Helmholtz Resonator

[merlin]

Got one on my cat back exhaust - odd looking thing.

Anyway - just wondering why these aren't more common? You can manaufacture them to effectively make all kinds of nice exhaust notes at low to mid rpm, they create a little back pressure too which helps driveability and then they just well up with air and do nothing when you're booting the crap outa the car.

Anyone know of any standard cars with them? Or do they only appear on aftermarket exhausts?

 

[Dogbreath]

Pretty much all standard backboxes have Helmholtz resonators built into them. A Helmholtz resonators is nothing more than an enclosed volume of gas that has been designed to resonate at a particular frequency, either to boost or cancel out a wave. e.g. the port on a reflex tuned sub is a helmholtz resonator.

You will also find them liberaly sprinkled around modern inductions systems, usualy appearing as a pipe branching off the main air inlet but leading to a dead end.

 

[merlin]

Thanks DB - I was looking for cross section diagrams of back boxes to see what's inside but couldnt find any.

What's the score with using them on the intake side of things? Is it just to get an induction note?

 

[Dogbreath]

It's generaly to reduce induction noise as much as possible on production cars, and also to give a better sounding engine note. The whole induction system is a essentialy a collection of pipes that will resonate at several different frequencies. They put Helmholtz resonators in to reduce the highest amplitude/most obnoxious frequencies down to acceptable levels.

If you look through the cross section of a typical production backbox, you will usualy find the resonataors take the form of one or two of chambers with nothing in them except either a perforated pipe going though the middle, or two separate pipes leading in/out.

 

[GT3]

There was one on my cars induction pipe, cut it off and hey presto sounds like its got an induction kit without having a crappy cone filter with heatsoak