MIcrophone Design II Reading

1. Directional Response

Different microphones respond differently to sounds arriving from different directions. Some pick up sound equally well from all around. Some are designed to pick up sound approaching from one direction while others pick up sound from the front and rear but not the sides. Many modern studio microphones combine both pressure-operated (PO) and pressure-gradient (PG) transducer design to produce varying patterns of directivity between omni-directional and figure of eight.

Common polar patterns:

• Omni-directional (all around)
• Figure of eight (bi-directional) front and rear
• Cardioid (unidirectional) heart shaped
• Super cardioid
• Hyper cardioid

A given microphones polar pattern is defined by the particular transducer design implemented in its capsule. A polar pattern is a form of two-dimensional contour map showing the microphones output (usually in dB) at different angles of incident of a sound wave. The greater the plot from the center the greater the output of the microphone at that angle.

2. Directional Issues

Directional microphones exhibit certain characteristics that are not seen in omnidirectional microphones. Figure of Eight and Cardioid patterns share two common characteristics:

Off-Axis response and Proximity Effect. Additionally for Cardioid patterns

Front-to-Back Rejection Ratio is an additional characteristic.

2.1. Off-Axis Response

Off-Axis Response

The off-axis response is also important to examine. A microphone always takes in sound from the sides too, the question is just how much and how good it sounds. In particular, directional microphones can, in their attempt to suppress sound from the sides, get an uneven off-axis response:

2.2. Proximity Effect

Proximity Effect

The proximity effect exaggerates the low-frequency content of an acoustic source when a microphone is placed near the source (within about two feet). The more directional the microphone, the greater the proximity effect will be. Also, the closer the mic, the greater the exaggeration will be. You have undoubtedly heard this effect with 'boomy' radio personality voices.

2.3. Front-to-Back Rejection Ratio

Front-to-Back Rejection Ratio

The variation of frequency response from the front of the mic to the back of the mic. As the on-axis focal point becomes tighter (ex. hypercardioid), the rejection will decrease. That is the trade-off between selecting a cardioid pattern or a tightly focused hypercardioid pattern.

3. Omni vs Directional

Microphones can have different directional characteristics. Omni's that pick up the sound all around them, and cardioids that mainly pick up the sound directly in front of them. Other directional patterns are: bi-directional/figure 8, super cardioid, hyper cardioid, and wide cardioid.

Generally a cardioid sounds appealing, since in a musical setting you only want the sound source and rarely want to record or amplify the surroundings. But directionality has a price and that sometimes is not worth paying.

From a focus standpoint, omni-directional patterns are the least focused. This is because the sound source being captured has to compete with all the sounds within the recording space. Bi-directional microphones begin to focus the image as sound on the side of the polar pattern in being rolled off. Cardioid type polar patterns offer the most focus, with the image getting tighter as the cardioid pattern goes from cardioid to super to hyper.

Directional microphones need to have a much softer diaphragm than an omni. This softness results in handling, pop & wind noise which puts a limit to how close you can get to a vocalist, even when using pop-filters.

A directional also suffers from proximity effect, which means that the closer you get to the sound source the louder the low frequencies get.

Additionally the off-axis sound of a cardioid is less linear than that of an omni. It is very hard to reduce the level of sound taken in from the sides without some coloration, and some directional microphones have a notably poor off-axis response. This means that sound entering the microphone from the sides and the rear are more or less strongly colored. This effect can be seen on the microphones polar pattern as ‘spikes’.

Multi-pattern microphones with both omni, bi-directional and cardioid characteristics will always compromise the sound quality. It may be very convenient to have a 3-in-1 solution, but the drawback is reduced performance in each mode. Due to the need of a pressure gradient design, a multi pattern microphone in omni mode has many of the weaknesses of the cardioid, such as popping, handling & wind noise and a less linear off-axis sound. In fact a multi pattern microphone in the same mode can have different characteristics depending on the frequency.

4. Large vs Small Diaphragm

Before choosing between a large and a small diaphragm microphone it is important to know the difference in features between them.

Large Diaphragm

Any microphone with a diaphragm larger than (and potentially including) 3/4" is considered to be a Large Diaphragm microphone. In general, Large Diaphragm microphones tend to have a "big" sound that engineers find especially pleasing where a little more character might be advantageous.

Small Diaphragm

Most professionals and manufacturers agree that any diaphragm smaller than 5/8" would be considered a Small Diaphragm. Generally speaking, Small Diaphragm microphones tend to do a good job of capturing high frequency content and transients. They will tend to have a bit more "air" to their sound and often have less coloration than large diaphragm microphones. Most of this is due to the reduced mass of the smaller diaphragm, which allows it to more closely follow any air disturbances it is subjected to.

Differences

A common myth is that large diaphragm mics capture more low frequencies than small diaphragm mics. Sometimes their coloration, due to being less precise in reproducing high frequencies, may make it sound like this is the case. A properly designed small diaphragm mic is more likely to be accurate throughout a wide range of frequencies, whereas the coloration of a large diaphragm mic can tend to enhance certain desirable characteristics in a sound, which sometimes amounts to more apparent bass or low end.

A small diaphragm has a higher self noise due to the fact that the small diaphragm is less compliant and therefore more sensitive to the bombardment of air molecules that causes some of the self noise of a microphone. And since the large diaphragm is softer than the small, it is easier to move and therefore more sensitive – even at very low levels. This means that the small diaphragm, because it’s stiff, can handle a higher sound pressure without clipping or distortion, but is less sensitive and needs more amplification, which also adds a little noise.

When reproducing very high frequencies, large diaphragm microphones have a more limited range than the small diaphragms. This is caused by three factors:

• A large diaphragm tends to break up and will no longer act as a true piston. This phenomenon is also recognized in loudspeaker technology and is the reason why loudspeakers are manufactured with different sizes of diaphragms to handle different frequencies.
  

• The weight of the diaphragm will attenuate the displacement of the diaphragm for higher frequencies.

• The diffraction around the edges of the microphone capsule will limit the microphone's capability to handle very high frequencies.

5. Grill Design

The grill design on a microphone has significant impact on its frequency response. The science behind this is beyond the scope this class but we can see this impact looking at 2 of the most common type of mics used - the Shure SM57 and SM58.

The Shure SM57 and SM58 are dynamic mics and are exactly the same in their electronic, magnetic, and diaphragm designs. The only thing that makes them different is the grill design. Looking at the frequency response graphs of each mic, the effect of the different grill on the frequency response can be seen.

6. Controls

Based on the design, certain switches may be available on the microphone. This is all driven by the manufacture.

Low Frequency Roll Off

This allows the user to cut low frequencies (typically the selection will range between 50 to 150Hz) by 10dB or so in situations where rumble or the proximity effect is a problem. We'll talk a lot more about what this means in the near future.

Pad

This switch will cut the output of the microphone (typically by 10-20dB). In rare cases, the switch may allow the output to be boosted.

Polar Pattern Select

A switch may allow omni-directional, figure of eight and cardioid polar patterns to be selected.

7. Windscreens

Windscreens

Windscreens are used to protect microphones that would otherwise be buffeted by wind or vocal plosives from consonants such as "P", "B", etc. Most microphones have an integral windscreen built around the microphone diaphragm. A screen of plastic, wire mesh or a metal cage is held at a distance from the microphone diaphragm, to shield it. This cage provides a first line of defense against the mechanical impact of objects or wind. Some microphones, such as the Shure SM58, may have an additional layer of foam inside the cage to further enhance the protective properties of the shield. Beyond integral microphone windscreens, there are three broad classes of additional wind protection.

One disadvantage of all windscreen types is that the microphone's high frequency response is attenuated by a small amount, depending on the density of the protective layer.

Microphone Covers

Microphone covers are often made of soft open-cell polyester or polyurethane foam because of the inexpensive, disposable nature of the foam. Optional windscreens are often available from the manufacturer and third parties. One disadvantage of polyurethane foam microphone covers is that they can deteriorate over time. Windscreens also tend to collect dirt and moisture in their open cells and must be cleaned to prevent high frequency loss, bad odor and unhealthy conditions for the person using the microphone. On the other hand, a major advantage of concert vocalist windscreens is that one can quickly change to a clean windscreen between users, reducing the chance of transferring germs. Windscreens of various colors can be used to distinguish one microphone from another on a busy, active stage.

Pop Filters

Pop filters or pop screens are used in controlled studio environments to minimize plosives when recording. A typical pop filter is composed of one or more layers of acoustically transparent gauze-like material, such as woven nylon stretched over a circular frame and a clamp and a flexible mounting bracket to attach to the microphone stand. The pop shield is placed between the vocalist and the microphone. The need for a pop filter increases the closer a vocalist brings his or her lips to the microphone. Singers can be trained either to soften their plosives or direct the air blast away from the microphone, in which cases they don't need a pop filter.

Pop filters also keep spittle off the microphone. Most condenser microphones can be damaged by spittle.

8. Tube vs Solid State

Before transistors, microphones used valves (tubes) as part of their electaronics. Valve microphones tend too have a narrower frequency range and are less transparent (their frequency curves are not flat) than modern solid state designs, but are favored because of the character and warmth (pleasant harmonic distortion) they can add to a sound. Solid state microphones are also referred to as FET microphones.

An example within the studio are two large diaphragm condenser:

9. Mic Choices at home

• There are all sorts of mods that one can do to their mics to convert them to sound like another perhaps more expensive mic.  Like the "turn your sm57 into an SM7B" tutorials you can find online.
• There are companies that make knock offs of very expensive microphones, like our Wunder CM7 at the studio (though it's still terribly expensive, it's a far cry from a real Telefunken)
• There are now mics with software that emulate a whole bunch of different types of microphones with software at the press of a button.  Slate has two mics that they make with downloadable mic models for extra cost.  Antelope has a few mics that model different expensive mics at a budget price.   These are just two companies that are currently building these mic modelers but be on the lookout for many more companies doing the same. 

Slate ML1 and software