Custom exhaust drone issue cure?

[Submariner1]

Calculation for resonant frequency of a closed tube:

Where v is speed of sound, L is length of tube, d is diameter of tube.

Yes diameter does matter.

OK I am confused
If one took and 850mm pipe say 50mm diam ( circa 33" x 2" )

Your calculation would be
343 m/s = V
Divided by
2x ( .850 plus 0.8x 0.050) i.e. 2x 0.890 = 1.78
343/1.78 = 192 Hz

To get down to say around 100 Hz with your formula one would need a pipe about 1.7 meters long thats 66" !!

See this where they ignore the diameter

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/clocol.html

F = V of sound / 4 L
343/ 4x 0.850
>
343 / 3.4 = 100Hz
This seems more like it. As the USA guys all said the real drone on a big V8 was about 100Hz and they all seem to use between 28" and 32" .

So my head says you must be right .... blow over different diameter bottles and the note changes ... yet that scientific link seems to ignore the diameter ???
But that calculation seems to mirror what the US guys do.

Apologies if I am being thick, but is your calculation right?
Although this US Link seems right, my brain cannot comprehend why in their case diameter does not factor.
Or is their some hidden logic, i.e the tubes must all be the same diameter for their calculation to be valid??

Love to understand this but its really hard for me to understand.

 

[LostKiwi]

Ah - my bad. I gave the open tube formula.

It should be 4 on the bottom line not 2.

In other words f=v/(4(L+0.8d))

 

[Submariner1]

Ah - my bad. I gave the open tube formula.

It should be 4 on the bottom line not 2.

In other words f=v/(4(L+0.8d))

Thanks that makes total sense.

 

[cl500fox]

cut outs are excellent got them on my w215 flick of a switch normal flick of a switch bedlam !!!
will get a video if anybody wants to hear them

 

[alexanderfoti]

Calculation for resonant frequency of a closed tube:

Where v is speed of sound, L is length of tube, d is diameter of tube.

Yes diameter does matter.

Sorry to drag out an old thread. What is L in the above equation, the length of the existing exhaust?

 

[LostKiwi]

L is the length of the resonating tube (in this case the heimholz resonator we are adding).

 

[alexanderfoti]

L is the length of the resonating tube (in this case the heimholz resonator we are adding).

Aha that explains why I cant figure it out.

Is there a calculation to determine the drone frequency, or do I need to drive the car around whilst recording it?

 

[LostKiwi]

You would need to measure the existing frequency and then calculate a the length required.
Or... make up a pipe with a sleeved overlap and vary the length to remove the drone.
Typically drones are around 100Hz or so

 

[alexanderfoti]

OK so I recorded me driving around and analysed the resulting recording. Peak frequency appears to be 86Hz and is strong until around 110hz.

So I think I will use 100Hz in my calculations.

Gives me a pipe length of 85cm....

That is rather long

 

[LostKiwi]

That sounds about right. US sites say about 33" - 36"

 

[alexanderfoti]

Cool! Spoke to the local place round the corner. £550 which is steep for the work.

Spoke to fast lane as recommended by OP. £250 tops. Much better! Will probably do the latter!

 

[LostKiwi]

Cool! Spoke to the local place round the corner. £550 which is steep for the work.

Spoke to fast lane as recommended by OP. £250 tops. Much better! Will probably do the latter!

£550 is definitely steep for what is essentially two lengths of pipe capped at one end welded to the existing system.

 

[alexanderfoti]

£550 is definitely steep for what is essentially two lengths of pipe capped at one end welded to the existing system.

I agree. I get the impression they are not 100% sure on the whole concept/execution so are pricing it high. I completely understand and would do the same thing if I where in that situation.

 

[alexanderfoti]

To finish off the calculations, I have two resonant peaks, one at 107.9hz @ -26db and one at 53.3hz at -33db.

Assuming an avg exhaust gas temp of 60 degrees C (near the rear backboxes) then the following calculation results:

speed of sound @ 60deg c (365.9) / freq (107.9) = 3.39M / 4 = 84CM

I understand the above calculation is for a 1/4 wave cancellation rather than a Helmholtz tube which I understand is normally 1/2wave?

 

[LostKiwi]

Your two resonant peaks are harmonics - note 53.3 is almost half 107.9....
Yes that is correct 1/4 wave cancellation.

 

[alexanderfoti]

Your two resonant peaks are harmonics - note 53.3 is almost half 107.9....
Yes that is correct 1/4 wave cancellation.

OK so my understanding is that one tube will deal with all other harmonics to a certain extent, but only in the frequencies higher than its cancelling.

So if I install a 84cm tube it it will only cancel 107.9hz and harmonics upwards of that but If I install a 171cm long tube it will deal with both.

There are obvious issues with 171cm long tubes though!