Microphones

With the plethora of microphones around you'd be surprised at how engineers all over the world seem to use the same mikes. Go surfing to all the studios and you'll find the same mikes in their mike list. I've got to state here that I'm not pushing any particular brand or type - I am not sponsored - so I'm only stating what I've observed over the years.

So how do they work? Basically all microphones have a diaphragm that vibrates when hit by sound waves. The vibration of the diaphragm is translated into an electrical signal that corresponds to the variation in the sound wave. That is why it is necessary to clean the diaphragms in your mikes on a regular basis as a build-up of dust, spit etc. will impede the vibration of the diaphragm and thus distort or colour the sound.

The Dynamic

In a Dynamic microphone, also referred to as a moving coil microphone, the capsule is rather like a speaker in reverse. The cone is the diaphragm and it has a coil attached that is suspended in a magnetic field. When the diaphragm vibrates the coil creates an electrical current. This is an entirely passive circuit as the magnet can be a permanent one so no external power is required.

The Condensor

On the other hand the Condensor microphone has two plates, one fixed and one moveable, that are each charged with a polarising voltage that creates a capacitor. The vibration of the plates creates a change in the distance between them which changes the capacitance and thus the sound wave is converted into an electrical current. In this case external power is required as there is an electrical circuit required to produce the polarising voltage. Because the current obtained is so small an amplifier circuit is also included.

Thus when using Condensor mikes an external power supply is required. This can be either a stand alone power supply for one or more mikes or it can be fed to the microphone from the console down the microphone cable and is commonly referred to as Phantom Power and is now standard at 48 Volts and all new consoles have that facility and usually consists of an on and off switch on the rear of the console or is an on/off option on each module. Incidentally, don't worry about sending phantom power to a dynamic microphone, it won't blow it up as the circuit is inactive in a dynamic mic situation.

The Electret

An Electret Microphone is also a Condensor microphone except that the charge on the plates is created by a permanent electrostatic charge. Therefore an external polarising voltage is not required but once again the voltage obtained is small so an amplifier is usually built in and powered by an internal battery. Electrets are often thought of as the cheap cousin to the condensor mike because the material required to hold the charge on the diaphragm is heavier but good electrets can sound fine.

The Pressure Zone

The PZM or Pressure Zone Microphone is also an Electret microphone except that it is mounted in a special housing near the pressure zone on the surface of a plate. This plate can be mounted on a flat surface like on the wall, floor or the lid of a piano. I have found that PZM mikes are not prone to popping and appear to have no proximity effect. They are typically used for pianos in concert situations where the lid can be closed to reduce spill and are also ideal as floor mikes in stage show productions.

The Ribbon

The Ribbon Microphone consists of a thin metal ribbon that is placed in a magnetic field. The vibration of the ribbon within the magnetic field induces a current that is proportional to the variation in the sound wave. This is also a passive circuit as the magnetic field can be created by a permanent magnet.

The Valve Microphones

Finally I must say something here about Valve Microphones. As mentioned before, the signal from the diaphragm in a Condensor microphone is small and must be amplified before it reaches the console where again it is amplified further. It is within this area that signal deterioration can easily occur and therefore the quality of the microphone must also be judged by the quality of the first stages of amplification. In a valve microphone the Condensor stage is a standard condensor system but the amplifier section uses a valve circuit to amplify the current as opposed to a transistor circuit used in later models. When I first started as an engineer in 1966 all the Condensor microphones were valve and each had its own power supply. The introduction of the transistor microphone eliminated the need for power supplies because phantom power was invented for the purpose.

The other major factor in those days was signal to noise. The average tape recorder had a signal to noise ratio of around 58db as opposed to the 70+ with today's analogue recorders.(Mainly due to the improvement in the surfacing of tape.) With such a low signal to noise ratio we were always careful about the high end of our recordings because if you had to add it later you sacrificed your noise and increased hiss. So when the transistor microphone came out we all remarked "Far out!" (it was the 60's) listen to that top end!!" and immediately used them instead because records were getting brighter then. What we were hearing was the difference in distortion between a valve and a transistor. A valve distorts in the 2nd harmonic first whilst a transistor distorts in the 3rd harmonic. The 2nd harmonic distortion is smooth, we can handle it but 3rd harmonic distortion is hard and harsh to our ear hence the difference between the two. The valve appears warmer like a valve Marshall does compared with a transistor version.

Today, on the other hand, the top end and noise is not a problem as modern analogue tape recorders have good signal to noise ratios and our mike preamps are also quiet yet from another aspect it is. The top end of digital is extremely bright compared with analogue tape due to the inherent distortion of frequencies above 7kHz created by the slow sampling frequency of 44.1kHz which in reality produces close to a square wave above 10kHz I find it produces what I call digital fatigue. Rupert Neve was recently reported as saying that we will need to sample at 24 bit/192kHz to equal analogue. (We will eventually) Meanwhile the warmth of the valve acts with the harshness of digital and produces a great compromise, hence one of the reasons for the popularity of valve mikes today.

Alternatively engineers today will put a mike through a valve preamp which is the second stage of amplifying a mic signal. Once again it is the soft clipping of the high end that produces that warm sound. What a lot of manufactures do today is the put a valve within a transistor circuit thus obtaining the soft clipping of the valve with the improved signal to noise of the transistor circuits. I've even seen an ad for a CD player that has a valve circuit in it!!

Polar Patterns

A dynamic microphone has a set sensitivity pattern called Cardiod Pattern or "heart shaped" or "Kidney shaped" pattern and the response looks something like this.

NOTE: This is not the curve for a SM57!!

Please note that this is not the response curve of a SM57, a SM57 is might tighter than this, it is only a demonstration. The line through the centre of the mike is called the Axis and when standing directly in front you are said the be On Axis as opposed to being Off Axis at the side and rear. In this example at 0 degrees there is full sensitivity, at 90 degrees the signal is reduced - 5db, at 120 degrees by 10, at 150 degrees by 20 db etc.

Condensor microphones have the added advantage of being able to alter their pattern from the standard cardiod and produce either a Figure 8 pattern or an All-round pattern.


FIGURE 8 and ALL-ROUND PATTERN

When using a Fig 8 mike you can place an instrument or singer on either side of the mike. With the all-round pattern you can place anyone anywhere as the pattern picks up through 360 degrees. Incidentally the all-round pattern does not exhibit proximity effect.

Hypercardiod

Finally there is the Hypercardiod pattern. This is like a cardiod pattern but tighter.

MS Stereo

MS stereo is short for Mid Side miking. It is recognised as being the truest form of stereo miking because it is not subject to centre lift in mono. When you join a stereo signal into mono the instruments panned to the centre (i.e. equal left/right) lift in the balance and is referred to as Centre Lift. MS stereo recordings don't have that tendency. You can buy MS Stereo microphones but if you've got a cardiod quality mike and condensor that will produce a Figure 8 pattern you are in business. Set them up like this.


The signal from the Figure 8 mike will need a mike splitter that splits into two mike inputs. This is the tricky part, To have a figure 8 mike it must be a condensor with phantom powering and if you split it and phase reverse, it will cut off the phantom power. You can purchase a transformer box like this:

The other way is to bring the Fig 8 mike up into a console and then take a feed from the direct out of that channel and bring it back in via a line input on another channel and phase reverse it.

Bring up the cardiod mike and pan it centre, now take the two splits of the figure 8 mike and pan one left and one right. Now reverse the phase of one of the splits. If you now have the cardiod mike pointing at what you are recording and you slowly add the fig 8 mikes you will hear the sound change from mono to wider and wider stereo as you add more of the fig 8 mike. The Cardiod mike is called the mid mike and the fig 8 is called the side mike.

When you mono this signal the left and right signal cancel each other and you are left with the mono centre signal which is a true mono. You can use MS Stereo for all sorts of things like overheads on a drum kit, ambience room mikes, stereo ACC guitars and pianos etc. You can always buy a MS Stereo mike but they are very expensive.

Proximity Effect

Anyone using microphones must understand proximity effect. When you get close to a microphone there is a rise in the low frequencies called the proximity effect. This low end boost can be 20+db boost at 50Hz!! A vocal mike like the Shure SM58 has a built in roll off to compensate for this because live performers like to sing close to the mike, but if you stand back from the mike it will start to sound thin, in other words if you want the SM58 to sound flat you must be close to it. Most mikes will have proximity effect so a low cut filter option is often supplied to compensate for it.

Microphone Phase Relationships

Before you record anything it is imperative that you check all your microphones for phase.

Two diaphragms in phase

Here the two diaphragms are moving in the same direction so they are in phase. Imagine them as two overhead mikes and they will both receive the signal from the drums in the same phase.

 

Two diaphragms out of phase

Here the two microphones are pointing opposite to each other yet their diaphragms are receiving the same signal. When the left mike's diaphragm moves in the other mike's diaphragm moves out.

As a result the two mikes are said to be out of phase and a phase reversal must be inserted or the two microphones will cancel each other. Officially they should cancel totally but they don't entirely in practice because each has a slightly different signal because of it's different position in the sound field. It will be most noticeable in the low frequencies so if you top and bottom mike a snare and don't use a phase reversal the sound will be thin and lack low frequencies.

 

Similarly, miking toms top and bottom the bottom mike will require a phase reversal. If your console doesn't have a phase reversal switch on it (funnily a lot don't) you should build some phase reversal plugs of your own. This can be done by simply making a male to female mike lead with pins 2 and 3 reversed. It's a good idea to paint them red or something so you know that they are phase reversal cables. You can also purchase pre-made phase reversal plugs from some retailers. Some people simply connect a male and a female cannon plug together with the leads reversed, paint them red and insert them into the mike lead before the mike patch point.

Checking your phase

A small note here - before you start recording it is a good idea to check the phase of all your microphones and cables. You can purchase small phase check boxes where you plug each end of the cable into it and if all three lights light up the cable is OK. At some stage it is worthwhile setting up all your mikes, select one mike as a reference, and getting someone in the studio to speak into your reference mike and each mike in turn to check that each mike is in the same phase and that all your cables are correct. You will notice immediately if one of your mikes is out of phase.

The Most Common Microphones

You can check all the mikes out at the websites of the manufacturers: