Questions about the Dowel or Straw Mod

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moray james

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Questions about the Dowel or Straw Mod
« on: 16 Jan 2003, 07:17 pm »
To simplify thinking about what a dowel or straw does when placed in front of a tweeter dome or small cone full range driver you might try thinking about it like this. The dowel kind of acts in similar to a phase plug. The phase plug stops one part of the cones output from "seeing" other parts of the cone so frequencies don't cancel or add together. Well the dowel stops you from hearing certian things which are going on that you do not want to hear.
   Lets look at the application with a dome tweeter. The top of a dome tweeter (concave or convex regardless of dome material) will go into resonsonance like it or not and it is in antiphase to the bulk of the domes output. This may sound bad to you but it is a physical fact of life. Another fact is that a 1 inch dome is physically to large reproduce high frequency over a wide dispersion angle because of wavelength size it just starts to get in its own way about 10 KHZ. You need to remember that resonance is the designers friend, you either learn to use it or it uses you (badly). So since the top of the dome goes into resonance we learn to design a dome which goes into resonance smoothly at the frequency you want and presto you now can develope a dome with a 20 KHZ antiphase reaonance (some dome tweeters actually design the dome's half round suspension to resonate at high frequencies because the dome is very ridgid ie: Celestion). This takes place right at the tip of the dome and due to its very small size the dispersion is very wide angle. Like it or not that's how they work. Nobody can make a dome which has no resonances. Ribbons and planars can pull off this trick with no resonances and this perhaps explains one of the reasons people find them special. Louis understands this and you can take a look at his two-way design which has a number of other very clever tricks which are not readily seen. I digress.
    This out of phase high frequency output from the top of the dome cannot be damped away or prevented, remember it's a designed in feature. You hear this output all the time but time is the key word here. While the resonance cannot be eliminated (and you don't necessarailly want it to stop as it IS your source of high frequency) you can make it go away in "time". Your brain is not only aware of frequency but also of time. This is what the dowel can do for you, if you reflect the resonant output from the top of the dome away from being fired straight at you (along with the bulk of the domes lower frequency nonresonant output) and bounce it off say the side wall of the room then presto again you win. Your brain still will eventually hear the high frequency bounce off the side wall BUT as long as there is delay the brain can both recogonise the waveform as part of the music but as a room reflection as is normal in any room and then the brain uses the delay info (time delay and output level) to calculate room acoustics. This modification actually makes it easier to asses ambient accoustics as well as to minimise high frequency smear of lower frequency output from the dome. Remember the resonant dome output is very high in "Q" and so usually louder than other non resonant dome output.
   Result; smoother sound with greater detail and resolution with improved high frequency response. This results in better stage and image in both depth and width of the stereo stage.
   Full range drivers have dust caps which can resonate like domes and also have suspension and cone breakup modes at high frequencies which also beam and so can by means of the dowel be deflected away from the listener.
   There are a number of modern designer drawer pulls (handels) which can be used over tweeter domes. Larger straight or inverted pulls can be used on cone drivers. In both cases the pulls act as very effective protectors. As I said this technique can be used to deflect resonance from domes, cones, the breakup points at the edges of wizzer cones and cone suspensions (rings required here). This is not new technology. Take a look at the Goodman Axiom 80 loudspeaker which is a wizzer cone full range designed in the 50's and still heald by many to one of the finest. You will also see a number of newer dome tweeters with buttons or tabs just over the top of the dome (also not a new idea) and or what look to be mounting rings (for the buttons) over the domes suspenssion.
   You can experiment very quickly and inexpensively with a little tape and a bic pen, crayon or soda straw. Remember if you use a straw or other hollow tube you need to either damp the tube inside with foam or fluff or tape off the ends otherwise the tube will whistle at some frequency like a flute. Have fun and experiment then share what you find. The closer the dowel to the dome the better. It will take you far less time that it took me to write this letter. Sorry this was so long but I think that it will give many of you a good solid understanding of what this modification does and why so you can apply this knowledge to other projects and ideas. Best regards Moray James.

blizzard

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Questions about the Dowel or Straw Mod
« Reply #1 on: 21 Jan 2003, 06:01 pm »
Hi Moray,
  Typically, are the dowels placed horizontally or vertically over the drivers?
       Thanks,
           Steve

moray james

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Questions about the Dowel or Straw Mod
« Reply #2 on: 22 Jan 2003, 08:05 pm »
Quote from: blizzard
Hi Moray,
  Typically, are the dowels placed horizontally or vertically over the drivers?
       Thanks,
 Hi Steve: you can try using a dowel either vertical or horizontal, I have found in most cases that vertical works well but please do experiment as it will give you a good idea of what your ceiling/floor reflections are doing. Regards Moray James.          Steve