Audible frequency of an air vent increases with distance: what's going on?

Question

I was just standing near a loud air vent that was roughly at eye level and pointing downward. I noticed something strange about the noise it was making. The further away from the vent I stood, the higher the pitch of the noise became. I initially thought this might just be the Doppler effect, but the effects persisted after I stopped moving and the pitch continually increased as I was walking away at a roughly constant rate.
I have no idea what would cause something like this to happen. Can someone explain why that would happen?

docscience · Answer

An interesting question. But more information is needed.

I'm assuming this is a heater or air conditioning vent. Is there hot or cold air coming out of the vent? In either case, is there a significant temperature change as you move away from the vent (significantly higher or lower?)

Although the change in temperature will affect the speed of sound as it propagates away from the vent, linearly it should not change frequency. But the temperature changes, perhaps the duct and grating geometries or combination thereof may be creating nonlinear distortion and dispersion of the sound waves as they move away from the vent.

Audible frequency of an air vent increases with distance: what's going on?

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