Car is so loud on inside. Helmholtz branch (1/4 wave) resonator?
 

Hey all,
I've been really struggling to be happy with my exhaust. It's a fantastic Artech unit, vband everywhere, 3" wahoo!

BUT man is it loud inside the cabin right at 3400rpm. This just happens to be where highway cruising and my ears are pounding when I exit the car.

Outside it's actually quite quiet.

I don't know what to think of it but in an attempt to mitigate the drone, I've tried replacing the resonator (in the cat location) with an actual metal substrate cat. It helped a tiny bit. I then added some sound deadening material to the parcel tray and rear trunk. It helped a tiny bit, but not nearly enough for me to be satisfied. My I often cannot hold a conversation and because this is my daily and I drive quite a bit, I'm seriously going deaf.

Has anyone gone about researching Helmholtz/branch resonators to alter sound attenuation? Just using an in-phone frequency analyzer I measured the deafening drone to be at 113.8Hz. A couple of times it'll be 320Hz but that could be due to limitation of the speaker and also road noise on highway. I'm not sure if I'm doing this correctly. But using some info found on forums (linked below) it could be calculated for the given RPM to be 113Hz. I am trying to find a source to back this up.

Then supposedly one takes the sound speed (function of exhaust temp measured at location of resonator), divide by frequency which gives you the wavelength. In this case for 113Hz with a guestimate of 80°C air temp, the wavelength will be about 3.33 meters. Divide this by four and that might be a length of capped pipe that'll hopefully reduce the drone.



Anyone have any insight? Or maybe it's something else that's causing this loudness inside the car? I've never been in another Miata so loud. loud.




Exhaust Resonance Tuning: Helmholtz/Branch Resonators - MR2 Owners Club Message Board

Toyota Supra MKIV : Powercruise Taupo 2009

Quarter Wave Tube - DiracDelta Science & Engineering Encyclopedia

Exhaust drone resonator fix - design and construction - Page 2

http://www.planet-9.com/987-cayman-a...struction.html

I installed the proper MT.net approved muffler so I never had that issue.

get an exhaust control valve. I haz one. it doesn't do much at idle, but does a LOT at over 2500rpm. like, the car sounds stock with it closed

we discussed this in the exhaust flapper thread and the thubawu thread. I've an apexi unit

Hmm. I wonder if Abe can create another axle back (for when I pop in a NB subframe) with new hanger an said mt.net approved muffler.

I'm also wondering if the 3.63 gears will drop me just under the resonant sweet spot at cruise, but it's still too much with the existing MagnaFlow muffler.

1. Top down.
2. Earplugs in.
3. Enjoy!

Of course, if you live where it rains . . . .

I've been planning on doing something similar in my subaru wagon, because it was* so damn drone-y when I first built my exhaust. For 'air' temp, something like what you chose 80°C is picked due to the temps in the resonator tube being (much)lower than the actual exhaust gas stream.

Diameter of the T branch correlates to amount of attenuation.

You look to be around 2.5-2.75 feet worth of T branch. Join the main pipe at a 90deg, and end the T with a flat wall. This is what I've read; never tried it.

I'd love to see you do it and see what difference it makes.

*75% of my drone went away by extending the tip past the edge of the bumper and making sure it wasn't pointed down even in the slightest (it's pointing up a few degrees).

My bad, whenever I looked up helmholtz, it always came up with intake related threads. This is the spring loaded flapper? I'm a bit weary about moving parts in the exhaust, but it may be an option.


Honestly, there are few days where I've taken the hardtop off (no soft top). I do have several sets of ear plugs in the car when I go on extended drives. FML I'm getting to old for this shit.

What motor mounts? With the AWR mounts and stock exhaust my car was very loud inside at right about that rpm, silent outside.

If not you can always use a different factor than 4 for the length of the tube if nearly 3 feet of t branch is too much. You just loose strength of attenuation.

I always have a set of earplugs in the console. I put them in before I start the car.

This is how OEMs do it.

I have a valve going in my car this winter when I get the new rear muffler installed, that valve is going in as well. Can't wait for the exhaust to be quieter.

The tip already pokes past the bumper and is just slanted end. I'm wondering if the muffler is just a bit compact. I just want to make sure I'm getting the right length. So far it seems about 0.75m is what's required for my particular target. That's pretty long. Not sure where I'm going to stick that unless curved and sharing space near the muffler. I just need to make sure I'm capturing the right frequency then I need to find space.



Currently I'm just on MazdaComp mounts. I'm wondering if I could get an exhaust shop to fab up a mount and a slip tube, tune it like a trombone before I commit to a length and have it welded permanently. This is all preliminary as I'm more worried about my fragged ball joints up front I just discovered today.

I do too, always wear them when the top is down and I'm going to be on the highway, even with that stock exhaust life in the old man edition. Makes it easier to hear the sirens and radio over the wind noise. And not to mention the wind noise on the highway is over 80db which is enough to cause hearing loss in sustained doses.

You want an awesome exhaust? Go copy EO2K's.
To date, his is the tamest I've experienced in a 3" exhaust miata.

Y8Spec has also been known to build axle backs with the mt.net approved muffler and he is local.

Thanks for the suggestions!

For future readers, it seems the MT.net muffler(s) of choice are the Magnaflow #12589 or #12579

#12589 is what I use in all my setups. Shoot me a PM and I will let you know where to get it the cheapest.

I probably should have read this before posting in my thread:facepalm:

I''ll probably lose my last cat, but you could try this. It's cheap. Get a pool noodle or a piece of leftover roll bar padding, and jam it between the passenger side inner wheel well and the gas tank. Cut it long enough it takes some effort to get it in there.

You are creating a damper for 2 panels. Not a bad idea, nothing to loose cats over.

I actually remember when we met up for the intercooler that when you exited the car you commented on how loud it was and how you could barely hear. Besides all the other suggestions have you also tried replacing the shift boot along with the sound deadening material their? I did mine a few months ago and when I did there was a very noticeable decrease in mechanical and exhaust noise getting into the cabin from underneath the car so much so that some of the old mechanical noises that were quite loud disappeared. The loud sounds I used to hear in the cabin were now only heard while driving past buildings with the top down or windows open. Just something to consider on top of doing other methods to get the best result. Probably not a full permanent fix but an idea.

Additionally: I know u are running high hp and I am not up to date on all the build threads so I am not sure of what transmission you use and if you retained the stock boot or not but either way that seems to be a problem area so a similar setup as far as sound deadening material should work.

So the running hypothesis at the moment is that the hangers I'm using is transmitting the noise into the trunk and cabin on the midpipe/muffler. The Miata Roadster shifter kit came with new sound deadening material for the boot and this has helped a lot with heat I used to feel radiating. But sound is unaffected. I also don't have that felt material under my carpet because base model. That would be a project for a new carpet install to get MLV and some CCF to decouple.

I have yet to switch back to rubber mounts with hose clamp and zip ties, but the car is definitely quieter on the outside. I took some acoustic foam and cut blocks off to pack under the package tray cover by the rear sway bar mount points and this surprisingly helped. I feel as though I placed the urethane mounts on the car for a reason because 3" pipe was banging around everywhere over wiggles and jiggles, but I may leave the rear most two mounts urethane while switching the rest back to rubber. Using the mega 4th mount made by Shuiend, and the stocker by the muffler tip, the urethane bushing should allow some transverse stiffness but due to stiffer boundary conditions being at the end of the car, and not attached to the subframe where it can vibrate midplane, shouldn't introduce as much vibration into the trunk where it may hit that nice ~113Hz resonance. I am planning to reupholster the package tray since I switched roll bars and will probably refresh the deadening there.

I stopped recommending polyurethane hangers to my customers because several of them had the hangers stretch due to them heating up. With the rubber ones and hose clamps, no more issues. I am not sure if that will change vibrations or not.

yes but that shop is snooty and charges a lot. prices double in winter because no garage!

Oh yea, I forgot to mention, I now run MT.net approved muffler. Car is quieter on the outside, but the same on the inside.

welcome to old age.

Mine with the SPRING LOADED FLAPPER is tamer!!

http://sites.psu.edu/asrpassion/wp-c...16/02/taco.jpg

It'll give 18psi wet dreams :giggle:

I'd actually add a flapper to mine, for the lulz, if someone would point out where I could buy one without cutting up an Evo8 muffler.

Here's the relevant spot in EO2K's thread. I looked it up because I was curious.

https://www.miataturbo.net/build-thr...9/#post1232036

Exhaust itself isn't loud though.

Truth, I need a non-Miata daily

find your local police surplus auction and grab a crown vic for under $500. I'm finding it to be a great life decision so far.

Have you tried blocking the two tunnels on either side fo the gas tank?
Apparently a lot of noise comes up through the trunk, bounces around, and gets sent into the cabin via those tunnels. I've read some threads of people filling them will insulation with good results.

Disclaimer: I haven't done this myself.... yet

I did, it helps a TON.

What did you use in it?

Some closed cell foam I bought at walmart. It was blue and cheap, in sporting good area.