Speaker Directivity Modifier

By Jay Mitchell

Common Myths and Corrections

Speaker directivity is the bane of a consistent guitar sound. That much has been generally recognized. However, there are two widespread audio myths regarding the causes and solutions of problem directivity:

1. High frequencies come from the center of a cone speaker, and that's why they beam.

False. If you could actually get the high frequencies to be radiated exclusively by the "center" of the speaker - say the dust cap - you'd find them being radiated over a greater angle, not a smaller one. A number of past cone speaker designs actually had compliance elements built into the cone in an attempt to create this exact scenario: the size of the cone effectively getting smaller at higher frequencies. They didn't work particularly well.

2. Placing an obstruction in front of the center of the cone "blocks" these "beaming" frequencies and makes the speaker's high-frequency radiation pattern broader.

False again. A "blocker" will change the on-axis response and directivity of the speaker, but the effect is not consistent over frequency or necessarily useful. The directivity of the speaker will actually be made narrower at some frequencies.

The mechanisms whereby an obstruction placed in front of a speaker changes its response and directivity are complex and counterintuitive. "Blockage" has nothing to do with it, however.

An obstruction will reflect sound back towards the cone, which will "re-reflect" the sound forward. The time it takes sound to make this extra trip means that the reflected sound will be delayed by some amount compared to sound that didn't make the extra trip. The response of the combined sound - slightly delayed plus undelayed - will contain interference ("comb filters"). The "phasiness" some folks describe when they install beam blockers is due to these comb filters.

A portion of the outgoing sound will diffract around the obstruction. This sound is also delayed and will cause an additional set of comb filters. The diffracted sound also has a different radiation pattern from that of the speaker, since it comes from the edge of a small disc. Taken by itself, the diffracted radiation has a strong beam directly in front of the disc. Combined with the other radiation (direct and reflected), the radiation pattern will vary widely with frequency - even more than the pattern of the speaker with no blockage. If you find these effects desirable - as a number of players apparently do - then there is nothing wrong with the use of "blockers," but they don't cause the directitivity changes their makers attribute to them.

If you want to alter the directivity of a guitar speaker in a favorable and frequency-consistent way, I've developed a means to accomplish that, and it can easily be tried by any reasonably competent DIY type. It is outlined below. I've used this method on my tube amps, and it works beautifully.

Solution

It is possible to tame the high-frequency beaming behavior of a guitar speaker while causing minimal change to its overall tone and volume. I (Jay Mitchell) developed this solution for my own use ca. late 2007 and have been using it in my amps since that time, with outstanding results. Because I own a company that manufactures loudspeakers, I originally gave some thought to producing a commercial product based on the development. However, I do not sell any of my products either directly to end users or through retail outlets, including music stores. For that reason, I would have had to develop completely new marketing channels. Additionally, as you will see below, the mechanical configuration of the device must be customized for specific cabinets. One size would not fit all.

Having decided to share the idea rather than trying to profit from it, I originally described it in some detail on the old Axe-Fx forum. There were a few questions, but apparently nobody tried the idea. I posted this information on The Gear Page almost a year later (November 2008), where it received much more interest. Since that time, several other players (at least a dozen or so) have tried the idea and reposted their results. The outcome was positive with a single exception as of this writing.

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 as little as 30 degrees of the speaker's axis. With the 3/4" foam doughnut in place, the on axis response and the response at 45 degrees off axis are very similar, 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.

Q&A

Q: How large should the modifier be?

A: Its o.d. should be a match for the baffle cutout (maybe even 1/16" smaller), so that it sits entirely within the cutout without being stuffed. If the cutout isn't round - some cabs have cutouts with a flat - then you should cut the foam to match the actual shape.

Q: Should it be kept from touching the speaker?

A:Yes.

Q: How much does blocking off that much of the cone reduce the overall volume?

A:None, if you're accustomed to playing in an off-axis position. Open cell foam in this thickness has almost no effect on sound passing through it at frequencies below ~1kHz. The doughnut will therefore cause no change in off axis response or sensitivity. It will reduce higher-frequency content directly on axis. If you're accustomed to playing with your cab aimed straight at your head, the doughnut will make it sound darker by reducing the on-axis sensitivity of the speaker in the higher frequency ranges. If you've adjusted your tone to remove the "ice pick" effect on axis, you will need to brighten it with the doughnut in place. The advantage you will gain is that your position relative to the cab will no longer matter so much: it will sound the same off axis as when you aim it at your head.

Principles of Operation

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.

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