User Manual

iLM8 – Live User Manual

Measuring Loudness

Human perception of the sensation of an audio signal is properly called “loudness”. As early as 1933,

research by Fletcher and Munson showed that human hearing sensitivity is largely based upon the

frequency of the sound, as well as the sound pressure level (SPL). As humans do not hear sound using

standard measures of intensity, measuring loudness based upon SPL or the root mean square (RMS)

voltage of the electrical signal will not agree with human perception. The Fletcher/Munson Equal

Loudness Curves show the limits human sensitivity of certain frequencies, especially low frequencies

(bass) at lower sound levels (Fletcher & Munson, 1933).

In order to create a loudness measure that more accurately represents the human perception of

loudness, frequency weighting (or equalization curves) are placed ahead of power measurements. This

introduces the measures Leq(A), Leq(B), Leq(C) and Leq(M) – with ‘Leq’ attempting to relate the

‘L’oudness of an ‘eq’uivalent amount of energy in a standard signal, typically a 1 kHz sine wave. The

most current weighting standard, ITU-R BS.1770 produces results known as LKFS that are referenced

to a dB FS scale. While certain Leq(x) measures match more closely to human loudness perception

than traditional SPL or RMS measurements, there are still many broadcast content types that cannot be

measured correctly with a Leq(x) or LKFS weighting method.

For example, a narrowband signal may be perceived to sound softer than an equally intense wideband

sound, depending on the relationship between rendering level, absolute threshold and signal content.

Additionally, content with large amounts of low-frequency energy is often inaccurately measured by

BS.1770 due to heavy low-frequency roll off (see weighting curves). This measurement will often vary

from a subjective measure, especially in the short term; however this variance is acceptable in a long

term measurement, as the values are smoothed over many seconds, minutes or hours. Small

deviations between the instantaneous perception of loudness is often acceptable when using a meter

solution to measure the loudness, as the meters usually offer an audio visualization method that human

operators can use. Combined with their ears, the operator can make any decisions about any corrective

action required.

Fletcher/Munson Equal Loudness Curves