Below and Beyond Hearing

Why it matters

Elephants rumble to each other across kilometres — below your hearing. Bats scream at the night — above it. Your ears own a window two octaves short of the universe's conversation.

The idea in plain words

The three bands. Tap them.

infrasonic < 20 Hz < audible < 20 kHz < ultrasonic

Why use ULTRASOUND (not audible sound) for medical imaging?

Short waves see small things: 2 MHz resolves millimetres

• below the window: infrasonic — earthquakes, elephants, storms — long waves that travel enormous distances
• above the window: ultrasonic — bats, dolphins, dog whistles — and every hospital's imaging room
• 8.12's list: ultrasound uses — sonar depth-ranging, medical imaging (sonography), cleaning delicate parts, flaw detection in metals
• Higher frequency → shorter wavelength → finer detail resolved
• And it's inaudible — no discomfort

Predict first

The audible range for a healthy human ear is about…

Two decades of frequency: 20 Hz (felt as much as heard) to 20 kHz (fading with age). Below: infrasonic. Above: ultrasonic.

• 20 Hz to 20,000 Hz — correct
• 1 Hz to 1,000,000 Hz
• 200 Hz to 2,000 Hz

What you do

Sweep the whole spectrum and park exactly on both edges of your hearing window.

Check yourself

A dog whistle seems silent to you because it…

The whistle is loud — just pitched beyond your window and inside the dog's.

Bats navigate in total darkness using…

Echolocation: emit ultrasound, read the returning echo — sonar, evolved millions of years before ships.

With age, humans typically lose hearing at…

The 17.4 kHz 'mosquito tone' ringtones teenagers use are inaudible to most adults — biology's practical joke.

• emits ultrasonic frequencies above 20 kHz — dogs hear to ~45 kHz — correct
• is too quiet
• only works for dogs' brains
• ultrasonic echoes of their own clicks — correct
• night vision
• smell
• the HIGH-frequency end first (20 kHz slides down) — correct
• the low end first
• both ends equally