# Whistle Design - Optimizing sound initiation pressure and volume

Assume you can use any whistle type, is there a way to choose the pressure point at which a whistle makes noise? For example I have a whistle that starts making noise at 2 psi but I would like to lower that to 1 psi. Would I change the area of the inlet, outlet? what feature would you change to accomplish this?

• Personally I would change the length of the whistle, specifically the inner chamber. Sound pressure decreases by half, each time the length of the whistle is doubled. Jan 11, 2018 at 9:43
• @loogle can you expand on that an post it as an answer? Jan 11, 2018 at 12:41
• @loogle - yes please explain your comment. do you mean that if I double the length the whistle will start making noise at half the psi the prior whistle needed to make noise? Math: Sound starts at = n = 2psi; Max sound at = m = 3psi; Whistle Length = L = 2 inches if L = 4 then n = 1psi? 2L results in n/2? Jan 11, 2018 at 16:54

## 2 Answers

The most serious and comprehensive information about whistles is provided by the folks who work on pipe organs: expensive instruments which endure critique by serious listeners, and which require maintenance where they are installed because of their size. A pipe organ's Flue Pipes are in fact no different from whistles.

A whistle is a noisemaker that uses no reed. In this sense, we might say that a brass player's lips and a singer's vocal cords are reeds, while a flute player is playing a kind of whistle.

A whistle has two components: a resonator and an air supply. The resonator's opening may be called its mouth, and the air supply must flow freely past the resonator's mouth in order for it to work.

The air flows over the resonator's mouth and sucks out a bit of pressure from the resonator by the Bernoulli principle. Once this pressure is lower inside the resonator, the free air supply flow changes course and starts packing into the resonator, increasing its pressure. This sends a high pressure wave through the resonator which may either bounce back (for a closed resonator) or depart (for an open resonator).

At some moment in time while the air is flowing into the resonator, the pressure inside increases enough to divert the airflow away from the mouth of the resonator. Once the flow is diverted away from the resonator, it begins to remove air from inside and lower the pressure, creating another pressure wave inside. Now the cycle repeats.

The sound you hear originates both from the opening at the top of the resonator and an opening at the bottom (for open resonators) where there is this fluctuating air pressure.

The sound is at a tuned pitch (or frequency) that is the result of the pressure wave bouncing from top to bottom, bottom to top in the resonator. The frequency is a simple matter of length with flue pipes, but I can't even explain what's going on in an ocarina.

How can I reduce the pressure of my whistle?

In order to make a sound, your instrument must initiate the aforedescribed cycle, and also convert the air source's energy into acoustic energy. Improving either (by initiating sound earlier or converting energy more efficiently) may enable your instrument to use less pressure. Here are some conditions which may affect either, in order of my guess for the most important first:

• Velocity of air supply (high velocity = earlier initiation of tone)
• Cross-sectional area of resonator near its mouth (smaller cross-sectional area = earlier initiation of tone)
• The resonator being stopped or open (stopped = earlier initiation of tone)
• Volumetric flow of air supply (high flow = more volume)
• Length along the flow direction of the resonator "mouth" (must be optimized)
• Length across the flow direction of the resonator "mouth" (must be optimized)
• Distance that the air supply travels freely (must be optimized)
• Laminarity of the air supply (unknown effect by me whether more or less sound)
• You might increase the laminarity by passing the air supply through a long, straight tube, for instance.
• Intended pitch of the resonator (higher pitch = more sound, to a point)
• Length of the resonator (shorter length = more sound, to a point)
• Atmospheric condition (ambient wind may "kill" the free flow of the air supply)
• Cross-sectional area of the resonator's other opening for open resonators (bigger rear opening = more volume)
• There's a reason tubas, gramophones, Alpenhorns, and megaphones have a flared end: the smaller volumetric velocity produced by the large cross-sectional area enables more sound energy to transfer into the open air, making the best use of the player's effort. This is called impedance matching.

As suggested elsewhere, dividing your airflow among multiple resonators may solve your issue. Good luck!

Further reading on pipe organ flue pipes ("whistles")

• Hi Elliot, Could I explain another environment/ set up. I'm interested to see if I can get a sound from it, if its possible. I've done some testing using what you mentioned but haven't been successful. I think mostly do to "Atmospheric condition" Jul 16, 2018 at 13:58
• Hi! Have you tried using a funnel to make the air flow more intense? Jul 16, 2018 at 14:07
• Environment: open area living room. 72 degrees Source of air flow: 24" by 24" Box fan with a 24" by 24" by 24"card board box taped to sucking air side of the box fan (this is here to create a air tight chamber). I am then going to cut a opening in the cardboard box in which to stick the whistle or anything that would make noise. The box fan can normally pull air through on the sucking side at 20mph 900 cfm. I tried using a beer bottle and get the air passing throw the hole I cut to make the beer bottle whistle. it didn't work. I tried just sticking a kazoo in the hole. nothing. Jul 16, 2018 at 14:15
• I have not used a funnel, I will give this a go Jul 16, 2018 at 14:43
• I think the sucking side may be harder to turn into a whistle than the blowing side, since the jet created by the hole in the box begins to spread out toward the fan blades once it enters the box (presuming the whistle is in the box). Or if you are placing the whistle outside, there's no jet anywhere on that side, so the mouth of the whistle won't do anything. I do suggest using the blowing side instead of the sucking side... if you want, you can make a manifold with several funnel jets for several whistles at the same time! Jul 16, 2018 at 15:01

A whistle works based on flow. For a given whistle design, a certain flow is required to generate the pressure oscillations which generate the sound. You can think of it like suddenly filling the resonator chamber with air, then draining it with the venturi effect, over and over. Below a certain flow rate, the flow is laminar and just exhausts without oscillation.

The volume of the chamber will change the freqency of the sound because it takes more time to fill and drain a larger volume. This will in turn change transmission and energy characteristics of the emitted sound. However, it will have a very small effect on minimum flow required to produce sound.

If you wish to have a whistle trigger off of pressure you will need to configure your system to have your pressure trigger point deliver the minimum airflow to the whistle.

To go from a low pressure to a higher pressure trigger point on the whistle, you can restricting the airflow with a throttling valve, orifice, or a small pipe. Alternatively, you could divert part of the flow to a different path or even a second whistle in parallel.

To go from a high pressure to a lower pressure trigger on the whistle, you will have to decrease the air passage area such that the operational pressure drop across the whistle is high enough for the lower pressure to deliver enough flow.

• do you want to make the whistle sound using the pressure of the wind, as for instance next to a moving vehicle? Jan 12, 2018 at 21:00