Taming the Beam

Sunday, November 1, 2009| by Will Chen

An Affordable Solution for Curing Speaker Directivity

 

It was one of those days. You know. You dial in a tone and it sounds great until you take a step to the left and suddenly your ear drums are about to explode. Unfortunately, a great many guitar amps, especially single speaker combos, suffer from fairly intense speaker directivity which causes the high end to be downright piercing when standing directly in front of an amp once it’s been dialed in based on what you as a player are hearing off- axis at a 45 degree angle to the speaker. I’d been researching solutions such as the Weber Beam Blocker, Fizzy Lifter, or Sound Deflect Flapjack which are all based on the design of placing a disc directly in front of the speaker’s dust cap to diffuse and disperse the highs of the amp in a larger pattern, eliminating the on- axis ice pick beam. However, I stumbled across an alternative method at The Gear Page, published by Jay Mitchell which has since been dubbed the "Mitchell Doughnut".

The Mitchell Doughnut is exactly what it sounds like; a doughnut with a 3” diameter hole in the middle cut out of acoustically absorbent foam, placed directly in front of the speaker under the grill cloth. Mitchell’s theory is based on acoustical physics which states that all parts of a speaker generate all frequencies, and while blocking the dust cap does alter the on-axis response and directivity of the speaker, the effect is not consistent over frequency. I was immediately skeptical as I’ve miced up a bunch of cabinets and when you move a mic even a hair off axis, the highs are attenuated. However, further reading explained that this phenomenon is due to the phase cancellation and comb filtering which occurs when one is close to a speaker rather than what would be truly heard in the real world (as none of us actually places our ears right next to a speaker). With a curious mind and this being such an easy and non-permanent mod, I decided to give it a shot on my Peavey Bandit 112.

I ordered the recommended foam from McMaster-Carr. Upon ordering, they do not quote you shipping and I was shocked to discover that shipping was more expensive than the foam itself. Rather than shipping it in a stiff envelope, it shipped in an oversized box. Live and learn... After its speedy arrival, I traced out a 12” diameter circle using a plate and a 3” diameter doughnut hole using a can and proceeded to cut it out with a pair of scissors.

Rather than using the spray adhesive recommended by Jay, I opted for double sided carpet tape since I had some in the garage. I placed little strips of tape all around the foam and pressed it to the grill cloth and then remounted the speaker.

The results? Incredible. This isn’t some subtle effect which requires a golden ear to appreciate. I was amazed at how even the response was now moving all over the room. I was now able to dial back the treble on the amp, yet still retain a nice crisp response as the highs were now being evenly amplified and projected with the rest of the frequencies. Every once in a while a piece of gear comes along which makes me wonder how I ever functioned without it. This is one of those moments and I can’t believe it is the results of a $5 piece of foam and less than half an hour of work.

For further reading, check out Jay’s original article at The Gear Page: http://www.tgpwebzine.com/?page_id=424

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