Tone vs hole size & spacing in a short tube

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    Hole Short Tube
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Discussion Overview

The discussion revolves around the relationship between tone production and the physical characteristics of a short tube, specifically a bottomless portafilter used in espresso making. Participants explore how the size and spacing of holes in the portafilter may influence the generated sound frequency, considering both theoretical and practical aspects of sound production in tubes and membranes.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant notes the production of a tone at just over 3000Hz when blowing into the portafilter and questions if this can be used to determine hole size and spacing.
  • Another participant suggests that the fundamental frequency of a tube is determined by its length and questions whether the portafilter can be accurately treated as a tube due to its shape.
  • A participant provides a theoretical formula for frequency based on tube length, asserting that the length is the crucial factor and suggesting that the holes and diameter are irrelevant to the frequency produced.
  • One participant shares personal experience with sound production in musical instruments, relating it to the phenomenon observed in the portafilter.
  • Another participant expresses curiosity about the relationship between the holes and sound production, comparing it to the physics of a teapot whistle, while acknowledging the complexity of that domain.
  • One participant reflects on their understanding of standing waves and expresses that the physics involved is more complicated than initially thought.

Areas of Agreement / Disagreement

Participants express varying views on the relevance of the portafilter's holes and shape to the sound produced. There is no consensus on whether the holes significantly affect the tone or if the length of the tube is the sole determining factor.

Contextual Notes

Participants acknowledge that the actual working length of the portafilter may be influenced by the method of blowing into it, which adds complexity to the analysis. There are also references to fluid dynamics and standing waves, indicating a range of physical principles at play.

JT Smith
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TL;DR
tone produced by blowing into a cylindrical tube with small, regularly spaced holes at one end
While trying to dislodge some stubborn coffee debris in the holes of a wet espresso bottomless portafilter by blowing into it I discovered that it produced a tone at just over 3000Hz, with harmonics. I wondered, is there some way to use that to determine something about the hole size and spacing? Or is it more complicated than that?
 
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JT Smith said:
Or is it more complicated than that?

I guess so! :-)
 
The fundamental frequency generated by a tube depends on the tube length via a very simple equation. However before going into details are you sure the portafilter can be treated as a tube? It looks more like a membrane? Where exactly are you blowing from?
 
I'm blowing from the top. The "tube" is about 6cm long. Diameter at the top, where it's open, is about 6cm. It's cylindrical but then tapers over the last 1.5cm near the bottom to 4.5cm in diameter. There are at least 150 holes spaced evenly about 2mm apart. I think the holes are about 0.25mm in diameter. It doesn't produce a tone when it's dry.
 
That's the theory you need: https://www.compadre.org/osp/EJSS/4492/277.htm
(Both ends open part)

Diameter doesn't matter. the one and the only one crucial thing to know is the length. You say "about 6 cm" for that. The formula gives "about 2800 Hz" in that case which is near to your 3000. Since you say it is "just over" 3000 I bet the length is "just below" 5.6 cm. I mean that is the part that resonates. Small holes and other stuff are irrelevant.

(Since it seems it is both ends open you will hear all the harmonics. If one end was closed you would hear 1500 Hz and only the odd harmonics)
 
JT Smith said:
It doesn't produce a tone when it's dry.

That also makes sense. I play traditional reed flute, when you wet it much easier to play...
 
Okay, thank you for the explanation. I know about standing waves in a tube but thought this was a different problem related to the holes, maybe akin to the whistle on a teapot. I was hoping it would be a clever way to measure the hole size. The actual working length is probably affected by my mouth/lips being inserted into the top to form a seal.

Thanks again.
 
JT Smith said:
maybe akin to the whistle on a teapot

Well that is really a complicated domain: :smile:
https://en.wikipedia.org/wiki/Physics_of_whistles

When practicing the reed flute first time and trying to make a proper sound; the first thing we were trying was to avoid "whistling" which is always a risk for a beginner. Distinguishing it from the proper tone is important.

Completely different physics there, it involves fluid dynamics not standing waves.
 
Yes, I'd looked at some of that prior to posting and realized it was too complicated for me to parse. Hence my question on this forum. Fortunately none of this matters when it comes to making espresso. I was just curious.
 

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