phase plugs

With most of the stuff I design, the only theory to be found is the logic buzzing around inside my head. My experiment lasted over a period of three months, not solid work, but going back and forth and measuring all the way.
This is how I arrived at the shape of my first phase plug. I was aware that the opening in the Turbo mid was at the edge of the cone. I was never convinced that the pressure on the cone was linear. so I had to formulate my own ideas. I drew up a shape that fitted into a PD121 with an inverted dome. All of our horn throats are smaller than the piston diameter, and if you took a line through the centre of the circular opening and calculated the cone area either side of the line, the figures would be the same. This is the way the throat was calculated, I was also satisfied that I had even pressure on the cone with the opening in the most logical position.
Next was the overall shape and position. I used a rough shaped plug and some blu tac. I slid the plug in and out of the horn throat measuring each movement untill I had the optimum response.
I also learnt that after the first part of the plug has done its job, the remainder of the shape didn't matter too much. I tried both long and short plugs and there wasn't a difference that I could measure. I therefore concluded that the nice bit on the front was there just to look cool.
As I can always visulize diffraction around a shape, I was not about to make a plug without a smooth bit to follow.
My view on a phase plug is that it is there to stop one side of the speaker seeing the other side. It is common Knowledge that when the size of the wave length reaches the piston diameter of the cone, the waves bounce across the cone instead of radiating from the diameter. The waves become directional and beam, which as it happens is not the point. What is significant, is that the waves that bounce across start to cancel each other out. The plug prevents this from happening and a higher response from the speaker is achieved.
I'm sure there could be a much deeper theory involved whith plugs, but this is how I did ours.
The results of all this can be heard from the guys who have our MX600's. I make no comment on their performance. I let the comments speak for themselves. Measuring was done with LEAP and LMS.

great stuff!

Tony Wilkes wrote: Come on Tony thats much too simple!! You should have padded the design process out a bit and then considered writing up a paper on it  Well done on the photo's. Tony

Bill Fitz works like that too I think , just builds it to see if itl work.

Nice to see a manufacturer making plugs that way - look good.

Stu, are you referring to the sliced PVC pipe method ? got link to some pics?

Cheers
Graeme

TONY.A.S.S. wrote: With most of the stuff I design, the only theory to be found is the logic buzzing around inside my head. My experiment lasted over a period of three months, not solid work, but going back and forth and measuring all the way. This is how I arrived at the shape of my first phase plug. I was aware that the opening in the Turbo mid was at the edge of the cone. I was never convinced that the pressure on the cone was linear. so I had to formulate my own ideas. I drew up a shape that fitted into a PD121 with an inverted dome. All of our horn throats are smaller than the piston diameter, and if you took a line through the centre of the circular opening and calculated the cone area either side of the line, the figures would be the same. This is the way the throat was calculated, I was also satisfied that I had even pressure on the cone with the opening in the most logical position. Next was the overall shape and position. I used a rough shaped plug and some blu tac. I slid the plug in and out of the horn throat measuring each movement untill I had the optimum response. I also learnt that after the first part of the plug has done its job, the remainder of the shape didn't matter too much. I tried both long and short plugs and there wasn't a difference that I could measure. I therefore concluded that the nice bit on the front was there just to look cool. As I can always visulize diffraction around a shape, I was not about to make a plug without a smooth bit to follow. My view on a phase plug is that it is there to stop one side of the speaker seeing the other side. It is common Knowledge that when the size of the wave length reaches the piston diameter of the cone, the waves bounce across the cone instead of radiating from the diameter. The waves become directional and beam, which as it happens is not the point. What is significant, is that the waves that bounce across start to cancel each other out. The plug prevents this from happening and a higher response from the speaker is achieved. I'm sure there could be a much deeper theory involved whith plugs, but this is how I did ours. The results of all this can be heard from the guys who have our MX600's. I make no comment on their performance. I let the comments speak for themselves. Measuring was done with LEAP and LMS.

And theres me thinking it was only me I find that if the phase bung is too long it will affect the dispersion, and a sharp point on the end will give it super pin-point accuracy (like an 'axe' through your head) 

Edited by mykey - 31 August 2007 at 5:28am

Yes ! they need to be solid, our first ever phase bungs were made of solid mahogany