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Direccionalidad de los parlantes: Solución


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Les dejo un escrito muy pero muy interesante de Jay Mitchell acerca de la direccionalidad de los parlantes, los "beam Blockers" y cómo solucionar ese tema.

Está en inglés...cuando tenga tiempo lo traduzco, por ahora les dejo el original. :)

 

Speaker directivity is the bane of a consistent guitar sound. That much is generally recognized. However, there are several audio myths that continually get repeated, and they should be be corrected:

 

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

 

Nope. If you could actually get the high frequencies to be radiated 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 directivity (the word "dispersion" does not actually apply to radiation pattern) broader.

 

Nope again. The "blocker" will indeed change the on-axis response and directivity of the speaker, but the effect is not consistent or necessarily useful. The directivity 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 in the next post. I've used this method on my tube amps, and it works beautifully.

 

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.

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.

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Lo lei hace un tiempo en The Gear Page a este hombre, tiene un post de como 50 paginas donde opinan sobre el concepto varios tipos que la tienen muy clara por ejemplo Bruce Egnater.

 

Aca hay fotos de la solución que da:

emptycabinside.jpg

emptycabwithfoam.jpg

 

mitchell-modifier1.jpg

mitchell-modifier2.jpg

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Está bueno a veces, yo uso un posavasos de una cerveza inglesa pegado con cinta, pero mas que nada como difusor de agudos para poder picarlos más sin que te maten, poder darle un toque mas de volumen al amp.

Igualmente con algunos parlantes y cajas no tiene sentido hacer esto, los parlantes tipo V30 y los de ese tipo en gral son bastante direccionales (tienen una presencia impresionante dentro del axis, pero apenas te corres un poco ya los perdiste). También pueden traer quilombos a la hora de microfonear.

 

Buen aporte,

 

Saludos!

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Yo había hecho un comentario hace mucho sobre esto. El tipo dice que poner algo adelante del domo, como esos beam blockers, en realidad sólo genera cancelaciones de frecuencias de manera poco controlable, el plantea exactamente lo contrario. Yo lo probé en mi 2 x 12, pero en ese tipo de cajas se da un fenómeno de direccionalidad por la disposición de los parlantes, además de la direccionalidad de cada cono, o sea que el efecto es menor. El plantea como ideal para esto las 1 x 12. Yo no llegué a ninguna conclusión definitiva, pero que funciona, funciona. Un problema es encontrar la espuma de la densidad/grosor adecuado. En el foro donde lo planteó por primera vez, a algunos que lo probaron les funcionó de maravillas, a otros no, a otros no les gustó el sonido... Quizás tendría que volver a probarlo en mi 1 x 12.

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Muy interesante!, será cuestión de probar, a mi me agrada bastante más el sonido cuando estoy lejos y mas o menos a 45º del eje del parlante. Averigüé y cuesta $ 2,50 cada plato de goma espuma de 28x2cm y de 30kg. x metro cúbico de densidad. Una ganga 8)

 

 

Donde se consiguen che? link? o forma especifica de pedirlos aca en arg... pa probar vio...

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