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Thread: designing muffler for formula cars......

  1. #71
    Thanks a lot Mbrit.
    That PDF was really awesome. It explained very clearly how to decide various parameters.

    But still, it is not clear that whether we have to go for higher TI or lower TI. Since TI increases with smaller attenuation of sound and effective muffler is which has higher attenuation. this clears that we should go for smaller TI, and in the literature in PDF it is mentioned, many times, to go with higher TI.

    what do you say in this case?

    Do you have any idea, generally, in what range of frequency, sound is produced in the cars we fabricate.

    Nishant Jaiswal
    nishant09iitd@gmail.com

  2. #72
    Senior Member
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    Mikey Antonakakis's long post on page 3 of this thread is excellent advice. The sound created by your primary firing frequency at the noise test rpm for your engine's stroke is the place to start.

    Let's say you have a CBR600F4i. Its noise test speed is 11,000 rpm. (11,000 rpm/60 s)(2 pulses/rev)=366.67 Hz. This is referred to in the auto industry as the "second order". Due to exhaust cam timing, multiples of this frequency will also likely be prominent in your exhaust's acoustic signature. I'm not a 4-cylinder guy, but I would focus on the 2-8 orders to find my frequency range to attenuate. You'll find that the 8th order frequency is not high enough for the transparency index play much significance. There is much more to be gained from adding volume within the muffler shell.

    A single's primary firing frequency is the half order and, at noise test speed, is severely squashed by the curve of the A-weighted dB scale. I think we'll be seeing more creative muffler designs in the future for singles. I've got one on the dyno right now.
    -----------------------------------
    Matt Birt
    Engine Calibration and Performance Engineer, Enovation Controls
    Former Powertrain Lead, Kettering University CSC/FSAE team
    1st place Fuel Efficiency 2013 FSAE, FSAE West, Formula North
    1st place overall 2014 Clean Snowmobile Challenge

  3. #73
    High-frequency noise from turbulence is, from most of the data I've seen, often a major reason FSAE cars fail noise tests versus second order exhaust notes - poorly fabricated headers or mismatched diameters, steps in the exhaust system, any small protrusions that cause turbulence will significantly increase the high frequency noise that you generate.

    It's really easy to tune out the 11k RPM drone with a properly designed band-stop filter. Chamber mufflers are excellent at this - but most FSAE teams don't use chambered mufflers because packed mufflers attenuate a broader range of high frequencies (and restrict less, to boot!) which are usually the problem, especially considering the A-weighting scale in which high frequencies have considerably higher gain (~ -10dB gain for 300Hz sounds compared to ~2dB gain for 3k Hz sounds, relating to the equal loudness Fletcher-Munson curves of the human ear)

    If you're going so far as to actually tune your exhaust note aesthetically(which in itself isn't a bad study, since most all manufacturers spend countless hours subjectively designing the "tone" of their exhaust systems) then you have even more study ahead, but from a simple "pass noise tech" standpoint, high frequencies generated by turbulence are killer, followed by the second order frequency.
    Wesley
    OU Sooner Racing Team Alum '09

    connecting-rods.blogspot.com

  4. #74
    Sir,
    We are running short of time for designing our muffler. We are falling in deciding the perforation parameters for Muffler Tailpipe. The graph we have got is for 10KHz frequency but our requirement is below 1000Hz. We are not getting the graph in our range. TI (transparency index) of pipe can be calculated according to various iteration of perforation diameter and bar size. We are working with material SS 321 pipe of thickness 1.5mm.
    Sir, please help us in this regard.


    Thanking in anticipation.

    Yours truly,
    Nishant Jaiswal
    nishant09iitd@gmail.com

  5. #75
    Not to sound like too much of a jerk, but if you have spent 17 months on muffler design and you are roadblocked by not having a graph (in a pdf someone else provided for you, no less) in the proper frequency range, then I just have to say, I don't think the real problem has anything to do with perforation parameters.

    Have you built something, anything yet?
    Dr. Adam Witthauer
    Iowa State University 2002-2013 alum

    Mad Scientist, Gonzo Racewerks Unincorporated, Intl.

  6. #76
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    1. Use a slightly smaller diameter perf tube than you think you need.
    2. Use a smaller-diameter "tailpipe" than that.
    3. Use a slightly higher-quality fiberglass than you think you need.
    4. Use a larger-diameter shell than you think you need.
    5. Make the muffler and call it a day.

    ^^Says the guy who wasted all day at work playing with mufflers in GT-Power...
    -----------------------------------
    Matt Birt
    Engine Calibration and Performance Engineer, Enovation Controls
    Former Powertrain Lead, Kettering University CSC/FSAE team
    1st place Fuel Efficiency 2013 FSAE, FSAE West, Formula North
    1st place overall 2014 Clean Snowmobile Challenge

  7. #77
    +1, I'm still proud (in a redneck sort of way) of the modified Harley muffler on page 2 of this post. It is too loud, primarily because it goes against all of principles 1-4 that Mbirt just mentioned, but it was free, and if it wasn't for having to grind off those stupid humongous mounting bungs we probably could have finished the job in an hour, which was a smashing success at #5. Plus it's chrome.

    If you want this problem to just "go away," do exactly like Mbirt says, and as long as it's sturdy and relatively well-sealed you should have something that works well, reliably, with no calcs involved.
    Dr. Adam Witthauer
    Iowa State University 2002-2013 alum

    Mad Scientist, Gonzo Racewerks Unincorporated, Intl.

  8. #78
    hey.....
    regarding exhaust tuning its not a perfect art....partially dependent on assumptions you take...there are parameters to determine, but there are no hard and fast rules to follow.
    first of all if you are designing a exhaust system i.e complete unit, then please tell me what approach you took for designing the manifold?. regarding muffler its all about wave dynamics...
    every thing is dependent on what power band you want your max power ....that will determine your dimensions for muffler ..in the same wat the convergent cone and the divergent cone has its own theory....and have specific angle acc to what you want from the engine...!! it cant be described in a para..

  9. #79
    We are designing our own muffler this year, the building of it has been a challenge not doubt. However in terms of theory, I would look at Transmission Loss (TL) and Insertion Loss (IL). The muffler you design will have a TL for any given frequency. I suggest looking
    into exactly what frequencies you want to attenuate for your engine and design your muffler around that frequency range so that you have adequate TL at that frequency.

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