Theres several ways to 'beam steer' Low frequency energy, heres a few..but not limited to:
'One in front one behind'
Delay a rear positioned bass loudspeaker speaker (say 1m behind) , invert polarity...and away you go, although you may find that there are lobes that appear in the negative half at around the 135degrees and 225degrees...which funnily enough makes the beam look 'cardioid', with good processing you can make it look more like a hypercardioid, which is good or bad...depends what you want to achieve.
'Inverted loudspeaker', look up D&B's CSA stack.
Rotate an element, Edit the Phase, EQ, and few other bits and bobs, rather complicated. requires FIR filters, so if your good with C/C++ and have the appropriate sound cards / DSP units to process through this can be achieved..but rather time consuming.
'End fire array / Logarithmic End fire array / Log end fire array with phase shift '
Use progressive delay, where by the very rear box is 0delay and the most front box is the last delay. The diffraction causes destructive interferance in rear projection and good summation in forward, I managed to get a minus 30dB drop in all the negative 180degrees areas (90 through 270). Down side is, you require ALOT of boxes, processing, amps and space..which makes this slightly infavourable to crew and alot of events. This has by far the best results sonically (IMO), and for the event licences. Although, I feel very sorry for the people in the front row, if you dont limit right, the forward summation can be very scary and ear ripping. So limit properly to protect the punters too!
Theres a fair bit of science behind all this aswell, My advice would be to read Bob McCarthys book, its green and called Sound Systems: Design and Optimization..very good, highly recommended..not just for sub stuff. Also look up Glen Ballou, Handbook for sound engineers, and if your lucky, Harry Olson's Acoustical Engineering. Reprint. Philadelphia, Professional Audio Journals. Page ,535
If you need specific stuff, Please feel free to PM me. Apart from that, theres stuff on prosoundweb and previous posts on here. Its very difficult to put it all in here, my dissertation was around 100pages long (Sorry Simon Lewis), but the maths is actually not scary for the basic cardioid stuff.. just space the subs (preferably as close together as possible, but usually you wont get closer than 80cm from the acoustic centre due to sub size, to be even better, space at a frequency in which you want forward summation to be 1/4 wavelength such as for 100Hz, 344 / 100 = 3.44 / 4 = 0.86cm distance for factor 1.4 (3dB) increase at 100Hz). Then go (assuming you've done spacing for 1/4 wavelength at given freq:
331.4 + (0.606 * temp in celcius) = C (speed of sound),
1/C = Speed of sound per Meter (1/344 = 2.91)
0.86 (distance) * 2.91 = 2.5ms delay on read speaker, and invert rear polarity.
its that easy
Play around on Meyersounds Mapp online, start by using CQ1's..I know they are not a sub, but because their prediction is a 'true omni' (unlike the HP700 as thats modelled for its actual predicted performance) its low frequency representation is a good starting point, before you play with the differences of 'omni resoponse' as frequency varies.... also note, just because theory teaches 'as you go up in frequency, the omni projection becomes narrower', does not mean to say 125Hz cannot be more omni that say 50Hz, its a matter of box construction, driver positioning and so forth. this means you may have to 'tune' the rear boxes / inverted boxes to get a tailored response, particularly to sort out unwanted boom / bass spill onto the stage.
Is it possible to create a cardioid response if a few subs are arranged in the right way and one or more have their phase reversed? Yes
Does the type of sub matter (ie front/back loaded, vented etc...)? Technically no, but easier, depends on your processing and what your trying to achieve
Are there any other ways of controlling bass dispersion? Yes, Bessel array, End fire array, Beam steering... (1/t, 2/t etc and steering the audio into an area desired)
This post has been edited by simonwest: 19 June 2008 - 11:09 PM
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