Physics
Acoustic Impedance
Quick fact
At a boundary between air and water, only about 0.1% of sound energy is transmitted; the rest is reflected back. This is why underwater sounds are nearly silent to someone above the surface.
Why this is interesting
Why can you hear a whale from miles away underwater, but a loud concert outside feels muffled when you plunge into a pool? The answer lies in a hidden property of every material: its acoustic impedance.
Read the full explanation
Understanding Acoustic Impedance
Think of acoustic impedance as the 'resistance' a material offers to the propagation of sound waves. Just as a heavy rope resists being shaken quickly compared to a light string, a dense or stiff medium slows and redirects sound energy. Technically, acoustic impedance (Z) is defined as the product of the medium's density (ρ) and the speed of sound in that medium (c), so Z = ρ × c. Each medium has a characteristic value: air has a low impedance (~400 Pa·s/m), water has a high one (~1.48 × 10⁶ Pa·s/m), and bone is even higher. When a sound wave encounters a boundary between two media with different impedances, part of its energy is reflected and part is transmitted. The greater the mismatch, the more reflection. This is why sound bounces off walls (impedance mismatch with air) and why a shout across a lake travels well because air and water are both low impedance? Actually, air and water are mismatched, so very little sound enters the water from air or vice versa.
A deeper explanation
The mechanism behind impedance is rooted in how sound waves transfer energy. A sound wave creates alternating compressions and rarefactions, and to keep propagating, each particle must push its neighbor. If the next medium has a different density or elasticity, the particle's motion is disturbed: some energy reflects back and some continues. The ratio of reflected to transmitted energy is governed by the reflection coefficient, which depends solely on the two impedances: R = ((Z₂ - Z₁)/(Z₁ + Z₂))². When Z₁ = Z₂, no reflection—perfect transmission. When they differ greatly, most energy reflects. This principle is why ultrasound gels are used in medical imaging: they match the impedance of the transducer and skin, allowing sound to enter the body instead of reflecting off the air gap. Similarly, in architectural acoustics, materials with different impedances are used to control echoes and absorption. Understanding acoustic impedance therefore explains everyday phenomena like why a person underwater barely hears sounds above, and it is a fundamental concept for designing everything from concert halls to sonar systems.