Hearing the wind “whistle” is a common phenomenon. While it may seem like the wind itself is making the sound, the noise is actually a byproduct of air interacting with physical objects in its path. This auditory effect is governed by fluid dynamics, where the movement of air causes vibrations that our ears interpret as sound. The type of sound produced, whether a sharp whistle or a low moan, depends on the physical characteristics of the object the air encounters and the speed at which it is moving.
The Core Mechanism: Helmholtz Resonance
The most familiar sound of whistling wind, such as the noise coming from a crack in a door or window frame, is explained by Helmholtz resonance. This occurs when a volume of air within a cavity vibrates at a specific frequency after air rushes over an opening. For example, blowing across the top of an empty bottle causes the air mass inside to oscillate, creating a distinct, resonant tone.
In the case of wind, a fast-moving stream of air passing over a small aperture, like a tiny gap in weather stripping, acts as the power source. This airflow creates a momentary pressure differential, forcing air into and out of the enclosed space. The air inside that cavity acts like a spring, while the air in the opening acts like a mass.
The resulting vibration is self-sustaining because the airflow continues to feed energy into the oscillating air mass. This process amplifies the sound waves at the cavity’s natural frequency, making a low-amplitude flow disturbance audible as a high-pitched whistle. This phenomenon is also responsible for the loud, throbbing sound sometimes heard inside a car when a single window is partially opened at high speed, known as wind throb.
Whistling vs. Howling: Understanding Vortex Shedding
Not all wind noise is a sharp whistle; a deeper, sustained sound like a howl or a moan is often the result of vortex shedding. This occurs when wind flows past a solid, non-aerodynamic object, rather than through a small opening. As air separates to move around an object—such as a utility wire, flagpole, or building corner—it forms a repeating pattern of swirling eddies, known as a Kármán vortex street.
These vortices are shed alternately from opposite sides of the object, creating a regular, alternating low-pressure zone in the wake. This periodic change in pressure causes the object and the surrounding air to vibrate. If the frequency of this vortex shedding matches the natural frequency of the object, the vibration is amplified, and the sound becomes louder.
This mechanism causes suspended telephone or power lines to “sing” in the wind. The difference between the high-pitched whistle of Helmholtz resonance and the lower howl of vortex shedding is determined by the size of the object involved. Vortex shedding is associated with larger structures, which produce lower-frequency sounds.
Pitch Control: How Size and Speed Affect Frequency
The specific pitch of the wind noise is determined by two variables: the speed of the air and the physical dimensions of the object or aperture. For both Helmholtz resonance and vortex shedding, a faster flow of air leads to a higher frequency and a louder sound. The frequency of the sound increases almost linearly with the speed of the wind.
When considering Helmholtz resonance, the size of the cavity and the diameter of the opening are the most important factors. A smaller cavity volume or a smaller aperture causes the air to vibrate at a higher natural frequency, resulting in a higher-pitched whistle. This is why a small crack in a window produces a higher pitch than a partially open car window.
For vortex shedding, the diameter of the object is the controlling physical dimension. Thin objects, like a narrow wire, shed vortices faster than thick objects, like a large chimney. This faster shedding rate translates into a higher-frequency sound, meaning thinner objects tend to produce a higher-pitched moan or hum.