How does the active exhaust work?

[Vlad Soare]

OK, each muffler has a valve that opens and closes, we know that.
But what exactly is it that makes it loud with the valve open and silent with it closed?
The way I'm imagining it is two mufflers in one. One half is just like any regular muffler, attenuating the sound as usual, while the other half is more or less open, allowing the exhaust gases to flow unhindered. The latter part is covered by the active valve. So ultimately you get a combination of exhaust sounds ranging from 100% muffled + 0% open pipe (i.e. quiet mode) to 50% muffled + 50% open pipe (i.e. track mode).
Am I right?
Has anyone ever opened up a stock AE muffler to see how it looks on the inside?

Sponsored

 

[Skye]

I hope to provide some insight and explanation to your question.

Sound waves can be characterized by both frequency (oscillations) and intensity (amount enegry carried). In general, lower frequency signals (think bass) travel farther than higher frequencies. Lower frequencies often contain or carry more energy that higher frequencies. In speech, higher frequencies are associated with the articulation of the spoken word, the intelligibility of it.

I take the muffler, resonator and associated tubing as the sound system for the car's engine. The frequencies and energy from the engine need to be quelled, as a completely-open exhaust would be ear-splitting. I also want the Mustang to have a certain signature note.

By using chambers, dividers and baffles ("to restrain or regulate") within the muffler and resonator, I can have some of those frequencies cancel themselves out. I can also use the internal nature of the muffler and resonator to alter the amount of energy those frequencies carry (increasing or decreasing).

The diameter and shape (round and bends) of the exhaust tubing also impacts sound. Exhaust tubes are similar to Radio Frequency waveguides: they accept certain ranges of frequencies while suppressing others.

IMO, the active flaps and vents of Active Exhaust are akin to you putting your hand close to your mouth when speaking. You're speaking with the same amount and type of energy as before, but some of that is now blocked or supressed by your hand. Remove the hand, and, while the speech itself has not changed, how it is received, has. Credit @Nightmonkey with detailed explanation below.

I reference sound cancelling above. Draw out a sin wave. Spin the paper 180 degrees around. Draw a second sin wave. You should notice the waves are equal, but opposite to one another. Do the math, +1 + (-1) = 0; this is how you can completely cancel the sound of a wave, how canceling headphones work, and a small insight into how I can shape the internals of the mufflers, resonator and an exhaust system to get certain sounds. It doesn't have to be a complete 180-degree cancellation. Partial cancellations can be done to get the desired sound. Think of sliding one of the sin waves left or right slightly to get a partial cancellation.

To increase sound, I do the opposite of above. Draw a sin wave. Keeping the paper in-place, draw another sign wave above it. 1+1=2. I can tune my exhaust system to add waves of the same frequency, intensifying their sound. Certain lasers (Dr. Evil quote symbols) work in a similar manner. Resonance.

 

[Nightmonkey]

The pipe behind the valve comes strait from the mid-muffler.
The pipe without the valve is only indirectly connected to the mid-muffler via the rear muffler.

The pipe in front of the valve is perforated; the gases flow into the rear muffler via the perforation.
The more the valve is closed, the more gas is forced through the rear muffler.


When it comes to sound, it's as @Skye describes an acoustic issue.
If it's all about loud and quiet, you can simply say the more gas that can flow directly out, the louder it exits.

So valve open means straight pipe, but the perforation still has an influence on the sound, without this it would sound different and may also be louder.
This effect is used by companies that offer a different rear silencer, in which the pipe behind the flap is an unperforated pipe, e.g. this exhaust:

 

[Prodigal]

I wish someone would post a pic of a factory AE with the valve present. I cannot see mine because of the rear diffuser hides them and I’m not removing it just to peek.

 

[Nightmonkey]

@Prodigal
To bad, they did not cut the muffler open:

 

[vanquishvzla]

Damn it… I thought little people lived inside the exhaust and they opened or closed the door…. You ruined for me

 

[CoolRod]

OK, each muffler has a valve that opens and closes, we know that.
But what exactly is it that makes it loud with the valve open and silent with it closed?
The way I'm imagining it is two mufflers in one. One half is just like any regular muffler, attenuating the sound as usual, while the other half is more or less open, allowing the exhaust gases to flow unhindered. The latter part is covered by the active valve. So ultimately you get a combination of exhaust sounds ranging from 100% muffled + 0% open pipe (i.e. quiet mode) to 50% muffled + 50% open pipe (i.e. track mode).
Am I right?
Has anyone ever opened up a stock AE muffler to see how it looks on the inside?

I haven't personally opened an active muffler but what you described is also my understanding of how it works. Essentially two mufflers in one.

 

[Prodigal]

@Prodigal
To bad, they did not cut the muffler open:

Awesome. Thank you. Question however, where is that flapper located? After the muffler? Inside the muffler? Does it allow the exhaust to bypass the muffler when opened? I’m not getting how it changes the sound level. It’s not as if it’s a set of electric dumps located just south of the header collectors.

 

[Nightmonkey]

Does it allow the exhaust to bypass the muffler when opened?

Exactly!
Just take a look inside with a flash light while in track mode.

 

[WD Pro]

I wish someone would post a pic of a factory AE with the valve present. I cannot see mine because of the rear diffuser hides them and I’m not removing it just to peek.

If you need anymore just ask - its off the car at the moment and the valves are held fully open by the actuators

WD

 

[Vlad Soare]

If you need anymore just ask - its off the car at the moment and the valves are held fully open by the actuators

Yes, I want to see what's inside. But I guess you aren't going to cut them open to satisfy my curiosity, are you?

 

[Mike Pfeifer]

maybe you could go in there with a borescope. Its not the same as cutting it open, but more info than nothing.

 

[WD Pro]

Yes, I want to see what's inside. But I guess you aren't going to cut them open to satisfy my curiosity, are you?

No ...

So valve open means straight pipe, but the perforation still has an influence on the sound, without this it would sound different and may also be louder.

This.

It's easy enough to suss out looking up the tail pipes

WD

 

[ARAMP1]

Holy Hell! I don't think anything on my car is that shiny. Is that stock and just polished up?

 

[MAGS1]

Borla cut them open

Sponsored