|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
In Reply to: Felt material around tweeters, why not in more speakers? posted by Sean H on July 29, 2002 at 09:53:52:
I've been recommending this simple expedient for several years. It's definitely worth spending the time and effort (with the placement and amount of felt) to tune the treble just the way that you like it.Obstructions around the dome cause secondary reflections that "roughen" the frequency response of the tweeter. Using the formula for determining the frequency of quarter-wave boundary nulls (N=1130/D x .3) (boundary nulls are just one aspect of the phenomenon - there are peaks as well), its possible to determine that an obstruction within 1 inch of the tweeter will cause a null at 4 kHz (other frequencies might experience a power increase and the peaks that result) everywhere in the room. An obstruction within 1/2 an inch from the tweeter will cause a null at 8.137 kHz everywhere in the room. These boundary effects (created by obstructions 1 inch and 1/2 inch from the tweeter - such as the fixing screws that bind it to the baffle) occur in a part of the treble that we are very sensitive to. The colorations are very audible, as well as visible. When graphed, these treble oscillations resemble the rough edge of a saw. Using felting prevents the secondary reflections that create these boundary effects and greatly smoothes the response of the tweeter.
I agee with you that many speaker designers don't bother to take into account (fully) how the baffle will interact with the speaker's treble response. 90% of speakers have exposed fixing screws around the tweeter that will roughen the tweeters response. Designers seem unaware of the problems that this creates. Other designers mount the tweeter in a wave-guide (horn). The wave-guide is intended to prevent baffle-edge diffraction effects, but occasionally these oscillations are created by treble interactions with the horn itself (one famous example comes to mind - that of the KEF Uni-Q driver, which over the course of 10 years, dispayed a power response null in the high treble created by interactions between the dome and its surround/horn/woofer - this problem was only corrected in the most recent iterations of the Uni-Q driver).
Furthermore, it seems many designers voice the treble using the baffle. Specifically, the designers use the baffle itself as a wave-guide. It will tend to brighten the sound of the tweeter, by concentrating its power in a forward direction (the same principal as using reflective material behind a flashlight bulb to increase the brightness and directionality of the beam it produces). Consumers can re-voice the relative level (the brightness) of the tweeter by experimenting with felting, which absorbs rather than reflects treble output. The ultimate expression of this effort (which I have tried) is covering the entire tweeter baffle with felt, which greatly reduces the relative level (brightness) of the tweeter output.
To sum up, anything in the immediate vicinity (less than a few inches) of the tweeter will interact with the tweeter's frequency and power response. Speaker designers don't seem to have full understanding or control over these interactions. Felting (by smoothing treble response and controling the reflectivity of the baffle) allows consumers to re-voice speakers just the way they like them.
Follow Ups:
Thanks for the reply. I actually read a number of your posts on just this subject and a other really good information you have provided about room acoustics. I actually tried paging you through a post to email me a few days ago, I actually somewhat solved my problem and you hadn't replied so I deleted it. I had two room boundaries that were equal, distance from woofer to side wall and to the floor of one of my speakers that was slightly causing a small null in the mid bass. Moving it a touch fixed it. However, I am still dealing with a nasty 50Hz bass peak in my room, which both my Paradox speakers and subwoofer are suffering from. I think treatments is my only hope. Your extremely informative posts have been great, thanks.All your points duly taken. I might start by way of a large amount of coverage to see what happens. I was stunned at what the large pieces of foam did in my experiment yesterday. The effect made the treble seem so sweet, even more integrated in the whole sound (coherence) and more focused too. Given the wide dispersion of this tweeter, it's mounting plate relectivity and the 8 mounting bolts, it seems a good candidate to benefit from tweaks like this. I'll report back with some findings for curiosity sake. Thanks again.
My computer was taken down all yesterday by a very malicious virus, so that's probably why I missed your page (got some functions - like internet - working again today). That 50 Hz room mode is a tough one to tackle. I don't put too much stock in treatments for room modes. Have you tried listening from a different position? You will hear a different spectrum of modes in a different position. Perhaps the 50 Hz mode will disappear. Alternatively, curing bad room modes might be as simple as opening a door (or a window) unto your listening space, as the change in room dimension seems to "spread" room modes to different (hopefully less obnoxious) frequencies.
Felting (by smoothing treble response and controling the reflectivity of the baffle) allows consumers to re-voice speakers just the way they like them.Are there any explanations or diagrams on how to do this? My lower end B&W tweeters could really use something like this.
Jeff
Actually, its just trail and error, but I have found that a felt-ring around the tweeter is probably the best starting point. Start with a small ring. If the speaker is still too bright, try a larger ring, and so on, until you have covered the entire tweeter baffle (if necessary). Be careful not to touch the tweeter dome or drip adhesive on it (I apply rubber cement to the felt and I wait until its tacky - semi-dry - before I adhere the felt to the baffle).
I'm still lost. I have no idea where to put the felt. Maybe if you pretend I'm a six year old (not too far off) then I could understand where the felt goes.
Jeff
Have you not been following this discussion? We've been talking about placing felt around the tweeter. The tweeter is mounted on the baffle. The tweeter is a circular dome. In order to apply felt around the dome (without touching or covering the dome), you will need to get a pair of sharp scissors and cut a ring out of your felt squares (obtainable at fabric stores). Start with a small ring, and as I outlined above...progress to an upper-baffle sized piece of felt (with a hole cut out in it for the tweeter) if necessary. Apply the felt using an adhesive like rubber cement (ideal because it will not damage the speaker's veneer). Let me know if I can make it any plainer?
OK, I was thinking it went inside the speaker and around the tweeter to reduce vibration but it actually goes on the outside of the speaker (wasn't sure what the "baffle" was) and covers part of where the tweeter sound is going out. If I'm getting it now.My wife is a seamstress. She has a room full of fabric samples so I can try just about anything.
Jeff
You have the right idea now. Apply your felt rings to that board that holds the drivers (aka the baffle). Be careful to cut the right diameter circle (hole) in the felt, so that all the area around the tweeter is covered (by the ring), but the felt does not actually touch the tweeter.
This post is made possible by the generous support of people like you and our sponsors: