34 replies to this topic

[Peter the Greek]

G'day all much wiser than I.

I finally got around the setting up my testing rig (Tascam card and calibrated mic), I've had trouble getting it all working. The step that is missing is calibrating SPL, for which i understand I need a basic SPL meter.

Can anyone point me in the right direction for a suitable one? I tried Dick Smith but they dont sell them anymore.

I assume I do need to calibrate the SPL and there isn't a way around this?

Is 1.5+/- accuracy acceptable?

Many thanks,
Peter

[automated]

Jaycar sell a couple of different ones, I can't comment on their quality / accuracy though.

[Craigandkim]

Peter,
I think you can use your calibrated mic with REW's SPL meter can't you?

SPL Calibration Procedure

• Select the Mic/Meter tab of the Preferences panel
  
• If using an SPL meter set it to C weighting and tick the C Weighted SPL Meter box in the Mic/Meter Preferences. Set the meter range to suit the measurement level used in the check levels process (the 80dB range is recommended for the Radio Shack meter). If using a mic and preamp leave the C Weighted SPL Meter box unticked
  
• Open the REW SPL meter by clicking the SPL Meter button in the toolbar then press the Calibrate button
  
• On the dialog which appears choose whether to calibrate the meter reading using your subwoofer or a main speaker driven by a calibration signal generated within REW, or to use an external test signal you provide, making the appropriate selection in the drop-down box and clicking OK.
  
  
• Enter the reading from your SPL meter in the calibration panel and press Finished when done
  

SPL Meter

The SPL Meter is an integrating, logging sound level meter that displays sound pressure level, equivalent sound level or sound exposure level based on the RMS level of the input channel. It offers A, C and Z weightings, fast or slow exponential filters, a high pass filter to suppress wind noise, and records minimum, maximum and unweighted peak levels. It takes into account both the soundcard and microphone calibration files and corrects its readings accordingly, allowing IEC class 0 performance when used with a calibrated microphone and SPL calibrator. Data recorded by the meter can be logged, graphed and saved to a text file.

[dor]

You don't need to calibrate the SPL unless you want the graph to read 'true'.

If you've loaded the calibration file for your mic then the response curve will be correct but not the level in relation to the values. To set the over all levels to match the scale values, yes you'll need a slp meter to calibrate.

Tandy or online are probably your best bets.

As the spl meter is used (in your set up) just for calibration of the Values ( y axis) a cheap meter is usually enough. You real only need to know was that measured at 80db or 100db, so +\- 1.5db would be fine for me.

[KenTripp]

Jaycar stock 3 SPL meters, @ $39, $99 and $399.

http://www.jaycar.co...er&form=KEYWORD

[automated]

G'day all much wiser than I.

I finally got around the setting up my testing rig (Tascam card and calibrated mic), I've had trouble getting it all working. The step that is missing is calibrating SPL, for which i understand I need a basic SPL meter.

Can anyone point me in the right direction for a suitable one? I tried Dick Smith but they dont sell them anymore.

I assume I do need to calibrate the SPL and there isn't a way around this?

Is 1.5+/- accuracy acceptable?

Many thanks,
Peter

I'm thinking about getting a calibrated mic, which one did you end up getting?

[Paul Spencer]

Strictly speaking it isn’t necessary to know the SPL level of your measurements. However, meters are cheap and nice to have anyway so it’s not a bad idea to get one. Jaycar have a cheap meter that is fine for the task:

http://www.jaycar.co...er&form=KEYWORD

And you can find the same meter on ebay.

Accuracy here isn’t critical as you will find that the level will jump around. When it comes to calibrating the level in REW you will see what I mean. Quickly you will realize that the accuracy of the meter isn’t the only variable. Meters will vary a bit more when it comes to peak SPL.

That particular meter is quite limited in what it will do but it’s fine for casual use.

[Peter the Greek]

I'm thinking about getting a calibrated mic, which one did you end up getting?

The specrum labs one.

I can't give an opinion on it as its the first one I have owned.....

Thanks for the info all, I'll have a play around with it later in the week....just need to get the darn thing working so I can play with my EQ....

I have a mate with a high end SPL meter, I'll just have to sweet talk him into borrowing it I think - his calibration method for subs is easy. 10-100hz on a CD, each played for 5 seconds. Record the db and then fiddle. Simple and highly effective. its also cool because you can physically hear the modes.

[Paul Spencer]

The high end meter won't be any different for use with REW. I can't imagine using your friend's method these days. I used to measure that way nearly 20 years ago, it was really tedious. A REW sweep that runs for a few seconds is not only quicker and simpler, it's also more accurate and gives far more information, rather than the fuzzy picture you will be getting, which is only mildly useful. 50 REW sweeps can be done quicly and give you a lot of information. It need not take long and it will give you information you won't find in a whole day day of using your friend's method. Taking measurements is not that hard. Being able to interpret them, and then know what to do with the results, that's the real challenge.

[Peter the Greek]

My understanding is room modes can take a while to escalate, thus a REW sweep might not fully capture the issue?

[Paul Spencer]

You must have seen waterfall plots. Those are showing time domain data, the decay of bass in the room.

[Peter the Greek]

I understand the decay point....or perhaps I dont.

What I have found is running say a 20hz tone (as an example) - it gets higher db the longer it plays, thus emphasising the mode. So with REW it runs a fairly quick sweep, I assume its sophisticated enough to notice these modes with such a sort period of time?

As I said, to the ear, I find it more noticable after the energy has been allowed to build

[Paul Spencer]

Room modes don't cause the SPL to go up, just like slap echo will be at a lower level compared to the original transient. Modes cause the bass to take longer to decay than the same source played outdoors. In fact, when I first added traps, the subjective impression was that the bass became more like outdoors bass, decaying quicker. This is reflected in cleaner waterfall plots and peaks and dips having a lower Q.

[Peter the Greek]

Ah I understand, so its probably the longer decay I can hear - that the mode causes. Thanks for the explanation Paul, much appreciated

[Paul Spencer]

Think of it as being like reverb, but because it's happening in the bass, our ears don't have the same sensitivity to time domain issues. Bass boom that we can easily blame on the subwoofer, can simply be modal ringing. It was a shock to find out how good my subs actually are, once set up in a trapped room. I was able to get a result that EQ couldn't touch. That is when I realised what a mistake it is to do what so many do with bass - to just choose one or two strategies and stop there. It becomes a question of EQ vs bass traps. The two give very different results. That's why I wrote the bass integration guide, to encourage people to use all the strategies, because they all actually work well together.

[svenr]

You don't necessarily have to calibrate REW. The measured responses will only vary in average magnitude. If calibration is required, you need an acoustic calibrator that fits to your capsule. Look at ebay for some cheap models.

[Peter the Greek]

Ok, so had a quick go with my second system - this doesn't seem right. It calibrated at 165db, which is obviously bollocks

[dor]

If you have a iphone, the spl app would be sufficient for setting levels.

Or set it by ear. Turn down your amp, start cal signal and increase the volume to the point where it's about the volume of a average noisy street and that's about 70-80db, calibrate to 75bd and you should be about right.

[Craigandkim]

Or if your having difficulty calibrating read the instructions I posted earlier?!!?

[Peter the Greek]

I did exactly that, something funny is going on with the soundcard

[dor]

Or if your having difficulty calibrating read the instructions I posted earlier?!!?

Step 5 will be a problem without a spl meter hence my suggestions.

[Peter the Greek]

I'll order one and just be done with it I think.

Having read Paul's post about the advantages of an active system I really need to pull my thumb out and get some accurate measurements going and start adjusting the EQ etc.

[Craigandkim]

Step 5 will be a problem without a spl meter hence my suggestions.

Ahhhh, not really because REW is taking the reading from the calibrated mic and displaying the SPL- enter the reading...job done.

[dor]

Ahhhh, not really because REW is taking the reading from the calibrated mic and displaying the SPL- enter the reading...job done.

What about mic pre amp gain?

[Craigandkim]

What about it?

PS. Dont mean to be abrupt, I have taken my readings through an external USB pre, with a calibrated Behringer.

Edited by Craigandkim, 29 February 2012 - 07:52 PM.

[dor]

It's why you have to calibrate(unless your very lucky).

The pre amp output level will be different from pre to pre just like cd players, two channel pre amps and phono stages. Variations in this signal is what REW interprets as different spl levels, so it needs calibration for accuracy.

A pre with a 2.2v output will max a 2v input with about a 90% output signal.

[Craigandkim]

Either way you look at it....if you already have a mic, you don't need to buy an SPL meter to accurately gauge an SPL reading with REW.

[dor]

I'd say the opposite if you want a accurate spl reading you must calibrate the mic\pre.

But if you only want a frequency response curve then spl calibration isn't 100% necessary if you've loaded the mic calibration.

[Craigandkim]

Well of course,
please re-read the REW instructions and you'll discover you can do the SPL cals without an SPL meter.

[Paul Spencer]

Either way you look at it....if you already have a mic, you don't need to buy an SPL meter to accurately gauge an SPL reading with REW.

What are you using instead? And how are you accounting for all of the gain settings involved?

[Arg]

Want to do a quick and dirty calibration with no SPL meter?

Rub your hands together about 6 inches (150mm) from the mic. Should be a little under 60dB (A weighted) or a little over 60dB (C weighted).

Is that helpful?

[dor]

great little tip, just tried it and yep 60db at 6 inches with a average rub. I saw 68db if I went real hard but it was hard work.

[Craigandkim]

Yep alright,
I understand what you're saying now...to get a definitive SPL reading you need the SPL meter.

This is how I have been taking measurements with REW, the Presonus USB pre and Behringer ECM-8000 mic:

1) calibrate the sound-card
2) Load mic calibration file from Cross Spectrum
3) Check signal levels in between -30db & -12db (measured -19db with 0db gain on presonus)
4) Using mic/pre leave C weighted meter un-ticked
5) Meter SPL calibration-(select external signal- pink noise generated by Integra)
6) Select calibrate on REW SPL meter
7) Record level and enter into SPL Reading calibration

After comparing the 75db signal level @ the listening position with a SPL meter & the mic there is a -19db difference (the mic measuring 56db).

So I assume I have been "lucky" with my SPL measurements being within the standard graph extremities and should probably respect the knowledge of others a little more than I have.

PS: Obviously I left out the taking of the actual sweeps with individual speakers, subs by hooking up to the Integra.

Edited by Craigandkim, 01 March 2012 - 06:39 PM.

[davewantsmoore]

Ok, so had a quick go with my second system - this doesn't seem right.

Longer time frame, and smaller frequency range .... I can't make out anytying there.

[Paul Spencer]

We're on the same page now. I used to not worry in the past but these days I like to know the actual levels I'm measuring.