Wavelength to frequency
A 40 Hz tone has a wavelength of 8.6 metres in air. In a room four metres across, that wave cannot complete a cycle before hitting a wall, which is why low frequencies produce standing waves, room modes and the familiar problem of bass that is deafening in one corner and absent in another. Room treatment for bass has to be physically large for exactly this reason — the wavelengths are.
Frequency is wave speed divided by wavelength. Sound travels at about 343 m/s in dry air at 20 °C, so a one-metre wavelength is 343 Hz. In water at 1,481 m/s the same wavelength is 1,481 Hz.
How to convert wavelength to frequency
The speed of sound in air varies with temperature, rising roughly 0.6 m/s for every degree Celsius. At 0 °C it is 331 m/s and at 30 °C about 349 — a 5% spread that matters for precise acoustic work and not at all for a rough figure. Humidity has a much smaller effect, and pressure essentially none, which surprises people who expect altitude to change it. What altitude actually changes is temperature.
Questions
f = v ÷ λ — wave speed divided by wavelength.