MICROPHONE TECHNOLOGY

PHANTOM POWER

First of all we need to consider & understand the operation of a capacitor microphone. A thin plastic diaphragm coated with gold or aluminium is stretched over a shallow hollow cavity which has a flat metal back-plate. These two plates form a capacitor of some 5 to 75pF {Nominally 20 to 30 pF}. A polarising charging voltage is provided by a DC source. Electrostatic attraction keeps the diaphragm taut, but sound waves that impinge upon it will cause a small variation in the capacitive value which varies in sympathy with the air pressure waves. The output impedance is very high at around 100Mohms. To avoid the losses of HF, a long cable cannot be used and a very adjacent pre-amplifier is required. The low mass and inertia of the diaphragm gives a very flat and wide response, while the output is high because of the pre-amp.
The need of power is a drawback and ways and means have been sought to provide the small but necessary power from within the amplifier with which the microphone is used. This is known as "Phantom Powering" The generally adopted standard is for a 48V DC supply. This is sometimes obtained within the actual microphone from a small battery. Otherwise it must come from the amplifier itself which might mean having extra wires. This is often overcome by the following circuitry.
The positive is taken to a centre tap in the input transformer or to the junction of a pair of resistors across the winding. Another similar method avoids the disadvantage of having to have the screen connected at both ends, a practice known to induce hum & noise pick-up. In this circuit both the signal wires are used to carry the dc power and the screen does not form a part of any circuit.

This is achieved by the circuit configuration known as A-B powering thus: See how cleverly the routes for the ac signal and the DC power is preserved over the same wires whilst the dc is blocked from the ac signal with scarcely any loading.

There is much more to know about microphones which all have their pros & cons. We have to weigh up their performance in several important ways: - Cost, performance, ruggedness, weight, size and pick-up qualities are the main considerations.
Since the latter may be the most obscure we will start with that. We may simply need a microphone that will pick up in an even way all around its site. Or we may need a performance that is more DIRECTIONAL. Here is a diagram that shows the most popular choices.

"Directivity" also Rejection ratio" - "Discrimination" - "Cancellation" and "Front to back ratio" are common terms used. Typical values are 15 to 20 dB which is about a twelvefold difference.

A is Omni-directional
B has a cardioid pick up
C is a super cardioid
D is a figure of eight velocity
E is a gun/interference tube type.
The latter is a very directional specialised type mainly used in noisy crowded rooms. You point it at what you want to hear. An open ended tube with a series of slots along one side is attached to a cardioid microphone. It operates by phase displacement cancelling down to a frequency that is a half wavelength of the tube length.

NOISE CANCELLATION
It often happens that unwanted background noise is picked up. There have been, and still are, novel ways to deal with it. Two microphones can be connected in antiphase to produce almost absolute cancelling. Announcements are made through one or the other from close proximity and this balance is overcome. Some capacitor units have two diaphragms facing front & rear. They each have a cardioid pick-up area. Polarising voltages can be variously switched off or reversed to form pick-up lobes.

EAR - 'ERE

HUMAN EAR - 'ERE
The final link in the sound reproduction chain is the human ear. How can you ignore the basic knowledge of it if you like or make music? It is a vital element to all those involved in audio engineering too. A read of this will cast doubt on Darwinian ideas of "natural selection".

DIRECTIVITY
Sounds from a source situated to one side of the listener arrive at the furthest ear fractionally later than the nearest. Thus there is a delay in phase which the brain interprets in terms of direction. Long wavelengths, (lower frequencies/notes), have less phase shift than the higher frequencies which fact is why it is much more difficult to locate the source of a low frequency note. The rattle of a box of matches is a bone-fide test used to demonstrate this.

Refer now to the drawing of the human ear below. The convolutions of the Pinner or outer ear produce reflections and phase delays which differ according to the angle of incidence with which the sound wave arrives. {A wave in this context is a variation in air pressure of a cyclic nature}. This also aids in direction location, especially, it is believed, in the vertical registration of the source. Again only the higher frequencies are effectively so identified. At mid frequencies the masking effect of the head plays a part by introducing small difference in amplitude.

IMPEDANCE MATCHING
The ear drum or tympanum vibrates in sympathy with the received sound and transmits the vibrations through three small bones called the ossicles in the middle ear. These are the hammer, anvil and stirrup {or stapes as some say}. The first two form a pair of pivoted levers that produces a nominal leverage ratio of 3:1, and the third communicates vibrations to the window of the inner ear. The ratio matches the mechanical impedance of the ear drum to that of the window, so obtaining optimum power transfer.
The tiny bones are held in place by small muscles which permit the pivotal positions, and hence the sensitivity, to alter. Thus the sensitivity is not linear, following a logarithmic law, being at a maximum for quiet sounds reducing to a minimum for loud ones.
This automatic volume control allows the ear to process an enormous range of sound levels being in the order of 10 to the power of 12 which is 1 million X 1 million = 1 trillion levels.
The Eustachion tube equalises the atmospheric pressure on both sides of the eardrum by venting the middle ear to the throat.

FREQUENCY DIVISION
The inner ear consists of a long liquid filled tube that is rolled up like a shell called the cochlea. A horizontal basilar membrane divides the tube along its length into upper & lower compartments except at the sealed far end where there is a connecting gap called the helicotrema. The compartments are termed the scala vestibule and scala tympani respectively.
Sound vibrations are applied to the Oval window at the entrance to the upper chamber by the Stirrup bone. From there they travel through the fluid to the gap at the end, then down into and along the lower compartment and back to its round window where they are absorbed. En route they pass through thousands of sensitive hair cells located on the upper surface of the membrane, which are linked to nerve fibres. These cells respond to different frequencies and are divided into 24 bands with one third octave spacings, starting with the highest band near to the entrance and the lowest at the far end. Individual bands occupy about 1.3mm of space, each being termed a bark.
The centre frequencies of the bands start at 50Hz for the No.1 and go up to 13.4KHz for band 24. Cut off outside of each band is sharp at the lower side but more gradual as the bands rise in frequency. The lowest band is 100Hz while the highest is 3.5KHz. The overall response in a healthy person under 30yrs is 16Hz to 16KHz with the girls generally having better HF hearing than the boys.

FREQUENCY RESPONSE
The frequency response of the ear is not flat, being at a maximum from 2 to 4KHz. The rest of the curve varies according to the sound level. At lower volumes the response to both treble & bass is less which is why some audio units boost these at low listening levels. Speech frequencies come well within the overall range but music encompasses greater range in both frequency & loudness. These contours show the sound pressures required to produce sensations of equal volume at various frequencies. They are known as "Equal Loudness Contours" and are the inverse of frequency response curves.
The contours are an averaged sample taken from an age group of 18 to 25 yrs.

HEARING DAMAGE
Temporary damage to hearing sensitivity results from exposure to loud sounds. It can become permanent if the exposure is prolonged. Damage is greater if the sound contains percussive energy bursts. Impairment is in the 4 kHz range (bark 18 in the cochlea) irrespective of the nature of the sound that caused the damage. As exposure extends the damage tends to reach down to 1 kHz.
Industry regs. give the following maximum exposures shown in the chart. It should be noted that disco music and headphone listening levels in excess of 100dBA can easily be realised. The dangers are obvious.

PRESBYCUSIS
This is an almost inevitable condition where hearing deteriorates with age. It starts slowly from 20 - 30 yrs and worsens over time. Exposure to loud noise plays its part. This chart shows the expected deterioration.

DE-Gaussing

Johann Carl Friedrich Gauss was a German mathematician & scientist 1777-1856 whose name was adopted for the CGS unit of magnetic flux density. {Magnetisation}. Sometimes bits of metal (iron/chrome) become magnetised when we don't want them to. It happens in tape recorders. If you bring those parts under the influence of a powerful "saturating" and alternating magnetic field, that is made to decay away relatively slowly , their magnetism will be removed. "De-Gaussed"
Magnetite is a naturally occuring ferrite commonly called Lodestone. It exhibits "ferrimagnetism" as opposed to "ferromagnetism". "Ferroxcube" is a tradename for similar man-made magnetic ceramic materials. A china that contains ferrous particles, mixed with other metals. It is very useful in conjunction with inductors at higher frequencies.

The Theremin

The theremin was invented in 1919 by a Russian physicist named Lev Termen. It is not like any other instrument, since it is played without being touched. Two antennas protrude from the theremin - one controlling pitch, and the other controlling volume. The electronic oscillators are tuned by hand capacity affects. As a hand approaches the vertical antenna, the pitch gets higher. Approaching the horizontal antenna makes the volume softer. They were originally built in the 1930's by RCA, GE & Westinghouse and found immediate use in the film industry because of their weird ethereal sounds. Robert Moog was building these in the 60's before the synthesizer.

Musical Electronics

Did you wonder where we are headed? Who would care if something more wonderful were not involved? To this very day, one of the most common discussions on audio amplifiers is always that of the guitar and transistors v Valves. Electric guitar players are convinced that the old valve amps are better. It is said that the transistors make the sound harsh ("tinny"), by emphasising odd harmonics. I have never really understood this claim. Although valves are more resistant to static discharge and rectifiers might give unintentioned compression, there are several things that put me off! Valves with their associated copper & iron transformers are heavy to carry & expensive! Overdrive effects are just as possible with transistors. I will grant it that valves are much more resistant to static damage but that is all. You tell me!

Speakers & Headphones

There once was a "Loudspeaker" made by the Leak company that was "electrostatic" in its operation. They were used for Hi-Fi in the early 1960's - so I suppose they were good. Very slim-line, they were expensive and looked like a radiator! We don't see them anymore.

I think all modern speaker "transducers" operate on an electro-magnetic principle. We want to move the air at various frequencies to hear our voices and our music. Long slow strokes of the diaphragm for the bass, very short fast strokes for the treble. That's one reason for the many different sizes and, indeed for the development of the "CROSSOVER" circuit used in Hi-Fi to drive them. We send the higher frequencies to the small one and the lower freqs to the larger one. Sometimes there's a "Midrange" unit (speaker).

All these units are constructed on the magnet & coil principle. They are the opposite of a generator. Here we have a varying applied (ac) voltage that generates a changing current in a coil. The ensuing magnetic field alters in sympathy and reacts with it's core magnet thus causing the coil assembly to move. A thin, (was paper - now often plastic), cone is attached to move the air. Too simple? Go and have a look at one!

We need these units to be very "responsive" & that means following the applied current in an exact copy. It ain't gonna happen folks! There's a raft of problems. Not least of these is the inertia, the impetus and the IMPEDANCE. {Got there!}.

The fact that the impedance varies with frequency is a big problem. If we know the impedance at a certain spot fequency, it still only gives us a basic idea of how far the impedance will range.

CROSS-OVER DRIVE
We can use different size units, coupled with a frequency steering & impedance matching network, but we still get an awful problem of "LINEARITY" in the drive current between frequencies. A novel, much used way around that is by a device within the amplifier known as NEGATIVE FEEDBACK. Were it not for this I doubt that true High Fidelity would be possible. Even then - but wait, enough unto the day etc. MORE SOON.

NEGATIVE FEEDBACK
The effective gain (amplification) of the amplifier is altered by taking a sample of the actual output and reducing gain when it is too much and increasing it when too little. We are able to do this via frequency filters if we want to, it's just the principle we need to grasp. If we take a sample of what is delivered into the speaker (load) we can level things out. There!

Glossary of Terminology

Right here's an apology: - I'm sorry! I really should have forseen that someone would want to know what these words mean - in this context. I can/will add to this list as we may progress. For now just these: -

RESISTANCE - This is a measure of how well a DC (& sometimes an ac) current can flow in a circuit. We say that when 1 Volt causes a current of 1 Amp to flow the resistance in the circuit is 1 OHM. This then, is OHMS LAW. To get POWER in WATTS multiply the current (I) by the Volts (V).

CONDUCTANCE is the reverse of that with a unit called a MHO (mho) Get it? Don't worry about this, it rarely crops up!

IMPEDANCE - Still measured in ohms this is the complex ratio of sinusoidal (ac) voltage to current in a component or circuit consisting of two parts. The real & the imaginary. The real part is the RESISTANCE which will dissipate heat, the REACTANCE part is "imaginary" and does not dissipate.

REACTANCE - Measured in ohms this is the ac only part of any circuit or component. Characterised by the storage of energy rather than by Wattful dissipation.

NOTE:- Bear in mind that most/all practical electrical/electronic components are not "pure" in the sense that they have elements of all in them. In that regard, and for most practical circumstances, we can regard RESISTANCE as being for both DC & ac, but IMPEDANCE as being for ac circuits which are much less than pure. Usually containing INDUCTANCE and/or CAPACITANCE.

RMS - Root Mean Square. DC is what you get from a battery. It has a fixed polarity. Ohms law works! In ac power they use slip-rings on the generator to collect the current instead of a commutator - as in a motor or DC dynamo. There are advantages which outweigh the cons. Less arcing, less brush wear AND because the polarity keeps reversing (50Hz here - 60Hz in America), it is much safer for us! Muscles don't get held in relentless contraction. You can alter voltages up or down with a transformer. You can't do that with DC. The shape of the reversing wave is sinusoidal - that is relating to the cyclic operation of the generator. We need to know when this varying, & reversing voltage has the same heating effect as an equivalent DC voltage. Hence an average of what power is delivered. There's a formula which relates to the ROOT of 2. That's 1.414. The peak voltage will have to be 1.414 times higher than its DC equivalent to give the same heating effect. That means that a a 240Vac supply will have a peak of 1.414 X 240 = 339.6 Volts. In BOTH directions! That makes the Peak to Peak (p to p) Voltage = 679.4
The wave shape is traced in time like the valve on a wheel as it rolls. If the axle is at zero, the valve goes higher and lower than that describing a sine wave. Rotations / second is Hz. Think about it!