# SPL Calibration, How Often?



## GeerGuy (Apr 26, 2011)

I have a Model 33-2055 Digital RS Meter that is at least 8 years old. 

Do these meters drift a lot over time? 

Can I still trust the readings from it?


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## nova (Apr 30, 2006)

That is a good question. I had never even considered it before :dontknow: and I have no idea what the correct answer would be. About the only info I can find on the subject is;

Radio Shack claims that under normal use, the meter shouldn't require recalibration. If adjustment is necessary, take the SPL meter to an audio professional who can calibrate the meter by connecting special equipment, such as a sound generator to the CAL jack.

I have no idea how you would even know if it needs to be calibrated. Perhaps someone else has more info?


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## GeerGuy (Apr 26, 2011)

I have a colorimeter that I store in a sealed Pelican case with desiccant. The 3 things that degrade the instrument are; light, heat and moisture. I know that the sensor used in an SPL is different, but any analog electronics will change and age with temperature and humidity, just not sure what impact it has on the measurement.

I have no way to calibrate or compare my SPL to a "reference" meter, but maybe someone has done this over the years...


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## Wayne A. Pflughaupt (Apr 13, 2006)

I expect that it’s not a cause for concern. I’ve dabbled in the pro audio field for well over 20 years (where room-measurements were commonplace long before the HT folks “discovered” it), and even worked professionally for a number of years, and I can’t say I’ve ever heard it mentioned that mics should be recalibrated after a certain amount of time. 

Sure, mics are affected by temperature, humidity, etc. which is why REW measurements taken months apart will often look different (Spring vs. Summer temperatures and humidity), but I've never heard of age as a factor. After all, mic elements are transducers much like speakers, and speakers aren’t reported to change their frequency response over time.

Regards,
Wayne


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## GeerGuy (Apr 26, 2011)

When I calibrate my speakers with my Model 33-2055 Digital RS Meter and the mic built into my Harmon Kardon 245 receiver they gave different results. This is why I am questioning my meter. Which one would you trust?


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## alexsabree (Apr 23, 2010)

Both will give very different readings. Overall, the RS meter is more reliable once you've added in the correction values.


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## Rodstintshop (Apr 26, 2011)

alexsabree said:


> Both will give very different readings. Overall, the RS meter is more reliable once you've added in the correction values.


Do your speakers not change over time I know I let mine play st a high volume for 10hours when I first got them and they sounded a lot better so I would think that over time with the center getting more play time that your spl would read different because the speaker becomes easier to move am I wrong please let me know I like to check mine once a month no longer than it takes what the hurt.


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## alexsabree (Apr 23, 2010)

In my opinion once a month is a little over the top. There is some truth to speakers "breaking in" or "loosening up" but it's not something that continues to effect your response month-after-month. Your varied responses are more influenced by outside variables like where people are standing, furniture moving over time and measurement inaccuracies.


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## informel (Jun 21, 2011)

Mic should not require recal, but an SPL meter will as everery electronic component have their variation over time. if you look at spec of resistor or capacitor, you will see that they have a variation over time express in %/year. good capacitor like glass capacitor have very low derating factor some have almost 0 like the COG cap (they should still exist, I haven't done real electronic design for quite some time).

But do not expect Radio Shack to use those parts (you would have to pay a lot more).


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## aackthpt (Jan 24, 2011)

Once a year is a typical period for re-calibration in most engineering environments. I'd guess that would be appropriate for an SPL meter.

However, when using the SPL meter as microphone I wouldn't expect calibration to significantly affect it. Unless you are doing something like verifying sound levels for some legal purpose, ensuring that you can hit reference level (unnecessary, if it gets loud enough for you without compression it's fine), or making measurements of subwoofers to compare or determine suitability (e.g. data-bass.com or Illka's tests archived here) then only shape of your measured response is important - not the absolute level.

If you're looking to get accurate SPL numbers from REW, I think the best way would *not* be to calibrate the SPL meter in REW with a hardware SPL meter (whose reading is jumping around), though it gets you close and is inexpensive (SPL meters being already owned by many enthusiasts). Rather, you'd get better results with an SPL calibrator (the Reed one at $150+ being about the least expensive one I could find with a few minutes' searching). Though at $150 I bet the calibrator doesn't come with a NIST-traceable certificate of calibration. And even if it did, you would still need to find a source to recalibrate (or at least check) your calibrator every year. The same calibrator use for REW could, of course, be used to check calibration of your SPL meter besides its use calibrating the REW chain.

I'd actually guess the RS meter wasn't exceptionally accurate when purchased; I don't know if they have accuracy/repeatability specifications, but I definitely know they don't come with NIST-traceable certificates. It's a totally wild guess but I'd be surprised if they were any closer than +/-2 or 3 dB SPL across all units, though I'd think any individual unit would have a better repeatability than that.


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## Guest (Nov 10, 2011)

BINGO! What he said. (Where is the LIKE button)

The Radio Shack SLM is not considered a professional piece and is absolutely not calibrated to any reliable accuracy at the factory. That being said, it's close enough for most. If you are using it to find relative level then the absolute SPL is not of concern. If you are using it for testimony in court, you had better be able to trace the calibration to NIST. In that case chances are you are not using the Radio Shack meter. For me, it's always been good enough. I don't carry it anymore as I use a Larson Davis CAL200 with my measurement mic and software.


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## aackthpt (Jan 24, 2011)

Welcome to The Shack, tigeraudio.

In practice, I generally use my preamp/interface with the input knob in the main REW chain (not the reference loopback) at maximum (and 0 in the driver gain). Therefore I found that I pretty much have a repeatable gain on the input side anyway, and while I still check the SPL with the meter regularly I have been stopping doing it every time as it is always within 1dB or less anyway. So while I can understand wanting to be careful, being less concerned over it is becoming my usual MO. :dontknow:


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## Guest (Nov 13, 2011)

You will have to pardon me. I don't really know what rew is exactly. Near as I can tell from less that 10 minutes of noodling its some kind analyzer using a sweep. I use smaart with 5 earthworks mics. I know it pretty well and its very easy to use and can even get around Systune which is much tougher at first and SIM3 but I was pretty lost looking at the rew demo. Can this software not use music as the source? Also I could not get it to update the trace in real time. How do I do that?


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## aackthpt (Jan 24, 2011)

tigeraudio said:


> You will have to pardon me. I don't really know what rew is exactly. Near as I can tell from less that 10 minutes of noodling its some kind analyzer using a sweep. I use smaart with 5 earthworks mics. I know it pretty well and its very easy to use and can even get around Systune which is much tougher at first and SIM3 but I was pretty lost looking at the rew demo. Can this software not use music as the source? Also I could not get it to update the trace in real time. How do I do that?


REW is just an acoustical analysis package. It does not use music as a source. It uses a logarithmic sine wave sweep, generally, to gather its data. It can't do multiple input channels like Smaart (I'm not at all familiar with the others you mentioned), but it can do some basic addition/averaging type operations on multiple FR curves from different sweeps. It is primarily meant to allow home audio enthusiasts to make small-room acoustical measurements at low cost, and notably also contains some aids for LF (subwoofer) equalization generally using MIDI communication to low-cost devices (such as Behringer DSP1124P). It's all in the help, though hopefully you found that since it sounds like you are running it already.

If you are looking to get a real-time updated trace, you can do that with the RTA window from the button at the top of the REW main screen. One would usually concurrently bring up the Generator, also a button at the top of the REW main screen, to send your desired signal. I have found the pink periodic noise particularly useful but I use the sine wave generator some to do mode mapping as well and I'm sure other signals it generates have their uses.


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## Guest (Nov 14, 2011)

At what level of spending are most here? Have you all heard of Xilica? Not much from Uli I would put audio through but lots of inexpensive alternatives.


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## aackthpt (Jan 24, 2011)

tigeraudio said:


> At what level of spending are most here? Have you all heard of Xilica? Not much from Uli I would put audio through but lots of inexpensive alternatives.


Nope, I personally had not previously heard of Xilica, but I can hardly speak for everyone on the forum. Their Neutrino units look attractive, but I couldn't find pricing and it appears to be a 1x1 unit. Possibly a good replacement for people needing basic functionality like Uli's feedback destroyers but I think in recent years most have moved up to the DCX or DEQ2496 - the DCX having the distinct advantage of being 2x6 and therefore very flexible for multiple subwoofer arrangements ala Earl Geddes or Welti and Devantier. The MiniDSP has become a popular alternative for the DIY crowd in the last few years. I've also noted more interest in the pro amp/DSP units recently.

I don't really frequent areas of the Shack outside the equalization/calibration area so I can't say much about them. But most of the home theater forums have a pretty good range of spenders, even some big-time DIY spenders. Especially the bassheads... I personally wouldn't call someone with multiple LMS Ultra 5400s a small spender. The equalization/calibration/DIY type areas will generally probably have more people who are either hardcore DIY or mid-fi as I think the super-high-end type people tend to pay a pro to design/calibrate their theatres for them - and most people looking into REW and such are way beyond the Bose/HTIB/Soundbar stage. But in these areas you also see people from acoustics companies catering to this market as well as the occasional circuit design engineer, pro audio guy, movie sound engineer, even some fairly well known speaker designers.

There are probably many on this forum who would be interested in inexpensive alternative to some of the Behringer products. Run some searches, and if they have not been discussed then by all means suggest them in the appropriate threads or start threads to discuss individual devices!


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