Sound in Technology, Medicine and the Environment

A healthy young human ear responds to frequencies from about $20\text{Hz}$ to about $20000\text{Hz}$. Ultrasound is simply sound above $20\text{kHz}$ — too high-pitched to hear, but in every other respect an ordinary longitudinal wave. It reflects, spreads, interferes, obeys $v=f\lambda$ and still needs a medium, exactly as audible sound does. The word sounds exotic, but nothing about the physics is new. The only thing that makes ultrasound special is the frequency: high frequency means short wavelength, and short wavelength means fine detail.

A transducer pressed against the skin does two jobs. It emits a very short burst of ultrasound, then falls silent and listens. Inside the body, every boundary between two different tissues — fat against muscle, muscle against bone, tissue against fluid — reflects a small fraction of the pulse back. The transducer times each returning echo, and that time becomes a depth using the relationship you already have.

No wave can resolve detail much finer than its own wavelength. Since $\lambda =\frac{v}{f}$ and the speed in tissue is fixed near $1540\text{m/s}$, the only way to shorten the wavelength is to raise the frequency. A $3\text{MHz}$ probe has a wavelength of about $0.51\text{mm}$; a $12\text{MHz}$ probe is near $0.13\text{mm}$ and resolves roughly four times the detail. The catch is that high frequencies are absorbed far more quickly — every cycle of travel costs the pulse a little energy as heat, and a wave with four times as many cycles per centimetre fades in a much shorter distance. A high-frequency probe therefore gives a beautiful picture of something shallow and nothing at all from something deep.

• Sonar — a ship times a pulse to the seabed and back through water at about $1480\text{m/s}$. Fishing vessels use the same instrument to find shoals, since a swim bladder full of gas is an excellent reflector.
• Non-destructive testing — a pulse is sent into a weld, a rail or a pipeline. An internal crack is a steel-to-air boundary, so it returns an echo from a depth where no echo should exist. The part is tested without being cut open.
• Ultrasonic cleaning — a bath driven near $40\text{kHz}$ so that the pressure swings form and collapse microscopic bubbles. The collapses scrub eyeglasses, jewellery and surgical instruments in crevices no brush could reach.
• Distance and level sensing — parking sensors, automatic doors and tank gauges all time an ultrasonic echo, and none of them add any noise to the room they work in.

Sound reaches your ear in a room twice over: once directly from the source, and then again and again as reflections from the walls, floor and ceiling. If a single reflection is delayed by more than about $0.1\text{s}$, the ear separates it and you hear a distinct echo. If the reflections arrive sooner and in vast numbers, they blur together into a smooth decaying tail called reverberation. Keep the two apart: an echo is one late reflection you can count, while reverberation is thousands of early ones you cannot separate, heard as the sound taking time to die away. Reverberation time is quoted as the time for a sound to fall by 60 decibels — that is, to one millionth of its intensity.

Acoustic treatment fixes this by replacing reflection with absorption. Panels of mineral wool or fibreglass go on the upper walls and hang from the ceiling as baffles, intercepting the reflections that would otherwise come back down to the floor. The direct sound from the speaker is untouched, so the room is no quieter at the source — but the reverberant tail is cut, and every syllable then stands on its own. Keep the distinction sharp: soundproofing stops sound entering or leaving a room and needs mass and sealing, while acoustic treatment controls what happens inside a room and needs absorption. Hanging panels will not quiet the room next door.

Noise pollution is unwanted sound at levels that affect health. Ontario workplace regulations cap exposure at $85\text{dB}$ averaged over an eight-hour shift, and because damage depends on the total energy delivered, every $3\text{dB}$ above that — a doubling of intensity — halves the time you may safely spend there. The effects reach well beyond hearing loss: sustained noise raises blood pressure and stress hormones, fragments sleep, and measurably lowers reading performance in classrooms beside busy roads. That is why municipalities write noise by-laws, why highways get sound barriers, and why aircraft noise contours shape where housing may be built.

• Distance first. Intensity obeys the inverse square law, so doubling your distance from a source cuts it to a quarter — a drop of about $6\text{dB}$ for no cost and no equipment.
• Time second. Halving your exposure time is worth as much as a $3\text{dB}$ reduction in level. A break away from the noise is genuine protection, not a break from protection.
• Earplugs — foam plugs rated near $30\text{dB}$, but only when rolled down and inserted deeply. A badly seated plug delivers a fraction of its rating.
• Earmuffs — a sealed cup over the whole ear, typically $20$ to $30\text{dB}$, easier to fit correctly and much quicker to take on and off.
• Headphone discipline — stay below about 60% of maximum volume and take a break each hour. If someone an arm's length away can hear your music, it is loud enough to be doing damage.