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Adventures with a sound meter

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Fellow TDR member Boldt's Wagon and I did some interesting experimentation yesterday in Pasco. BW has a very fine DTT transmission setup, along with 215 injectors, BHAF, no cat, and a free flowing muffler. I have a stock filter housing w/K&N and 3 inch straightpipe.



It took precisely 120 seconds to determine that his truck has a loud exhaust drone... And our sound levels proved it. Although his club can 2nd gen truck has better isolation from outside noises, he recorded 2 to 3 db louder internal noise levels at 60 mph in direct.



Both of our trucks are auto's, both 3 inch exhaust, club cab, 3. 54 gears, but his is 2nd gen 12V, mine 93 1st gen. He has about 250 HP at the rear wheel, my truck is stock. Allyn's truck is definitely quieter until the engine reaches about 1800 rpm, and that point the noise escalates until his is louder than mine. It has a "boom" or drone that never really changes pitch, but drops in level while coasting.



Boldt's Wagon's truck recorded a quite steady 80 DB while cruising 60 out of OD. My truck recorded a somewhat unsteady 78 DB over the exact same road and speed. Certain stretches of road would drop to 77, but some bumps and rough spots would bump it up to 79 or 80. We did not measure sound levels IN overdrive.



Boldt's Wagon's truck has a long straight section of pipe that runs from the downpipe to the muffler. By reading the sound meter, walking around outside the truck while running the engine at the speeds it booms the worst confirm that there is strong sound levels at the middle of the truck that are almost equal to the level of noise right at the outlet of the exhaust. On the other hand, my truck has more than 10 DB louder noise while standing 6 or so feet from the outlet of the exhaust pipe.
 
I checked the sound levels against PW's rig with a Radio Shack sound meter because I was sure the noise had increased and I had the dreaded "drone" when I installed the flow thru muffler and MHAF. Today I bought six 3 inch muffler clamps and cut 3 pieces of 1 1/2 x 1/8 inch angle iron to strap onto the straight sections of pipe in front of the muffler.



The sound level now maximizes at 2200 rpm in 3rd and is 76 dBA with occasional 75 dB. This is a 4 to 5 dB reduction for the angle iron strap on. I recollect that 3 dB is an apparent two times solund level change. The sound level decreases both above and below the 2200 rpm. The sound level is 72 dBA at 1900 rpm in 3rd or 1750 rpm in OD. With the engine off, I have the radio playing at 69 dBA.



For trucks with the drone problem, the previously posted strap-on fix works, data provided with the help of PowerWagon. Now PowerWagon can install a flow thru muffler to reduce his noise levels for the long trip to Alaska.
 
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While at the muffler shop getting that broken rear hanger strap fixed, I did some examining of the undersides of my truck... I'm going to try clamping some stuff on the straight part of my exhaust pipe too.



I found out that Radio Shack still sells the exact same meter you have... but it's 39. 95 now.



Also, since my truck seems to have a lot more ambient noise (not exhaust... diesel clatter, mostly) I'm going to try adding a little more soundproofing to it. I'm going to be putting a thin sound-stop pad between the engine and firewall and transmission and firewall. It's a heat isolator as well, so it'll help cool the interior in the summer while towing. If I have the money, I may put some b-quiet products on the floor and low on the back wall to see if it isolates any better inside.



Also, I'm going to use a few square feet of thier self-attaching soundproofing over the oilpan and lower engine block. I suspect that quite a bit of noise is transmitted to the transmission housing and into the cab from it.
 
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