Hello TrueRTA users!
Bruce is reporting measurement results that he thinks are the result of "bugs" in TrueRTA. I will address his complaints one at a time. I take all bug reports seriously as they help to improve the product.
Originally Posted by bgavin
I am a 33+ years professional programmer, and the corollary to this is a feature is a bug with useful side effects.
This is the sort of denial one expects from Microsoft. It does nothing to correct the problem, so let's please stay focused on the bug.
Here's the first problem restated:
1) TRTA geneartes a 100 Hz sine wave
2) Separate SPL meter reads 105 SPL
3) TRTA on-screen meter reads 106 SPL
4) TRTA on-screen data point reads 105 SPL
5) Export to TXT shows 100 Hz at 105 SPL
This is a point measurement, not broad band. The sine wave is 40dB higher than background. If TRTA is accumulating across the entire spectrum, it is still hearing 100 Hz at 40dB higher intensity than background.
I always calibrate TRTA to the SPL meter.
Do I calibrate TRTA to the Screen data point, or Screen Meter value?
Question: one is wrong, one is correct. Which one?
What Bruce is reporting here as a "bug" is actually normal behavior for any spectrum analyzer. Let me explain...again, and in more detail this time.
There are two distinct measurements being made by TrueRTA. There is a Narrowband measurement and a Broadband measurement. It is normal for them to be different. Sometimes the difference is small and sometimes it is large depending on the signal being measured.
The Broadband measurement is the reading from the TrueRTA SPL meter. This meters all the audio signals from 20 Hz to 20 kHz. The external SPL meter is also making a broadband measurement.
The Narrowband measurement is the reading taken from the individual RTA bar at 100 Hz. This reading only represents the energy within this narrow 1/24th octave wide band of frequencies.
Even though the narrow band at 100 Hz contains most of the measured energy, opening the bandwidth of the measurement to the full audio spectrum will add a slight bit more energy from all the additional frequency bands resulting in a higher SPL reading. This additional energy that is included in the broadband measurement is the difference between the readings of 105 vs. 106 dB SPL. This difference is perfectly normal and fully expected by those experienced with such measurements.
Note: When you perform TrueRTA's SPL calibration TrueRTA's SPL reading will instantly be adjusted to match the SPL of your calibration source. Most often this is a mic calibrator placed over the mic to be calibrated but you can also calibrate from another SPL meter. Just be sure to use an "unweighted" setting on your external meter when using it for calibration.
For those who want a little more depth here is a simple mathematical example showing how the two measurements are different:
Let's say we have one frequency "bin" with a signal level of 1.0 V. Define this level as 0 dB. Adjacent this bin are five bins on either side with "noise" at a signal level of .1 V (-20 dB).
The Narrowband measurement in this case would be the signal level of the single central band or simply 1.0 V or 0 dB. Let's name the Narrowband signal level "N".
N = 1.0 Volts = 0 dB
The Broadband measurement would consist of the combination of all the frequency bins. Let's name the Broadband measurement "B" and calculate it's value.
B = Sqrt(L1*L1 + L2*L2 + L3*L3 + L4*L4 + L5*L5 +L6*L6 +L7*L7 +L8*L8 + L9*L9 +L10*L10 + L11*L11 )
B = Sqrt( 0.1*0.1 + 0.1*0.1 +... + 1.0 +...+ 0.1*0.1 +0.1*0.1)
Where L1, L2 etc. are the signal levels at the individual frequency bins.
B = Sqrt(.01 + .01 + .01 + .01 + .01 + 1.0 + .01 + .01 + .01 + .01 + .01)
(note the five .01 signals on either side of the central 1.0 signal)
B = Sqrt( .05 + 1.0 + .05 )
B = Sqrt( 1.10)
B = 1.0499...
B = 20* Log (1.0499)
B = 0.414 dB
So in this simplified exmple we see a 0.414 dB rise in signal level for the wideband measurement versus the narrowband, single bin, measurement. The additional frequency bins of a broadband measurement will always increase the signal level.
You could also do a benchtop experiment with a variable width filter that would make the effect of bandwidth on signal level quite clear. Imagine this, you feed a pink noise source through a variable bandwidth filter and play the output signal over your speakers. Turn the volume up to say 90 dB SPL with the full bandwidth pink noise. Next start sweeping the filter (perhaps a crossover) upward from 20 Hz so that you start chopping off the low frequencies. The apparent sound level would decrease as would the reading of an external SPL meter. Narrowing the bandwith has lowered the signal level.
I hope from this it is clear that a single frequency measurement on the RTA will always be somewhat lower than the broadband SPL measurement seen at the SPL meter at the far left. Only in the ideal case where the noise in the adjacent bins was absolutely zero would the two measurements be the same.