Exhaust noise solutions

[richzx]

So for low down noise you need a big muffler to keep it quiet is that right? I don't mind a bit of noise over 4000 rpm but its the driving around town in the lower rpm range that I want it whisper quiet. I have a jap spec rear muffler that quitened my 1JZ down heaps from the cannon I had previously, the middle muffler is a 3" Redback straight through that is oval in shape & I'm thinking this is what needs replacing to quieten it down further. What do you think?

[abently]

So for low down noise you need a big muffler to keep it quiet is that right? I don't mind a bit of noise over 4000 rpm but its the driving around town in the lower rpm range that I want it whisper quiet. I have a jap spec rear muffler that quitened my 1JZ down heaps from the cannon I had previously, the middle muffler is a 3" Redback straight through that is oval in shape & I'm thinking this is what needs replacing to quieten it down further. What do you think?

Yes, a round muffler is A LOT better then an oval shaped muffler. And the bigger the better.

The rockwool packing density affects the frequency range your targetting so maybe sample your exhaust noise digitally if you have the time, just to be sure which frequency range you need to attenuate before you go having a muffler built to your specs. Most Truck/Diesel exhaust shops can knock a reasonably large one up for about $250 which is good value for money for what you will get.

[richzx]

I wouldn't put a round one in there it wouldn't fit and the round one I had was incredibly loud hence its removal and replaced by an oval muffler at the rear. The middle muffler is oval and straight through so I'm wondering if this is where the residual noise is coming from.

[YLD-16L]

I'd like to hear more specifics as well, as I'm building a custom exhaust for my 4AGTE which will be 2.5". I'm basically after the same setup, as quiet as possible without too much restriction. Many of the 2" 4AGE exhausts I've heard are really loud yet the factory 2" SX exhaust is really good with a 2" diameter cat, ~12" long muffler, ~12" long resonator and a huge rear muffler. This is what I would like to use as a guide, however finding a resonator has been really hard. Would it be better to keep looking or just use another muffler? The two things that have been hardest to find in fact are a 2.5" resonator and a centre-in/centre-out muffler in 2.5".

My 4AGTE is very quiet, like Draven I always get comments at events that it's too quiet, people can't hear it. It is 2.5" the whole length from turbo to tip, one stainless steel Australian made Hi-Tech brand centre in centre out 2.5" resonator, one Hi-Tech stainless muffler centre in centre out 2.5" straight through and one Hi-Tech baffled 3" tip. The whoel system is now nearly 9 years old and works the same as the day it was fitted. It came in well under the dB test requirements.

I ran the exhaust without the muffler and tip and just a straight through 2.5" pipe and my missus said she could hear the car one suburb away, so it does make a big difference as normally it can't be heard once I'm half way down the street.



As for the specific thread topic, I've been playing with a 1J single turbo 3.5" exhaust external gate plumbed back into exhaust Hilux and this thing runs a single straight through4" truck muffler (quite long but not sure of the exact measurement). It's loud, too loud for long trips but not too obnoxious that you'll get pulled over on the street if driving sensibly. Definitely too loud for a sleeper.

[abently]

I wouldn't put a round one in there it wouldn't fit and the round one I had was incredibly loud hence its removal and replaced by an oval muffler at the rear. The middle muffler is oval and straight through so I'm wondering if this is where the residual noise is coming from.

Without recording the sound digitally, with and without the centre muffler (i.e. straight through connector or drop the rear section and bolt it up) you won't ever know how well the centre muffler is working. It will be doing something but probably not as much as you need it to.

[TERRA Operative]

That scan would be sweet Absently. I have an AE101 Levin, and have room for a centre muffler/resonator and the exhaust splits to two at the rear wheels to end at two mufflers which can both be a decent size.
At the moment it drones something chronic (aftermarket bits by previous owner), I'm hoping I can quiet it down a little and keep it sounding 'beefy'.

[abently]

m=(r3/r1)^2

I can put some up of the perforated straight through sizing vs transmission loss, though this one states best case scenario occurs with r1=r2, just doesn't take into affect a perforated tubes affect.

[oldcorollas]

that would be this one? (edit: does that load up for people? it does, but i have journal access anyway...)
http://www.sciencedirect.com/science...ece712fefba778

Journal of Sound and Vibration
Volume 272, Issues 3-5, 6 May 2004, Pages 1125-1133
Letter to the Editor
Sound attenuation in dissipative expansion chambers
M. B. Xu a, b, A. Selamet , , a, b, I. -J. Lee a, b and N. T. Huff c

if people want the PDF of the paper, PM me.
but it is just a paper about dissipative loss in ducting and silencers, but is more about the calculation than design.

there must be better papers around?

have you seen any that use similar modelling for baffles, and direction changing etc?


edit2:

1. Introduction
Recent improvements in fiber properties combined with their broadband acoustic dissipation characteristics make such materials potentially desirable for implementation in silencers. The acoustic modelling of simple silencer geometries, such as circular or rectangular cross-sections, has been established and classified depending on the assumption of locally- or bulk-reacting lining. Morse [1] used a locally-reacting model to investigate the sound transmission in pipes with absorbing material on the inner walls. By using bulk-reacting model for linings, Scott [2] studied the transmission of sound in infinite rectangular and circular ducts; Ko [3] investigated the characteristics of sound attenuation for rectangular, annular and circular ducts; Cummings and Chang [4] examined the effect of mean flow on the modal attenuation rates.

The foregoing investigations [1, 2, 3 and 4] have targeted infinitely long ducts, thereby excluding the effects associated with the expansion/contraction of a finite chamber. In order to fully understand the acoustic performance of a finite-length dissipative silencer, it is essential to investigate the effect of discontinuities across the expansion/contraction of the expansion chamber. Cummings and Chang [5] analyzed the sound attenuation of a finite length dissipative silencer with mean flow by using a mode-matching technique. Axial acoustic particle velocity and pressure were matched across the expansion/contraction of the chamber, and a good agreement between predictions and measurements was observed. Peat [6] developed a transfer matrix formulation for the bulk-reacting dissipative silencer from the match of average acoustic pressure/velocity across the silencer discontinuities. By using one-dimensional analytical approach, three-dimensional BEM, and experimental methods, the acoustic attenuation was discussed for both a single-pass dissipative expansion chamber [7. A. Selamet, I.J. Lee, Z.L. Ji, N.T. Huff, Acoustic attenuation performance of perforated concentric absorbing silencers, SAE Noise and Vibration Conference and Exposition, SAE Paper No. 2001-01-1435, Traverse City, MI, April 30–May 3, 2001.7] and a hybrid silencer [8] consisting of two single-pass dissipative expansion chambers and a Helmholtz resonator. A pod silencer, which is a lined circular duct with a cylindrical pod inside, was also investigated; and both the four-pole parameters and transmission loss were obtained by plane-wave analysis [9].

In [5, 6, 7 and 8], the diameter of the airway in the dissipative expansion chamber is always set equal to that of the inlet/outlet pipes. In [9], this diameter is different than that of the inlet/outlet pipes, however, only a plane-wave analysis is provided. Using a two-dimensional analytical approach, the present study examines the effect of fiber thickness, chamber diameter, and material properties on the acoustic performance of dissipative silencers. An analytical approach is proposed based on the solution of eigenequation for a circular dissipative expansion chamber. The acoustic pressure and particle velocity across the silencer discontinuities are matched by imposing the continuities of the velocity/pressure integral over discrete zones at the expansion/contraction. The analytical results are shown to agree well with the numerical predictions and measurements.
.
.
.
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3. Results
A dissipative expansion silencer has been fabricated with r1=2.45 cm, and L=25.72 cm. The characteristic impedance and the wavenumber of the absorbing material in the present study is given as [10] (equations)


with Z0=ρ0c0 being the characteristic impedance of the air, and f(Hz) denotes frequency and R(mks Rayls/m) the resistivity of the absorbing material.

The transmission loss results from the analytical approach with different number of higher order modes are compared with BEM predictions and measurements in Fig. 2 with r2=3.50 cm, r3=8.22 cm, and R=4896 Rayls/m, which corresponds to a filling density of 100 g/l. The details of the BEM and experimental approaches are elaborated in [8 and 11]. With N8, all three sets are in good agreement.

Fig. 3 depicts the analytical transmission loss results for different fiber flow resistivity for r2=3.50 cm and r3=8.22 cm. Higher fiber resistivity leads to higher transmission loss at medium to high frequencies, while the lower resistivity improves transmission loss at low frequencies. The fiber flow resistivity may therefore depend on the application.

With R=4896 Rayls/m and r3=8.22 cm, Fig. 4 examines the effect of variation of fiber thickness, r3−r2, on the transmission loss. The results for the empty silencer are also included. The overall performance of the silencer is generally improved by increasing fiber thickness. Thus, for maximum attenuation, the diameters for the airway and inlet/outlet pipes in the dissipative silencer would be the same.

With r2=r1 and R=4896 Rayls/m, Fig. 5 investigates the effect of variation of area ratio m=(r3/r1)2 on the transmission loss. Higher area ratio improves the performance of dissipative expansion silencer at low to medium frequencies. Such a trend may be reversed locally at higher frequencies and higher area ratios. Currently, an effort is under way to develop an analytical solution for dissipative chambers with empty space between fiber filling and outer housing.

[oldcorollas]

their reference 8 seems more relevant?
http://www.sciencedirect.com/science...82d6c&ref=full

Journal of Sound and Vibration
Volume 262, Issue 3, 1 May 2003, Pages 509-527
2001 India-USA Symposium on Emerging Trends in Vibration and Noise Engineering
Acoustic attenuation of hybrid silencers

A. Selamet, , a, b, I. J. Leea, b and N. T. Huffc

this seems to have better calculations perhaps?

Applied Acoustics
Volume 69, Issue 2, February 2008, Pages 173-178
Transmission loss prediction on a single-inlet/double-outlet cylindrical expansion-chamber muffler by using the modal meshing approach

C.J. Wu, a, , X.J. Wanga and H.B. Tanga

http://www.sciencedirect.com/science...bcabae3809b8e7

[abently]

Oh hang on you can upload pdf's....

Unfortunately there's not one paper anywhere which covers all muffler designs.

I think most of these are suitable for 90% of cases, after that its simulation time, which is quicker then messing about with TMM's.

[abently]

their reference 8 seems more relevant?
http://www.sciencedirect.com/science...82d6c&ref=full

Doesn't exactly show the effect of different sizes, not to mention they use a 32cm diameter muffler as the main example.. lol

I can imagine the comments you would get from passers by wondering what on earth that thing is..

[oldcorollas]

Oh hang on you can upload pdf's....

can't upload ones that big on toymods.. and shouldn't make them publically downloadable.. etc.. but sharing via email is... not so bad.

[oldcorollas]

random (publically available) links for starter material
http://myfwc.com/boating/airboat/Section3.pdf
http://www.cari-info.org/actes2006/78.pdf

and funny inglish
http://www.insinet.net/jasr/2007/676-680.pdf

this seems more practical
http://www.scielo.br/scielo.php?pid=...rttext&tlng=en