Equal-loudness contour

Introduction

An equal-loudness contour is a measure of sound pressure (measured in decibels), over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tonessound pressure. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours. By definition, two different sounds in a given equal-loudness contour are perceived as equally loud by the human ear, even though they can have different physical sound pressure levelssound pressure levels.

History and Development

The concept of equal-loudness contours was first introduced by Fletcher and Munson in 1933. The first equal-loudness contours they measured demonstrated that our sensitivity to sound varies with frequency and sound pressure level. This discovery was significant because it revealed that the human ear does not have a flat frequency responsefrequency response, but rather, it varies depending on the loudness level and the frequency of the sound.

Fletcher-Munson Curves

The first scientifically reliable equal-loudness contours were researched and published by Fletcher and Munson in 1933 and are known as the Fletcher-Munson curves. These curves were derived from the average responses of several subjects. The curves show the varying absolute intensity of a pure tone that has the same apparent loudness to the average human listener as does a 1 kHz tone. The Fletcher-Munson curves are a significant tool in the study of psychoacousticspsychoacoustics, the psychological study of hearing.

Robinson-Dadson Curves

In 1956, D.W. Robinson and R.S. Dadson produced a new experimental determination of equal-loudness contours using headphones, and their results were standardized as ISO 226 in 1986. These curves, now known as the Robinson-Dadson curves, showed substantial differences from the Fletcher-Munson curves, particularly at low frequencies and high sound pressure levels.

ISO 226:2003

The most recent standard is ISO 226:2003, which is considered the most accurate representation of equal-loudness contours. This standard was based on the results of several international studies. The data for this standard was collected from 12 laboratories in 11 different countries. The standard defines a set of equal-loudness contours for various sound pressure levels ranging from 20 to 120 phons in steps of 10 phons.

A photograph of a sound pressure level meter with a frequency spectrum analyzer.

Importance in Audio Engineering

Equal-loudness contours are crucial in many aspects of audio engineering, particularly in the design of loudspeakers, the tuning of concert halls, and the creation of equalizers. By understanding how the human ear perceives different frequencies at different volumes, audio engineers can create systems that produce a more accurate and pleasing sound.

Conclusion

The concept of equal-loudness contours is a fundamental aspect of psychoacoustics and plays a crucial role in our understanding of human hearing. It has significant implications in various fields, including audio engineering, music production, and acoustics.