Absorption silencer question

[cmsapms]

Is an absorption type silencer's noise reducing ability related to the internal area of the perforated tube?

In other words, take two silencers of identical external dimensions; say 600mm long, 180mm diameter). Both have inlet/outlet diameters of 65mm. Silencer A has a straight 65mm perforated pipe through the middle. Silencer B has a pipe that starts at the 65mm inlet, splits smoothly into two 45mm pipes and then rejoins smoothly into the 65mm outlet - the two smaller pipes are 25mm apart. The perforation size/spacing is identical throughout and the same type/density of absorption material is used in both silencers.

Which silencer would be quieter?

[CrutyRammers]

I'd guess at a, because it has a greater volume of wadding. The consensus generally seems to be that length is most important, followed by girth...as it were.

[Peanut Gallery]

I would vote that the number of holes would be a large deciding factor, the more the quieter.

Pipe a - 600mm long, 65mm diameter = 122.522mm square surface area.
Pipe b - 600mm long, 45mm diameter = 84.823mm square, X 2 = 169.646mm square.

Note, my calculation above uses the amazing technology of having the pipe split into 2 instantly, and then join up again instantly at the back of the silencer. As this has not been invented yet, you will loose a fair bit of length doing this.
(Yes, you could split into two before the silencer and join back into one after)

However, once you start trying to get as smoother flow through as possible, I would vote for the straight through 65mm pipe.

Just my thoughts, please don't come back to me if your neighbours give your lawn the sausage treatment for having a loud exhaust. - There are lots of people far more knowledgeable than I with regard to exhausts!

[feef]

I don't know the maths, but I'd also expect that once beyond a certain thickness, the wadding would absorb less and less of the energy so, assuming sufficient wadding in the first place, two pipes should be quieter than one for the same 'volume' of perforations in the tube

This is more intuitive than scientific however and I'd be interested to know if I'm on the right track

[E-bmw]

I am pretty much in agreement with most of the above.

With absorption silencers, my understanding is that, every perforation allows a tiny bit of the primary sound wave front into the wadding & drops the impact of the wave front slightly, so for every perforation (assuming wadding is good) the sound wave front will be reduced slightly.

I may be wrong though!