Experiments in sound mitigation
Mar 24, 2023 20:28:41 GMT -5
90GTVert, 190mech, and 1 more like this
Post by rollingbender on Mar 24, 2023 20:28:41 GMT -5
This thread is ultimately a part of the “transformation of a Chinese scooter” thread but I thought that this little side trip would make a better thread all it’s own.
I am definitely not a member of the church of loud-pipes-saves-lives. When I was an out of control teenager, I discovered that if you are going to be doing stupid stunts in a vehicle, the last thing you want is a loud exhaust to draw attention to your shenanigans. So, I’ve always had quiet vehicles. I want the scooter to be as quiet as any granny’s car on the road.
I’ve done a ton of research on the subject over the past few months including a lot of YouTube videos of people trying (and usually failing) to quiet gas generators, riding lawnmowers, and UTV’s. I think I have a design plan that will get the scooter motor exhaust noise to be the least of the noise coming from the engine. It remains to be seen (and heard) what noise is actually the intake and the case itself.
My design will be 3 chambers. The first chamber going into the muffler will have the intake pipe. The intake pipe will be slotted and scooped, hopefully causing the maximum amount of disruption of sound waves and absorbing the pulse wave from combustion. Here’s what I have come up with as a way to make this tube. First I did a layout dividing the tube into 6 equal sections. Then I sawed through the 1/3 of the tube on each side leaving 1/6 of the circumference intact. Move down the tube along its length 1/6 of the circumference and over 1/6 and do it again. I just happened to do it 7 times on my test piece for a total of 14 slots.
Then I took a hammer and a punch to the tube and bent in one side of each slot.
Thus creating a tube with a bunch of different “scoops” inviting hot, fast moving gases and sound waves to exit the tube. The intake tube will then dead end at the end of the first chamber and two more similarly slotted and scooped tubes will pick up those gasses (now traveling a bit slower and with less of the pulse and also the sound waves that manage to survive being bounced off each other and carry this mess into the second chamber.
The second chamber will be similarly piped as the first chamber but only in reverse…two slotted pipes coming in, one slotted pipe going out.
The third chamber will be essentially a classic glass pack design. The single pipe going through the third chamber will be perforated thusly…
The perforated pipe, as it travels through the third chamber, will be wrapped in a comfy blanket of high temp fiberglass wool.
All of this will be encased in a 10” long, 3-1/2” OD steel pipe. The intake end will be flat with 3 threaded holes for attachment to a standard GY6 header pipe. All of the internal piping will be .825” ID.
I found a bunch of steel discs that were drops from a local manufacturer laser cut in holes for whatever they were making. I’ll be using those as the end caps and walls between the chambers. I’ll have to cut all of the necessary holes in these plates on the CNC mill so everything lines up nice, fits tight, and doesn’t look like something some guy slapped together in his garage.
I am definitely not a member of the church of loud-pipes-saves-lives. When I was an out of control teenager, I discovered that if you are going to be doing stupid stunts in a vehicle, the last thing you want is a loud exhaust to draw attention to your shenanigans. So, I’ve always had quiet vehicles. I want the scooter to be as quiet as any granny’s car on the road.
I’ve done a ton of research on the subject over the past few months including a lot of YouTube videos of people trying (and usually failing) to quiet gas generators, riding lawnmowers, and UTV’s. I think I have a design plan that will get the scooter motor exhaust noise to be the least of the noise coming from the engine. It remains to be seen (and heard) what noise is actually the intake and the case itself.
My design will be 3 chambers. The first chamber going into the muffler will have the intake pipe. The intake pipe will be slotted and scooped, hopefully causing the maximum amount of disruption of sound waves and absorbing the pulse wave from combustion. Here’s what I have come up with as a way to make this tube. First I did a layout dividing the tube into 6 equal sections. Then I sawed through the 1/3 of the tube on each side leaving 1/6 of the circumference intact. Move down the tube along its length 1/6 of the circumference and over 1/6 and do it again. I just happened to do it 7 times on my test piece for a total of 14 slots.
Then I took a hammer and a punch to the tube and bent in one side of each slot.
Thus creating a tube with a bunch of different “scoops” inviting hot, fast moving gases and sound waves to exit the tube. The intake tube will then dead end at the end of the first chamber and two more similarly slotted and scooped tubes will pick up those gasses (now traveling a bit slower and with less of the pulse and also the sound waves that manage to survive being bounced off each other and carry this mess into the second chamber.
The second chamber will be similarly piped as the first chamber but only in reverse…two slotted pipes coming in, one slotted pipe going out.
The third chamber will be essentially a classic glass pack design. The single pipe going through the third chamber will be perforated thusly…
The perforated pipe, as it travels through the third chamber, will be wrapped in a comfy blanket of high temp fiberglass wool.
All of this will be encased in a 10” long, 3-1/2” OD steel pipe. The intake end will be flat with 3 threaded holes for attachment to a standard GY6 header pipe. All of the internal piping will be .825” ID.
I found a bunch of steel discs that were drops from a local manufacturer laser cut in holes for whatever they were making. I’ll be using those as the end caps and walls between the chambers. I’ll have to cut all of the necessary holes in these plates on the CNC mill so everything lines up nice, fits tight, and doesn’t look like something some guy slapped together in his garage.
