Pitch and loudness are subjective properties of sound. Pitch is closely correlated with frequency, and loudness is closely correlated with amplitude. However, under certain circumstances, slight changes of pitch can be created by changes of amplitude, and changes of loudness can be created by changes of frequency. The ear's uneven response is part of the explanation for these latter phenomena. In the pitch standard known as concert pitch, the note A4 (the A above middle C) is set to a frequency of 440 Hz.