Why some waves pass through walls and others don’t
Your phone gets signal inside a wooden house but struggles in a concrete basement. Sunlight streams through a window but not through a brick wall. A thin sheet of aluminum foil can block a radio, yet you can still hear someone talking behind it. All of this comes down to three things that happen when a wave meets a material.
Reflection, absorption and transmission
When a wave reaches the boundary between two materials, its energy splits:
- Reflection — part of the wave bounces back at the boundary.
- Absorption — part of the energy is turned into heat inside the material.
- Transmission — whatever is left continues out the other side.
Energy is conserved, so the three shares always add up to 100%: R + A + T = 1.
Impedance: why waves bounce
How much a wave reflects depends on how different the two materials look to that wave. Physicists describe this with impedance. For sound, the acoustic impedance is Z = ρc: the density of the material times the speed of sound in it. At a boundary, the fraction of the wave’s amplitude that reflects is
r = (Z₂ − Z₁) / (Z₂ + Z₁)
and the reflected share of the energy is r². Air has a very low impedance and concrete a very high one, so almost all sound energy reflects from a concrete wall. That’s why ultrasound scans need gel on the skin: without it, the thin layer of air would reflect nearly everything.
Absorption: how thickness and frequency matter
Inside a material, the wave loses energy as it travels. The intensity falls roughly exponentially with distance:
I(d) = I₀ · e^(−2αd)
where α is the attenuation coefficient and d the thickness. Doubling the thickness doesn’t halve the transmitted wave; it squares the fraction that gets through. Attenuation usually grows with frequency, which is why higher-frequency Wi-Fi (5 GHz) fades behind walls faster than 2.4 GHz.
Why metal blocks radio
For electromagnetic waves, what matters is how the material responds to electric fields. Metals are full of free electrons. An incoming radio wave makes them move, and their motion creates a wave that cancels the incoming one inside the metal and reflects it outside. The wave only survives a very short distance, called the skin depth — a few micrometers for copper at radio frequencies. Sound, on the other hand, is a mechanical wave and doesn’t care about free electrons, so it can pass (weakly) through a thin metal sheet.
Try it yourself
In the wave propagation simulator you can choose acoustic or electromagnetic waves, pick one of eleven materials, and sweep the frequency and thickness. Energy bars show reflection, absorption and transmission live, together with the wavelength, impedance and attenuation.
Three experiments to start with:
- Pick concrete and electromagnetic waves. Find the frequency at which transmission drops below 10% for a 20 cm wall.
- Switch to copper and compare sound with radio waves at the same thickness.
- Choose soft tissue and acoustic waves and sweep into the ultrasound range, as in medical imaging.
Key points
- Every wave meeting a material is split into reflected, absorbed and transmitted parts.
- A large impedance mismatch means strong reflection.
- Thicker materials and higher frequencies usually absorb more.