OK, back from Thanksgiving dinner with inlaws. Here's the deal:

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.

To answer a couple questions: to try the idea, you could temporarily attach the doughnut to the front side of the grille. Just make sure to center it over the speaker cutout. If your cab has metal speaker protectors, you can attach the doughtnut to those.

The spacing of the foam off of the speaker cone is not critical, but you do not want it to make contact with the cone, and you also do not want there to be a flanking path (i.e., a large gap around the outer edge of the doughnut) that sound can take without passing through the foam.

In order to understand why/how it works, you have to understand a bit of the geometry and acoustics in a cone transducer. The cone is driven very near its center (where the voice coil former is attached to the cone). The mechanical excitation (the vibration of the cone) is not instantaneous. It moves outward in the cone with its own characteristic velocity, which is geater than the velocity of sound in air. As the excitation moves outward, the air in contact with the segment of the cone that is moving is also excited, and sound is radiated from that segment. If the angle of the cone is "just right" for the properties of the cone material, the sound that is radiated from each section will be almost perfectly in time with the sound that was radiated earlier, from a portion of the cone that is set further back.

When all this sound adds up out in front of the cone, you get a coherent wavefront to a relatively high frequency (as high as ~5kHz), but only on axis. At off-axis positions, the synchronization falls apart very rapidly, to the extent that a typical guitar speaker will see its output above 1200 Hz fall off by as much as 15-20dB within just a few degrees off axis. The effects on directivity due to mis-synchronized off-axis arrivals are much less at lower frequencies, because the wavelengths of sound are larger than the cone, and the speaker is therefore not "beamy" at these frequencies.

Placing an absorber doughnut in front of the cone has almost no effect below 1kHz, because the material is too thin to have a significant effect at lower frequencies. This is good, since, as we've seen above, the speaker's behavior below 1kHz is not a problem. At higher frequencies, the radiation from the outer portions of the cone is progressively absorbed, whereas the portion of the radiation from the center of the cone which moves straight forward is allowed to pass through the opening in the doughnut unattenuated. The result is that there is a net reduction in high-frequency content on axis, but the high frequencies that do get through are now radiated by a virtual source - the opening in the doughnut - which is much smaller than the actual speaker, and they are therefore radiated over a greater angle. The speaker's directivity is now essentially the same over the entire range of interest for electric guitar, and you're out a few bucks for a piece of foam and a few minutes to cut out and attach the doughnut.

Bilder hade varit tacknämnligt [:)]

[url="http://www.theboogiecats.com/"][/url]<font color="pink"><i>Rockabilly är inget annat än en tidig form av Rock'n Roll'"]</font id="pink">

Så här gjorde jag på frugans fiolförstärkare.

Blir det även spridning av ljudet så att säga...

Borde det inte kunna funka med att klippa en bit ur ett gammalt liggunderlag? De är ju av nån slags skumplast eller skumgummi.

Erik Runeson

Liggunderlägg kommer nog inte att fungera så bra. Det skall vara skum med öppna porer, sådant som en gång var populärt som frontskydd på hifihögtalare.

Alex
http://www.tubewonder.com
http://www.youtube.com/LouderAndMore - Rum K33 Fuzz Guitar Show 2008

Hmmm...jag förstår inte riktigt varför ni ändock inte kör med de "vanliga" beamblocker...skulle ni uppleva att de "äter" diskanten så är det välan bara till att lacka beamblockern så att ljudet/diskanten studsar mot den

Om jag förstått rätt, så påstår killen som skrivit tråden att hans lösning är att föredra framför en beamblocker, eftersom den ger ett mer konsistent ljud vid olika vinklar, en bättre spridning. Jag kan givetvis ha missat något här, <font color="red">men beamblockern ger tydligen inte en lika bra spridning av diskanten som en sådan här lösning skulle åstadkomma. </font id="red">