When will an airflow cause whistling?

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Discussion Overview

The discussion revolves around the conditions under which airflow through a slit or gap may produce a whistling sound. Participants explore various factors influencing this phenomenon, including geometry, air velocity, and acoustic properties, without reaching a consensus on a definitive approach or solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how to determine if airflow through a gap will cause whistling, expressing uncertainty about the shapes and air velocities involved.
  • Another participant suggests that in dust collection systems, keeping airflow below 5000 feet per minute may reduce noise, but acknowledges uncertainty regarding open orifices and the influence of the angle of incidence.
  • A different participant states that aeroacoustic simulations may be necessary to understand whistling, noting that factors such as orifice geometry and airflow details are critical, as well as the oscillation of the shear layer at the orifice.
  • One suggestion is to consult terminal air device catalogs to find similar devices and their acoustic properties.
  • A participant shares a personal experience with nasal cannulas that produced variable whistling sounds, indicating that the phenomenon can be unpredictable.
  • Another participant mentions the concept of Helmholtz resonance as relevant to the discussion.

Areas of Agreement / Disagreement

Participants express varying opinions and experiences regarding the factors that contribute to whistling sounds in airflow, indicating that multiple competing views remain and the discussion is unresolved.

Contextual Notes

Participants highlight the dependence on specific geometries and airflow conditions, as well as the potential need for simulations, which suggests limitations in the current understanding of the phenomenon.

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Is there any good approach to determining if an amount of air passing through a given slit or gap will cause a whistling sound? I'm unsure with what shapes whistling might become an issue, and also at what air velocities.
 
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I don't know. But generally in dust collection (which is allowed to be loud) we try to stay below 5000 fpm through ducts as above that you tend to get lots of noise. Through an open orifice, I have no idea off the top of my head. It also depends on the angle of incidence between the air path and the orifice (think of a flute or soda bottle)
 
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There isn't a good approach of which I am aware. To really figure it out you will likely need some aeroacoustic simulations. It's going to depend on the geometry of the orifice, slit, or gap, the geometry of the vessel through which the air is flowing, and the details of that air flow. The whistling is caused by the oscillating of the shear layer that forms at the orifice. The amplitude and frequency of those oscillations as well as whether or not the surroundings are capable of resonating with that frequency are going to determine how audible any whistle is going to be. Depending on the geometry in question, there are going to be a few rules of thumb to help roughly determine the frequency, but that won't give you much information about whether it is going to be audible.
 
My suggestion would be to flip through some terminal air device catalogues, find some devices similar to what you are picturing, and see what their acoustic properties are.

Try: www.titus-hvac.com
 
It isn't restricted to commercial stuff. I've had cannulas (nasal prongs) for my oxygen equipment that whistled so loudly in my nose that I couldn't go to sleep. It seemed to be at their whim: sometimes they did and sometimes they didn't. When they did, I would bend and twist and beat them until they got down to a level that I could eventually ignore.
 
The effect is called Helmoltz resonance
 

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