Hertz Meter Reading Device

You'll have to place the mic or meter right up to the sub's cone(sealed) or port. Otherwise you'll only be measuring how the sub interacts with your room.

What you're looking for is called a 'Real Time' or 'Audio Spectrum' Analyzer(http://en.wikipedia.org/wiki/Real_Time_Analyzer) You can get software for a computer now that will enable you to analyze the SPL output of your speakers, in your room, at all(what are considered) audible frequencies:(http://www.trueaudio.com/rta_faq1.htm) Another very affordable method is the 1/6th octave RTA that the Behringer DEQ2496 provides(http://pro-audio.musiciansfriend.com/product/Behringer-DEQ2496-http://cgi.ebay.com/B-K-SINE-SQUARE-WAVE-GENERATOR-MODEL-E-310B_W0QQitemZ130323305278QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item1e57dfd33e&_trksid=p3286.m20.l1116UltraCurve-Pro-Mastering-Processor?sku=182484) The low freq cutoff for most RTAs is 20hz though. A greater problem of getting accurate measurements at the lower freq is the response(below 100hz) of most "calibrated" microphones. Mics that are actually accurate at the freqs you are interested in are expensive, and the testing device you are using has to accept the correction curve data of the mic. ie:(http://www.earthworksaudio.com/27.html) (http://www.linearx.com/files/pdf/M51_Mic_Brochure.pdf) To test for output at 11hz; you'd probably need a signal(sinewave) generator like this(http://cgi.ebay.com/HEATHKIT-AUDIO-GENERATOR-MODEL-G-2-SINE-AND-SQUARE-WAVE_W0QQitemZ160355544791QQcmdZViewItemQQptZBI_Signal_Sources?hash=item2555ef6ed7&_trksid=p3286.c0.m14) or this (http://cgi.ebay.com/B-K-SINE-SQUARE-WAVE-GENERATOR-MODEL-E-310B_W0QQitemZ130323305278QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item1e57dfd33e&_trksid=p3286.m20.l1116), an SPL meter(which again will have a mic that's inaccurate at low freqs) and a volt meter so you can be certain you are remaining consistant in your signal between your reference freq and the freqs for which you are testing. Hook the generator up to one of your preamp's inputs(with the voltmeter across the +/- outputs for calibration) and test away. There are many signal generators(more expensive) that have built in voltmeters BTW. The low freq cut off for Redbook CDs is 20hz, and there's VERY little on vinyl, outside of some direct-to-disc pipe organ music that even goes down to 16hz. You won't hear anything at those sub-bass freqs, but the visceral effects are fun.

Ojgalli,
Saint Sans organ piece goes to 16hz. A recording of it comes as part of the 'demo' disk with my sub.
I played it 1x and it is scary. Whole house rumbled, plates rattled and you could see the window reflections vibrate.
So yes, there IS music that goes that low, but so low that unless you just gotta have it, there is almost not point.
And I played it 'safe' not turning it up near to 'normal' listening levels.