Noise Damage and Age-Related Hearing Loss Revision Notes | A Level AQA Physics

1. Why Hearing Loss Matters: The Physics Behind the Problem

Your ears do not treat all frequencies equally. A 100 Hz tone at 60 dB SPL sounds noticeably quieter than a 1 kHz tone at the same level — your auditory system is less sensitive at low and very high frequencies. This frequency-dependent sensitivity is captured beautifully by equal loudness curves, and when hearing is damaged — whether by noise or by age — those curves shift in characteristic, predictable ways.

Understanding those shifts is not merely academic. It tells you precisely which sounds a person struggles to hear, why speech can become difficult to follow even when it seems loud enough, and how the physics of sound perception connects directly to real-world disability.

2. Equal Loudness Curves: The Baseline You Need to Know

Picture a graph with frequency on the x-axis (from about 20 Hz to 20 kHz, often on a logarithmic scale) and sound pressure level in dB SPL on the y-axis. Each curve on this graph connects all the frequency-level combinations that a normal listener judges to be equally loud. These are the equal loudness curves, also called Fletcher-Munson curves or ISO 226 contours.

The reference point is always 1 kHz. If a 1 kHz tone at 40 dB SPL is the reference, the equal loudness curve at 40 phons traces out how many dB SPL you need at every other frequency to match that perceived loudness. At low frequencies (around 100 Hz), you need considerably more SPL — perhaps 60 dB or more — to sound equally loud. Near 3–4 kHz, the ear is most sensitive, so you actually need less SPL than at 1 kHz to achieve the same perceived loudness.

The key feature to fix in your mind: the curves are not flat. They dip (meaning less SPL needed, so greater sensitivity) around 3–4 kHz, and they rise steeply at low frequencies. At very high loudness levels the curves flatten out — the ear becomes more uniformly sensitive when sounds are very loud. This non-uniform shape is what makes hearing damage so perceptually significant: damage at a specific frequency region shifts only that part of the curve upward, creating a characteristic distortion of the listener's sound world.

Notice how the 20-phon curve dips most dramatically near 3–4 kHz — that is where the ear canal resonance boosts sensitivity most strongly. This region is precisely where noise damage tends to strike first, which is why the consequences are so significant.

3. Noise-Induced Hearing Loss: The 4 kHz Notch

When the ear is exposed to excessively loud sound — whether a single very intense event such as an explosion, or prolonged exposure to industrial noise or loud music — the hair cells of the cochlea can be damaged or destroyed. Hair cells do not regenerate in humans; damage that causes a permanent threshold shift is irreversible.

The hair cells most vulnerable to noise damage are those tuned to frequencies around 4 kHz. This is partly because the ear canal resonance means that sounds entering the ear are amplified most at this frequency, so the mechanical stress on those hair cells is greatest. The result is a highly characteristic pattern: a sharp loss of sensitivity centred on 4 kHz, with relatively preserved sensitivity at lower and higher frequencies on either side.

On an equal loudness curve diagram, NIHL appears as an upward notch in the curve around 4 kHz. A higher curve means more SPL is required to achieve the same perceived loudness — in other words, the person is less sensitive at that frequency. The notch shape is the diagnostic hallmark: sensitivity dips at 4 kHz but recovers somewhat at 8 kHz, giving the curve a distinctive V-shaped dip when plotted as a hearing threshold audiogram.

In terms of lived experience, a person with NIHL will find that sounds containing energy around 4 kHz — including many consonant sounds in speech such as 's', 'f', and 'th' — become harder to distinguish. Speech may seem loud enough overall but lack clarity, because the high-frequency detail that separates consonants is lost. This is why people with NIHL often say they can hear that someone is talking but cannot make out the words.

The severity of NIHL depends on both the intensity of the noise and the duration of exposure. Regulatory guidelines (such as those from the Health and Safety Executive in the UK) use the concept of noise dose — combining level and time — to set safe exposure limits. Repeated exposure without adequate protection progressively deepens and broadens the notch, eventually affecting a wider frequency range.

4. Age-Related Hearing Loss: Presbycusis and the High-Frequency Decline

Where noise damage attacks a specific frequency region, age-related hearing loss — known as presbycusis — follows a different pattern. Rather than a notch, presbycusis produces a progressive, broad decline in sensitivity at high frequencies, typically beginning above 2–4 kHz and worsening with age. The equal loudness curves shift upward across the high-frequency end of the spectrum, and this shift becomes more pronounced as a person ages.

The pattern is gradual and bilateral (affecting both ears roughly equally). In the early stages, only the highest audible frequencies are affected — a person in their 40s might notice difficulty hearing sounds above 10–12 kHz. As the condition progresses into later decades, the affected range extends downward, eventually encroaching on the frequencies most important for speech intelligibility (roughly 1–4 kHz).

On an equal loudness curve diagram, presbycusis appears as a family of curves that are normal at low and mid frequencies but curve sharply upward at high frequencies. The higher the frequency, the greater the SPL required to reach any given phon level — meaning the person needs sounds to be much louder at high frequencies before they can perceive them at all. At very high frequencies, the curves may not be measurable at all because the sounds fall below the person's threshold of hearing entirely.

5. Comparing NIHL and Presbycusis: Patterns on the Equal Loudness Curve

Now that both conditions have been introduced, the most important skill for the exam is being able to compare their effects on equal loudness curves precisely and use the correct terminology. The table below consolidates the key differences.

A critical point for exam answers: both conditions raise the equal loudness curves in the affected frequency region, meaning more SPL is required to achieve the same perceived loudness. The difference lies in which frequencies are affected and the shape of the shift. Examiners will award marks specifically for identifying the notch shape in NIHL versus the sloping high-frequency decline in presbycusis.

6. Interpreting Equal Loudness Curve Shifts: What Does the Person Actually Experience?

Reading a shifted equal loudness curve is a skill in itself. When a curve shifts upward at a particular frequency, it means the person needs a higher SPL at that frequency to perceive a sound of a given loudness level. Equivalently, sounds that were previously audible at that frequency may now fall below the person's threshold — they do not hear them at all.

Consider a person with a 4 kHz notch from NIHL. Before the damage, a 4 kHz tone at 20 dB SPL might have been clearly audible. After the damage, the equal loudness curve at 20 phons has shifted upward at 4 kHz — perhaps to 50 dB SPL. That same 20 dB SPL tone is now below threshold and inaudible. The person has not lost the ability to hear all sounds; they have lost sensitivity specifically in that frequency band.

For presbycusis, the experience is different in character but follows the same logic. High-frequency sounds — birdsong, the ring of a telephone, the sibilant consonants in speech — require progressively more SPL to be heard. In a quiet room, a person with moderate presbycusis may follow conversation reasonably well because they can compensate using context and lip-reading. In a noisy environment, where background noise masks the already-weakened high-frequency cues, speech intelligibility collapses rapidly.

7. Exam Technique: Scoring Full Marks on Hearing Loss Questions

AQA questions on this topic tend to fall into two categories: those asking you to describe the changes to equal loudness curves, and those asking you to explain the perceptual consequences or compare the two types of hearing loss. Each command word demands a different response structure.

For describe questions, focus on the shape and location of the curve shift. State which frequencies are affected, state the direction of the shift (upward), and use the word 'notch' for NIHL or 'high-frequency decline' for presbycusis. Do not explain causes — the command word only asks for what happens, not why.

For explain questions, you must link the curve shift to a perceptual consequence using a causal chain. A strong answer structure is: state the shift → state what this means for the SPL required → state what sounds become inaudible or harder to hear → state the consequence for the listener. For example: the equal loudness curve shifts upward at 4 kHz, so a higher SPL is required at that frequency to reach threshold; sounds with significant energy at 4 kHz (such as certain consonants) may fall below threshold and become inaudible, reducing speech clarity.

For compare questions, you must address both similarities and differences. A common error is to describe each condition separately without explicitly drawing the comparison. Use comparative language: 'whereas', 'in contrast', 'both conditions... however...'. Examiners award marks specifically for comparative statements, not for two separate descriptions placed side by side.

One final point worth consolidating: the spec explicitly excludes physiological changes from the required content. You do not need to describe the cellular or anatomical mechanisms of hair cell damage or cochlear degeneration. Your focus should remain on the physics of the equal loudness curve — the shape of the shift, the frequencies affected, and the perceptual consequences in terms of what sounds can and cannot be heard.