sound meter help

i borrowed a sound meter from work (radio shack 33 2055). the RS site doesn't have the users manual (it has a link to it, but target doesn't exist). so i'd like some help on using it.

i want to measure car cabin noise before and after new tire install.

that said, what weighting do i use? A or C? the former i believe mimic human hearing at low volumes while the latter measure everything or is that only at high volumes? i did a quick search on google that it didn't help much.

lastly, any ideas on how to position it in the car, handheld, or it doesnt matter. thanks for any help!

EDIT: it looks like using A weighting meter will shut off after flashing "LO" in the display. i'm guessing the sound level is too low to measure, however, it works fine with C. not sure how that works or what frequencies etc are at work.

The weighting is over a given time I can't remember which is faster. Just turn it to one of them and clap one will read lower and more consistent and the other will spike up and fall off faster. For metering what you doing in the car it should matter. I would suggest taking the reading at the same speed and the same stretch of hi-way since road types can make a huge difference in noise. As far as placement that's up to you, but personally I would put it up near head height, and again make sure it is in the same place both times.

there's a seperate function for response time, slow and fast. the dbA vs dbC is a different button thingy, unless A and C weighting also have a damping or averaging over time affect.

yup, i'll try the same stretch of road, speed, etc.

You should probably use C weighting and slow response. Here's some more information:
http://www.fredsoundofmusic.com/soundmeter.html

Btw, a sound meter is a great tool to have for home theater buffs. /ubbthreads/images/graemlins/grin.gif

The meter I have (though not a RS) has a selectable hi/low sensitivity setting as well as the A/C Weighting; if the sound level is too low, the LO indication is displayed, telling you it's below the range of the meter.
C Weighting picks up all the low frequencies which predominate in the car, so it'll read much higher than on A.
The low frequencies are less bothersome than the high, so the C readings may exaggerate the noise level, but if you can't read it on the A setting, it's still useful for comparison purposes.
When you get the LO reading, is it at idle, or actually driving? My meter reads to below the threshold of hearing (mine, anyway...), so it'd be surprising that it's below the meter's range, unless you're sitting in an anechoic chamber-