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Old 09-20-2012, 03:00 PM
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Accoustic Resonance of an open tube HELP

So I am trying to make sense of the different varying formulas that I have found. Some of the formulas are not fully explained hence my questions. This is a "musical exhaust" related topic.

I am attempting to figure out what the formula is to determine the frequency of a stainless steel pipe (the one constant) with air pushed through it with these following variables.

( A ) Length of pipe
( B ) Wall thickness of pipe
( C ) CFM going through the pipe
( D ) Inner Diameter of pipe

Also I have read that overblowing a pipe will cause the frequency of the note to jump an octave. At what point is it considered to be overblowing a pipe? Is there a formula for the max cfm prior to the said overblowing? I am working with 57132.3958333 cfm btw

this is the closest thing i could find.
Acoustic resonance - Wikipedia, the free encyclopedia

it does not say what the value of the integer is supposed to be.
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Old 09-20-2012, 04:42 PM
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I happen to know someone who rebuilds church organs.

Is that how much cfm coming out of the cylinder, at what rate, and pressure?

You want an actual frequency of tone at 1 5/8" diameter pipe at what length?
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Old 09-20-2012, 05:46 PM
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It sounds like your trying to make sense of something. If it is back pressure you can use a manometer to determine what pressure you have. A port in front of the muffler with a gauge attached is a easy way. It is common for people sat 2-3 psi is excessive back pressure.

Musically if you wanted a E or A note to be sounded that is a different. You cannot have length,diameter, and pressure a variable.
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Old 09-20-2012, 09:42 PM
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"embouchure" is what determines the pitch in a brass instrument,that is how octaves are raised,,,,sorta,that combined with variable length pipe. The horn amplifies the vibration of the players lips. The volume of air varies the sound level,the variable vibrations change the pitch and the tube length
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Old 09-21-2012, 02:19 AM
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What would be the formula then with a regular pipe? no variations just that particular length and diameter etc.
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Old 09-21-2012, 02:22 AM
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Quote:
Originally Posted by spinn View Post
It sounds like your trying to make sense of something. If it is back pressure you can use a manometer to determine what pressure you have. A port in front of the muffler with a gauge attached is a easy way. It is common for people sat 2-3 psi is excessive back pressure.

Musically if you wanted a E or A note to be sounded that is a different. You cannot have length,diameter, and pressure a variable.

Lets talk exhaust - what if we could give the exhaust of an engine based on the size of the piping a note. An engine is just an air pump. If the only thing you heard was the exhaust what note would it give for example of x length, y diameter, etc.
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Old 09-21-2012, 06:13 AM
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Horns are different. Like a buggle there is mouth peice articulation. If you were cruising at constant speed you could tune your exhaust to make a note. Did somebody tell you a F makes more efficient power than an A?
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Old 09-21-2012, 10:23 AM
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Horns are different. Like a buggle there is mouth peice articulation. If you were cruising at constant speed you could tune your exhaust to make a note. Did somebody tell you a F makes more efficient power than an A?

not about efficiency although one technically can tune the header frequencies to correspond to pulses apparently. this is beyond the header. This is just about producing a particular sound out of exhaust not considering a muffler. The muffler has baffles and cause the exhaust system to act like a closed end organ.
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Old 09-21-2012, 10:59 AM
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I am working with a 336.6 CID sbc. At 7000rpm VE is at 83.8. Peak VE is at right around 5500rpm being at 92.1

RPM is a factor of cfm, it doesn't make sense to me that cfm is not a factor in the sound and the rpm is seeing that they are related. The higher the rpm the higher the cfm and vise versa.

The formula should technically be able to find the frequency giving the cfm comming out of the engine. valve sizing as far as I can see and all other factors of the engine cause the ve to be where it is at, so ve is the only number that should matter there. which again is a factor of the cfm.

Also one should be able to even out the pulses of the exhaust running first through an x pipe back into a single pipe. That single pipe is the one that matters in this formula.

As for the formula given I understand that "N" is an integer being "1,2,3" what I don't get is what it is supposed to represent. It says the resonant node, are they trying to say the frequency that we are attempting to achieve, or something else that I am missing here?

For example. if f=nv/2(L+.3d) and the req we are looking for is say again middle c (261.626 hz) at 20c, which would make speed at 343 mps (meters per second) the length is say about 1 meter and the diameter is .0254 meters (1 inch) then f would equal 44546.0087328hz which makes no sense to me that appears to be very high. A 3 foot open pipe with 1 inch diameter openings. Unless that happened to be the limit of the pipes ability to make x amount of HZ with max amount of allowable cfm to travel through the pipe. Which this does not tell us when it is too much cfm for the pipe and then the octave raises.

Show me where I am wrong here, because I feel like I am missing something.
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Old 09-21-2012, 12:20 PM
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Quote:
Originally Posted by blight View Post
I am working with a 336.6 CID sbc. At 7000rpm VE is at 83.8. Peak VE is at right around 5500rpm being at 92.1

RPM is a factor of cfm, it doesn't make sense to me that cfm is not a factor in the sound and the rpm is seeing that they are related. The higher the rpm the higher the cfm and vise versa.

The formula should technically be able to find the frequency giving the cfm comming out of the engine. valve sizing as far as I can see and all other factors of the engine cause the ve to be where it is at, so ve is the only number that should matter there. which again is a factor of the cfm.

Also one should be able to even out the pulses of the exhaust running first through an x pipe back into a single pipe. That single pipe is the one that matters in this formula.

As for the formula given I understand that "N" is an integer being "1,2,3" what I don't get is what it is supposed to represent. It says the resonant node, are they trying to say the frequency that we are attempting to achieve, or something else that I am missing here?

For example. if f=nv/2(L+.3d) and the req we are looking for is say again middle c (261.626 hz) at 20c, which would make speed at 343 mps (meters per second) the length is say about 1 meter and the diameter is .0254 meters (1 inch) then f would equal 44546.0087328hz which makes no sense to me that appears to be very high. A 3 foot open pipe with 1 inch diameter openings. Unless that happened to be the limit of the pipes ability to make x amount of HZ with max amount of allowable cfm to travel through the pipe. Which this does not tell us when it is too much cfm for the pipe and then the octave raises.

Show me where I am wrong here, because I feel like I am missing something.
As you first said, RPM matters because it sets a frequency and that is the frequency (or some harmonic or sub-harmonic thereof) that you want your pipe to resonate at if you want the max volume.

As you said, CFM doesn't matter. Again look at a pipe organ. The "Church" or "Bach" style pipe organs work at low pressure and lower CFM than a high pressure "theater organ" (e.g., Mighty Wurtlizer), but both use the same length pipes to produce a given note.

In tuning a pipe, the pipe typically tunes to the frequency based on its length, but capping the pipe reduces this length by about half. With a bunch of baffles in the muffler, it's hard to say which of these it would use.

"Overblowing" is typically reserved for wind instruments and is a technique done using the muscles and positioning of the mouth/lips (embouchure), similar to a glissando. You can actually change the pitch based on the way in which you blow into/across the mouthpiece and the force of which it is done. I don't think that this is something that can be easily accomplished in the exhaust system.

Listen to from 15 to 19 seconds and you can hear the effects of "overblowing" to produce a glissando on a clarinet. You can easily overblow the next octave up. I don't think you can easily accomplish this in an exhaust system.

Yahoo! Video Detail for Gershwin plays Gershwin: Rhapsody in Blue
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