Question: The team set out to investigate whether the vibrato rate of classical singers showed significant acoustic change following vocal warm-ups.
Results: After completing a 25 minute vocal warm-up singers with a high vibrato rate had a significant reduction in rate; additionally singers with slower rates had an increase in vibrato rate.
Thoughts: Unfortunately this study does not include details about the 25 minute warm-up procedure that was followed. Twenty-five minutes is a significant warm-up and could cause vocal fatigue in singers. Additionally vibrato rate and extent preferences change over time and by genre. If you have a singer that has an uncommonly fast vibrato rate it may be worth trying an extended warm-up; however, without the warm-up procedure mileage will likely vary significantly.
Reference: Moorcroft, L., & Kenny, D. T. (2012). Vocal warm-up produces acoustic change in singers' vibrato rate. Journal of voice : official journal of the Voice Foundation, 26(5), 667.e13–667.e6.67E18. https://doi.org/10.1016/j.jvoice.2011.10.007