Loudness Level and Frequency

Two sounds having the same intensity may not have equal loudness.
Loudness is a subjective quality that depends on
- sound pressure level;
- frequency;
- spectrum;
- duration and amplitude envelope;
- environmental conditions etc.
The phon is a unit of loudness for pure tones:
- compensates for the effect of frequency;
- sounds having the same number of phons sound equally loud;
- number of phons is the dB SPL at 1000 Hz:
  $\fbox{\parbox{5.5in}{ \begin{displaymath} \mbox{phon} \triangleq \mbox{SPL in dB at a frequency of 1000 Hz.} \end{displaymath}}}$

``Music 175: Loudness'' by Tamara Smyth, Department of Music, University of California, San Diego (UCSD).