subwoofer calibration

I'd like to pull your brains on subwoofer calibration.
 
I installed a subwoofer (ADAM Sub 7) a while ago in my studio, following the traditional recommendations on placement (somewhat off-center, same distance as satellites, etc.) and I did the "crawl around and listen" thing. I'm actually quite happy with what the system sounds like and how mixes turn out. It nicely clears out the lower mid range when switching from 2.0 to 2.1 while not changing the level of the lower end.
 
Anyway, the voices in my head keep telling me that there should be more to it than crawl around and be happy. So, I'd like to do some proper acoustic measurements like I did for the rest of the studio, but can't find much useful literature telling me what to expect and what to aim for in the calibration process.
 
I wonder if any of you guys can point me somewhere for further reading, provide some insight in your own calibration efforts or maybe even share some measurement plots of a properly calibrated system???
 
Cheers.
 
Rob
 

Hi bitflipper,

Your advice is spot on, as always

I did a number of measurements (using REW) this morning and it turned out that regarding subwoofer placement and sub level the calibration by ear gave pretty much the best obtainable settings; so the "ear calibration thing" actually works quite well. SPL meter actually showed only about half a dB difference between satellites and subwoofer. 

Nevertheless, from your recommendations I could get a somewhat flatter response by lowering the crossover frequency from the recommended 85 Hz to about 80 Hz. Biggest impact, however, had the phase inversion switch on the sub, which basically ironed out the dip at the cross-over.

Interestingly, subwoofer placement doesn't seem to have much of an impact at all in my control room; this may sound strange at first, but in the past year I put several months of work into design, construction, bass trapping and building a RFZ. So it's good to see REW plots confirming a pretty flat response across the bass range.

BTW, this re-design and major construction effort was triggered by a discussion I had with you and Bristol_Jonesey on this forum about 1 1/2 years ago; the subject back then was ARC and the outcome was about 25K spent rebuilding the studio with proper acoustical measures in place. Unfortunately I don't have the website online yet (I have that done by a pro) but once it's online I'll PM you to get a glance to see where some simple discussion on this forum lead to ...

ARC I bought and used meanwhile in the poorly treated old mixing environment. It has served me well ... and since I already own ARC, I think I should give it a try in the new control room to see if it'll be the cherries on the cake or just empty calories ...

Cheers, mate.

bitflipper


Aside from speaker placement and bass trapping, really all you can do beyond that is to fiddle with the crossover frequency.


Even though a subwoofer only has to handle an octave or two, it's very hard to design a box that's truly flat over that span when you're talking low (< 60Hz) frequencies. You can't do much about the low end of its range, but you can decide where to set the crossover for the upper end. 

You'll find that there is some upper frequency where the sub starts to wimp out. On a quality unit like yours that might be well over 100Hz, but on my cheaper M-Audio sub it's around 70Hz. Since my main speakers are reliable to about 46Hz, I let them handle as much of the low end as they're capable of, and set the crossover to around 55Hz. This yields a flatter response than when I previously had the crossover set to 70Hz.

I discovered the same thing with my BM5a's. Much better to leave them flat. I was getting really screwy mixes when I tried to let the sub handle everything below 80hz.
 
Regards,
Dan
 

Oh sure, go and get scientific on us, Jeff. We all know that trial-and-error is the approved method of Famous Professionals the world over. I read it on Gearslutz so it must be true.

ARC won't compensate for antinodes, unfortunately.

Zo, I have the same monitors as you (P11A), but I couldn't afford the matching sub. That would indeed be an epic combo. 

One last observation...most peoples' mixes would probably improve if they just shut the subwoofer off and HPF'd everything at 50Hz. :)

The bottom line with a sub, the frequencies in your room to your ears at your listening point should be at the same volume. One easy way to set your crossover point is to simply grab a MIDI keyboard and output a Sine Wave in the bass range. 

Without the sub present, start with a bass note in a semi-low bass range. Descend notes slowly, listening to each to determine if they are all even in apparent volume. Soon, at the lower limit of your speakers, you should hear a roll-off. Depending on how fast your subwoofer crossover frequency rolls off (let's assume -3db per octave), when you reach a lower note that seems to be about 3db lower than the note before it, find out what frequency that is, and start your subwoofer cross-over point about there. Fine-tune until none of the notes between your speakers and the sub differ in volume down to about 30Hz. 

I have a set of Polk Audio 2.3TLs from 1989, which can audibly go down to 25Hz with no sweat. So, I know in my semi-treated room, if the notes through my system are pretty flat, you can get this too if you choose the cross-over frequency correctly. 

Hopefully the crossover will be lower than 50Hz. If you need to cross-over at a higher frequency, direction starts to become a problem, and you may need  two subwoofers to better represent accurate bass imaging. To survive with only one subwoofer, unless you are mixing in surround and the sub is your .1 reference, you do not want to have a sub sounding monaural. You shouldn't be able to quickly pinpoint where it is in the room if it's deployed correctly.

Best of luck!!

Be careful using sinewave tones in the bass range. An oscillator is better for that sort of application rather than a keyboard synth.  Bob Katz suggests using pink noise, it is much better. It reduces the possibility of standing waves and nulls and things giving you incorrect readings on level etc..

The Katz article I referred to is logical and has an organised approach to sub woofer alignment.