# SPL meter odd behavior



## HeavyGuy (Apr 24, 2011)

Hello, everyone. 
I'm relatively new to the science of acoustics but I'm learning a lot and need a bit of help.
So I managed to get my hands on Extech 407730 SPL meter for measuring the frequency response of my speakers. 

First of all, when SPL meter is connected to my soundcard there is some weird noise, hard to describe like electric, metalic buzzing. mp3 CLICK . Is this normal?

Another issue is with cables that connect the meter to PC. When using 1m long wire (3.5mm) to connect the SPL meter, windows shows that microphone's input volume is maxed out even in complete silence. When the SPL meter is connected using at least 2 short pieces of wire together in series input volume is quite low. Has it got something to do with too less wire resistance?

I also found some odd behavior using REW. After calibrating mic _REW's SPL meter_ shows different dB readings than my actual SPL meter. REW's readings are higher and the numbers don't seem to drop under ~60dB, even in silence when my SPL meter shows below 40dB.

The last issue is SPL meter's readings sometimes seems kind of "laggy" when the device is connected to computer. For example if there is a short loud noise followed by silence, the SPL meter's readings drop quite slowly, but when I unplug the stereo jack from SPL meter, the readings drop instantly.


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## EarlK (Jan 1, 2010)

> Take some pictures of the cables & adapters you are using to interface the SLM to the computer ( then post the pics ).

> Clearly show the ends of the cables .


:sn:


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## EarlK (Jan 1, 2010)

Here's the type of adapter that I recommend you use right at the output of the ( Extech ) SLM .



From this adapter to the next set of adapters ( going into the computer ) you use a regular single male RCA to male RCA ( as an extension cable between computer & SLM ).



Going into the computer ( & assuming you are using the typical soundcard of a laptop ) you'll need something like this ;



Get the above adapters ( or something representing this very concept ) .

:sn:

ps ; 
> You *can not use *a single cable like the following ( & get away with it ) to patch the SLM directly into the computer . 
> All you'll do is short out the signal in the front end of the computers' soundcard and then you'll get goofy symptoms such as you are describing .


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## HeavyGuy (Apr 24, 2011)

Yes, I'm using some sheap standart 3.5mm cables cablesPICTURE ( CLICK )
I should get a pair of better quality cables by Monday, but are they really the ones responsible?


Ohhh, thanks EarlK. Didn't think that using 3.5mm cables instead of RCA cables would make any difference.


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## EarlK (Jan 1, 2010)

> Didn't think that using 3.5mm cables instead of RCA cables would make any difference.


> It's not the type ( or within reason, the length ) of cable that matters here .

> What matters is the SLM outputs a *mono signa*l and that signal needs to* remain mono* all the way into one channel of the computer . 
> I simply gave you a cabling topography that accomplishes that dictate .

> I don't think you've read ( or at least digested ) the  * REW Cabling and Connection Basics . *

> You won't get very far using sophisticated software like REW, without first understanding basic signal routing .

:sn:


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## HeavyGuy (Apr 24, 2011)

I found a set of RCA cables and adapters in my shelf and tried connecting the SLM the proper way but the input volume is much lower.



I'm not sure these cables were originally meant to deliver audio signal but that's what I could find.


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## HeavyGuy (Apr 24, 2011)

Tried experimenting with lots of different adapters and cables but still haven't got anywhere.
It seems that the SLM _3.5mm mic out_ outputs signal using left channel and computer can pick up that signal *only* if SLM is connected to the left soundcard input (not right. weird?). the problem is that input volume is maxed out in silence and REW's SPL meter displays "over". Where is the problem? Is it in the SPM itself?


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## EarlK (Jan 1, 2010)

> Tried experimenting with lots of different adapters and cables but still haven't got anywhere.
> It seems that the SLM 3.5mm mic out outputs signal using left channel and computer can pick up that signal only if SLM is connected to the left soundcard input (not right. weird?). the problem is that input volume is maxed out in silence and REW's SPL meter displays "over". Where is the problem? Is it in the SPM itself?


> Your meter outputs a mono signal, on a mono type receptacle ( called a "jack" ) .

> So quit plugging into that jack a stereo plug with the ( erroneous ) assumption that you'll get the full available voltage outputting from the SLM ( we have no way of knowing if your approach will work apart from contacting the maker of the meter or actually making some comparitive AC voltage measurements ).

> In my second post ( first pic ) I displayed a link for the necessary adapter that you need to get to plug into your SLM ( it's a mono adapter ). 
> An adapter like this will guarantee that you get full voltage outputting from meter to computer .

:foottap:

*Anyways :*

> This is all a really pointless excercise ( in signal routing ) since there's no generic soundcard calibration file available for your Extech meter / meaning ; without a meter calibration file you can't believe much of what you see on the computer screen,( assuming you figure out the signal routing for your Extech ) .

> I recommend that you buy a Radio Shack type SPL meter ( with mono RCA out ) that you can easily interface to a laptop using the other 2 adapters displayed within my second post .

:sn:


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