
Oorspronkelijk geplaatst door
Jay Mitchell
Here's how to make your own speaker directivity modifier:
Cut out a doughnut-shaped piece of acoustically absorbent foam. The diameter should be the same as the speaker cutout in the baffle in your cab, and the diameter of the center hole should be ~3". Attach it to the rear side of your cab's grille using spray contact adhesive (e.g., 3M Super 77, available at Lowe's). Spray a light coating of adhesive on the foam only, and press it against the grille cloth within about 30 seconds of spraying it. You can easily remove the foam with no ill effect on the grille material, if you decide you don't like the effect.
The material you want is open-cell polyurethane foam in sheet form, and there are a number of sources for it. McMaster-Carr is one. I use acoustic foam that the company I own purchases for use in my loudspeaker designs, but that is a matter of convenience. I have tested and subjectively evaluated two thicknesses: 1/2" and 3/4". The limit on maximum thickness is the thickness of your baffle, so make sure you don't exceed that.
With the material I use, the 3/4" doughnut produces the most consistent response at different angles. A 12" speaker has huge variations in its response above ~1200 Hz within just 10 degrees of the speaker's axis. With the 3/4" foam doughnut in place, the on axis response and the response at 40 degrees off axis are almost identical. This is a huge improvement.
If you think about the subject of directivity, you'll easily recognize that there are two ways of saying the same thing: when you say a speaker becomes "beamy" at high frequencies, you're also saying that its on axis response is much brighter than its off axis response. For example, if you equalized the response to be flat on axis (a hypothetical exercise, as that's never what you actually want from a guitar speaker), you'd find that the response off axis falls off pretty rapidly at higher (> 1200 Hz) frequencies.
The reason for the preceding paragraph is to point out that making directivity more consistent over frequency requires that either the on axis or off axis response change. The foam doughnut causes a change in the on axis response, while leaving the off axis response alone. This means that, if you've tweaked your tone with the speaker aimed at your ears, it's now going to sound darker, and you'll need more treble, presence, and/or midrange, depending on the design of your amp's tonestack and other tone-altering circuits. If you're placing your amp on the floor facing the audience, the response you hear will change little or none, but the response the audience hears will now match what you've been hearing all along.
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