| A parametric equalizer allows you to specify the center frequency for the boost/cut and the bandwidth - how wide a frequency range it affects. |
Using light-wave propagation as an analogy, the walls of any room typically reflect some frequencies (colors) better than others. Often walls are relatively transparent to bass but tend to mirror or absorb treble frequencies. Worse yet, such walls often come in parallel pairs (ceilings are often parallel to floors, etc) and so consequent wave reflections are likely to cause what are called standing waves, whereby the wall pairs accumulate wave energy something like a closed organ pipe of the same length. Acoustical amplification of such frequencies is a natural result inasmuch as the walls reflect those frequencies well. These are acoustical distortions in the frequency-response curve that only a good equalizer can accurately compensate for. If an equalizer is to compensate for these acoustical discrepancies, it must have bands whose center frequencies appropriately match the various overly prominent frequencies. When any band's amplitude is to be adjusted, the CENTER-FREQUENCY is one of the most important PARAMETERS characterizing the band, because it determines whether the given band is appropriately located to adapt the electrical response to the acoustical environment. In a parametric equalizer such as the 2x3EQ, the user can select the center-frequency parameter from a wide and continuous range, thereby insuring that the de-emphasized (or boosted) band is appropriately centered for optimally neutralizing the acoustical peculiarities.
The job of a microphone is essentially to transform the sum of all the sound pressure fluctuations heard by the microphone at each given instant, to transform that sum of pressures into a proportionate electrical voltage. This pressure sum includes not only sound contributions arriving from the singer, instrument, etc (original voice/s), but ALSO includes those contributions returned to the mic from the speaker system, which each mic also hears summed with those of the original voice/s. For some frequencies the speakers' pressure sum "heard" by the mic arrives at the mic IN PHASE with the pressure of the original voice/s at that frequency. This is essentially constructive wave interference, and it causes POSITIVE FEEDBACK that reinforces the sound-pressure of the original voice/s, and therefore boosts the mic's output at those frequencies. But wave pressures of other frequencies from the speaker system will reappear at the mic OUT OF PHASE with the original voice/s. Their wave interference is essentially destructive, introducing NEGATIVE FEEDBACK that partially cancels the source pressure from the original voice/s and likewise decreases the mic's electrical output.
Positive feedback essentially boosts the mic output of those frequencies where it occurs, while negative feedback de-emphasizes the mic output of those frequencies where it occurs, and the drivers' audible responses are affected accordingly. This effect is hardly restricted to microphones, but extends to any transducer-instrument combination whose electrical output is influenced by the speakers' sound. The feedback that sustains the vibration of an electric guitar string is a familiar example of this, where the driver pressure alone can sustain the string's vibration even with zero-licks in. This is a clearly a noteworthy response given the zero input stimulus to the strings. Of course, such feedback may or may not be desirable, but it clearly demonstrates that wave interference influences response even in transducers not intended for direct transform of sound pressures. Not surprisingly, acoustical instruments with electric pickups and the resonating cavities are likewise influenced by the frequency-sensitive phase difference between the instrument's vibrating source and the returned speaker pressure, a phase difference that will for any transducer, depend critically upon the frequency.
With all the acoustical influences, prospects for wave reflection, etc., the bandwidths over which the acoustical feedback remains positive (or negative) is pretty unpredictable for the arbitrary performance setting, and if (for a typical example) it becomes necessary to boost a broad band that's largely absorbed by the room's walls, but such a boost introduces feedback ring at some troublesome frequency within that broad band, then it becomes desirable to superimpose two bands of the EQ, one band within the other, to introduce a narrow notch within a boosted broader band. Such options are simply impractical with graphic equalizers, which could only provide rough approximations of the needed equalization curve. The BANDWIDTH PARAMETER as well as the CENTER FREQUENCY PARAMETER can be precisely selected for each band with any fully parametric equalizer, such as our 2x3EQ.
| A parametric equalizer allows you to specify the center frequency for the boost/cut and the bandwidth - how wide a frequency range it affects. |
Whether any given frequency returned from the speakers will interfere constructively, destructively or neither depends most fundamentally upon the relative phase angle between the original voice/s and the wave returned from the driver/s as heard by the microphone. The number of wavelengths separating the transducer from the virtual driver is the essential criterion determining this relative phase angle. (Here I say virtual driver because in general there are more than one driver, but the transducer ÒhearsÓ only the phasor sum of their sinusoids, thus seeing a net single source with a single phase angle.) A variety of acoustical features are significant in affecting that essential criterion, the most significant ones including the sinusoidÕs wavelength (which is inversely proportional to its frequency) and the location of the transducer relative to the virtual driver. The significance of this latter parameter explains why the location of a microphone influences the frequency at which it will feed back. Any phase shift introduced by the amp and driver likewise affects the relative phase of the original voice/s and returned wave.
For any one performance environment, the physics problem of how to set up the equipment to get perfect reproduction directly from unequalized signals is more likely to be impossible than merely formidable. Indeed, in many environments the possibility of an ideal equipment arrangement for acquiring perfect reproduction of sound from flat electrical source/s is impossible without some sort of electronic equalization to neutralize the acoustical features of the performance environment. These acoustical problems are much more likely to be solved by working en vivo with the aid of good electronic tools; i.e., a good parametric equalizer, such as the 2x3EQ made by Collier Electronics.
Given a parametric EQ having sufficient versatility, therewith one can electronically boost any band of frequencies whose sound is largely absorbed by walls, or whose electrical reproduction is being partially canceled by negative acoustical feedback. Meanwhile any specifically selected band of frequencies can be de-emphasized or notched if its sound is otherwise being inappropriately reinforced by resonance of room-walls or by positive acoustical feedback. This not only prevents feedback whistle, but also accommodates louder, crisper and flatter overall mic reproduction.
The parametric EQ is a vital tool for achieving truly professional-quality sound in arbitrary acoustical environments. The electronic means for achieving parametric equalization have not formerly been available except at relatively high costs for most performers, primarily due to the lack of technological developments well suited for such applications. But due to an electronics technique that I discovered, published and patented**, the Collier 2x3EQ can be produced at the modest list price of only $450, at last making such capabilities available throughout the performance industry. We at Collier Electronics think you'll find our equalizers impressive.
Bob Collier
*If the phase is reversed by the amp, then the virtual separations at
which the acoustical positive feedback occurs will be essentially one-half
wavelength PLUS any integral multiple of one wavelength.
**
An instructive discussion of the abilities of a parametric equalizer
was presented for Harmony Central by
Scott Lehman of M.I.T..
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