[pnMedia]

I run a Lexicon MC-12 with three passive (identical) THX Certified subs:Stereo L/R + LFE. Each sub is powered by a discrete power amp channel, also identical. The system is calibrated to 75 SPL @ 0dB Reference. Crossovers are set to the THX 80Hz spec.

I use the MC-12's internal Pink Noise generator for calibration.

When passing the calibration Pink Noise through the subs, the L/R sub output sounds like low freqiency bass with no indication of Pink Noise. When the Noise passes through the LFE sub, it sounds much different. In fact it sounds much more like narrow-band Pink Noise mixed with low-end bass. It's not a placement issue (all three subs are below and span across the bottom of the screen).

I pulled the driver from the cabinet and inspected it. From what I can tell the driver and everything else is fine. There is no indication of any speaker damage whatsoever. In fact when playing source material with LFE activity there is no indication of any problems with the driver.

And so the question is - why does the Pink Noise sound so different when passing through my LFE sub vs. the L/R subs? Is this to be expected, or is it possible there may be an obscure problem with the speaker that has eluded me?

Thanks.

-paul.

 

[BassThatHz]

I created an all-channel identical 7.1 THX Pinknoise flac file.

Try that in VLC over HDMI from your HTPC to your processor.
https://drive.google.com/file/d/0BwUrIAjh6sJ3N2M3TDRWVlVjVTA/view?usp=sharing

If this one sounds different across channels you can be assured it isn't the data.

The system is calibrated to 75 SPL @ 0dB Reference.

THX Reference is 85dbA at -20dbFS at the LP from each individual speaker (as I understand).
As I understand, for subwoofers, it is 95dbZ at -20dbFS.

75dbA is merely a convenience thing for people or speakers who can't actually handle a REAL 85dbA if treble and 95dbZ of bass.

The weighting scale is kind of important: Z, C, A. (Big difference...)

In any case, identical amps and cones and boxes shouldn't sound different (ignoring room acoustical issues).

 

[PrimeTime]

It may be relevant to point out that subwoofers and LFE are not equivalent.

The LFE calibration is 95 dB at -20 dBR. That is for the LFE subwoofer channel, not the L/R subwoofers (if any) which are considered to be part of the full-range L/R system and are calibrated accordingly.

It's curious that, when investigating the behavior, the OP chose to disassemble the LFE sub rather than just swapping the sub connections.

 

[pnMedia]

I created an all-channel identical 7.1 THX Pinknoise flac file.

Try that in VLC over HDMI from your HTPC to your processor.
https://drive.google.com/file/d/0BwUrIAjh6sJ3N2M3TDRWVlVjVTA/view?usp=sharing

If this one sounds different across channels you can be assured it isn't the data.

Thank You. I pulled it down ...

THX Reference is 85dbA at -20dbFS at the LP from each individual speaker (as I understand).
As I understand, for subwoofers, it is 95dbZ at -20dbFS.

75dbA is merely a convenience thing for people or speakers who can't actually handle a REAL 85dbA if treble and 95dbZ of bass.

The weighting scale is kind of important: Z, C, A. (Big difference...)

Understood. I guess I should recalibrate ...

In any case, identical amps and cones and boxes shouldn't sound different (ignoring room acoustical issues).

Makes sense. However I have a feeling there's more to it based on the MC-12's Bass Management setup (L/R Sub + LFE). I'll continue to investigate, so thanks again.

-paul.

 

[pnMedia]

It may be relevant to point out that subwoofers and LFE are not equivalent.

The LFE calibration is 95 dB at -20 dBR. That is for the LFE subwoofer channel, not the L/R subwoofers (if any) which are considered to be part of the full-range L/R system and are calibrated accordingly.

It's curious that, when investigating the behavior, the OP chose to disassemble the LFE sub rather than just swapping the sub connections.

Thanks, and good point on my method of investigation. I've been inside the L/R sub cabinets before. Besides checking for damage I wanted to make sure the LFE driver was an exact match. I can swap the connections. 'Will do that later today.

Thanks Again.

-paul.

 

[pnMedia]

I called Lexicon. According to the engineer that I spoke to - LFE pink noise is full range hence the difference in tonal characteristics as compared to the L/R subs that are crossed over. He also suggested that I run all three subs as mono in parallel. Interesting suggestion, considering when I first bought the MC-12 (11 years ago) the standard recommendation for three subs was L/R + LFE.

Note that in case you are not familiar with the Bass Management in the MC-12, it's highly flexible. And so if I decide to try what the Lex engineer suggests, any LFE activity will be routed to the mono subs as expected. Now he has me wondering if this is the best way to set things up. I guess I'll be testing this and see how it works out.

I'm using Lexicon NT Power amps rated at 125 watts per channel. I hope this is sufficient to drive three subs in parallel. Otherwise I can run two in paralell off sub output (1) on the MC-12 and run the third off sub output (2), also set to mono ...

-paul.

 

[PrimeTime]

The usual AVR bass management in SMALL mode will route everything to one sub. Which is a lot for one sub to do. LARGE mode is more like a cinema setup, which is apparently what you were doing. But since your LFE subwooferr is not more powerful than the L/R channels, it makes sense to gang the three together to share the LFE burden.

With this arrangement (LFE mixed with the L/R bass), you might get too much LFE with the normal LFE +10dB calibration. LFE level may need to be reduced somewhat for proper integration here.