The Human Hearing Range Revision Notes | GCSE AQA Higher Physics

1. Why Can't We Hear Everything?

Right now, sound waves are hitting your ears that you can't hear. Elephants are communicating with deep rumbles below your hearing range. Bats are echolocating with high-pitched squeaks above it.

Your ears work like a selective radio receiver. They can only tune into certain frequencies of sound waves. This isn't a design flaw — it's actually perfect for human survival and communication.

Understanding frequency limits explains why some sounds are inaudible, why hearing aids work, and how animals use sound in ways we can't naturally detect.

2. The Human Hearing Range

Think of your ear as having a frequency window. Below 20 Hz, sound waves are too slow and deep for your ear drum to respond effectively. Above 20 kHz, the waves are too fast and high-pitched for your inner ear to process.

This range isn't the same for everyone. Young people can often hear up to 18-20 kHz, but this upper limit decreases with age. By adulthood, many people lose sensitivity to frequencies above 15 kHz.

The 20 Hz lower limit exists because very low frequencies don't vibrate your eardrum efficiently. Instead of hearing them as distinct sounds, you might feel them as vibrations in your chest or body.

3. Beyond Human Hearing

Elephants communicate using infrasound frequencies as low as 5 Hz. These deep rumbles can travel for kilometres through the ground, allowing herds to coordinate across vast distances. Earthquakes and volcanic eruptions also produce infrasound.

At the other extreme, dolphins and bats use ultrasound frequencies up to 100 kHz for echolocation. Medical ultrasound scanners typically operate between 2-15 MHz (millions of Hz) to create images inside the human body.

These frequency ranges exist because different applications need different wavelengths. Infrasound's long wavelengths travel far but carry less detailed information. Ultrasound's short wavelengths provide precise detail but don't travel as far.

4. Why These Limits Exist

Your ear's frequency limits aren't random — they're perfectly matched to human needs. Most human speech occurs between 85 Hz and 8 kHz, well within our hearing range. The fundamental frequencies of musical instruments also fall mostly within our audible spectrum.

The physical structure of your ear determines these limits. At very high frequencies, the eardrum and inner ear cannot respond efficiently enough. At very low frequencies, the ear's overall mechanical response becomes less sensitive, so sounds below about 20 Hz are not normally heard.

Evolution shaped our hearing range for survival advantage. We needed to detect approaching predators (medium frequencies), communicate with other humans (speech frequencies), and locate food sources. Hearing every possible frequency would overwhelm our brains with unnecessary information.

Other animals evolved different ranges based on their needs. Bats need ultrasound for precise echolocation in darkness. Elephants use infrasound because low frequencies travel further through dense vegetation and ground.

5. Calculating Sound Wave Properties

Sound travels at approximately 340 m/s in air at room temperature. Using the wave equation, we can calculate the wavelengths of sounds at the limits of human hearing.

Worked example: Finding Wavelength at Hearing Limits

Calculate the wavelengths of sound at 20 Hz and 20 kHz in air (v = 340 m/s).

1. $v = f \lambda$ so $\lambda = \frac{v}{f}$ — Rearrange to find wavelength
2. $\lambda = \frac{340}{20} = 17$ m — Low frequency gives long wavelength
3. $\lambda = \frac{340}{20,000} = 0.017$ m = 1.7 cm — High frequency gives short wavelength
4. Ratio = $\frac{17}{0.017} = 1000:1$ — Wavelengths differ by factor of 1000

Answer: λ = 17 m at 20 Hz, λ = 1.7 cm at 20 kHz

Notice the huge difference in wavelengths. The lowest audible frequency (20 Hz) has a wavelength of 17 metres — longer than a bus. The highest audible frequency (20 kHz) has a wavelength of just 1.7 cm — smaller than your thumb.

6. Real-World Applications

Understanding hearing ranges has practical applications in technology and medicine. Hearing aids are designed to amplify frequencies within the human audible range, particularly the speech frequencies between 500 Hz and 4 kHz where most communication occurs.

Medical ultrasound uses frequencies between 2-15 MHz because these penetrate tissue effectively while providing good image resolution. The high frequency allows detailed imaging, but the waves don't travel as far as lower frequencies would.

Audio engineers use frequency limits when designing speakers and recording equipment. There's no point making speakers that reproduce frequencies below 20 Hz or above 20 kHz for human listeners — it would waste energy and add unnecessary cost.

Animal researchers use the fact that different species have different hearing ranges. They can study elephant communication using special microphones that detect infrasound, or bat echolocation using ultrasound detectors.

7. Exam Technique and Common Mistakes

Exam questions often test whether you know the specific values for human hearing limits. Always state 20 Hz to 20 kHz — don't round to "about 20" or give vague ranges like "very low to very high".

When explaining why processes work over limited frequency ranges, link the physical mechanism to the frequency limits. For hearing, mention that the eardrum and middle ear bones have physical constraints that prevent them from responding to all frequencies.

In calculations involving the wave equation, watch your units carefully. Frequency might be given in kHz (multiply by 1000 to get Hz) and wavelength answers might need to be in cm or mm rather than metres.