
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
MultiWay Conventional loudspeakers with crossovers 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
5th November 2009, 02:47 AM  #1 
diyAudio Member
Join Date: Dec 2005
Location: west lafayette

Wave transformation, Higher order modes?
Over the weekend, I was trying to visualize the wavefront expanding through a horn and had some ideas. I was wondering if this phenomenon is what Dr. Geddes refers to as Higher Order Modes.
There are 3 possible dimensions in which a wave can propagate (x,y,z). Waves initially propagate in one dimensions (x). If the wavefront is forced to expand, antiparallel wavefronts will be generated. Visualize an oscillating piston located within a cylinder. Neglecting resistance at the boundaries (sides of the cylinder), the wave will propagate unimpeded. However, once the wave reaches the end of the cylinder (an impedance mismatch exists at this boundary), an abrupt transformation of the wavefront occurs. A secondary wavefront is created at this boundary, which propagates antiparallel to the original wavefront (ie a reflection). The primary wavefront is then forced to expand in 3 dimensions. Let us now visualize an oscillating piston located within a waveguide. As the wavefront propagates down the waveguide, it is constantly expanding and thus secondary antiparallel wavefronts are constantly being generated. Are these secondary antiparallel wavefronts the "Higher Order Modes" that Dr. Geddes attempts to solve with a resistive element within the waveguide? These secondary antiparallel waves interact with the wave source. At low frequencies, the effect is primarily reactive and is why transducers are very inefficient. At high frequencies, the effect is primarily resistive. The reactive force bends the initially flat wavefront into 3 dimensions (sphere) and is why transducers are omnidirectional at low frequencies. The reactive force is similar to the effects of a capacitor in that it dissipates energy over time. A source which minimizes the reactive force and allows the resistive force to approach a constant value may reproduce a square wave most effectively. Is this a correct observation? I would much appreciate feedback.
__________________
"It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them." Last edited by thadman; 5th November 2009 at 03:07 AM. 
5th November 2009, 03:23 AM  #2 
diyAudio Member
Join Date: Dec 2005
Location: west lafayette

A source which provides the most gradual transformation to 3 dimensions may satisfy this requirement (minimize reactance, allow resistance to approach a constant value). We have two available functions which provide a constant rate of expansion (ie linear), a 2nd order function or an exponential function. The exponential function will never provide a vector which is orthogonal to the initial vector, thus a 2nd order function is required (ie roundover).
__________________
"It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them." Last edited by thadman; 5th November 2009 at 03:44 AM. 
5th November 2009, 12:53 PM  #3 
diyAudio Member
Join Date: Aug 2004
Location: US

A trip to the library might help. Check out Morse and Ingard, Theoretical Acoustics. Chapter 9.2, Higher (order) Modes in Ducts. Horns are just ducts of non constant cross section.
__________________
John k.... Music and Design NaO dsp Dipole Loudspeakers. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Effect of changing C but not L in higher order xovers  madisonears  MultiWay  2  2nd August 2009 02:06 PM 
Planar bass wave to take care of room modes ??  JinMTVT  Subwoofers  12  4th October 2007 06:15 PM 
The power of transformation  SY  Tubes / Valves  9  6th July 2005 12:47 PM 
Higher order crossovers is it possible with LM4780?  diysmartdeep21  Chip Amps  0  10th February 2005 01:17 PM 
Superpower amps  higher current better than higher voltage  Circlotron  Solid State  34  28th June 2002 05:01 AM 
New To Site?  Need Help? 