Sound fundamentals: wavelength, frequency, and speed through media

One-line orientation

Before STC, NRC, and IIC ratings make sense, the exam expects the underlying physics — the wavelength–frequency–speed relationship and how fast sound moves through air versus water versus steel.

Key points

• Sound needs a medium. It propagates as a pressure wave through air, liquids, or solids — it cannot travel through a vacuum.
• Frequency (f): the number of complete wave cycles passing a point per second, in Hertz (Hz) — perceived as pitch.
• Wavelength (λ): the distance between two consecutive in-phase points on the wave (e.g., crest to crest), measured in feet.
• Speed (v): how fast the wave travels through a medium, in feet per second (ft/s).
• The wave relationship: v = f × λ. Because v is essentially fixed by the medium, frequency and wavelength are inversely related — high frequency = short wavelength, low frequency = long wavelength.
• Speed depends on the medium — denser/stiffer carries sound faster:
  • Air: ~1,125 ft/s at 68°F (~1,128 ft/s is the commonly cited round figure).
  • Water: ~4,862 ft/s.
  • Steel: ~19,570 ft/s.

Confusions / comparison

Sound speed by medium — remember 1× · 4.3× · 17.4×

Bars share a true zero; values are representative at room-temperature reference conditions.

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Speed is not controlled by density alone. It depends on elastic stiffness and density; steel’s much greater stiffness dominates this comparison.

Medium	Sound speed (approx.)	Why
Air	~1,125 ft/s (68°F)	Low density, low stiffness — slowest
Water	~4,862 ft/s	Denser than air — faster
Steel	~19,570 ft/s	Dense and stiff — fastest

Quantity	Symbol	Unit	Note
Frequency	f	Hz (cycles/s)	Perceived as pitch
Wavelength	λ	ft	Crest-to-crest distance
Speed	v	ft/s	Set by the medium; v = f × λ

Related

→ Acoustics: STC / NRC / IIC (dB rules, octave bands, single-number ratings) · Sabins & RT60 (absorption and reverberation) · Design strategies (mass vs stiffness; structure-borne vs airborne; low- vs high-frequency control).