the aligner does not overwrite any existing files. You can place this folder basically anywhere on your computer, and you can call this whatever you want; however, you may not re-use the input folder as the output folder.

Obtaining acoustic models

Download an acoustic model

Pretrained acoustic models for several languages can be downloaded directly using the command line interface. This is the master list of acoustic models available for download.

Train an acoustic model

You can also train an acoustic model yourself directly on the data you’re working on. You do need a fair amount of data to get reasonable alignments out. If an existing acoustic model already exists, it might be worth simply using that directly on your data or you could try adapting the model to your data. There are many cases, however, when training a new acoustic model is the best path forward.

The mfa train command takes 3 arguments:

  • path to the input folder with the audio files and utterance-level transcripts (TextGrids)
  • name of the pronunciation dictionary in the pretrained_models folder or path to the pronunciation dictionary
  • path to the output folder (it still runs an alignment at the end)

In other words, mfa train has the same syntax and requirements as the mfa align command, but without the acoustic model specification.

mfa train --clean ~/Documents/input_spanish ~/Documents/talnupf_spanish.txt ~/Documents/output_spanish_textgrids

Once again, I’m using the --clean flag just in case I need to clean out an old folder in Documents/MFA.

If you would like to reuse the acoustic model, you can find all of the trained models (there are several – one for each layer of training) in the associated folder in Documents/MFA/ and any folder ending in _ali. I would recommend using the acoustic model from the last training and alignment pass, which is the sat2_ali folder. sat2_ali stands for the second round of Speaker Adaptive Training — alignment pass.

The mfa model save acoustic command takes one required argument, which is the path to the model you want to save, and an optional (but highly recommended) argument which is the new name of the acoustic model. The optional argument is specified after the --name flag:

mfa model save acoustic --name guarani_cv ~/Documents/MFA/mfainput4guarani/sat3_ali/ 

Obtaining dictionaries

The pronunciation dictionary must be a two column text file with a list of words on the left-hand side and the phonetic pronunciation(s) on the right-hand side. Each word should be separated from its phonetic pronunciation by a tab, and each phone in the phonetic pronunciation should be separated by a space. Many-to-many mappings between words and pronunciations are permitted. In fact, you can even add pronunciation probabilities to the dictionary, but I have not yet tried this!

One important point: the phone set in your dictionary must match that used in the acoustic models and the orthography must match that in the transcripts.

There are a few options for obtaining a pronunciation dictionary:

Download a dictionary

This is the master list of pre-existing pronunciation dictionaries available through the MFA. Click on the dictionary of interest, and if you scroll to the bottom of the page, it will tell you the name to type in the mfa model download dictionary command.

mfa model download dictionary spanish_latin_america_mfa

NB: you must add any missing words in your corpus manually, or train a G2P model to handle these cases.

Generate a dictionary using a G2P model

Grapheme-to-phoneme (G2P) models automatically convert the orthographic words in your corpus to the most likely phonetic pronunciation. How exactly it does this depends a lot on the type of model and its training data.

The MFA has a handful of pretrained G2P models that you can download. This is the master list of G2P models available for download.

mfa model download g2p bulgarian_mfa

You’ll then use the mfa g2p command to generate the phonetic transcriptions from the submitted orthographic forms and the g2p model. The mfa g2p command takes 3 arguments:

  • name of g2p model (in Documents/MFA/pretrained_models/g2p/)
  • path to the TextGrids or transcripts in your corpus
  • path to where the new pronunciation dictionary should go
mfa g2p --clean bulgarian_mfa ~/Desktop/romanian/TextGrids_with_new_words/ ~/Desktop/new_bulgarian_dictionary.txt

You can also use an external resource like Epitran or XPF to generate a dictionary for you. These are both rule-based G2P systems built by linguists; these systems work to varying degrees of success.

In all cases, you’re best off checking the output. Remember that the phone set in the dictionary must entirely match the phone set in the acoustic model.

Train a G2P model

You can also train a G2P model on an existing pronunciation dictionary. Once you’ve trained the G2P model, you’ll need to jump back up to the generate dictionary instructions just above. This might be useful in cases when you have many unlisted orthographic forms that are still in need of a phonetic transcription.

The mfa train_g2p command has 2 arguments:

  • path to the training data pronunciation lexicon
  • path to where the trained G2P model should go
mfa train_g2p ~/Desktop/romanian/romanian_lexicon.txt ~/Desktop/romanian/romanian_g2p

Create the dictionary by hand

This one is self-explanatory, but once again, make sure to use the same phone set as the acoustic models.

Tips and tricks

  • Add -h after any command to get a help screen that will also display its argument structure. For example:
mfa align -h
mfa train -h
  • Use the --clean flag each time you run the align command
  • Don’t forget to activate the aligner
  • Make sure the output folder is empty each time you run the align command
  • The input and output folders must be different folders
  • Many users have trouble reading/writing files to the Desktop folder. If you’re having issues using the Desktop, just switch to a different folder like Documents, or Google how to change the read/write permissions on your Desktop folder
  • And a little more on conda:

If you have created multiple environments, you can list all of your environments using the following command:

conda info --envs

You can delete an environment with the following command, where ENV_NAME is the name of the environment, like aligner. Make sure that you have deactivated the environment before deleting it.

conda deactivate
conda env remove -n ENV_NAME
  • You can inspect the details of any local acoustic model or dictionary using the mfa model inspect commands. One of the important outputs of this is the assumed phone set:
mfa model inspect acoustic english_us_arpa
mfa model inspect dictionary english_us_arpa
  • You can get a list of the local acoustic models and dictionaries in your Documents/MFA/pretrained_models/ folders using the mfa model list commands:
mfa model list acoustic
mfa model list dictionary
  • You can check for problems with your corpus using the validate command. Such problems might include words in your TextGrids that are not listed in your dictionary, phones in your dictionary that do not have corresponding acoustic models, as well as general problems with your sound file (like a missing waveform – this has happened to me when an audio file got corrupted during download). Note that checking for audio file problems takes a lot longer, so I frequently add the optional --ignore_acoustics flag.

The three arguments required by the validate command are:

  • path to the corpus
  • name of the dictionary
  • name of the acoustic model

If you don’t yet have an acoustic model (i.e., you’re running the validate command on the data you’re using to train an acoustic model), just put in any acoustic model as a placeholder and ignore any corresponding warnings about phones in your dictionary that are not in the acoustic model.

mfa validate ~/Documents/myCorpus/mfainput4english/ english_us_arpa english_us_arpa 

mfa validate ~/Documents/myCorpus/mfainput4english/ english_us_arpa english_us_arpa --ignore_acoustics

words that are not in your dictionary and problems with your sounds

  • And finally, there is even more to the MFA than just what I’ve put here! Definitely poke around the MFA user guide to learn more and find special functions that might make your workflow even easier.


CC-BY-SA 4.0


BibTeX citation:
  author = {Chodroff, Eleanor},
  title = {Montreal {Forced} {Aligner}},
  series = {Linguistics Methods Hub},
  date = {2023-01-22},
  url = {},
  doi = {10.5281/zenodo.7591607},
  langid = {en}
For attribution, please cite this work as:
Chodroff, Eleanor. 2023, January 22. Montreal Forced Aligner. Linguistics Methods Hub. ( doi: 10.5281/zenodo.7591607