Axiom Home Page Forums General Discussion Hearing Things Dr Floyd Toole's Talk at Mcgill University

We watched The Hobbit 'Battle of the Five Armies' Blu Ray last evening at -15 Db with Audyssey OFF.

Sounded very good with nice clean dialogue & CC content from my VP160.

As for the movie, it was well done; however, I do tire of long continuous battle & fighting scenes similar to The Transformer series that drags out the carnage for far too long It's just too much sometimes...

TAM

Hi Fred -- This is an interesting / excellent observation. Let me elaborate.

All room modes (standing waves) will be intrinsically position dependent (they represent sine/cosine type spatial transitions -- or more complicated functions in oddly shaped rooms -- between pressure maxima at the room boundaries). Another way to say this is that energy conservation implies that a peak at one location must be compensated by a null elsewhere -- it can't be peaks everywhere in a zero-sum game. However, your counter example emphasizes a really critical feature: for very long wavelengths, the position dependence may be so broad (spanning across all of the listening positions in some cases) that it can really be interpreted as a global/constant effect for practical purposes, at least over the relevant confined listening area. The wavelength at 56 Hz ( note wavelength = speed of sound / frequency, where speed is ~ 343 m/s) is around 6 meters, close to twenty feet, so this caveat certainly applies.

The lowest room mode, taking parallel walls, will occur when the wall separation is half a wavelength. For a typical 20 foot dimension the lowest room mode would have a wavelength of 40 feet, or about 12 meters, which corresponds to a frequency in the ballpark of 30 Hz. In this case, there would be a null at the midpoint, with accentuation (an amplitude peak) at the boundaries. The second mode would be a full wavelength wall to wall with peaks at the boundaries (as always), but also at the dead center, in the interior listening area. The wavelength of 6 meters would correspond to a frequency right around the value you noted (say 56 Hz) and it would have positional minima (nulls) at the quarter wavelength locations, i.e. 5 feet from the front/back wall. As long as you were within, say, the inner 8 feet or so of the room, it would be pretty fair to call this a constant / global effect, as long as no one was seated close to the locations of the nulls. The third harmonic would correspond to the room dimension being 1.5 wavelengths, yielding a wavelength of 20 feet * 2/3, which is about 4 meters, corresponding to a frequency of about 85 Hz. Being an odd harmonic, there would be a null at the midpoint, which is prime listening real estate. It might just correspond to your experience (although there are effects from all three room dimensions that are more complex)

There is a nicely done writeup (which spared me from some trouble) here.

Cheers - DSQ

To add a (hopefully interesting) note, the comment of Andrew's that I was referring to is from last year in his excellent Sound and Vision interview on subwoofers here.



I wouldn't want to put words in his mouth, but it seems safe enough to extrapolate a bit. With multiple subs, placed at different locations, the way in which they excite the room modes, interact with the boundaries, and transmit sound to your location may be distinct. These two sources will interfere and tend to cancel each other's imperfections to some extent. The difficulty of the problem is that it does not exist just in the global frequency domain, but additionally varies locally from point to point. With two perfectly co-located subs, you will double the pressure output. The power is proportional to this quantity squared, so it goes up by a factor of 4. 10*Log_10 ( 4 ) = 6 dB. But, since the output of the two is spatially identical, the 6 dB boost is contributing to exactly the same peaks in exactly the same locations, and not improving the smoothness at all. If all that one wants is power, then doubling the number of subs will actually quadruple the power delivered at a location where peaks coalesce, at least for the selected frequencies. Of course, there is a price for this. Doubling the number of subs must, on average, precisely double the delivered power. This is not 6 dB, but 10*Log_10 ( 2 ) = 3 dB. You can't get 6 dB everywhere, it violates energy conservation, and the dynamic imbalance between peaks/nulls will be made worse at the spots where you do get it (for certain frequencies). If the subs are not co-located, then the interference may have the effect of spatial averaging, which would be a good thing for smoothness of response in various locations, limiting the artificial inflation or negation of frequencies that interact in particular ways with the room geometry.

Also relevant is the next Q/A:



Cheers - DSQ

Its fantastic to have you here DSQ. Excellent, well presented and informative posts. Cant wait to hear a little more about your space and maybe have you pick mine apart once I get rolling on setup.

Serenity -- very kind & glad to be here. I should put some pictures of my build up at some point. Its slow work, as I tend to go in little fits and bursts as time and energy allow. I've got a name picked out ... "The Limited Edition". I have a large collection of film (mostly StarWars and Rings) statues, and the current (now nearly concluding) phase of the build is shelving.

Look forward to some info on your build/setup as well, and happy to comment, but please don't mistake me for an expert on the more practical/applied aspects of listening room optimization (and certainly not on loudspeaker design). The underlying physics I generally do understand, but the application of those basic concepts to real environments is very specialized and requires experience more than anything else -- in that regard I am mostly a novice.

Cheers - DSQ