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1983 280ZX Turbo
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1,062 Posts
Discussion Starter #1 (Edited)
Today I pulled out my faithful old Radio Shack sound level meter and went for a drive.

A couple of years ago I switched from rubber suspension bushings to poly, and I was convinced that my 83 ZXT is perceptibly louder inside as a consequence.

The meter measures the noise level either of two ways:
  • A-weighted, 500-10,000 Hz, which is the primary human hearing range and is the most used rating for workplaces and general sound levels. The human ear doesn't hear low frequency sounds as well as higher frequencies.
  • C-weighted, 32-10,000 Hz on this meter, a wider sample but not as representative of human hearing.
The results? (Only A-weighted results are relevant...)

At in-town speeds of 30-40 MPH, the A-weighted reading is about 72-78 dB and every time I hit a crack or expansion joint it jumps to over 80 dB.

Highway sound levels are similar, but slightly higher and any time I go over pavement with expansion joints or cracks it jumps up another 10 dB; A-weighted readings are consistently about 76-86 dB. Anything over 80 is concerning, and anything but perfect pavement causes sound to top 80dB and stay there as long as cracks are present.

No wonder I think the daggone thing is loud inside. Even at the A-weighted readings, the sound level is close to 80dB, the level where hearing damage can begin with 2 hours of exposure.

The real sound problem is all low end frequency, and I think that's the result of the poly bushings - every bit of rumble and low end road noise is transmitted into the passenger compartment. I have a bluetooth radio in the car that links to my cell phone, but I can't answer a phone call traveling above 50 MPH because I can't hear the caller and they can't hear me on mic over the noise level. The carpet and sound deadening in the car are original, as the car is very well preserved from new.

I expected a harsher ride, but it never occurred to me that the noise level would increase so much from the bushings. I'm thinking of going back to rubber bushings, as I don't really autocross this car and I honestly don't need them.

Has anyone else installed poly bushings and noticed a noise problem?
 

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There are other factors to consider as well. First of all I hope you don't have poly bushings on the T/C rods, if you do you need to change those back to rubber. The poly bushings can cause the T/C rod to snap. I've seen it happen many times. They definitely transmit more noise on that part. Next is what kind of shocks do you have and are they working good. Some tires are significantly noisier than others. It wouldn't hurt to add some Q pad or Dynamat to the floor, rocker panels, and the rear strut tower area. I've got poly bushings in my 82 turbo and while I did notice a little bit of noise increase, it's not significant though. Z man of Washington
 

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1983 280ZX Turbo
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1,062 Posts
Discussion Starter #3
Thanks for the comeback. The T/C bushings are going back to rubber right away, as well as the sway bar bushings - they're all easy to get at.

Shocks are Tokicos and I'd guess they have about 30K miles on them. I have some Noico mats coming and will install them under the passenger carpet and on the back deck. The strut towers are an excellent suggestion as well, thanks.
 

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Super Moderator
1983 280ZX Turbo
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1,062 Posts
Discussion Starter #4
I just finished adding Noico sound mat in the rear hatch area, all the way though the passenger compartment. I pulled the seats and the carpet, removed the mouse fur padding under the carpet, and covered the entire passenger area including all but the top of the transmission hump with the Noico stuff. I haven't done the rear strut towers (didn't pull off all the stuff required) or the inside of the doors.

Sound levels have dropped. Now at 80 MPH on road that's not very smooth the sound level is a fairly steady 76 dB, and going over expansion joints or bumps makes very little difference. I can also hear a difference, although the interior isn't what I'd call quiet at highway speed. Speeds around 60 MPH are closer to 70 dB, which is definitely lower.

Here are a couple of photos of the passenger compartment sound treatment:

107882


107881
 
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