MODIFICATION: Noise Abatement
There is too much noise in the interior of the car. Particularly under load or at highway speeds the exhaust resonates. It is similar to that of a single engine airplane. I can not hear the turn signals click or tick. I can not have a conversation with a passenger. I can not hear the radio or music. I become fatigued during long drives. I cannot hear the engine or components if they are not operating properly.
Deaden all resonating surfaces and increase sound absorbing surfaces
(1) Absorb sound on the interior surfaces (felt).
(2) Deaden sound in the interior body panels (tar panels).
(3) Eliminate noise paths from gas tank to interior (foam).
(4) Eliminate resonance (wrap) of the muffler.
(5) Eliminate resonance (wrap and shield) of resonator.
(6) Add sound absorbing material (tech mat) to hood and trunk.
Excellent drivability, comfort. I feel the car is more solid and performs better (yep, I know it is only sound but it is how I feel).
DETAILED DESCRIPTIONS OF STEPS:
Absorb high frequency vibrations with added carpet layers. To do this I bought 1/4" felt padding (approximately 6 square yards polymer jute carpet padding, $2.97 per square yard SKU 687978 "FELTPAD" in the carpet department at Home Depot). Remove carpets and use as a template to cut the pad. Cut pad slightly larger where overlap with other carpets is possible. Install under carpeting wherever possible. Stuff scraps into all open body cavities. Layer as many layers as possible without causing occupant discomfort or carpet bunching. I used a tape measure and Sharpie marker to make templates. I used FRESH razor blades for each cut, cut from the backing side, and it was easy and the cuts were clean.
Absorb low frequency vibrations with the addition of tar mats to the body panels. The header runs directly underneath the driver right leg and terminates before the handbrake where the catalytic converter is. This is a hot and noisy area. To fix this I bought QPads (Duramat, 2 packs, 6 per pack, adhesive backed, blue release paper, tar panels 12"x12" and "1/16" thick, Levine Auto $26.00) to absorb heat and dampen noise and apply to floorboards and all resonating surfaces. Notice I have started at the body seam. In front of this area is is not necessary due to existing dampening material, multiple layers of body material and gussets.
Eliminate any open noise paths from the exterior and underbody to the interior. I found the gas tank is inside a body cavity that is open to the underbody and road. I bought "Great Stuff" (insulative foam, Home Depot $2.45 can) and foamed the gas tank area in place. Be sure to put a drainage hose in before foaming, I suggest a 7/16" fuel line hose (about 2 feet, about $1.50 per foot, Levine Automotive). After it has cured, wait 8 HOURS or else it will make a mess, peel the excess foam away (see photo). It peeled away crisply and I cleaned it up with a razor blade. I had very little excess and I used the entire can.
Then I taped all openings with silver metallic HVAC tape (1 roll, Home Depot). Others have used "Great Stuff" to fill the empty body cavities in the fenders and wheel wells and it has been noted that this traps moisture, promotes rust and in some instances I have been told it has deformed exterior body panels. Perhaps the new door and window stuff will work better but I am not experimenting futher with my car.
Eliminate muffler resonance without changing exhaust performance characteristics. I have a Jackson Racing muffler and I do not wish to reduce the exhaust efficiency but I do want to reduce the occupant noise. I think a layer of insulation on the exterior will dampen some of the noise coming out the sides and reduce the resonance by chaning the mass. I bought "Exhaust Wrap" (asbestos cloth 2" wide 50 feet in length, Advance Auto $42.88) and wrapped the muffler with 2" Exhaust Wrap and fastened with hose clamps.
(5) SHEET METAL
Eliminate resonator resonance without changing exhaust performance characteristics. I have a Jackson Racing resonator and tail pipe. I used the same 2" Exhaust Wrap and wrapped the resonator and fastened it with wire.
The resonator seems to the be the largest source of noise in the interior. To help reduce this further I wrapped the resonator with Aluminum sheet and fastened with wire. Then I wraped it with hose clamps, drew tight and then removed wires. The tighter I made the clamps, the more noise the resonator produced through the sidewalls so I backed off on them one turn. This also confirmed there is substantial noise coming through the sidewalls of the resonator. My next step is to put sound absorbing materials on the heat shield directly above the resonator and attached to the underside of the trunk (see photo). I'll probably use the same stuff I will insulate the hood with. As an update, I bought the hood insulation stuff and after looking at it I don't think it will work (it won't stay in place in all conditions).
Someone from the Nutmeg Miata Club noticed my tailpipe was only long enough to extend a little past the rear bumper facia and commented that they had heard about the tailpipe noise resonating in the below trunk area of the car (behind the rear axle). They suggested moving the tail pipe trim back enough to extend past the extreme rearward limit of the bumper (or about 2 inches). To test the theory I wrapped the "coffee can" with sheet aluminum (6"x12") and secured it with two large hose clamps extending the tail pipe about 2.5 inches, and went for a drive. Although it still has the same great sounds, and it can still drone at certian RPMs, that overwhelming drone is completely gone! Apparently the noise escaping from the tailpipe resonates in the below trunk (behind rear axle) area, and at certain speeds, or gets sucked back into the under car area (or something). Anyway, moving the tail pipe back 2.5 inches was enough to eliminate it. I'm going to the local muffler shop next week to put in a small extension just before the tail pipe trim. I suggest people try this first.
As a side note, I also feel this proves that at certian (highway) speeds, air is sucked back under the car, and a diffuser and perhaps skirts would likely improve handeling and performance. Perhaps my next project.
Eliminate engine noise. The supercharger whines when under power and this can be easily eliminated with hood insulation.
In the final analysis (or... my next Miata)
(1) I feel the transmission tunnel is the greatest source of heat and an additional source of noise for the driver. (3) A good layer of QPad around the transmission tunnel, and another good layer on the parcel shelf would beat most of the occupant nosie and heat.
(2) The resonator (last in the exhaust system) is the greatest source of noise and the noise comes up though the parcel shelf and gas tank area and obviously out of the pipe itself. I originally suggested buying a different resonator and muffler, but I have found this is not necessary. I recently discovered moving the tip of the tailpipe back a few inches eliminated the resonant effect in the cabin at highway speeds.