Bell effect and flat lower register

[adms]

As a trumpet design rather than performance question, does anyone have any experience or done any experiments with the bell effect to adjust the flatness of the low register? (See http://hyperphysics.phy-astr.gsu.edu/hbase/Music/brassa.html for more about the bell effect).

I built a plastic bell that sounds good second-space A and above, but awful in the low register up to G. It's possible this is due to the material, but the resonance of the plastic (how stiff, etc.) should affect *timbre* not whether the note is flat or sharp. In other words, the plastic might eliminate the overtones, but the main note should be in-tune as evidenced by people who put a mouthpiece and a funnel in an appropriate length garden hose and make a reasonable note.

Any ideas why the bell effect wouldn't be working? Any other suggestions for why only the low register is flat (sometimes half a step) while the center of the range sounds decent? (The high range is a little sharp, but passable).

I've racked my brain trying to figure out what's going on acoustically, but running out of ideas.

Thanks for any brainstorms,
Adam

[JayKosta]

What is the construction of the rest of the instrument?
Is the plastic bell being use on a 'tuning bell' (or removable bell) trumpet.

Is the taper rate and size of the plastic bell similar to a conventional trumpet bell? And what is the total length of the plastic bell section?
_________________
Most Important Note ? - the next one !
KNOW (see) what the next note is BEFORE you have to play it.
PLAY the next note 'on time' and 'in rhythm'.
Oh ya, watch the conductor - they set what is 'on time'.

[adms]

The instrument is brass. The plastic bell adds an additional wrap to a standard trumpet bell to shorten the instrument while maintaining the overall tubing length. I shortened the mouthpipe as well, and this material was added to the length of plastic bell. Overall length is of combined tubing is 54". Plastic bell accounts for about 25" of the total. I also have a plastic 3rd slide that bends backwards. Taper rate should very closely simulate the brass bell that was removed.[/img]

[Tpt_Guy]

Are there any air leaks?
_________________
-Tom Hall-

"A good teacher protects his pupils from his own influence."
-Bruce Lee

[adms]

As the 3rd valve and bell are friction fitted in place rather than soldered, leaks are possible, but adding electrical tape to the joints to seal them didn't alleviate the problem.

Right now, middle C is somewhat flat, D thru G is sometimes a quarter tone or more flat, and A and above are OK, or can be at least reasonably lipped in tune. I can *technically* lip the low notes into tune, but only with substantial effort.

Does anyone know if some non-linear acoustics means that plastic parts should be shorter than brass ones for the same resonant frequency? Are ptrumpets and tromba plastic trumpets 54" or something else?

Thanks,
Adam

[adms]

Does anyone know of a simple spectrum analyzer application that I could use to compare the harmonics of the plastic bell to the metal one?

[JayKosta]

[adms]

The mouthpipe could be contributing, true, but I would think the mouthpipe would contribute more to the mouthpiece effect, i.e. lowering the pitch of the high harmonics, rather than causing the low frequencies to be flat, which is related to the bell effect.

I can piece the brass parts back onto the horn and do some tests to see.

The taper on the bell is an open question, and a fair one. I did try to mimic the conical angle of the cornet's original bell tail, but adding length to it obviously would alter the taper as the length is now stretched by ~2 inches compared to the original.

With the current plastic bell being 3D printed, and taking about 34 hours per iteration, I was hoping there might be experiments I could do with the current hardware that would point me in the right direction before I printed another bell to test.

So far, if it is the taper, I could make the bell taper match the original and transition to a cylindrical bore for the added length, but the amount of taper is so slight, this is probably very nearly the same shape as the existing bell tail.

As an alternative geometry question, does anyone have a sense of how bends or radiuses in tubing would alter the perceived acoustic length? If you took a 54" wound trumpet and compared it to a straight herald's trumpet, would the lengths be identical or would one be slightly shorter than the other to compensate for how the sound wave travels in a bend?

Thanks,
Adam

[Beyond16]

[Beyond16]