Has anyone tried to make some Helmholtz resonators to reduce exhaust drone ? Basically you cut a hole in your manifold pipe. Then add a extra capped leg about 26" long. Go to youtube and search Synapse Engineering exhaust drone .
It has been my experience that exhaust "drone" comes from naturally-occuring resonance in the exhaust system being transferred to the body and/or frame by a direct route such as solid-mounted exhaust hangers. Changing the frequency of the exhaust system resonance - as it would appear a "dummy leg" would do - might eliminate the drone...but so will properly attaching the pipes to the vehicle with hangers which utilize flexible components of one sort or another to isolate the pipes from the frame/body. My $.02.
I've got the rubber mounts, using Dynomax super turbos 14". Getting low speed/low rpm drone 1000-1800 rpm or so (my tach is not working right off abit)
Turbo mufflers are notorious for droning, or having that tinny sound at idle that I hate. Switching to a good pair of glasspacks, and the problem should be solved. Might be cheaper and easier then cutting and welding in a bunch of helmholtz resonators. I have used helmholtz style resonators to quite intake track noise on very off topic off road vehicles (long snorkles, and such), but never exhaust issues. More than likely you will need a couple of resonators of different lengths as the droning is most likely to be caused by a couple of different frequencies.
H and X pipes rarely make much difference in Drone. And it is not a case of improper mounting either, that can cause sympathetic vibrations, but it is not truly Drone. I design exhaust systems for a living and can tell you that different mufflers in stock late model cars can drone you right out of your mind, with all the stock isolators/mounts in place - and the OE's do a very good job of hanging the exhaust these days. The issue is compounded by low RPM that we see in Over Drive transmissions that have cars cruising down the freeway under 2,000 RPM. 1/4 wave pipe resonators can definitely make a difference as can Helmholtz resonators (which are not the same thing). Length on a 1/4 wave will vary depending n the RPM that the drone is occurring - in most cases somewhere around 29" is what I have found to work.
So what is the difference between 1/4 wave pipe resonators/and Helmholtz resonators ? All of my " drone " is 1300-1700 rpm. At the speeds you are at driving around neighborhood 15-35 mph. Once I get above 1800 rpm/40mph sound cleans up. Sound good at idle & at cruising speeds.
The 1/4 wave is generally just a J-Tube ~ a piece of tubing bent like a 'J' that welds into the exhaust and is capped at the end. A true Helmholtz is aimed at 1/2 wave cancellation and therefore requires more volume and a chamber (or multiple chambers) to cancel different waves and attenuate sound at different frequencies. The diameter of the pipe isn't very critical, although larger will broaden the RPM range effectiveness slightly. I have used as small as 1.5" and up to equal the main pipe with similar results. It is not about the volume of air moved in and out, but rather the cancelation of the wave. As for location, further forward will cancel more sound waves then further back, BUT if you don't get the length right, the rear most can sometimes catch a drone that the mismatched size doesn't. Honestly, the position isn't all that critical, but forward is better. 15-35 MPH is not the common area that Drone is dealt with, as we are usually just passing through that RPM band, not constantly driving it. Drone is best described as a dissonant sound that creates a thrumming sound - a fluctuating wave pattern. This is what a 1/4 wave resonator can solve. It is different than just a Loud sound at particular RPM. That is usually best cured with muffler design and or an H-Pipe smoothing the pulses. Because the 1/4 wave is a tuned length based on the speed of the exhaust, they work over a fairly narrow RPM band and fortunately drone usually occurs at a narrow band as well. You are talking about a fairly wide RPM range to kill with simple 1/4 wave. The math tells me that you need 38" for the 1200 RPM area and about 29.5 " for the 1700 rpm area. So a good compromise will be about 32-34" depending on which end of the RPM range is most annoying. Hope this helps.
Very interesting information. I have not experienced drone on any of the cars I have owned, so this is good info to keep in mind if it goes ever occur. Are certain makes or models more prone to this or is it dependent of exhaust system type or modifications?
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