The effect on equal loudness curves and the changes experienced in terms…

The idea

Equal loudness curves join points of different frequency and intensity that sound equally loud. The lowest curve shows the threshold of hearing, which is most sensitive around 2–5 kHz and rises sharply at low and high frequencies. Hearing loss alters these curves: the threshold curve moves upwards, so quiet sounds become inaudible, and the curves flatten because loudness grows abnormally quickly once sound exceeds the raised threshold. Noise-induced loss typically produces a dip around 4 kHz and reduces sensitivity at high frequencies, while age-related deterioration raises thresholds most at high frequencies. Both effects compress the usable dynamic range between threshold and discomfort, so a person may struggle to follow speech in noise and may find loud sounds uncomfortable.

What earns the marks

• Describes equal loudness curves as showing combinations of frequency and intensity that are perceived as equally loud.
• Explains that the threshold of hearing is the lowest equal loudness curve and is frequency dependent.
• States that hearing loss raises the threshold curve, making quieter sounds inaudible.
• Explains that the equal loudness curves become flatter or closer together, indicating a reduced dynamic range.
• Distinguishes noise-induced loss, often with a high-frequency dip, from age-related deterioration, which mainly affects high frequencies.
• Relates the changes to practical consequences such as difficulty hearing speech or increased discomfort at high intensities.

Common mistakes and how to put them right

• Assuming equal loudness curves are horizontal.

  Put it right: They are strongly frequency dependent, especially near the threshold.

• Confusing loudness with intensity.

  Put it right: Equal loudness curves compare perceived loudness, not equal intensity.

• Stating that hearing loss only shifts the threshold without changing the spacing of the curves.

  Put it right: The curves also flatten as dynamic range is reduced.

• Describing physiological mechanisms such as hair cell damage when the question excludes physiological changes.

  Put it right: Focus on the perceptual and curve-based effects.

Exam tips

• Sketch or interpret the equal loudness curves with frequency on a logarithmic axis and intensity level on the vertical axis.
• Use the threshold curve to explain why some quiet high-frequency sounds become inaudible first.
• Compare a normal set of curves with a hearing-loss set by describing both the upward shift and the reduced spacing.
• Refer to the dynamic range between threshold and discomfort when explaining everyday listening difficulties.