The characteristics of musical sounds help to distinguish one musical sound from another. These are pitch, londness and quality.
The term pitch is the characteristic of musical notes that enables to classify a note as high or low. It is a subjective quantity which cannot be measured by an instrument.
It depends on frequency. However, there does not exist any one-to-one correspondence between the two. A shrill, sharp or acute sound is said to be of high pitch. But a dull, grave and flat note is said to be of low pitch. Roaring of lion, though of high intensity, is of low pitch. On the other hand, the buzzing of mosquito, though of low intensity, is of high pitch.
The loudness of sound is a subjective effect of intensity of sound received by listener's ear. The intensity of waves is the average amount of energy transported by the wave per unit area per second normally across a surface at a given point.
There is a large range of intensities over which the ear is sensitive. As such, logarithmic scale rather than arithmetic intensity scale is more convenient.
It is the characteristic of sound waves which enables to distinguish between two notes of the same pitch and intensity but sounded by two different instruments.
No instrument, except a tuning fork, can emit a pure note (a note of one particular frequency).
In general, when a note of frequency n is sounded, in addition to it, notes of higher frequencies 2n, 3n, 4n ... may also be produced. These notes, have different amplitudes and phase relations. The resultant wave form of the emitted waves determines the quality of the note emitted.
Quality, like loudness and pitch is a subjective quantity. It depend on the resultant wave form.