Production and interpretation of equal loudness curves.

The idea

Equal loudness curves show how the sound intensity level needed for a listener to judge a tone equally loud changes with frequency. To produce them, a reference tone at 1 kHz is set to a chosen level, such as 40 dB, and the listener adjusts a test tone at another frequency until it sounds equally loud; repeating this across frequencies gives one curve. Each curve joins points of equal perceived loudness, labelled by the 1 kHz level in phons. Interpretation requires reading the vertical gap between a curve and the 1 kHz line: at low and high frequencies the ear is less sensitive, so a larger intensity level is needed. The curves also flatten at high levels, showing that frequency sensitivity matters less for loud sounds.

What earns the marks

• State that a reference tone, usually 1 kHz, is set to a fixed sound intensity level and the listener adjusts a comparison tone until the two are judged equally loud.
• Describe repeating the adjustment at many frequencies to obtain a set of points that form one equal loudness curve.
• Explain that each curve is labelled by the sound intensity level of the 1 kHz reference, measured in phons, so the curve represents one perceived loudness.
• Interpret a curve by comparing the required intensity level at a given frequency with the value at 1 kHz, noting that greater sensitivity means a lower required level.
• Use the curves to explain why low and high frequency tones need higher intensity levels than a 1 kHz tone to be perceived as equally loud at moderate levels.
• Recognise that the curves become flatter at higher levels, so the ear's relative sensitivity to frequency is less pronounced for loud sounds.

Common mistakes and how to put them right

• Treating an equal loudness curve as a graph of loudness against frequency.

  Put it right by identifying the axes as sound intensity level against frequency, with each curve representing constant perceived loudness.

• Assuming that equal sound intensity level always means equal loudness.

  Put it right by using the curves to show that equal loudness generally requires different intensity levels at different frequencies.

• Reading the phon label as a frequency or as a sound intensity level in dB.

  Put it right by stating that the phon value is the sound intensity level of the 1 kHz reference tone for that curve.

Exam tips

• When describing production, name the reference frequency and explain the listener's adjustment task rather than describing a purely instrumental measurement.
• When interpreting, quote a frequency and read the corresponding intensity level from the curve, then compare it with the 1 kHz value to support the conclusion.
• Use the flattening of the curves at high levels to explain why loudness perception depends less on frequency for loud sounds.