Help . Helmholtz Resonator exhaust tuning problem

[goldviper]

Hello , i have a small 155cc single cylinder bike with glass pack muffle and I am trying to do a Helmholtz Resonator (not quarter pipe) in exhaust pipe (Pic 1: dimension model , Pic 3: installed in middle ) to reduce some noise.
i use my phone app to check where is the noise frequency (pic 4: red is max db and yellow is dB when engine idle) and choose 300Hz. I find some online calculators and spreadsheets online. They have vary different answers (not much) so i double check and try one (pic 2). I choose that Speed of sound because i think that average air temperature in my Resonator ( i watch some youtube tutorial clips, and then guess some number) not too high, around 100C.
After install, i test with my phone again to see the number (Pic 4) , i surprise that the noise reduce not in frequency in calculate and much lower (or not?). In my mind , i thought if i want to reduce noise in that lower frequency , you need much larger Volume or longer pipe . You can also see my simple redraw graph to compare (Pic 5). Overall the Db line is higher but it reduce greatly at around 100Hz to 150Hz and 400Hz. But the 300Hz noise shift to lower Hz but still high. The graph after 10km ride is improve a bit but it could be the damping effect of hot glass pack muffle ?

The real world sound i hear it abit lower, i feel . It not all for nothing i guess .

Could you explain the result and graph? Was i estimated the temperature and speed of sound too low , and instead reduce the 400Hz noise ? but then why it also reduce at 150Hz? Is reduce noise at 300Hz better than 150Hz and 400Hz to overall "loudness' or what i did here is a happy accident ?
Thank you very much
Sorry for bad english

image link https://imgur.com/a/ianQctD

[MarJay]

I studied mechanical engineering at university and this is beyond my knowledge. I assume this is a four stroke engine? I may be able to help a bit more if it's a two stroke, but four stroke exhaust resonator tuning is a little beyond my knowledge tbh.
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[goldviper]

yes, it 4 stroke engine. All the thing i experiment here all start from this Blog https://ironhydroxide.blogspot.com/2017/09/drone-warfare.html
A guy try to cancel drone noise from car engine. I thought this kind of thing not too new because it happen in aftermarket car and bike exhausts system a lot, for both noise reduce and engine performance.

[Robby]

I don't know how to fix it, but it is worth mentioning that I have sometimes seen similar things on the intake side of small single cylinder bikes/scooters. I don't recall seeing one on an exhaust.

So you may get better results doing it on the intake. To get a quieter exhaust it's usually just down to making the gases travel further.

[WD Forte]

I've seen them on small engine intakes too.
Only got a passing awareness of them, but on the Op's exhaust
there doesn't seem to be much restriction on the neck when compared to the down pipe and resonating cavity.
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[Kawasaki Jimbo]

My ZX6R has them on the intakes (so enough of the ‘small’ ) but I don’t know if they’re for mor powah! or noizes.