Have you ever noticed that the frequency balance of a speaker changes as you move from side to side across the front? This phenomenon is called "beaming," and it is important to understand because it affects everything you do -from micing up a guitar amp to sitting in front of your studio monitors.

We all know that when you connect a signal source to a speaker, sound is produced. But how? To put it in simple terms, when a voltage appears across the speaker terminals, the current flows through the voice coil and this generates an electromagnetic field around the coil. Since an audio signal is AC, or alternating current, the electromagnetic field generated alternates it's polarity and is either attracted to or repelled by the fixed magnet of the speaker. This is what causes the cone to move in relation to the signal. In proper polarity, a positive voltage causes the cone to move outwards, and a negative one causes the cone to move inwards, like a little piston. Speaker designers actually refer to the cone, voice coil and magnet structure as a motor. The piston moves air equally across its entire surface. The cone shape is for structural strength and has little effect on the radiation pattern.

Frequency is inversely linked to wavelength in direct proportion. As the frequency increases, the wavelength decreases. (This relationship is clear if you notice that the frequency of a vibrating string doubles as the length is halfed. The halfway point on a guitar is the 12th fret, and the pitch at the 12th fret is an octave higher than the open note. An octave in musical termimology is the doubling of a frequency.)

Imagine two sine waves, one cycle each, where the wavelength of the complete cycle is exactly the same length as the diameter of a speaker, a 12" sinewave on a 12" speaker. Now imagine these waves positioned so that one is eminating from each edge of the cone. If viewed from the side, directly across the speaker, they look like one single cycle. If you shift youe vantage point and look at them from a 45 degree angle, you will see that the positive half of one is overlapped with the negative half of the other. These signals at that frequency and at that angle are 180 degrees out of phase, in other words they cancel out. In the case of our 12" speaker, this occurs at 1125Hz. Below this frequency, complete cancellation is impossible at any angle. Above this frequency the cancellations are harmonically related and these cancellations produce multiple comb filter effects, rolling off the higher frequencies and giving the darker character to the tone off axis. This is why speakers -and microphones- all exhibit beaming, or directionality, at higher frequencies.

In order for a speaker to reproduce frequencies from 20Hz to 20kHz without beaming, it would need to be less than 1/4" in diameter. A speaker this size is smaller than the wavelength of a 20kHz signal. This means that the same frequency could never emanate from two points on the surface that are spaced greater than the smallest wavelength, and cause cancellation --in other words, no beaming. In reality such a driver would not be able to move sufficient air to produce any meaningful low end. You need a larger surface area.

The problem here is that a larger cone cannot be made stiff enough to withstand the extreme directional changes it is subject to when reproducing a broad spectrum of sound. The cone will form distinct vibrational patterns across it's surface, known as "breakup". Many guitarists prefer using a lower wattage driver specifically for the brighter and richer sound that cone breakup gives.

 

Copyright © 2005 Chuck Zwicky

 

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