SpectrumLab Electric vs Acoustic System Measurements

[Ricci]

I finally got around to setting up SL again and calibrating it. It's been a long time. I've been meaning to do this for a while. I think I have the relative SPL close between the electric and the microphone measurement but it still may require some tweaking. Also I'm not sure if I'm using the current version or whether my settings match up with anyone else. Might need to do some tweaking there as well. I did a comparison of the dragon crash from HTTYD last night. I don't have familiarity or the setup to do a strictly digital capture of the signal on the disc so the electrical signal here is what is coming out of my Onkyo 886 with a 200Hz Xo vis a splitter, so it does have a bit of extra roll off below 5Hz. The second graph is mic'd at the main seat at REF playback or close. My signal chain rolls off below 5Hz so I only get down to around 5-6Hz solid. Signal chain is -3dB at 6Hz. I don't quite get the full effect down below 6Hz but I'm ok with that. You can also see a sharp null that I have at 115Hz. It was kind of fun doing this so I may capture some more since I've got it setup again.

 

 

[3ll3d00d]

I can make a filter for your mic cal file if you need one. Just send it over.

[Ricci]

I can make a filter for your mic cal file if you need one. Just send it over.

 

The mics are either EW's M30's or ACO Pacific 7052 depending on what's handy. I never use cal files with either. Actually neither company provides what you would normally call a cal file. It's a data sheet with sensitivity information and verification of the frequency response. I'd have to make the cal file to load. Most of the correction needed is above 15kHz.  They roll off some below 5Hz but not enough to worry about. If I recall both are something like -3dB at 3Hz.

 

Now my SC is about -0.5dB at 10Hz and roughly -10dB at 2Hz if I recall. Have to check. Been a long time since I looked at that cal file. That one could be worth looking at but if it's all within a couple dB down at 5Hz I'm not going to worry about it. In my experience some cal files are inaccurate and give greatly exaggerated results because they are themselves.

[3ll3d00d]

Well then I guess you don't need one then!

[SME]

It's kind of funny.  I was just thinking about this same issue today.  I'm using a Cross Spectrum calibrated UMIK-1, and the cal files provide data down to 5 Hz.  Mine doesn't drop off too dramatically down there (-4.5 dB vs. 1kHz), which gives me at least some confidence in the data.  But what happens below 5 Hz?

 

At least I own the Motu A16 with DC-coupled outputs.  This means, I can loopback from the A16 to any other input including one of its own to measure its impulse response and create an inverse filter for it.  So in theory, I can at least get perfectly flat electrical measurements.

 

But yeah, no clue what to do about the mic.  Does anyone else here actually use a mic with valid calibration data going below 5 Hz?  Is this really a big deal even?  Bossobass Dave seems to speak extension down to 3 Hz being desirable.

[Bossobass Dave]

The ACO Pacific rig (includes cartridge, pre-amp, power supply and BNC cables) is sepc'd to 2 Hz (+/-2dB).

 

Regarding the frequency response requirement for playback, it comes from my initial scrutiny of the DD guidelines from Y2K onward:

 

 

Dolby Digital Professional Encoding Guidelines Encoding
4-2
shows one possible configuration. Since inclusion of the LFE channel is optional and the listener determines its reproduction from a decoder, essential low-frequency information should not be mixed exclusively to the LFE channel. Alternative practices exist within various industries so it is important to check the source and accompanying documentation.
Table 4-1
Channel-to-Track Layout Example
Channel L R C LFE LS RS Lt Rt
Track 12345678
Channel Levels
The following text is in accordance with the ITU-R recommendation and SMPTE standard referred to in the above section. For consumer and DVD production studios, relative channel levels assume each speaker delivers identical acoustic sound pressure levels to the listener. This excludes the LFE channel, which is intended for reproduction at +10 dB SPL (with respect to the main channels within the same 3 Hz to 120 Hz passband).

 

As time progressed and so did our measurement capabilities, we found (and see the data in the MWB list) that content included frequencies to <1 Hz.

 

There have been debates over SpecLabs linear presentation vs REWs log sweep presentation which claim that <10 Hz content shown in SL is grossly exaggerated, but my own data on the subject, which included looking at the phenomenon of driver excursion requirement of 4X per octave as content frequency decreases, supports the idea that SL spectrographs are far more accurate than the opposing side claims.

 

I contend that SL spectrograph comparisons between the digital loop and the mic'd-at-the-seats versions are THE metric for gauging accurate in-room playback. They show frequency response non-linearity (the most important metric to your listening pleasure), Harmonic Distortion, Intermodulation Distortion and Spectral Contamination (which is present in the SL caps during the GTG "incident" and suggests problems upstream of the drivers).

 

I know of no better tool and wish more people who have measurement capability would engage in the exercise and post their results.

 

To be clear on what I personally believe, due to the exponential excursion requirements when diving an octave from 5 Hz to 2.5 Hz (essentially 3 Hz), I have evolved to feel that reference level playback capability to 5 Hz is a respectable goal in-room and puts one in a very elite group.