Contents
Introduction to Loudness
Loudness refers to the perceived volume of sound as heard by the human ear. It may differ from the actual sound pressure level (dBSPL), which objectively quantifies sound intensity.
Historical Context and Standards: Equal-loudness Contour
The concept of the equal-loudness contour comes from studies on human loudness perception. The standard curve was proposed by Fletcher-Munson and is often cited in academic and professional discussions. Their findings were published in a 1933 paper titled “Loudness, its definition, measurement, and calculation.” Further research and updates have led to ISO 226, published since 1961, with the latest revision incorporating studies from Japan, Germany, Denmark, the UK, and the USA in 2003. A successor is currently under development.
This is the 2003 revised edition of ISO 226, and for comparison, the original Fletcher-Munson curve is underlined in blue
Characteristics and Observations
When examining the equal-loudness contour, one can notice that the human perception of loudness across different frequencies becomes more uniform as the volume increases. Generally, at 85dBSPL, low and high frequencies are perceived more uniformly.
Units of Measurement: Phon and Sone
Phon is a unit of loudness based on the equal-loudness curve and is equivalent to dBSPL at 1kHz. For instance, a sound at 10 Phon is equal to 10dBSPL at 1kHz and 75dBSPL at 20Hz. Both would be perceived as equally loud. Another unit, Sone, is defined in relation to Phon; 40 Phon is defined as 1 Sone. Therefore, a 10 dB increase to 50 Phon would be 2 Sones.
Loudness and Duration
Loudness is also affected by the duration of the sound. Sounds lasting less than 100 milliseconds are perceived as quieter the shorter they last.
Human Perception and Dynamic Range
Humans typically perceive a 10dB increase in sound as twice as loud, and a 100dB increase as four times as loud. These are average figures and may vary from person to person. The dynamic range of human hearing is roughly between 120 and 140dB. Generally, casual listeners hit their limit at around 125dB SPL, while trained listeners can go up to 135-140dB SPL. Prolonged exposure to sounds above 120dB SPL can cause hearing damage.
Just Noticeable Difference (JND)
The Just Noticeable Difference (JND) for loudness is about 1dB, typically measured at 1kHz with a sine wave in a typical noise environment. This implies that minor adjustments, such as raising the volume of an instrument by 0.5dB during mixing, would be hardly noticeable.