Through attention paid to the sympathetic sensations of tone, the listener may carry on mentally a running commentary on the throat actions of all those whose voices are heard. Continuing to use the word empirical in the sense thus far adopted, it may be said that the summary of the impressions conveyed in the sympathetic sensations of tone constitutes empirical knowledge of the voice. In other words, empirical knowledge of the voice is an understanding of the operations of the vocal mechanism, obtained through the attentive listening to voices. Let us consider first the running commentary on the throat action, mentally carried on by the listener. This mental commentary is an inseparable accompaniment of the listening to the voices of others, whether in speech or song. As we are concerned now only with the problem of tone-production in artistic singing, our consideration will be limited to the critical hearer's observation of the tones of singers. Let us imagine two friends to be seated side by side in the concert hall, listening to the performance of a violin sonata by an artist of about mediocre ability. Suppose one of the friends to be a highly trained musical critic, the other to be almost unacquainted with music of this class. Let us now inquire how the tones of the violin will impress these two hearers; and further, let the inquiry be limited strictly to the matter of tone, leaving out of consideration all questions of composition and rendition. As a matter of course, the tones of the violin will impress these two listeners in widely different ways. The untrained observer will greatly enjoy the beautiful tones,--supposing of course that he be gifted with a natural fondness for music. But so far as musical value is concerned, all the tones will sound to him practically alike. For the trained hearer, on the other hand, every note drawn by the performer from his instrument will have a distinct value. Some of the tones will be true in pitch and perfect in quality. Some will vary slightly from the correct pitch; others will perhaps be in perfect tune, and yet be marred in quality by faults of scratching, thinness, roughness, etc. When the two come to compare notes at the end of the performance the trained critic will be utterly unable to convey to his friend his impressions of the player's technique. Vividly clear as it is to the critic, his understanding of tonal values is lodged solely in his cultivated ear. This understanding cannot be imparted in words; it must be acquired by experience in actual listening to music.