The 1.8T project just got a full 76mm exhaust system (read about it here). But it’s wasn’t perfect so I designed a Helmholtz Resonator for the exhaust to make it even quieter. These are the results and what I’d do differently.
What is a Helmholtz Resonator?
In the simplest terms it is an enclosed volume (chamber) connected to the exhaust pipe via a neck. The size of the neck and the chamber effect what frequencies the chamber can suppress in the exhaust system.
The Problem
What problem was I trying to solve with an exhaust Helmholtz resonator?
The 76mm turbo back system on the 1.8T project was super quiet but because the system only used absorption mufflers, below 200hz was a bit too noisy/boomy. The problem frequencies were between 96hz and 141hz according to the Spectroid app.
To try and reduce the sound here I thought I’s use a Helmholtz resonator in the exhaust.
Design – Helmholtz Calculator
I used this calculator to work out the chamber and neck size of the Helmholtz exhaust chamber.
Helmholtz Results
While it works when the exhaust is cold, it seems to lose it’s effectiveness as the engine gets warmer and the faster the car is driven.
At cold it’s good, hot not so good.
It’s still noticeable better than without the resonator, the drone when lifting of the gas with the car in gear show the biggest sound reduction, but the drone is still there.
And before 140hz was the problem frequency at cruise and now it’s 95hz.
Changes
Using the Helmholtz calculator here, I though I was being ultra conservative with the operating temperature ie I planned for the exhaust to be 60C and the frequencies I targeted were also conservative. I said the problem frequencies were 95-140hz so I targeted 120-125hz.
I think what I’d change is the frequency I’d target. I’d move it down to 115hz. This would lead to a bigger chamber volume or longer neck.
And I’d also target a higher temperature, say 90C instead to target 115hz. This would also lead to a longer neck or a bigger chamber.
Why These Changes?
Because I think it’s better to be a bit higher on the temp than too low because I could always insulate the chamber to get the temp higher.
And on the frequency front, 95hz is a problem all of the time. Whereas 140hz is only really a problem at higher RPMs. If I targeted 115hz instead of 125hz I would get more suppression more of the time.
My Helmholtz Dimensions
As I said I wanted to be able the target 120-125hz and 60C.
Neck diameter was 46mm internally.
Neck length was 11cm.
Chamber diameter was 73mm internally and 55cm in length.
These numbers give a targeted frequency of 117.6hz.
And I assumed I would get a +/- 15hz (30hz total) frequency window. So I was hoping this would hit frequencies from 102hz to 132hz.
In reality, at operating temp, ie cruise, the window seems to be centered around 130-140hz
Possible Better Measurements.
Given that, I think it would be better to target the following numbers.
115hz again.
But at 110C (high temps give bigger chamber results in calculator) temperature instead of 60C (we can always insulate out chamber to get the temp up later if we need to)
Neck diameter will be a bit longer at 12cm because 11cm led to it being a bit close to the fuel tank.
Chamber internal diameter the same at 73mm.
Bolting these numbers into the calculator gives us a new chamber length of 63cm (2.6 litres instead of 2.3 litres before)
The bigger chamber gives the Helmholtz resonator more power. And as I said, if the temps are a bit lower than expected, we can wrap the chamber and neck in exhaust insulation to get the heat up.
Helmholtz Resonator Summary
This has been my experience of the using a Helmholtz resonator to quieten exhaust.
It’s not a night and day difference but it made the car nicer to drive.
Would I do it again?
Yes, but with the changes I mentioned.
I used 76mm exhaust pipe for my resonator but if you can use a larger diameter cylinder, like 4/5 inch I think that might be beneficial.
And better to be a bit high on the temp in the calculations than too low.