# Radio Shack SPL meter broken?



## crom0123 (Feb 14, 2012)

Hi,

I have a analog old Radio Shack SPL meter that has started acting weird.
The needle gets stuck at +3db as soon as connect the spl to the soundcard,even without 9V baterry, just the meter ON.
The needle moves and acts properly if I don't connect it to the soundcard.
All these happened after I switched the line-in to mic-in in soundcard settings (to increase my low input levels). I have a Xonar DX soundacard which has the mic-in and line-in output shared.
Any suggestion will be greatly appreciated.

Thank you,
crom


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## JohnM (Apr 11, 2006)

When the input is set to mic in it puts out a voltage to power the mic, that back feeds the RS meter. The meter is a line level device, the input should be set to line in. Make sure the input volume is turned up and the correct input is selected in REW.


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## crom0123 (Feb 14, 2012)

Thank for your help but my spl meter is still not acting normally. I did set up the souncard input as a line in. Is there any procedure to check if my spl is working properly? 
Why I'am saying that? Last evening I took a couple of measurement, everything looked OK, except low input levels: -31db when I measure the sub + mains and around -35 when I measure only the sub.
The spl range was set at 80db and the AVR volume was adjusted so that spl reads 75db when checking levels in Preferences REW Panel. 
At the end of the measurements I noticed the needle was stuck at +3db (I tried hard not to clip the meter this time).
I attached my full system scope graph, my soundcard debug file.
Are my measurement reliable with these low levels?

Thank you very much for your help and advice,
crom


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## JohnM (Apr 11, 2006)

crom0123 said:


> I noticed the needle was stuck at +3db


That is caused by having the soundcard input on mic setting, you *must* set the input up as line, not mic.

Levels around -30 dB FS are not a problem, but you would usually see higher with the meter on the 80dB range, so make certain the soundcard line input volume is all the way up.


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## crom0123 (Feb 14, 2012)

The soundcard input is set to line-in 100%. (please see attached).
SPL range was set to 80db, and AVR volume set so the SPL show around 75db when checking levels.
I tried all the options available/setting combination (Java 41,48 ASIO 41,48) and there is no way to rise the input level. I spent at least 5 hours trying different settings and at least 10 doing reading on this issue. No luck yet.:crying:
I found a very similar issue/soundcard on the forum at http://www.hometheatershack.com/for...15584-asus-zonar-hdav-1-3-sound-card-rew.html. I just didn't get the resolution of the issue.:dontknow:
Is there a solution/fix to my issue or maybe I have to buy another soundcard? I would like to get reliable measurements.
I really appreciate any help.

Thanks a lot,
crom


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## crom0123 (Feb 14, 2012)

Attached is one of my latest room measurement file.

Thanks,
crom


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## EarlK (Jan 1, 2010)

You need to check your soundcard for DC voltage offsets .


 Set your ASUS soundcard to "Line In"

 Use a multi-meter ( setup to measure DC voltages under 10V )
 Probe between the outer shield & the central receptacle/pin ( on a female RCA connector ) of either input .
 If you get a DC voltage reading that is more than a fractional Volt ( when your card is setup for Line Level input ) , then your soundcard has either a DC voltage offset problem or the phantom voltage meant for external mics can't be shut-off ( rendering this sound-cards usage with the RS SPL meter, a "no-go" ) .

- One solution ; buy the ubiquitous Behringer UCA202 ( a USB soundcard that uses the computers generic USB drivers). 
- The UCA202 is well behaved ( meaning, it doesn't output any DC voltage of consequence via it's input connectors / which would "peg" the meter movement of the RS SPL meter if present ) .

:sn:


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## crom0123 (Feb 14, 2012)

Hi EarlK,
I will check my soundcard's DC voltage as soon as I get home.

Thanks,
crom


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## crom0123 (Feb 14, 2012)

I have found online the requested DC voltage:
http://www.hardwarecanucks.com/foru...s-xonar-dx-7-1-pci-e-sound-card-review-2.html

*Output/Input Full-Scale Voltage: 2Vrms (5.56 Vp-p)*
It doesn't look good to me...
Thanks


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## EarlK (Jan 1, 2010)

> *Output/Input Full-Scale Voltage: 2Vrms (5.56 Vp-p)*


The quoted measurement is not the same measurement that I'm referring to . ( So don't panic yet ! )

Above, is an AC voltage & it refers to how much voltage the front-end of your card takes ( ie; the Voltage that is necessary to run the converters ( A to D ) into a full-scale (100%) digital conversion ) . 

We are looking for a DC off-set from the soundcards connections ( unfortunately present, on many laptop computers ) due to margin-able Power Supply design .

:sn:


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## crom0123 (Feb 14, 2012)

Hi EarlK,

Thanks for your input. Unfortunately Electronics is not my field.
I have found an identical issue here:
http://www.hometheatershack.com/forums/rew-forum/46059-low-input-level.html
Apparently I'll need a different soundcard if I want reliable measurements.

Regards,
crom


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## JohnM (Apr 11, 2006)

The measurement you posted looks fine. If the card has 2V rms input sensitivity then levels around -32 dB from the RS meter would be normal and OK for measurement. If the card is putting out a mic supply (which it might do based on sensing the impedance of the source, regardless of the mic/line setting) the voltage may only be on one channel, so try connecting your meter to the other channel (remember to tell REW you are using the other channel in the Soundcard preferences).


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## crom0123 (Feb 14, 2012)

EarlK said:


> You need to check your soundcard for DC voltage offsets .
> 
> 
> Set your ASUS soundcard to "Line In"
> ...


I checked my soundcard for DC voltage offsets.
The voltage reading for "Line In" input was 1.8 mV. (or 0.0018 V)
When I set the input to "MIC In" the voltage reading was 3.07 V.
Is my souncard good for taking accurate and reliable REW measurements? (It was a $100 soundcard)
:wave:Regards,
crom


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## crom0123 (Feb 14, 2012)

JohnM said:


> The measurement you posted looks fine. If the card has 2V rms input sensitivity then levels around -32 dB from the RS meter would be normal and OK for measurement. If the card is putting out a mic supply (which it might do based on sensing the impedance of the source, regardless of the mic/line setting) the voltage may only be on one channel, so try connecting your meter to the other channel (remember to tell REW you are using the other channel in the Soundcard preferences).


I connected my meter to the other channel (as you advised me, thanks :T) and I was able avoid spl meter's needle getting stuck at +3db.
At the end I was able to raise the input level at -27.7db by:
1. selecting ASIO Xonar drivers
2. lowering SPL range at 70db from 80db 
3. setting the AVR's volume so that the reading on SPL was 74db when checking levels (using mains speaker test tone, not the sub-woofer's)
I observed 3 things:
A. When I run the measurement the headroom numbers are now green  not blue
B. SPL's needle gets pegged for a fraction of second (Low Frequencies <100Hz are 10db higher than the rest of the spectrum). Am I hurting the SPL meter by that?:dontknow:
Another thing I'm not sure how to interpret are THD numbers/distorsion. What's an acceptable range for distorsion? Isn't my THD a little too high?

Thank you very much,
crom:wave:


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## EarlK (Jan 1, 2010)

> Is my souncard good for taking accurate and reliable REW measurements? (It was a $100 soundcard)


There won't be any problem using this card ( attributable to DC offsets ) .

Making, "Accurate & Repeatable Measurements", is an acquired skill ( so the rest is up to you ) . 

:sn:


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## crom0123 (Feb 14, 2012)

Thanks:wave:


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