I am doing speech recordings for an upcoming study to measure loudness and frequency of speech in people with motor disorders. We have a method for all of the recording already, but are having trouble playing a calibration tone at a known dB level. Would we do this by measuring with a sound level meter as close to the sound source as we can. We'll be using the same sound level meter to measure the calibration tone at a fixed distance from the microphone that is recording the participant.
There are some speech intelligibility metrics (e.g. Articulation Index, Speech Interference Level), that would have some reference loudness levels for speech. Check some standard acoustics texts and ASTM standards on speech intelligibility for further info.
You can use a sound calibrator like Bruel & Kjaer 4231. It's an old model. There must be new ones now.
https://www.bksv.com/en/products/transducers/acoustic/calibrators/4231
Set up the calibration tone source at equi-distance from the microphone and the sound level meter. This distance should be the same as the distance from the speaker's mouth to microphone during recording (usually ~15cm). The calibration tone is typically a 1000Hz pure tone that can be generated using Audacity (free software). See a more detailed guide by Anders Asplund here: http://www.clinsci.umu.se/digitalAssets/50/50362_andersa0902.pdf
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