Methods to quiet trunk noise?

[ro9397]

Finally drove my complete frame-off restored M80 Thursday (GM Fast Burn, 700R trans). Outside the car exhaust noise is moderate and sounds solid.

Excess noise entering the passenger compartment is easily identified from the rear/trunk area because almost every square inch of the interior surface is lined with Dynamat (I know, it’s heavy).

Installing the two missing 6x9 speakers in the rear deck lid will not help much because only the speaker cones divide the trunk and passenger compartment.

I’m an acoustics person. The body metal is thin. My best guess is:
1. All trunk panels resonate
2. Substantial exhaust noise filters through all trunk panels
3. The air space within the trunk resonates when its resonant frequency matches the exhaust and even multiples of it.

Please vote/comment on my proposed solutions:

1. Install two 6x9 rear deck speakers (minimal expected improvement).
2. Damp all interior trunk panels including the lid with currently most-accepted spray-on nose/vibration control. What is best spray-on product for this use?
3. Damp bottom/exterior trunk panel as above.
4. Carpet the trunk...OEM? Aftermarket? Custom? Glue-down seems like a bad solution, but maybe Velcro. Is there carpet material that will form-fit by applying a heat gun?

TIA!

 

[DOUG213XRJ]

Post a few photos, PLease.

 

[ro9397]

Hi Doug!

I'll post pics next week when I get back to the wrench working on it, only ten minutes drive. It was detailed last week and frankly it looks gorjalicious.

 

[Doober]

I'd spray some deadener (or stick it) to the insides of the quarter panels, they are the most flexible in the trunk area. The floor is stamped and doesn't flex too much, and the lid has its reinforcements. You could probably spray the whole thing and it will help, and also build a divider that divides the trunk from the back of the rear seat, but also leaves air space for the speakers. Another option would be to build a panel that fits against the back of the seats (not creating a separate compartment for the speakers) and put some sound deadener on it... you could likely just use a piece of sheetmetal with some short screws to mount it to the braces behind the seat, just be sure to have something between the sheet & braces or it may be another source of noise (i.e. rattling).

 

[ro9397]

I love your sage advice, thanks all!

Talked late last night to my wrench, one of the nicest, most fair, and most honest humans on this earth...when the subject came up he reminded me the rear end is borderline (maybe the only mechanical device not new/rebuilt), and a LOT of the described noise is the rear end.

Thanks again for taking the time to type from your busy day.

Also talked to a great audio tech who is also a great wrench...he recommended a particular local audio shop...I've been to this shop and indeed it looked impressive...

Will post images next week when we pickup the M80...as mentioned it was super detailed, carpet shampoo'd, buffed out, the car is smokin hot.

Jimbo

 

[malibuplotter]

You also might want to consider building a x member to tie the rear framerails together, not having anything turns the rear rails into a tuning fork.

 

[ro9397]

Wow...Impressive to meet another acoustics nut...sweet!

Funny you mention that: just today a particular humming sound seemed to come and go in unison with RPM, possibly exactly what you describe.

Is your X-member removable or permanently welded? My wrench Jordan will love this idea.

 

[El Coolmino]

Show us or describe your tail pipe arrangement. Being an inch short for asthetics will do horrendous things for trunk noise