Hi Neil and other Rotorheads;
Sorry for the delayed reply, had a busy week with my kid in a hockey tournaments, as well as my own beer league. I love this season.....
I was using an AeroTurbine 2525 several feet downstream of my modified stock turbo until recently. It's a long story why and how, but I had this muffler inside a housing to keep it out of the air stream for aerodynamics and to keep it out of the cowl. Hard to describe and it was a half-baked idea, that I would not do again, but it was successful in that I had no visible muffler hanging below that aircraft and it was very quiet. I had strived to have a quiet aircraft and was quite satisfied with this aspect and many other pilots on the airport grounds had made very positive comments about how quiet it was when I would taxi past and also on take-off and climb out. I only ever had one cranky old guy say that it was loud, but it was clearly a case of it being different and not like a conventional aircraft noise, so to him it was obviously not good and must be loud. I think we can dismiss this one complaint.
Unfortunately I've not got any proper decibel measurements from a proper meter, but I did install a decibel meter app on my phone and took some measurements from inside the cockpit during a full power run while tied down just before I removed the muffler, for some before and after comparisons.
Why did I remove it? Well again it's back to that darn half-baked muffler housing idea. The radiant heat from the muffler would radiate into the cockpit alongside my leg and despite having the muffler ceramic coated as well as firewall insulation on the housing it would get a little warm on hot summer days. That and I decided that it was time for my aircraft to go on a diet and start shedding some pounds, so I decided that it was time to see how bad it was without a muffler as I had never run it before without a muffler. Before the AeroTurbine I had a home made version of a SpinTech.
Anyways the not-so-hard data that you're looking for.
With the muffler in place I saw a max of 83db, without it I saw a max of 89db. According to the app this is equivalent to an alarm clock?
Keep in mind that this from a cell phone app that I had just downloaded moments before the test and had made no attempt to calibrate it so really can only be used for before and after comparison.
So it was only a 6db increase in noise. It has always been possible to talk easily with the pax without the use of headsets during cruise flight. So with such a small increase in noise, I modified the exhaust pipe to come straight out of the rear of the cowl and went for a flight. One of my old cowboy buddies has a small ranch right under the departure end of one of the runways, so I notified him I was going for a flight and to go outside to take a listen and tell me how it sounds. I didn't tell him why, but he knows that I've always wanted to make sure I was quiet. So afterwards I stopped by for a beer and first thing he says is that it sounds quieter now. WTF? Since he's not very technical, I'm thinking again that it's just a different tone and he has interpreted it as being quieter.
So I have a co-worker who lives under the downwind leg of the circuit and he is much more technical minded (he converted his own truck to electric about 10 years ago) so his opinion is probably more relevant. However since he's below the downwind leg, it's at a point where I have reduced power, so is already much quieter that when climbing over my other friends place. Anyways this guy has previously reported to me that he feels my plane is a little quieter than most, but mostly it is a different tone that is easy to tune out as he finds it less obnoxious than many of the GA planes that fly over. So I asked him a few weeks after I made the change if he'd seen me flying over recently and how did it sound? He said he hadn't noticed any difference, but wasn't paying attention to the sound anymore. So while that's not very scientific, it tells me that any increase in sound or change in tone hasn't been enough to make anyone take notice. So for all that reason, I think I'm going to make this a permanent weight loss.
So clearly the stock turbo does provide some muffling. Another thing to note is that the thin wall 321SS pipe is about 30" long and has 5 elbows in it as it weaves it's way out. I'm not sure that would have much effect, but I imagine it's better than a straight pipe.
I see Dave has also responded and while I believe he has a stock turbine housing, the turbine and compressor wheel are not stock at all, so I don't know how much, but I suspect this would have some effect on sound.
As far as the longevity of the AeroTurbine, It showed no signs of damage at after about 25 hours of flight and probably another 15 hours of ground running. That was before I sent it out to have it ceramic coated and I then put about another 10 hours on it (ground and flight) before I removed it. It shows no sign of damage or flaking of the ceramic coating. I'll probably use it on an old Jeep that my kid and I are rebuilding for him when he turns 16.
Based on my experience, I would clearly chose to pack the useful weight of a turbo, than that of a muffler. Although that comes with it's own challenges as well.