Equal Loudness

Loudness is a subjective indicator of the perceived noise level expressed in phons. The loudness level in phons is the sound pressure level in decibels of a 1 kHz tone having the same perceived loudness as the tone being evaluated.

Thus, a 1 kHz tone with a loudness level of 30 phons is equally as loud as a 1 kHz tone with a sound pressure level of 30 dB referenced to 20 µPa. However, a signal frequency of 100 Hz requires a sound pressure level of 44 dB referenced to 20 µPa to provide the same loudness level.

The following figure shows equal loudness curves.

Figure 61. Equal Loudness Curves

1378