Sound and Hearing

Objective: Describe compression and sound speed in media.

Sound is a mechanical disturbance generated by vibrating sources and carried through matter. In a fluid such as air, sound is primarily a longitudinal pressure and density wave with compressions and rarefactions. It cannot travel through an ideal vacuum. Sound speed depends on medium properties and conditions, including temperature in air; it is not simply set by how loud the source is. For an air sound wave with speed about 340 m/s and frequency 170 Hz, wavelength is about 2 m. A solid may carry sound differently than air. The particles oscillate locally while acoustic energy propagates.

Objective: Distinguish physical wave quantities from perception.

For ordinary audible sounds, higher frequency generally corresponds to higher perceived pitch, while greater intensity often corresponds to greater loudness, though human perception depends on frequency, duration, and context. Sound intensity is power per area (W/m²) and decreases with distance from a source in a simple spreading model. Amplitude relates to wave energy and intensity but is not itself a decibel value. Decibels use a logarithmic ratio, so a 10 dB intensity-level increase corresponds to a tenfold intensity increase under the standard definition; it does not mean ten times perceived loudness. Pitch is not speed, and a louder sound in the same medium does not automatically travel faster.

Objective: Interpret reflected sound and changing observed frequency.

An echo is reflected sound heard after a delay. If an acoustic pulse travels to a reflector and back in time t at speed v, one-way distance is d = vt/2, because the sound traverses the path twice. A 0.4 s round trip at 340 m/s gives a reflector distance of 68 m. The Doppler effect changes observed frequency when source and observer move relative to each other: approach generally raises observed frequency and recession lowers it in the common stationary-medium sound model. The source’s emitted frequency need not change. Echo strength and timing depend on surfaces and environment; Doppler reasoning should identify which objects move and the medium frame.