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Dear John,
I hope you don't mind me writing directly to you. I've been posting on Romy the Cat's audio board about the pros and cons of a bass horn in my new Barn Conversion. I'm probably not going to go ahead with it for various reasons. But I was thinking of installing Bass Line arrays below 40hz against the short wall of a 10m by 7m double height room with vaulted cieling. Probably 6 or 8 15" drivers in a sealed box per side. But - at this is the question, Romy seems sure that Line arrays will only work properly when used on the long dimension of the room. Is this correct? Intuitively I can't see why...
Your thoughts on this would be most welcome

Bass horns:

Yes, they do work and can work extremely well.  Probably the best authority on the planet when it comes to bass, period, is Tom Danley.  His first commercial success was a product that had to fit in a Land Rover and call elephants when it was first discovered that the communicated over great distances in the 20 Hz region.  It was the Packiderm 6 and was the predecessor to the Intersonics ServoDrive subwoofers that came out in the late 70's.  ( www.servodrive.com)  Since then, he created the acoustic suspension chamber that created perfect crystals on the space shuttle.  It requires 175 dB SPL.  He has been involved with two sound companies since ServoDrive, Sound Physics Laboratories (SPL) and he currently designs product for Danley Sound Labs (www.danleysoundlabs.com).
At Danley, his mid-high products are the best passive speakers in the professional sound reinforcement industry, in my opinion.  Let alone that he makes the best subs in the industry.  This link will give you an idea of Tom's world:
www.danleysoundlabs.com/forum/forums/thread-view.asp?tid=47&posts=1&start=1>>

Just Google-search Tom Danley and lab horn and see if you can dig up the article where somebody built-in Tom's lab horn in a rather large-scale (multiple drivers) application in a large room.  It would be 2 -5 years old by now.  It would be very similar to your application.

Bass Line Arrays:>>

More important than what wall they go on is what is the ratio of the room dimensions.  Regular room dimensions, like 10 X 20 X 40 aggravate isolated room modes where the room literally rings at low frequencies (think cathedrals).  There are ratios recommended by acoustical consultants that distribute modes to prevent this.  However, if you are stuck with your dimensions, then narrow-band notch filters can prevent your system from exciting them.  The trick is finding someone that can discern room ring modes from localized standing waves (In England, my friend Peter Mapp comes to mind).  They are not the same and you must look at time vs.. frequency, not frequency vs. dB to find them.  >>
The first mistake most make when doing a bass line array is stacking them together.  In order to act as a line source there are two requirements:>>
1) The array must be sufficiently long to provide the -3 dB/doubling of distance for more level from less cabinets at a given distance.>>To do this in your barn, I would suggest going from wall-to-wall across the short wall.  The reason for the short wall is two-fold: a) They will be aligned with the mid-highs on the same wall without the need for a digital delay to align them.  b) If you go wall-to-wall, then the side walls, if massive enough, will act as extended virtual sources, making the line array virtually infinite.  The more massive the side walls and the one behind the speakers, the better.  >>
2) The array elements must be within 1/2 wavelength of the highest frequency they reproduce.  >>
If you are using 15" woofers, I would crossover into your mid-highs at about 90 - 125 Hz.  At 100 Hz, 1/2 wavelength is 1128/100 = 11.28  11.28/2 = 5.64 ft. = 1.7 meters   So, at 7m, you could put 5 cabinets across the front wall on the floor (a 1/4-space loading for 12 dB more output than in free space) with the first and last in opposite corners.  This is a proper line source at long wavelengths.  If you want more level, use pairs instead of single cabinets.
If it were me, I wouldn't bother designing my own cabinets and selecting a driver with the right parameters.  Leave that to the home theater tweaks that convince themselves that expensive AC cords make a difference.  I'd use something like Danley TH-115's for level or TH-112's or DTS-20's for extreme lows (http://www.danleysoundlabs.com/DANLEY_productspagehome.html).  His old company SPL also has a couple of models I'd consider, like the B-Deap32 for level or the Contra Bass for extreme lows (www.servodrive.com/products.html).  >>
Danley Sound Labs mid-highs array together very well too.  But, in my opinion, the best sounding non-arraying high-output mid-high sound-reinforcement speakers in the industry are from a new company called KV2.  They will absolutely smoke products like JBL, EAW, and Meyer at half the price.  They are powered and they are as clean as studio monitors.  I handle them so I've heard them against all comers.  I call them loud Genelec's on a stick.  They are as low in distortion as many good studio monitors are and get stupid loud.  KV2 has a distributor in elace>ngland. lace>See if you can get a listen to them.  For very loud boxes, you will love them. www.kv2audio.com/Products/index.php>>

Hope this helps!

John A. Murray

Your email is fascinating and hugely interesting. Thanks for being so free with your expertise.
And thanks too for introducing me to Danly. The DTS-20 looks quite incredible. Just might solve the problem at a stroke.
I just have one more question. If, as you suggest, I use the short wall in a horizontal array (my side walls are 30 inches thick stone!) How do I deal with stereo? One channel on top of the other? or mono in the middle and 2 either side as stereo in a single array?
Also it seems to me that if I use both corners and the roof 'corner' (the barn will be open to the rafters) below 40hz then that would constitute an array too? And maybe load the room every well? Ok, I'm sorry that's 4 questions.

James,
There is no stereo for subwoofers because we cannot perceive stereo at extremely long wavelengths.  Our ears are 5 - 6 inches apart.  Even from the side the longest wavelength that will have even a 90-degree phase difference (1/4 wavelength) relative to each ear is that of a 564-Hz signal (6 inches = 1/2 ft.  1/2 ft. X 4 = 2 ft. 1128 ft./2 ft. = 564 Hz).  If the phase difference is less that 90 degrees, it becomes more difficult to discern the directional location of the source.  You will say, "But I can localize where my subwoofer is when it's playing, even with my eyes closed."   Not without the short wavelength components of harmonic distortion, you can't.  Try playing a very low distortion sub at a relatively low volume.  Blindfold someone, spin them and see if they can tell where the sub is....

Long answer to a short question, mono-sum your subs.  That's why there's a 1 in 5.1

Concerning including the barn's "roof corner," as an array component, I think you've missed something here.  First, depending on your sub type, use them up to 80 or 125 Hz as the manufacturer recommends.  They are the best component for that frequency range.  Your mid-high box's LF speaker will not do that range better than the subs, especially if it's at a high volume.  Second, a bass line source array would be a straight line of sources on 1.7m centers.  Sticking a sub in the roof would not be part of a line array across the front wall at floor level.  Since it is up high and farther from most listeners, it would also begin to smear the bass in time.  However, you could put subs all over the place to even out ringing room modes, but the bass would loose definition, tightness, impact.  This is a time smearing, distorted impulse response issue.  Just put them across the front wall and, if needed, have a good tech measure and notch out any bothersome room ring modes with 1/6th- to 1/10th-octave filters.

And don't listen to any smoke-and-mirrors shaman that tells you rubbish about narrow-band filters causing phase distortion.  If the signal in the room has a narrow (2-3 Hz wide) ring mode, it has a nasty phase shift that mother nature packs along with it.  The filter dampen the amplitude of the ringing and improve the phase.

That is what proper equalization does as well.  If a speaker has a peaking resonance, equalization removes the amplitude problem and cancels the peak's phase shift with the opposite shift in phase.  All IIR filters, be they transducers or electronic filters, have a minimum-phase component that is the amplitude's partner.  All good speakers are equalized because no transducer, as a fixed-diameter moving piston, has a flat frequency response.  Mother nature says no, despite what any company's marketing department may want you to believe.  In the home high-fi world, they just put the EQ filtering components inside the speaker cabinet on the crossover network board.  That way "he who abhors EQ and all its nasty phase shift" is blissfully ignorant.

As for putting what I'm saying on that message board, go ahead.  I haven't had any hate mail for a while now.......

Cheers!

John A. Murray

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