Hello everybody, I'm newly registered here at Tech Talk even though I am a long time and lurker and PE customer. I'd like to first say hello and share that I've gained a lot of information and inspiration from all of the knowledgable posters here over the years. I hope that this is the start of more active role in this community!
I am posting here because I would like to hear from anyone who has any experience with high frequency diffraction control using felt. I would be remiss not to mention my gratitude to Dave's Speaker Pages for the study he has done regarding felt diffraction amelioration for tweeters. This page has been my motivation to experiment with the technique myself.
I have been working on (yet another) Reference series MTM system for music and HT usage. The design uses the familiar RS28F-4. I am at the crossover simulation / emulation stage with these towers. What has causing me grief is trying to understand the effect of edge diffraction on the high frequency response; I am trying to understand what frequency response features are characteristic to the system, and what features are position-dependent acoustical anomolies; of course I want to avoid attempting to 'solve' any acoustical problems through electrical network means.
I have done some simulation of the cabinets (The Edge) and gained a basic appreciation for the signature of the diffraction to be expected from the geometry of the design. From my measurements I can see the familiar series of peaks and valleys caused by diffraction, albeit at a lower frequency than was predicted. The top blue curve in the graph below is the on-axis raw response of the tweeter in the cabinet.
My next course of action was to try to "even out" these undulations using the prescribed pressed wool felt treatment around the tweeter. Unfortunately, my first attempt has not been as successful as I hoped for. In the picture, you can see I am using 5/8" F13 felt, leaving a 3"x3" window around the tweeter.
I have attached a graph of the frequency response at three angles in the anticipated listening region - the blue curve are without the felt, the red curves are with. From the graphs, I feel that using felt in this way suppressed the first "hump" of the diffraction effects; if all of the measured off-axis responses looked like the 10 degree case, I would be satisfied. However, a big dip at 3 kHz starts to develop at the listening angle increases.
This is the reason I am seeking your opinions and prior experience with this sort of technique. I would have just jumped in and started experimenting with different placements and whatnot myself, except that a) the felt is stuck on with an adhesive backing, and b) the felt is not the cheapest stuff in the world to experiment with. Thus, I would like to understand what is happening here and try to go about changing the felt in a informed and deliberate manner.
From the website I linked to above, I feel that I probably placed the felt too close to the tweeter dome and may be suffering from "horn effects" due to this. Perhaps the felt shouldn't be as symmetrical as well for best results? Maybe using slightly thicker felt is causing a problem? This is why I'm asking you for your input before I go cutting up more of this felt!
If anyone is interested in any sort of incremental measurements, I would be happy to contribute to the collective knowledge base here. Thanks for reading what I'm sure is the longest first post in history!
Regards,
Dan
I am posting here because I would like to hear from anyone who has any experience with high frequency diffraction control using felt. I would be remiss not to mention my gratitude to Dave's Speaker Pages for the study he has done regarding felt diffraction amelioration for tweeters. This page has been my motivation to experiment with the technique myself.
I have been working on (yet another) Reference series MTM system for music and HT usage. The design uses the familiar RS28F-4. I am at the crossover simulation / emulation stage with these towers. What has causing me grief is trying to understand the effect of edge diffraction on the high frequency response; I am trying to understand what frequency response features are characteristic to the system, and what features are position-dependent acoustical anomolies; of course I want to avoid attempting to 'solve' any acoustical problems through electrical network means.
I have done some simulation of the cabinets (The Edge) and gained a basic appreciation for the signature of the diffraction to be expected from the geometry of the design. From my measurements I can see the familiar series of peaks and valleys caused by diffraction, albeit at a lower frequency than was predicted. The top blue curve in the graph below is the on-axis raw response of the tweeter in the cabinet.
My next course of action was to try to "even out" these undulations using the prescribed pressed wool felt treatment around the tweeter. Unfortunately, my first attempt has not been as successful as I hoped for. In the picture, you can see I am using 5/8" F13 felt, leaving a 3"x3" window around the tweeter.
I have attached a graph of the frequency response at three angles in the anticipated listening region - the blue curve are without the felt, the red curves are with. From the graphs, I feel that using felt in this way suppressed the first "hump" of the diffraction effects; if all of the measured off-axis responses looked like the 10 degree case, I would be satisfied. However, a big dip at 3 kHz starts to develop at the listening angle increases.
This is the reason I am seeking your opinions and prior experience with this sort of technique. I would have just jumped in and started experimenting with different placements and whatnot myself, except that a) the felt is stuck on with an adhesive backing, and b) the felt is not the cheapest stuff in the world to experiment with. Thus, I would like to understand what is happening here and try to go about changing the felt in a informed and deliberate manner.
From the website I linked to above, I feel that I probably placed the felt too close to the tweeter dome and may be suffering from "horn effects" due to this. Perhaps the felt shouldn't be as symmetrical as well for best results? Maybe using slightly thicker felt is causing a problem? This is why I'm asking you for your input before I go cutting up more of this felt!
If anyone is interested in any sort of incremental measurements, I would be happy to contribute to the collective knowledge base here. Thanks for reading what I'm sure is the longest first post in history!
Regards,
Dan
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