Acoustics & Controlling Exhaust Noise
There are three basic approaches to controlling exhaust noise.
Containment reduces the transmission of sound waves by confining them. Many economy mufflers silence exhaust noise restricting the flow of the exhaust gas by using small diameter or "plugged" flow tubes. This is typically not desirable because when exhaust gasses are confined to the vehicles power and fuel economy suffer.
Absorptive devices convert sound energy to heat high-frequency sound waves collide with material that flexes and vibrates easily.
The material absorbs the sound rather than passing it on a resonance or bouncing it.
Glass packs are a typical example of sound reduction by absorptive control, and as they work best on absorbing high-frequency sound, result in a deep exhaust note.
Reactive devices reduce, change or eliminate noise by manipulating the sound waves.
Louvred or Perforated Flow Tubes;
The common application of a reactive sound device is the louvred tubes found in "Tri-Flo" or reverse flow mufflers. The purpose of these tubes is to steer sound waves against each other and use their own energy to cancel each out
There's one variation of reactive sound control that provides us with a couple of different devices that reduce low-frequency exhaust noise (drone). The first of these devices is referred to as the Helmholtz tuner.
The Helmholtz tuner operates on the theory that if you have exhaust gases pulsing through a constricted area under high pressure and high velocity, and then pass hose gases into a large area, you will reduce the pressure of the velocity of the gases, and in turn reduce the low-frequency exhaust note.
The Helmholtz tuner is usually the large empty "chamber'' that is sealed from the gas flow from all other components except the tuning tube. The tuner is usually a tube that is welded to the side wall of the flow tube and as a gas pulse approaches the tuner the pressure in the chamber rises until it reaches equilibrium.
Once the pulse has passed the tube the pressure in the chamber is higher and so a pressure wave is sent back, the gas flow resulting in destructive interference.