Defects of Hearing
Hearing damage doesn't show up in a physics exam as a diagram of the inner ear — it shows up as a shifted curve. Whatever the underlying biological cause, both noise-induced damage and age-related hearing loss leave the same kind of fingerprint on an individual's equal loudness curves, and reading that fingerprint is the actual skill being tested here.
Defects as shifted curves, not shifted physiology
The equal loudness curves introduced in the previous lesson describe a "normal" healthy ear. When hearing is damaged — whether by noise exposure or by ageing — an individual's own equal loudness curves change shape relative to that normal baseline. The physical mechanism behind the damage (exactly what happens to the structures inside the ear) is not needed here; what matters for this topic is entirely how the curves themselves are affected, and how to read what a changed curve implies about a person's hearing.
In both cases, the underlying pattern is the same kind of change: at the affected frequencies, a higher intensity level than before is now needed for a sound to be perceived as audible, or to be perceived as being as loud as it used to sound. Graphically, this appears as the curve shifting upward at those frequencies — more decibels are now required to reach the same point on the loudness scale.
Noise-induced hearing damage
Exposure to loud sound, particularly when prolonged or repeated, can damage hearing in a way that is not uniform across all frequencies. Typically, the resulting shift in the equal loudness curves is most pronounced over a specific frequency range — often a mid-to-high frequency band — rather than spread evenly across the whole audible range. At the affected frequencies, someone with noise-induced damage needs a distinctly higher intensity level than a person with normal hearing to perceive a sound as being at a given loudness; outside the affected range, their hearing may remain close to normal.
This uneven, frequency-localised shift is itself diagnostically useful: the specific frequency range where the curve has shifted upward most can point toward noise exposure as the likely cause, distinct from the more broadly high-frequency pattern typical of age-related loss described below.
Age-related hearing loss (presbycusis)
A separate and very common pattern is the gradual hearing loss associated with natural ageing, sometimes called presbycusis. Here, the equal loudness curves shift upward gradually over time, and the shift is typically most pronounced at the higher end of the frequency range. Practically, this means higher-pitched sounds become progressively harder to hear clearly as a person gets older, while lower-frequency hearing is comparatively better preserved — again, a frequency-dependent pattern rather than a uniform loss across the whole range.
It's worth being explicit: nothing about the biological cause of noise-induced damage or presbycusis needs to be known for this topic. What's examined is the ability to describe how the equal loudness curves change (an upward shift, typically frequency-localised) and to interpret a given curve or pair of curves — reading off, in decibels, how much less sensitive an affected ear is at a particular frequency compared with normal hearing.
Key takeaways for The Ear
- The ear detects sound via three regions: the outer ear collects and funnels sound to the eardrum, the middle ear's ossicles mechanically transmit and pressure-amplify the vibration to bridge the air–fluid impedance mismatch, and the inner ear's cochlea is where vibration is eventually converted to nerve signals.
- Intensity level, $IL = 10\log_{10}(I/I_0)$ dB with $I_0 = 1.0\times10^{-12}\text{ W m}^{-2}$, is the logarithmic scale used to describe sound intensity physically, chosen because of the huge range of audible intensities and because it tracks perceived loudness better than a linear scale.
- Equal loudness curves show that perceived loudness depends on frequency as well as intensity level: the ear is most sensitive in the mid-frequency range and needs higher intensity levels at low and high frequencies to sound equally loud.
- dBA weighting corrects raw dB measurements using the general shape of the equal loudness curves, so a single reading better reflects perceived loudness — used especially in noise exposure and hearing damage assessment.
- Noise-induced hearing damage typically shows up as an upward shift in a person's equal loudness curves, often concentrated over a specific mid-to-high frequency range.
- Age-related hearing loss (presbycusis) typically shows up as a gradual upward shift most pronounced at higher frequencies, making high-pitched sounds progressively harder to hear.
- In both cases, the physiological mechanism of damage is not required knowledge — what's examined is reading and interpreting the resulting shift in the equal loudness curves.
- Together, these four lessons build a complete physical picture of hearing: mechanical structure and impedance matching, the logarithmic scale used to quantify loudness, the frequency-dependence of perceived loudness, and how hearing defects are characterised as measurable changes to that same frequency response.