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Drone reduction via side branch resonators - results
My FBO '11 F with PPE headers and ISS DES exhaust had a pronounced drone from about 1,550 to 2,250 rpm, especially under load. I like being able to hear the engine, but with others in the car (wife, clients, etc) the drone could be a bit obnoxious, bordering on unprofessional. I've been considering changing to a dual mode exhaust, swapping back in stock mufflers, etc, but those solutions achieve noise reduction via additional exhaust restriction. Nothing wrong with that, especially with a dual mode, but I wanted to try something much more affordable than a new catback first. I'd also been kicking around the idea of adding quarter wave resonators (aka side branch resonators) to cancel out a portion of the offending drone without adding any restriction to the exhaust. I'm not an engineer, but here's the layman's description of a side branch resonator: It is a tube that is T'd into the exhaust, and is capped at the far end. The length of the capped resonator is calculated to be 1/4 of the total wavelength of the offending drone wavelength. Finding the drone wavelength involves some assumptions about the temperature inside the resonator (affects speed of sound in the res) and a few other factors. But fortunately, the resonator is somewhat forgiving on these assumptions and is still effective across a decent rpm range even if your assumptions are not spot on. The theory is that the exhaust gasses themselves do not travel through the resonator since it is capped at the end (thus no additional exhaust restriction), but the sound waves DO travel down the res, reflect back off the capped end of the res, and re-enter the exhaust 180 degrees out of phase, thus cancelling out the drone wavelength. Google will take you to many threads on these, but the thread below is one of the more comprehensive threads I found. It describes the theory and math (and addresses the assumptions) much better than I could here. One caution as you do your research: some people refer to these as a helmholtz resonator (which includes a bottle-shaped chamber on the end of a short neck - like HKS's SSK mufflers) or as a sub-set of helmholtz resonators, but a quarter wave resonator is different animal than a helmholtz. The theory for noise cancellation for helmholtz's is similar, but the math is different and considerably more complicated.
The other factor that determines the effectiveness of a side branch resonator is the diameter of the resonator. Most people try to use a res that is the same diameter of the exhaust itself, which will provide maximum attenuation (noise reduction) of the calculated wavelength. I've added one of these to my supercharged Tacoma - it has a 3" single exhaust and I used a 3" diameter resonator and it eliminated the drone ENTIRELY and quieted down the whole exhaust by a very noticeable amount (sounds stock now, and it definitely did not before). I did not want that level of attenuation on the F because I still want to be able to hear that it's a V8. My ISS DES exhaust is dual 2.5", so I figured I'd try 2" diameter resonators to kill most of the drone/noise, but not all of it.
Well, today I had two 27" long, 1.75" diameter side branch resonators added to my F exhaust and I am happy with the results. I don't know if the relationship between resonator diameter and % of sound reduction is linear or not, but based on my very unscientific "ear dyno" I think it must be at least close to linear. The 1.75" pipe is 70% the diameter of the 2.5" pipe, and it includes a non-mandrel bend, so let's call it 60 - 65% the diameter of the exhaust, and it seems like the drone was about cut in half, maybe a bit more. I ended up using 1.75" instead of 2" due to packaging concerns. I had spoken to PPE and they thought they could probably get a 2" res snaked around the mufflers, but they quoted me 5 hours of labor to do so, and they are booked solid for a couple more weeks and could not get me in for a while. I used another shop locally that got me in right away, but the fabricator there was not comfortable trying to fit 2" resonators in the space available. He said it may work, but he couldn't guarantee it would not hit/rub on other parts of the car, so we went with 1.75". Other exhausts may have more or less space than I have with the ISS DES. My total cost: $100. I'm going to try to add a pic of where mine are located.
This isn't for everyone, especially if you are averse to modifying your existing exhaust or if you want the variability of a dual-mode exhaust (I still think the PPE dual mode would be my ultimate exhaust). And if you want an exhaust that's louder than stock, you won't want to use a 2.5" resonator. But if you want a solution to reduce or eliminate drone or have a very quiet exhaust without adding restriction to your aftermarket exhaust, it may be something to consider.
Spent 100.00 to make car quiet after getting rid of quiet oem exhaust, all while making it heavier and increasing the chance for the exhaust to hit the undercarriage?
Sorry if I'm misinterpreting your last statement. But are you saying this actually quieted down the exhaust outside of the drone range as well?
On the F - hard to say, and without before/after dB readings, I know it's all subjective. On the Tacoma, the 3" resonator definitely reduced the entire exhaust volume, both inside and outside of the drone range - absolutely reduced the entire exhaust sound, no dB readings required to determine that. On the F it may have quieted it outside the drone range as well, but if it did, it wasn't by much. Maybe the best way to describe it on the F is that it took the harsh edge off the drone in the drone range - it's still there, but is diminished and much more comfortable - and the exhaust MAY be a little quieter everywhere. I THINK the difference in results between the Tacoma and the F are due to the smaller diameter of the resonator on the F (~65% the diameter of the exhaust) vs. the Tacoma (100% the diameter of the exhaust).
But placement may have been a factor as well? On the Tacoma the resonator T's into the exhaust at about the front of the bed vs behind the rear axle on the F...
If we could have fit in 2" resonators, I'd have done it, and would still recommend that (and/or use mandrel bends vs regular crush bends).
Spent 100.00 to make car quiet after getting rid of quiet oem exhaust, all while making it heavier and increasing the chance for the exhaust to hit the undercarriage?
I dont get it
Getting rid of OEM exhaust doesn't mean you were looking for sound only. Seeing that he has PPE headers, obviously he's going for performance. Ive had the louder ISS QES exhaust only with no headers and it was extremely loud outside and especially inside. Rattled everything. Headers with certain exhausts, drone is pretty crazy. I fully understand where the OP is coming from. This is why I'm holding back from exhausts on the RCF. i drive professional clients to lunch sometimes and its down right embarrassing and unprofessional with a loud exhaust. But of course i want the performance aspect.
Ive had a few cars where i did full bolt ons and there was zero drone or loudness inside the car . ISF is not one of them.
Spent 100.00 to make car quiet after getting rid of quiet oem exhaust, all while making it heavier and increasing the chance for the exhaust to hit the undercarriage?
I dont get it
I bought the car with the ISS exhaust already on it. And as I stated, it was fabricated with 1.75" resonators vs 2" specifically to ensure that it does not hit the undercarriage - and in fact it does not. I don't track the car, so the very minimal weight gain is a non-issue for me. Ultimately, I'm not concerned whether you "get it" or not. Some of us are trying to reduce drone, and this is one option.
Awesome glad to hear. With headers these cars are LOUD. So even if it didn't quiet it down some I don't think it's a big deal at all. My XForce is coming, this year, I hope lol. Will be doing headers and then adding GESI cats and these QW resonators as needed. Unless that Greddy SS gets released and then will hold off on spending the money on the cats.
Awesome glad to hear. With headers these cars are LOUD. So even if it didn't quiet it down some I don't think it's a big deal at all. My XForce is coming, this year, I hope lol. Will be doing headers and then adding GESI cats and these QW resonators as needed. Unless that Greddy SS gets released and then will hold off on spending the money on the cats.
Question , i have just got my ISF which already equiped with ISS Forged Des exhuast only which been installed since 4 years ago as iam experiencing some drone between the RPM range 1k to 25k . Is there any one experiencing the same & is there any thing i can do to redues the drone .
I have the same "technology" built into my custom Unobtanium exhaust on my RUF. Wouldn't have it any other way. Outside the car it sounds like a Cup car... inside no drone and completely liveable. There is actually quite a bit of science into "helmholtz resonators" which basically the pipes isolate and reflect certain frequencies to cancel out drone almost like noise cancelling headphones.
Every exhaust I do going forward will have the same stuff. Get to have my cake and eat it too!
I have the same "technology" built into my custom Unobtanium exhaust on my RUF. Wouldn't have it any other way. Outside the car it sounds like a Cup car... inside no drone and completely liveable. There is actually quite a bit of science into "helmholtz resonators" which basically the pipes isolate and reflect certain frequencies to cancel out drone almost like noise cancelling headphones.
Every exhaust I do going forward will have the same stuff. Get to have my cake and eat it too!
Awesome write up Tom2769! I have been searching around and wanted to get rid of the drone on my custom exhaust set up too. Glad to hear this works well for you.
01-14-17, 10:43 AM
Drone reduction via side branch resonators - results
