pilgrim
Super Moderator
This week 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, but (EDIT) I will just quote the relevant one.
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?
At in-town speeds of 30-40 MPH, a pretty steady A-weighted reading is about 72-78 dB and every crack or expansion joint boots it to over 80dB. 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 at 76-86 dB. But anything over 80 is concerning.
No wonder I think the **** thing is loud inside. Even at the A-weighted readings, the sound level is close to 80dB, the accepted level where hearing damage can begin with 2 hours of exposure.
The difference in scale measurement 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?