Home Speaker Asylum

General speaker questions for audio and home theater.

Re: Felting

12.92.215.231

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.


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