If a sound wave's wavelength increases, the perceived pitch or frequency of the sound decreases. This is because wavelength and frequency are inversely proportional: as the wavelength increases, the frequency decreases, and vice versa.

Mathematically, the relationship between wavelength (λ) and frequency (f) is expressed as:

λ = v / f

where:

λ is the wavelength in meters (m)

f is the frequency in Hertz (Hz)

v is the speed of sound in meters per second (m/s)

As the wavelength increases, the distance between adjacent peaks or troughs of the sound wave becomes larger. This corresponds to a slower oscillation or vibration of the sound waves, resulting in a lower perceived pitch.

Conversely, if the wavelength decreases while other factors remain constant, the frequency increases, and the sound becomes higher in pitch. This principle applies to all types of waves, including sound waves, electromagnetic waves, and water waves.