Sound Production and Speed

You see lightning, then seconds later hear the thunderclap. You yell across a canyon and your echo takes a moment to come back. You can hear an approaching train through the rail long before you hear it through the air.

All these everyday experiences point to the same fact: sound is a mechanical wave with a finite speed, and that speed depends on what it travels through. Light is essentially instantaneous on Earthly scales (~$3 \times 10^8$ m/s). Sound is much slower (~340 m/s in air). The mismatch is what creates the delay between the lightning flash and the thunder.

Sound Is a Longitudinal Pressure Wave

When a speaker cone pushes forward, it compresses the air molecules right in front of it. Those molecules push their neighbors. The neighbors push their neighbors. A wave of compression travels outward from the source. Behind each compression, the air spreads into a region of lower density called a rarefaction.

Sound requires a medium. No particles, no compressions, no sound — which is why there’s no sound in the vacuum of space, despite what every sci-fi movie ever made would have you believe.

Speed of Sound in Different Media

The speed of sound depends on two properties of the medium:

• Stiffness (how strongly molecules push back when compressed). Stiffer = faster.
• Density (how much mass is being moved). Denser = slower (more inertia).

In practice, solids are much stiffer than liquids and gases — by orders of magnitude — so the stiffness effect dominates and sound moves fastest in solids.

Temperature and Sound Speed

In a gas, higher temperature means molecules move faster on average — and faster molecules transmit compressions more quickly. Higher temperature → faster sound speed in air.

A useful approximation: sound speed in air rises by about 0.6 m/s per 1°C. At 0°C, $v \approx 331$ m/s. At 20°C, $v \approx 343$ m/s. The MCAT usually gives 340 m/s, or specifies the value to use.

Why Speed Matters Clinically

The speed of sound in soft tissue (~1540 m/s) is the basis of ultrasound imaging. The machine sends out a pulse, times how long the echo takes to come back, and uses $d = vt$ to calculate the depth of the reflecting structure (organ surface, fetal heart, tumor edge). Different tissues have slightly different speeds — and slightly different reflectivities — which is what produces image contrast.

The same echo-timing principle works for sonar (mapping the ocean floor), seismic surveys (finding oil and gas), bat echolocation, and dolphin echolocation. All of them rely on knowing $v$ accurately so they can convert “echo arrival time” into “distance to reflector.”