Sentence:  Isshiki found that if a singer is asked to <WSD>phonate</WSD> Ee, Oo, and Ah at the same loudness he will actually make the Ah much louder than the other two. This is because he gauges loudness by the effort he is using, and in this experiment the subglottal pressure was the same for Ah and Oo, but the Ah was louder because the mouth was more open. 
Meaning of the word marked with <WSD> tags:  To use the voice to make (specific sounds).