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Wavelengths and Directivity: Notes and Links
When I walk to the left and right of a monitor wedge, it seems like the tone changes. Is that a mystery, or is it simply because the higher of the frequencies emanating from the LF element make a narrower "cone" that may not match up with the wider dispersal pattern of the HF horn? Each given frequency is like a musical note, and in many ways its behavior can be predicted and modelled. Those of us who started out in bars may not have been taught that. We were too busy doing hand to hand combat, and trying to do a show with equipment that we barely knew how to run. Part of the process of being involved with Prosoundweb.com has, for me, been meeting some heavy-duty theorists. Maybe I am better than they are at taking the bass player to the hospital, but I am fascinated by the concepts that the theory guys have shared with me. The best of them are the kind of people and teachers that can boil it down to a sketch on a napkin. While being a part of our live chats with the likes of Dave Rat, Dave Gunness, John Tripeny, and Robert Scovill, I noticed that they are always thinking scientifically about how a given wavelength works, both in the original design of speaker cabinets, and how they are subsequently placed in a show environment and interact with one another, when us hooligans start sending big rude noises through them. As Ken Berger puts it, what we are dealing with sometimes are "the prevailing feelings that the guys who only listen (to tune a system) think that they are better, and the (schooled) 'engineers' feel that they are cowboys winging it. It is the meeting of these two minds that are the most powerful, as one without the other is not seeing the whole picture (or sound field, if you will.)" Some links that describe and illustrate the physical dimensions of each frequency: http://www.bamberglab.com/ta_freqwave.htm http://www.mcsquared.com/wavelength.htm And, regarding standing waves, a related subject: http://www.cord.edu/dept/physics/p128/lecture99_35.html Through speaking with Mark Gander of JBL, I learned of the existence of their Sound System Design Reference Manual. It is in its third edition and was originally written in 1977 by George Augsburger. Please check out this excellent overall resource, which includes a section on LF directivity. It exists as a pdf. file, requiring Acrobat Reader to view it or print it out, which can be downloaded at the same site, or in our Study Hall. The Manual also covers power relationships, voltage, wave propagation, fundamentals of room acoustics, the concept of gain, and much more. You can find it on the Web in the JBL Professional Technical Library at : http://www.jblpro.com/pages/tech_lib.htm We have all heard the horror stories about the FOH guys who peer at their laptop and obediently institute equalization on a system that makes it sound horrible, but the (wrongly obtained) information on their screen is telling them is correct. For me, it has been quite a relief to hear Sam Berkow and Jamie Anderson say that if it sounds bad, change it back to the way it sounded better, by all means. That needs to remain paramount. Cheers - Chris Kathman Editor, Live section |
