Aermore exhaust horn tube length

[Mark Nunn]

I have an Aermore number 2 that is missing it's longest tube. Does anyone have the overall length of the tube? I can make a guess but it would be out of tune.

PXL_20220905_135138139.jpg[/attachment [attachment=1]PXL_20220905_135120061.jpg

[Craig Correll]

Mine measures 11 1/4" for the longest tube on the No.2 Aermore. Next smaller is 9", next smaller is 7 3/4" and smallest is 5 3/4"
Hope this helps.

[Allan]

I doubt that an inch or two would make much difference to the 'tune', but it might as well be correct if going to the trouble of making a replacement.
My pedals are spring loaded and hooked to the whistle with a solid rod linkage. I can play a 'tune' by altering the pedal height and the throttle settings while driving. Many use a kickdown pedal designed to hold an exhaust cutout open. That restricts the fun you can have. Driving in a city canyon you can get a wonderful 'tune' as you take off from traffic lights and then change into high gear.

Allan from down under.

[Scott_Conger]

I suspect that an inch or two off of length will in fact make a very large difference in being a harmonious chord and not. That said, also I suspect that like a trombone, you should be able to estimate the length of the missing part and then tune it with an adjustable stop, making the final length correct for the chord played. Should be an interesting project.

[Mark Nunn]

I have a long piece of tube and considered making an adjustable plug for tuning. I have not done a tube-length/frequency analysis since college. That was a long time ago and I'm hoping to go the easy route.

Craig, I will measure my shorter tubes to see how they compare to yours. Thank you.

[namdc3]

Mark, the short answer is 13” for your particular exhaust horn.

Fulton/Aermore was inconsistent through the years, and there are short No. 2 horns/whistles and long No. 2 horns/whistles. Yours is a long and has 1-1/8” OD tubes. It is the same in both tube lengths and diameters as most No. 1s. The short No. 2 has 1” OD tubes, the longest of which is 11-1/4”. The long No. 2 and No. 1’s OAL is about 14-3/4”, and the short No. 2’s OAL is about 13”.

The three horns/whistles at left are all No. 2s. You can see at right that base nut styles, end cap stamps, tube opening styles, and even the direction in which the tube lengths cascade around the center post changed through the years.

[namdc3]

This photo shows what I believe is every size, with the possible exception of the 8” OAL motorcycle whistle if they ever made it. It is listed in some advertisements, but I haven’t come across one. They are probably out there somewhere. Generally, the size listed for the T was the second from left, an 11-1/4” OAL whistle, which is sometimes listed as a No. 3 (not stamped as such that I’ve seen). Later, they began listing the No. 2 as the size for the T.

[RajoRacer]

I believe I have the 8" motorcycle one - I'll open up the showcase tomorrow.

[namdc3]

Steve, last we emailed each other yours was 9-1/2” OAL, which I suspect is the 10” motorcycle version. The advertised lengths vary somewhat from the actual lengths. This photo shows two that I believe are 10” versions. While yours is slightly shorter than these two, I would expect an 8” to be just shy of 8”. You gave me your tube lengths, too, and they match these. I’m don’t truly know, though. Clearly I’m in the market if one should arise.

[Allan]

I had no idea that each horn was tuned to "an harmonious chord". That would demand a particular length for each tube. What effect would altering the revs of the engine, the power of any acceleration, and the variation of the amount of exhaust gas delivered to the horn?

Allan from down under.

[RajoRacer]

You have a better memory than I do for sure ! Save me from pulling the case out from the wall !

[Mark Nunn]

Thank you for this great information. I took the measurements given here and other threads to make a judgement of what I need to make. I went back to the basics of frequency as a function of tube length. The fundamental frequency of a closed tube is,

f = (speed of sound) / (4 x tube length) Solving for length is, L=c/4f where c is speed of sound and f is the desired frequency.

This gets you close to the needed length. The actual sound node is beyond the end of a closed tube. The difference for round tubes is close to 0.3 x inside diameter (ID). The length I need is: L=c/4f - 0.3 ID

The length is not the overall length of the tube. It is the length from the cut notch to the inside stopper at the end of the tube. Now, what frequency do I use? I measured my small whistle with no number on it and the larger #2 whistle (except for the missing tube). I ran the calculations and found the notes or pitch of the tubes.

The small whistle metrics with an internal diameter of .904" are:

Resonant length / frequency / Note
3.83" 881.5 HZ A5
5.56" 607.2 HZ D5#
6.77" 498.7 HZ B4
8.5" 397.2 HZ G4

The larger #2 whistle metrics with an internal diameter of 1.038" are:

Resonant length / frequency / Note
5.58" 607.2 HZ D5#
7.77" 434.5 HZ A4
9.35" 361.1 Hz F4#
11.185" (estimate) 293.7 HZ D4

I used an online tone generator to test these and they sound really good to me. This is where I will start for a replacement.