Reduce Exhaust Drone by Building a Helmholtz Resonator

What's up guys? I hope you're having a fantastic day today. What I'm going to show you how to do today is to build a Helmholtz resonator for your exhaust. Now, you're probably wondering, what is a Helmholtz resonator? Well, you know that annoying drone you have sometimes with an aftermarket exhaust system when you're driving down the highway? Well, the Helmholtz resonator will actually cancel out certain frequencies in your exhaust system so that it won't be so annoying on the highway. So that's what we're going to learn how to do today. Stay with us. We're going to do that right now. Alright guys, so what you're going to have to do now is basically just go for a drive and go the speed you normally go on the highway and figure out what frequency is resonating at that speed where you're cruising. Yeah, now I'm just getting distracted guys. I'm having fun driving around in the truck here. Alright guys, so this is how you figure out the frequency of that drone. I have my camera right here and I'm using an app called AudioTool. And you can see right at about 130 Hz, I have the file pulled up on here. Let me zoom in and show you. So it's actually going right now. You can see my voice as it's happening. But, right there at 129 Hz it says where I put the cursor. That's how I found the drone frequency on the truck. Okay guys, just real quick I'm going to show you the equation for calculating up the Helmholtz resonator for the exhaust. So, check this out. Here's your equation. So that's your equation right there. And what this is, is your resonant frequency equals speed of sound over 2 pi and the square root of area over volume times length. Now the way you calculate this is, I'll try to draw this nice and big so you can see it. This is the volume of your cavity, which is this right here. And this right here is going to be the length of the neck. And then the cross-sectional area of this neck is A. And that's pretty much it. To calculate the speed of sound, I mean you can just look it up, but we used approximately 600 degrees to calculate our speed of sound. So there it is right there. After understanding the math, I went ahead and used a calculator online in order to finagle the size and frequency of the cavity. And here's the rudimentary drawing that I have right here. We're going to go ahead and use this as our plan. And really, this is all that you need anyway. Just something to show you what you're doing. For this build, we're using a piece of 2 inch crush bent pipe we had done locally, a piece of 4 inch pipe left over from the exhaust system, and a piece of heavy gauge sheet metal, which we also bought locally. To start, we need an 8 and a half inch long neck. Make sure you always double check your measurements. As you can see, we've already made a mistake and drew up the wrong size. The correct way to measure the neck is to measure it through the center. Make sure you put an X through the ones that you're not going to cut. Here we're using the thinnest cutoff wheel we could find at Home Depot. Right here we're switching to our sander in order to sand off any rough edges on our pipe. I've decided to use an 80 grit sandpaper on this cutoff wheel. But if you like brute force, just go for the 40 grit sandpaper. Give it a quick test and just make sure that it's flat enough. We've cut it, now we're going back to the sander to smooth off any rough edges. Since I didn't cut this pipe very straight, I'm going to need to sand it a lot. Carefully trace your pipe so you can get the right size for the cap. I decided to use the cutoff wheel to cut out the circles as well. We did test out our jigsaw, but as you can see, we don't have the correct blade. So I would recommend getting a metal cutting blade and don't do what I did. So we're going to go back to the cutoff wheel for this project. Dang, look at those sparks fly! This is how you get work done in the workshop. Go ahead and test fit the circle that you just cut out. That's looking pretty good. Go ahead and mark where you'd like to cut your hole for your 2 inch pipe going into your cavity. Since metal cutting gets extremely hot, I like to keep it oiled a little bit and keep the tool lasting a little bit longer. Kick it up to high speed if you want some theatrics. Always test fit your parts after you cut them. That's looking pretty good. Nice and tight fit. As you can see, we've marked our pipe for two more cuts. Crush bent pipe is a little bit smaller than the original pipe. So we're using the 2 inch pipe to cut out a small adapter so that we can weld on the slightly smaller pipe that was crush bent. Since most of the parts are cut, we're test fitting everything. And that's the joint that will be welded. Always refer back to your plan to make sure you cut everything properly. What I'm doing now is cutting off a piece of pipe to attach to the exhaust system as a spacer. We'll need a slight spacer in order to have the clearance for the Helmholtz resonator to line up parallel with the exhaust system. Each side of our pipe was a different size, so we're carefully marking the second cover plate. And then we're going to cut it. While the exhaust system was on the truck, we carefully marked this 2 inch hole so that all of the parts for the Helmholtz resonator would fit underneath. I don't see any other way of doing this safely, and I didn't want to ruin a brand new exhaust system. That would be too expensive. Always make sure you oil up your pedal. Woo! Thank you. After you've cut this hole, it's as simple as finding a friend and having him help you weld that thing up. So here's all the pieces welded together. As you can see, we used an exhaust hanger and clamp on one side to hold it together. And we used a coupler and exhaust clamps on the other side to hold it together so it can be easily removed later. In my truck, the engine runs at 1800 RPM on the highway. And right at 1800 is where we calculated the frequency for this Helmholtz resonator. The droning is greatly reduced at 1800 RPM, as well as slightly above and below that RPM. Another interesting thing I learned is that below 1800 RPM, even down as low as 1200 RPM, it did quiet the exhaust system a little bit. So it's safe to say it does affect the overall sound at low RPM. Dang, look how big that exhaust tip is. Alright guys, so we're all done for today. And this resonator is really going to help you. Especially if you're cruising at 70 miles an hour and you have this terrible droning in the low frequencies. Probably around 130-140 Hz. This resonator is really going to make a big difference. So I hope you guys enjoyed this video. Please subscribe, please like the video, and share it with your friends. So, that's it.