Default 4 stroke muffler outlet too small? Whats inside?
loquin
Well-Known Member
It's really a hybrid three-stage silencer.
Each 3 dB represents a doubling of the sound energy, so your 7 db increase is more than 4 times the sound energy.
The fact that the holes aren't deburred is usually intentional. The sharp edges/protrusions add turbulence, and act to shift some of the sound energy up in frequency. (you'll get some 'whistling' as the exhaust pulses move past the sharp edges) But, higher frequency sound is more easily reduced by low-pass filters, so, these burrs make the later stages a little more effective at attenuating the sound.
- Stage one is the 6 holes in the side of the inlet pipe in the open portion of the muffler. This is a side resonant silencer, and is designed to absorb sound at a specific frequency.
- Stage two is the open chamber portion, and is a type of diffusing silencer. It's designed to only pass those frequencies which resonate in the chamber, and is a type of band pass filter. These two stages are combined into one open chamber, rather than being separate, so, this is what makes this a hybrid silencer. It's probably less effective than adding a separate, divided chamber, though. (The 'step' in cross sectional area of the chamber where the inlet pipe ends acts as a type of wall, however, and reflects some of the sound energy back, and acts like a type of wall...)
- Stage 3 is the portion with the holes, and it's not a spark arrester. This is a side resonant silencer stage, and it is intended to absorb sound at a specific frequency, such as the such as at the fundamental exhaust pulse frequency of the engine. As such, it is a type of notch filter. Now, the total area of the 8 holes is apx. 0.61 square inches, and the area of the 1 inch pipe that they're in is apx. 0.79 square inches. As the total area of the side holes gets closer to, and then exceeds the area of the pipe that they're in, it's performance shifts away from the side resonant silencer towards a diffusing silencer.
Each 3 dB represents a doubling of the sound energy, so your 7 db increase is more than 4 times the sound energy.
The fact that the holes aren't deburred is usually intentional. The sharp edges/protrusions add turbulence, and act to shift some of the sound energy up in frequency. (you'll get some 'whistling' as the exhaust pulses move past the sharp edges) But, higher frequency sound is more easily reduced by low-pass filters, so, these burrs make the later stages a little more effective at attenuating the sound.
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pst wanna know a secret, the best muffler for both noise reduction and perfomance, its the Briggs round tube 4 hp muffler, weld it on the end of a 17-22" long header.
Its quieter than a "gutted" can muffler and flows very well.They are cheap too, I pay about 2 bucks for them.
Its quieter than a "gutted" can muffler and flows very well.They are cheap too, I pay about 2 bucks for them.
professor
Active Member
Good info here.
Loquin, I'm impressed with your specific knowledge!
And Bob- thanks for the reminder of how good those little muffs are.
Loquin, I'm impressed with your specific knowledge!
And Bob- thanks for the reminder of how good those little muffs are.
I
ibdennyak
Guest
Loquin, thanks for the specific info. I've been trying to design a muffler for a generator, and your knowledge just explained a lot of things.
and Old Bob, that helps too. Gonna try that on one of my bikes. Ummm, just how old are you anyway?
and Old Bob, that helps too. Gonna try that on one of my bikes. Ummm, just how old are you anyway?
loquin
Well-Known Member
I bought a chapter of an engineering textbook online; the book deals with two-stroke engine design, The chapter I bought, pertains to silencing. It was a tough read (since ibdennyak: it's been many moons since I took any engineering classes, or calculus classes,) but I've summarized/condensed it to an article which I'll be posting, along with a program to calculate the silencing characteristics of various muffler configurations. (the original was published by the textbook author in GWbasic; I'm in the process of converting it to Visual Basic) Stay tuned.
Old Bob: The bulk of the silencing from the extended pipe is because the length of pipe forms a tuned chamber, which will pass sounds with frequencies which resonate at the pipe length... When you play the trombone, and you lengthen the tube, the notes produced are lower. The same effect is in play with the relatively long chunk of pipe. Then, the muffler at the end silences these frequencies some, as well as further reducing the sounds not passed well by the pipe. As you accelerate, and the RPM's climb, you be getting peaks in the sound at some RPMs, then it will drop off, and an octave higher, the sound will peak again... The trick is in picking a pipe length which doesn't resonate at the speed where you want to cruise.
Old Bob: The bulk of the silencing from the extended pipe is because the length of pipe forms a tuned chamber, which will pass sounds with frequencies which resonate at the pipe length... When you play the trombone, and you lengthen the tube, the notes produced are lower. The same effect is in play with the relatively long chunk of pipe. Then, the muffler at the end silences these frequencies some, as well as further reducing the sounds not passed well by the pipe. As you accelerate, and the RPM's climb, you be getting peaks in the sound at some RPMs, then it will drop off, and an octave higher, the sound will peak again... The trick is in picking a pipe length which doesn't resonate at the speed where you want to cruise.
I
ibdennyak
Guest
Hey thanks Lou, especially for the *condensing/summarizing* part. I did fool around with an article concerning 2 stroke ultralites in Scandinavian countries that are flat out banned from flying unless they are under a certain db level. I did have minor success at sound deadening, but the darn muffler ended up weighing almost as much as me.
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I'm well aware of that, I have a dog eared copy of Phillip Smiths in my library, the only book I don't have is Gordon Blairs 4 stroke
book, I have everything else including Blairs 2 stroke software and excellent books on two strokes along with wave tuning software for engine development and a muffler program.
With that said, the length of pipe silences only from the volume of the pipe not the length. The length of the pipe will effect the pulse wave activity and the inertia effects.
You could silence an engine with an empty can large enough.
You do actually want the pipe tuned to produce peak torque at cruise speed,IE resonanting, to gain max fuel economy, peak torque generally coincides with the lowest BSFC.
There really is no trick in picking pipe lengths, they can be calculated conveinently for a starting point thats very close to optimum.
The 4 hp briggs muggler reduces the sounds pressure level of the Honda exhaust note whether its mounted on the head or any length of pipe.
loquin
Well-Known Member
Gotta disagree about the silencing not depending upon the length of the pipe/chamber, and only the volume. It's just not so.
The harmonic frequencies of sound inside a three-dimensional object are definitely related to the length of the object, AND are related to the diameter of the pipe (and to the height and depth of a rectangular cross section)
These are sounds with wavelengths 2 times the lengths, 1 times the length, 1/2 the length, 1/3 times the length ... 1/N times the length and diameter (or width, and height) of the chamber.
In fact, in loudspeaker design, it's very important to avoid having the length of the sides be related, else the resonant frequencies can reinforce each other and contribute to bad sounding speakers. It's actually a good idea to have the internal length, width and height of the speaker enclosure be related by the golden ratio (apx. 1.618:1) to minimize reinforcement of resonant frequencies and their harmonics.
The harmonic frequencies of sound inside a three-dimensional object are definitely related to the length of the object, AND are related to the diameter of the pipe (and to the height and depth of a rectangular cross section)
These are sounds with wavelengths 2 times the lengths, 1 times the length, 1/2 the length, 1/3 times the length ... 1/N times the length and diameter (or width, and height) of the chamber.
In fact, in loudspeaker design, it's very important to avoid having the length of the sides be related, else the resonant frequencies can reinforce each other and contribute to bad sounding speakers. It's actually a good idea to have the internal length, width and height of the speaker enclosure be related by the golden ratio (apx. 1.618:1) to minimize reinforcement of resonant frequencies and their harmonics.
