NeMo has scripts to convert several common ASR datasets into the format expected by the nemo_asr collection. You can get started with those datasets by following the instructions to run those scripts in the section appropriate to each dataset below.

If you have your own data and want to preprocess it to use with NeMo ASR models, check out the Preparing Custom Speech Classification Data section at the bottom of the page.


Freesound is a website that aims to create a huge open collaborative database of audio snippets, samples, recordings, bleeps. Most audio samples are released under Creative Commons licenses that allow their reuse. Researchers and developers can access Freesound content using the Freesound API to retrieve meaningful sound information such as metadata, analysis files, and the sounds themselves.


Go to <NeMo_git_root>/scripts/freesound_download_resample and follow the below steps to download and convert freedsound data into a format expected by the nemo_asr collection.

  1. We will need some required libraries including freesound, requests, requests_oauthlib, joblib, librosa and sox. If they are not installed, please run pip install -r freesound_requirements.txt

  2. Create an API key for at

  3. Create a python file called and add lined api_key = <your Freesound api key> and client_id = <your Freesound client id>

  4. Authorize by run python –authorize and visit the website and paste response code

  5. Feel free to change any arguments in such as max_samples and max_filesize

  6. Run bash <numbers of files you want> <download data directory> <resampled data directory> . For example:

bash 4000 ./freesound ./freesound_resampled_background

Note that downloading this dataset may take hours. Change categories in to include other (speech) categories audio files. Then, you should have 16khz mono wav files in <resampled data directory>.

Google Speech Commands Dataset

Google released two versions of the dataset with the first version containing 65k samples over 30 classes and the second containing 110k samples over 35 classes. We refer to these datasets as v1 and v2 respectively.

Run the script to process Google Speech Commands dataset in order to generate files in the supported format of nemo_asr, which can be found in <NeMo_git_root>/scripts/dataset_processing/. You should set the data folder of Speech Commands using --data_root and the version of the dataset using --data_version as an int.

You can further rebalance the train set by randomly oversampling files inside the manifest by passing the –rebalance flag.

python --data_root=<data directory> --data_version=<1 or 2> {--rebalance}

Then, you should have train_manifest.json, validation_manifest.json and test_manifest.json in the directory {data_root}/google_speech_recognition_v{1/2}.


You should have at least 4GB or 6GB of disk space available if you use v1 or v2 respectively. Also, it will take some time to download and process, so go grab a coffee.

Each line is a training example.

{"audio_filepath": "<absolute path to dataset>/two/8aa35b0c_nohash_0.wav", "duration": 1.0, "label": "two"}
{"audio_filepath": "<absolute path to dataset>/two/ec5ab5d5_nohash_2.wav", "duration": 1.0, "label": "two"}

Speech Command & Freesound for VAD

Speech Command & Freesound (SCF) dataset is used to train MarbleNet in the paper. Here we show how to download and process it. This script assumes that you already have the Freesound dataset, if not, have a look at Freesound. We will use the open-source Google Speech Commands Dataset (we will use V2 of the dataset for SCF dataset, but require very minor changes to support V1 dataset) as our speech data.

These scripts below will download the Google Speech Commands v2 dataset and convert speech and background data to a format suitable for use with nemo_asr.


You may additionally pass --test_size or --val_size flag for splitting train val and test data.

You may additionally pass --seg_len flag for indicating the segment length. Default is 0.63s.

You may additionally pass a -rebalance_method='fixed|over|under' at the end of the script to rebalance the class samples in the manifest.

  • ‘fixed’: Fixed number of sample for each class. Train 5000, val 1000, and test 1000. (Change number in script if you want)

  • ‘over’: Oversampling rebalance method

  • ‘under’: Undersampling rebalance method

mkdir './google_dataset_v2'
python --out_dir='./manifest/' --speech_data_root='./google_dataset_v2'--background_data_root=<resampled freesound data directory> --log --rebalance_method='fixed'

After download and conversion, your manifest folder should contain a few json manifest files:

  • (balanced_)background_testing_manifest.json

  • (balanced_)background_training_manifest.json

  • (balanced_)background_validation_manifest.json

  • (balanced_)speech_testing_manifest.json

  • (balanced_)speech_training_manifest.json

  • (balanced_)speech_validation_manifest.json

Each line is a training example. audio_filepath contains path to the wav file, duration is duration in seconds, offset is offset in seconds, and label is label (class):

{"audio_filepath": "<absolute path to dataset>/two/8aa35b0c_nohash_0.wav", "duration": 0.63, "label": "speech", "offset": 0.0}
{"audio_filepath": "<absolute path to dataset>/Emergency_vehicle/id_58368 simambulance.wav", "duration": 0.63, "label": "background", "offset": 4.0}

Preparing Custom Speech Classification Data

Preparing Custom Speech Classification Data is almost identical to Preparing Custom ASR Data.

Instead of text entry in manifest, you need label to determine class of this sample

Tarred Datasets

Similarly to ASR, you can tar your audio files and use ASR Dataset class TarredAudioToClassificationLabelDataset (corresponding to the AudioToClassificationLabelDataset) for this case.

If you would like to use tarred dataset, have a look at ASR Tarred Datasets.