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Quieting impact harshness/boom

Quieting impact harshness/boom

(OP)
I have a 2005 Honda Accord sedan, 51k, and am attempting to quiet the car both for my ears and as a learning exercise. Mostly I want to soften the impact harshness/boom over bumps. (Tires are 65-series H Michelin MXV4 which are quiet for cruising). I gather that impact boom is more difficult to control, as it's usually whole-body resonances. Still, subjectively it does seem like the impact sound "comes from" the wheel that had the impact (probably the higher frequencies, while the lower freqs are still whole body?) What, if any, after-treatments are effective for impact harshness/boom?

I've done some diagnostics. Attached is a spectrum analysis of ~1 minute of 30-50mph driving over variably rough roads and raised road markers. (This is plain dB, and the dB scale is not absolute since this is via a mic and Audacity.) The harshness is clearly worst between 25 and 60 Hz. I know 1/4" of any insulation won't do anything for that. I've also tap-tested some body parts (mic'd outside the vehicle to ~isolate that panel's signature):
Roof: a few major peaks 100 - 140 Hz
Trunk: peaks 100 - 220 Hz
Doors: numerous peaks 60 - 150 Hz

My plan is first to baseline the vehicle with repeated recordings over known bumps at fixed speed. I'll record audio wav files for FFT and stream dBA or dBC from an Extech 407736 to an Arduino Due (daq at ~100 Hz sampling). The Extech can do dBA or dBC (not dB), but I'm not sure which is best for this test. I gather dBA is more for hearing loss and low dB levels, but seems to be the standard for car noise.

I'm currently thinking:
1. CLD on body structure that I can ~easily get to: rocker panels, inside structure near wheels, lower B-pillar, roof through the dome light (?). I'm unclear about doors for boom, but will do them anyway because of their spectra.
2. MLV (over 1/8" CCF) on floorpan (and up) and the backseat back to block the trunk.
How effective would these be for impact harshness/boom?

Any advice for treatments and/or test methods is most appreciated.

RE: Quieting impact harshness/boom

My gut feel is that nothing you've proposed will help the low frequency.

Options are

1) redesign the suspension.

2) softer bushings in the suspension

3) reduce the tire pressure

4) change the tires.

Sadly 1 and 4 are the best options in my experience.

Cheers

Greg Locock


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RE: Quieting impact harshness/boom

(OP)
Thanks, Greg. I've been wondering if any would help, too. My tires are already on the cushy side -- actually I'd like +1" for better handling. If CLD & MLV won't help, I wonder how Honda made the TL quieter using the same platform and +2" wheels. Honda notes many things compared to the prior TL, but only a few of them would help low freqs (and they don't say what the upgrades are from the Accord).
http://www.honda.com/newsandviews/article.aspx?id=...
http://www.honda.com/newsandviews/article.aspx?id=...
Of those that may help the lows, I could add a front shock tower brace and stiffen the parcel shelf.
Vs the Accord, I wonder if they up-gaged some sheetmetal for higher body frequencies?
Or maybe their low freq response is similar, but human perception of impact boom is better just by cutting the higher frequencies?

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