Calibration dispute !

[MDS]

Then again, he is assuming that it won't track the input to the output properly. Just remember that this guy is an EE. But he asked me questions that I don't think people encounter on these forums everyday.

As a matter of interest, how accurately does the SPL meter track the input to the output ? Does anyone know ? Since he brought up these questions in the first place, I am a little interested to know.

Thanks.

--Sincerely,

The SPL meter measures the sound pressure level. Period. It doesn't track anything.

 

[mtrycrafts]

Yes, but this depends on output. But at what point does this become audible ? I mean, 2% THD doesn't, to me, seem like something to worry about. 10% THD, on the other hand, is something else.

So with a 30 hz tone passing through the amp to the speakers, it will largely still be a 30 hz tone. At reasonable levels, there should be no overtones present but at extreme output levels, harmonics will be added from the speakers (and amp).

But is this a point of concern ? That we don't know, as a poster suggested, that a tone from the amp will not be exactly the same on the output end ?

If I put in a 30 hz I expect to measure output at 30 hz.

--Sincerely,

At low frequencies THD is very difficult to hear, that is one reason why 10% is used when subs are tested. Some test have shown much more is needed. At mid band, 1kHz-4kHz, the amount is much less but, in both cases, it also varies by content.
That 30Hz will be 30Hz with very small amount of harmonics, very small. Not of concern.

I am still not sure what your boss is trying to measure?

 

[mtrycrafts]

As a matter of interest, how accurately does the SPL meter track the input to the output ? Does anyone know ? Since he brought up these questions in the first place, I am a little interested to know.
Thanks.--Sincerely,

It appears more and more that your boss is interested in knowing the input signal and output sound and any deviation between them?
That needs other instrumentation for sure, not an SPL meter.
But, I am still not sure why he wants to know this? Another set of speakers will have a different result.

 

[Vaughan Odendaa]

mtrycrafts, I think this is what my manager (not my boss) was trying to get across. That the input will not read via the SPL meter accurately to the output.

But how valid a point is he really making ? As you mentioned, if the input voltage is 1 khz, then I also don't see how the output would be grossly affected.

He seems to think that the SPL can't read the input properly. I don't know that means in the grand scheme of things.

--Sincerely,

 

[Vaughan Odendaa]

My boss has a RTA and stresses that it's far, far more accurate at low frequencies compared to my Radio Shack. I don't know how accurate that statement is, but I do know that it it at least more accurate than typical SPL meters down low.

Do you know what frequencies are used in the warble tones for subwoofers in the av receivers ? I mean, it isn't our job to calibrate a subwoofer's output to certain frequencies, it's the mixing engineers job to make sure that the soundtrack has ample, consistent and well utilized low frequencies across the band.

Pink noise is what is used worldwide for sub calibration, correct ?

--Sincerely,

 

[mtrycrafts]

mtrycrafts, I think this is what my manager (not my boss) was trying to get across. That the input will not read via the SPL meter accurately to the output.

As MDS, if memory serves me, indicated, the spl meter read in air sound pressures only, not input voltages.

As you mentioned, if the input voltage is 1 kHz, then I also don't see how the output would be grossly affected.

The output will be 1kHz with some harmonics of very low values, inconsequential in the sub region at most volume levels.

He seems to think that the SPL can't read the input properly. I don't know that means in the grand scheme of things.

It won;t read the input voltage is correct, as the input even to the voice coil is electrical signals, not air pressure vibrations that the spl picks up and translated to electric signals and in turn to a value.

 

[mtrycrafts]

My boss has a RTA and stresses that it's far, far more accurate at low frequencies compared to my Radio Shack. I don't know how accurate that statement is, but I do know that it it at least more accurate than typical SPL meters down low.

It could be, most likely it is; however, the RS meter has correction values on the net.

The RT accuracy is also depends on its mic and design, its band capability as it is not a full band, all frequency RTA from 20Hz-20kHz but perhaps 1/3 octave or some other octave fraction. So, it would be just as off inbetween the bands as well. The RS meter would read the sound pressure of a 33Hz signal well, but a 1/3 octave may not as it is off the center fr.


Do you know what frequencies are used in the warble tones for subwoofers in the av receivers ?

Nope.


I mean, it isn't our job to calibrate a subwoofer's output to certain frequencies, it's the mixing engineers job to make sure that the soundtrack has ample, consistent and well utilized low frequencies across the band.

Yep, someone should EQ it for sure, but that in itself is not that easy. That has been discussed here. An spl meter is not really that great either in the low band nor pink noise but now you are also at the mercy of the room EQ interaction. Software driven analysis is what is needed:

http://www.hometheatershack.com/roomeq/



Pink noise is what is used worldwide for sub calibration, correct ?

--Sincerely,

Many do but it is not very accurate from what has been discussed on this board and the above link will show.

 

[vierling]

Understanding The Spectrum

If You have an RTA or spectral analizer, You begin to understand sound once You "see" how the freq´s work in the audible spectrum.
In a PC or Mac based Analysis Software You can get info, that You would never see in the SPL meter or low-price-resolution RTA´s.

Imagine that You are measuring the spl to calibrate your sub.
You have White or Pink Noise, but the speaker´s freq response is 10 dB higher in 80 Hz, the Spl meter will measure those extra 10 db´s, without noticing the peaking frequency.

There are great High-resolution analyzers based in a computer, which are cheap, easy to use and have a lot of extra tools that You can use.
* Real Time Analysis
* Noise Generator ( Pink & White noise, or choose any freq You want )
* Spectral analysis
* Equipment measurement ( amp´s, receiver´s, eq´s.. )
* Delay times
* levels ( SPL )
* Harmonic distortion calculation
* Room acoustics analysis

All You need is:

A measurement mic. I Use Bruel & Kjaer, but there´s a great option from Behringer ( ECM-8000 $65 ) that will give excellent results.

Microphone preamp with phantom power ( needed for the condenser mic´s ) Behringer ( MIC-800 $50 ) ,also check Tascam, M-audio, Creative...

PC or Mac with line in, or USB ( depends of Your preamp or external sound card )

Mic stand ( camera tripod & Gaff tape should work for a Home T )

Cabling ( XLR-Miniplug, Miniplug-RCA, XLR-XLR ) To interconnect the PC with the preamp, receiver and Measurement mic.

THE BASICS:
*Computer Line out goes into one of the receiver´s line in. ( Noise Generator )
*Measurement Mic goes into the mic´s preamp and receives Phantom Power from it.
*Preamp´s line out goes into the computer´s line in ( or USB )

The software shoots the generated signal ( Noise or freq ) into the receiver.
The Mic measures the noise that goes out from the speakers, and the software make´s the required analysis.

HOW TO GET STARTED:
http://www.siasoft.com/pdf/Started-with-SmaartLive.pdf


http://www.behringer.com/MIC800/index.cfm?lang=ENG

http://www.behringer.com/ECM8000/index.cfm?lang=ENG

SMAART LIVE (30 Day demo ):
http://www.siasoft.com/

SPECTRA RTA ( Demo ):
http://www.soundtechnology.com/RTA132.html

You may download the demo version ( Smaart Live has 30 days ! ) and start to do some experiments.
You can measure the signal that goes out from the computer ( If You Go to
the Recording controls in the sound card properties, Select "Wave out", or "Stereo out " ) and see how the freq response fluctuates when You play any sound signal in the computer ( Music files, or the noise generator that is included in the software )

If You wanna go Pro, read about the transfer function, phase, and spectrogram.

 

[jaxvon]

It should also be noted that it would be beneficial to have your measurement microphone lab calibrated in order to ensure flat response and accurate measurement.

 

[Vaughan Odendaa]

Well, I have the Radio Shack meter. Can this meter be calibrated ? And if so, who would do that ? Who would I contact for calibration ?

--Sincerely,