Oh such a perfect phase… 3 of 3

Following post 2 of 3, let’s analyze now the second arrangement, when in the capture are involved microphones with different type of transduction (condenser + moving coil or condenser + ribbon). As we’ve seen in the post 1 of 3, their phase is already 90° out by default. According to “phase” terminology, 360° corresponds to full wavelength; 180° to half wavelength and 90° to 1/4 wavelength. So they are 1/4 wavelength out.

The peak of the condenser mic (in red) occurs when the electromagnetic (in green) is at null and the next negative peak of the electromagnetic corresponds to a null of the condenser. And so on… peaks and nulls are 1/4 wl apart . This provides us with the correct solution when positioning such microphones and looking for perfect phase of one frequency: we must separate them 1/4 wavelength of the target frequency. Not only that: since the phase of the electromagnetic (moving coil or ribbon) is “ahead”, the condenser is the one to be positioned closer to the source to compensate the difference.

If we refer to the same 1 meter separation we set up when checking out microphones of the same type (tuning frequency 340 Hz) and replace one of them so that the closer to the source is a condenser and the farthest is an electromagnetic, we obtain radically different results.

1 meter is to be regarded now as 1/4 wavelength of 85 Hz: that’s the tuned frequency (this means you can tune at lower frequencies using a lot less real estate!). The coincident positioning is to be avoided (at least until you get familiar with the matter and learn how to implement tools such as IBP by Little Labs or Phazer by Radial); what was the coincident positioning is now a 1/4 wavelength-apart positioning, with the condenser closer to the source.

Let’s look at this from a different starting point. Say you have positioned the different microphones as coincidents: there is no chance to have perfect phase for any of the frequencies. Period. As the sound wave propagates from the source towards the microphones, condenser translates variations of sound pressure at any frequency with a 90° or 1/4 L delay. Or, if you prefer: electromagnetic translates 1/4 L ahead. So… repositioning is a must. Move the condenser closer to the source, move the electromagnetic farther away, keeping in mind that the distance is 1/4 L of the “tuned” frequency. The positive combination of energy will reinforce that frequency.

On top of this let’s analyze further implications: first, because of the needed separation, as you tune (i.e. get 0° phase) to 85 Hz, you get total 180° phase cancellation at 255 Hz, i.e. the frequency three times higher (3rd harmonic).

3of3Fig2

That’s because 1 meter corresponds to 3/4 wave length of that higher frequency, which gets canceled out because the condenser picks up the negative peak exactly when the electromagnetic picks up the positive peak.

3of3Fig3

Second, the separation also corresponds to 5/4 wave length of an even higher frequency: the 5th harmonic (five times higher), 400 Hz, which is perfectly in tune with the fundamental.

3of3Fig4For a number of reasons like microphone’s sensitivity and consequent preamp gain involved, or rather sonic results, it’s common practice to have an electromagnetic closer to the source and a condenser placed farther away. Right? OK. No problem… Think back to what we learned about microphones of the same type: they can be either coincidents, or 1/2 wavelength apart with polarity reversed or else full wavelength apart both with the same phase.

Having learned this and knowing now that with different type of microphones a 1/4 wave length separation is mandatory, you’d have to adopt one of the following two options:

  • move the condenser back 1/2 wave length and invert its phase
  • move the electromagnetic back 1/2 wavelength and invert its phase

The first option is often the prefered one, with the electromagnetic closer to the source and the condenser 1/4 wave length behind it and with phase inverted.

You can go ahead and build the positioning pattern: with respect to the source

CONDENSER

ELECTROMAGNETIC

CONDENSER, polarity reversed

ELECTROMAGNETIC, polarity reversed

CONDENSER   and so on…