Sunday, October 22, 2006

A Drum, a capsule, and a banjo - a multipart tale of Tizz - Part Three

Before I get started on the inevitable discussion of the construction and operation of the condenser capsule, a tympanic membrane, let me soften the blow a bit by letting you know that there are still many great pieces of recorded music that I love that were nonetheless made with condenser microphones. Back in the dark ages of the 70's there were many sonic shortcomings in music not the least of which were other sources of noise such as the surface noise of vinyl records, some which I still have and occasionally listen to on my Empire Troubador turntable. I have one particularly crackly copy of "Stand Up", which is a precious treasure.

This is a shot of a typical condenser capsule from an inexpensive microphone. It is well made actually, and certainly a reliable device, capable of detecting sound and converting into an electrical signal for an extended time.

A quick tutorial on how it works: The membrane represents an acoustic impedance mismatch to air. Soundwaves carried through the air impart a force onto the membrane, a varying pressure, which causes it to move. Part of the membrane (not shown here) is coated with a conductive material and one lead is brought to it forming one-half of a variable capacitor. Another lead is bright to the backplate making the other half. A charge is placed across the two halves: either a battery or a DC supply, perhaps 48 Volts or more is present across the two halves. The result is a charged capacitor, that can move.

Now the magic happens: As the membrane moves, its ability to couple electrons, or charge, from one section to another is modulated, and a varying voltage can be detected at the terminals that corresponds with the motion of the membrane. This voltage is then applied to the grid of a tube or the gate of a FET and amplified. It is important to note that the moving membrane does not generate any signal by itself, it only modulates a static, or DC charge. This creates a very sensitive device as the membrane is thin and it takes only the tiniest of movement for it to vary the charge and be heard. In fact it can be made so sensitive that random air molecules can be heard!

Obvious comparisons to the drum can be made, and there are differences, too. The drum is struck by a drumstick, the membrane of a condenser is moved by air. Sometimes that can be a very sharp wavefront with a lot of "kick" like from a kick drum, not surprisingly.

Remember the snare? Those air particles can become agitated and in some instances will produce a noise that rides under the signal. It won't show up in a spectrogram, and it's not harmonic distortion, so it doesn't reveal itself there, either. What it does is create a kind of noise floor that varies with the amplitude. It can be a problem when, for instance, multiple tracks of a vocal are used, as that noise accumulates, and mud and a certain haze or veiled effect can be noticed.

But the number one noise effect has to do with lateral modes, the energy that does not move in and out, but ricochets left and right, and takes time to die down. That's the source of the hissy sound you hear on badly recorded vocals that rides along with the natural breath and throat sounds heard in close mic recording and makes it sound harsh. When it's bad, we refer to it as tizz. Tizz can be subtle, pernicious, and contribute to edgy sounds in horns and violins too.

Tape used to mitigate that effect, but no more, as most music production has gone digital, like it or not. I think the need to overcome what has been revealed - an undesired inherency - is what is driving the continued efforts to make LDCs sound better, and why some cost more than $10,000.

2 comments:

Anonymous said...

YOU SEEM TO BE SAYING THAT THE CONDENSER MICROPHONE HAS THIS PROBLEM AND THAT RIBBONS DO NOT HAVE THE PROBLEM. IS THAT TRUE?

SO YOU ARE SAYING CONDENSER CAPSULE IS LIKE A DRUM, YES? AND IT MAKES NOISE THAT CAN BE HEARD.

GT

Bob Crowley said...

Yes

That's what I am saying. The condenser capsule is constructed exactly like a drum, and acts like one when excited by sound waves. No matter how tight you tension it, the same lateral propagation of energy exists which contributes to noise that rides under the signal. In addition, the drum head and the capsule both behave unevenly to any force that is off-axis. It turns out the a perfectly on-axis force is an exception: most sounds do not come from the exact center.