Loudness is a subjective quantity and perceptions of loudness vary from person to person. Different people describe different intensity sounds as having the same loudness – it seems that for many people pitch or frequency is an important factor in perceptions of loudness. This is illustrated in the diagram below, which shows curves, each one of equal loudness as perceived by a hearer. The curve labelled MAF is the minimum that would be heard.
The following points should be noted:
The ear is most sensitive to frequencies between 2 and 5 kHz.
At low frequencies, the sound needs to be at a higher intensity before it is heard.
The diagram above is the response for a normal ear. Age or disease might alter the response.