1. The Festival Speech Synthesis System [Computer program], Version 2.1 (2010). http://www.cstr.ed.ac.uk/projects/festival/
2. The Snack Sound Toolkit [Computer program], Version 2.2.10 (2012). http://www.speech.kth.se/snack/
3. Speech Signal Processing - a small collection of routines in Python to do signal processing [Computer program] (2015). https://github.com/idiap/ssp
4. Bohm, T., Audibert, N., Shattuck-Hufnagel, S., Németh, G., Aubergé, V.: Transforming modal voice into irregular voice by amplitude scaling of individual glottal cycles. In: Acoustics 2008, Paris, France, pp. 6141–6146 (2008)
5. Blomgren, M., Chen, Y., Ng, M.L., Gilbert, H.R.: Acoustic, aerodynamic, physiologic, and perceptual properties of modal and vocal fry registers. J. Acoust. Soc. Am. 103(5), 2649–2658 (1998)