Bonneville America Forums Forums Technical Forums Exhaust Exhaust Acoustics

I've been considering this topic for some time, and I think I'm beginning to understand how bike exhaust systems affect the sound of the exhaust note. For example, glasspacks usually have a cylindrical chamber surrounding the central flow path. Exhaust gas can enter the chamber, packed with sound-deadening fiberglass, only through a pattern of small holes. I surmise that because the holes are small, they create a low-pass filter that passes low frequencies best. Thus, those frequencies - including the rumble we like to hear - are quieted the most by the fiberglass in the chamber. Higher frequencies encounter more resistance from the small openings, and are attenuated less. This is the reason glasspacks are so quiet at idle, but become louder (and "poppier") as combustion pressures or RPM increase.

This makes me wonder what a pipe would sound like if all the exhaust had to pass through a large plate with a network of small holes (without fiberglass). Would the plate attenuate the more obnoxious high frequencies, but pass the lower frequency energy that sounds so good?

Anyone here tried any experiments along these lines?

The answer is yes and resulting in an extremely annoying sound and I like loud pipes. Brokenfixed had a set of Dunstalls on his bike and that is what they were open with a perforated metal plate near the end of the pipe for the exhaust to escape through. I hated the sound.

Well, that's not quite what I meant, so I probably should have been clearer. This can't be accomplished with a small plate near the end of the pipe. It wouldn't have enough surface area. Without a lot of surface area, the holes have to be too large, otherwise flow suffers. If the plate has a really large surface area, the holes could be made much smaller, attenuating the upper frequencies without restricting flow. I was thinking about a plate that would be nearly the length of the entire pipe. Imagine a long, skinny oval, drilled with a few thousand holes. It would be welded into the pipe at a diagonal, so all gas entering the pipe has to go through the plate. I think that might sound a little better.

You're going to have a couple of variables in here to consider as the length of the tube is as large a consideration as the type of baffling especially when it comes to the length of the wave you're trying to enhance or suppress. In general short pipes will put out more high end and longer pipes will put out more lows; the baffles will adjust somewhat for frequency nodes as well as damping the overall volume.

I didn't understand but it seems that is what the plate in Tom's pipes did. It was a baffle with no large hole but a series of 1/8" to 1/4" holes drilled all around the edge. I would have to see them again for the exact configuration and hole size. It had a sharp higher tone sound that I found annoying, he seemed to like it though.

I have FMF "powercore" aftermarket performance exhaust systems on all of my Yamaha dirtbikes, and a DG preformance system on my Yamaha quad. All of them four strokes of course.
The one thing they have in common is the inter baffle. They are ALL the free flow design, like old glasspacks.(I know, as I repack them all the time)
Now we can't argue that performance gains are what these manufactures are after, not necessarily sound. And all of them have a straight threw baffle, the same diameter as the header pipe.
I made my own baffles from a sheet of expanded metal from Home Depot, and cut the ends from some 18 ga. file cabinet doors. Wraped them with the same packing material I use for the other mufflers.
I LOVE THE SOUND I NOW HAVE! and imaginary or not, I beleive the acceleration is better.
Thought I post a couple pics showing the stock baffles side by side with mine.