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I have a 65 coupe and unfortunately the floor of the cabin coupled with the size of the cabin creates a "drum" that resonates at the same frequency as my ears.
I guessing this is about 25 Hz, low end of what is hear-able. My wife does not notice - just seems to effect me. I assume this is a freak thing, but figure I would put this up on the forum to see if anyone else had come across this issue. If I can't solve it, I will sell the car.
I thought about lead foil or a thick layer of rubber under the carpet. Dynamite is not sufficient.
What driving conditions create the resonance? If it's related to the exhaust, consider an H pipe between the pipes just aft of the transmission cross member. There are good aluminized clamp on sets available from Summit Racing. If you put rubber under the carpet, use hard, high density closed cell foam; the kind that gets used fro carpet backing used on gloss tile floors.
If you drive a midyear coupe with the side windows down and vent windows closed you get that "thrumming" effect....many of these cars do this. If that's the case cracking a vent window a bit should clear it up.
If you think its truly some kind of harmonic working through the body maybe a cargo floor mat will dampen the effect. You just throw it in the back and then cover that with a cargo mat cover - no real installation required....if nothing else its nice insulation -- Mid America Motor Works sells these two items...
I have both in my 63 and it really cuts down on drivetrain noise and a bit of heat barrier for the A/C in the cockpit too and you can pay a bit more and get a nice logo on the cover....
Last edited by Frankie the Fink; 08-31-2018 at 06:02 PM.
x2 Thick heavy mats placed in various spots will help
Some 60s era GM cars came with a few pieces of heavy material...cant recall if my A body had rubber or that tar like material but recall a diagram in the assy manual saying it was to abosorb harmonics.Never knew this til I pulled the original carpet and jute padding up.
Always wondered if the pipes werent round or smooth if that would change the harmonics at all. Or having a heat shield above the pipes out of a thick gauge material.
Cant hurt but doubt anything will totally get rid of it save for one hell of a big case turbo muff.
If you can press your hand against the floor (or other surface) and the noise is reduced, an application of damping material such as Dynamat will help.
The 94 through 98 Mustang convertibles had a 25 lb "vibration dampener" at the front frame to prevent shaking and harmonics. So -- even more modern cars can exhibit the problem...
I guess that I am a little dense, but I need some info on the application of the sound deadening mats. Do you pull up your carpet and apply these mats underneath, or do you use adhesive and apply underneath to the underside of the body? It's been a tough week, and I don't necessarily get the drift of how you use these mats.
Last edited by William Buckley; 08-31-2018 at 08:09 PM.
You just lay the acoustic mat over your existing carpet in the cargo area, foil side down......nothing else required - no adhesive and no removing or lifting anything...it fits very snugly.
Then if you buy the cargo mat "cover" which is carpet with the logo, you just lay that on top of the acoustic mat....for a total thickness of prob 3/4" or so.... It really quiets things down...and weighs very little..
If you don't want it in for some specific reason - like judging, you can just pull these items out -- no problem..
Last edited by Frankie the Fink; 08-31-2018 at 08:57 PM.
When new my SWC had a low frequency resonance at 2000 RPM coming from the back of the luggage compartment. I knew it was the large flat panel behind the jack well because it didn't make the noise if I was hauling a couple of cases of beer or other fairly heavy cargo, so I basically ignored it because I didn't cruise at 2000 very often. It wasn't until I bought an AIM that I realized the plant was supposed to cement a couple of rubber pads on the top of the frame cross member below the panel to provide some support and vibration damping, and when I removed the body from the frame, they weren't there, so check your car.
Those pads raise the natural frequency of that panel to above normal driving revs.
The plant also didn't install the rubber grommet in the upper radiator support, which meant the radiator was not electrically isolated from the ground plane so the radiator was the anode in a galvanic cell and it ate through the original Harrison aluminum radiator after about 12 years despite biennial coolant changes.
It doesn't matter if the frequency is 25, 50, 100 Hz, or whatever. Let's just call it a low frequency resonance. Flat panels are worst and can act like a drum skin. That's why I call it "drumming", and such panels usually have stiffening ribs molded (plastic) or stamped (metal) into the panel.
It would help if you could isolate the drumming to a specific location. The floor pan is a single piece from the cowl to the luggage compartment kickup. Probing with hand pressure might help locate it as placing a force on the area will tend to make mitigate the resonance.
My issue is the 25Hz; I don't think some realize how low a frequency that is... There is not much in a car that will cause that IMO; That's below what some high end sound systems can produce...
If you drive a midyear coupe with the side windows down and vent windows closed you get that "thrumming" effect....many of these cars do this. If that's the case cracking a vent window a bit should clear it up.
If you think its truly some kind of harmonic working through the body maybe a cargo floor mat will dampen the effect. You just throw it in the back and then cover that with a cargo mat cover - no real installation required....if nothing else its nice insulation -- Mid America Motor Works sells these two items...
I have both in my 63 and it really cuts down on drivetrain noise and a bit of heat barrier for the A/C in the cockpit too and you can pay a bit more and get a nice logo on the cover....
I have this problem, and it stops when I close both side windows. This is not practical in my car because I like the breeze and the exhaust sound with the windows down.
Here are the items in my car with the mat and cover in place and then the cover piece folded back, then the acoustic mat folded back and the the edge of both pieces showing the thickness — very plush.
Last edited by Frankie the Fink; 09-01-2018 at 11:59 AM.
If you drive a midyear coupe with the side windows down and vent windows closed you get that "thrumming" effect....many of these cars do this. If that's the case cracking a vent window a bit should clear it up.
That sounds like an aerodynamic effect. my GFs RAV4 will do that if both back windows are down, but the front window up, you get some uncomfortable low frequency pule pressure on the ears.
Doug
That sounds like an aerodynamic effect. my GFs RAV4 will do that if both back windows are down, but the front window up, you get some uncomfortable low frequency pule pressure on the ears.
Doug
It is And modern cars suffer from it. Several models have subtle deflectors to try to reduce the effect.
It is And modern cars suffer from it. Several models have subtle deflectors to try to reduce the effect.
My coupe seems to work like an amplifier on road noise and the most minute vibration. Going to order this baffling next week! Thanks for the information, Frankie!
This sounds like, no pun intended, "Helmholtz Resonance" "Vortex shedding in the shear layer over the cavity opening (sunroof or side window) couples with the cabin acoustics, leading to a self-sustained oscillation of shear layer and cabin pressure." The worst I've ever experienced this was in a friends 80's-90's Pontiac minivan.
This sounds like, no pun intended, "Helmholtz Resonance" "Vortex shedding in the shear layer over the cavity opening (sunroof or side window) couples with the cabin acoustics, leading to a self-sustained oscillation of shear layer and cabin pressure." The worst I've ever experienced this was in a friends 80's-90's Pontiac minivan.
This is the 65 coupe- continued
In my case, the drum is due to a flat panel reverb effect from road noise.
I put a piece of "horse stall pad" ( 1" thick really heavy rubber) on the cargo floor - didn't help much, but made me realize the "drum" was actually the back panel.
If I apply a lot of pressure to the back panel and then thump on it, the sound is sufficiently diminished. So it seems the vertical flat panel is the cause. I took the car over to the local sound/upholstery guy and he supplied me with some 3/4" carpet pad which I glued in over the dynamat type stuff that I had previously put on the back panel.
I re-installed the carpet and the problem is reduced but still there.
At this point, I'm pretty sure that the frequency is low enough that it needs to be addressed with basic mechanical thinking. So going forward I'm going to experiment with stuffing something between the gas tank and the back panel. Another thought was to actually glue 1" angle across the panel for the outside.
didn't help much, but made me realize the "drum" was actually the back panel.
If I apply a lot of pressure to the back panel and then thump on it, the sound is sufficiently diminished. So it seems the vertical flat panel is the cause.
Aaron
That's why they added the riveted-on vertical channel-shaped steel reinforcement to the gas tank side of the center of the rear wall of the underbody for 1964.
P.S. I'd post a photo, but the new photo uploader isn't cooperating with my computer.
08-31-2018, 04:40 PM
Low Frequency cabin resonance
