The sensitivity pattern of a bidirectional microphone (red dot) viewed from above.

A ribbon microphone is a type of dynamic microphone that uses a thin aluminum or duralum ribbon placed between the poles of a magnet to generate voltages by electromagnetic induction. Ribbon microphones are typically bidirectional, meaning they pick up sounds equally well from either side of the microphone.

In the dynamic microphone, the diaphragm is attached to a light movable coil that generates a voltage as it moves back and forth between the poles of a permanent magnet.

In ribbon microphones, a current is induced at right angles to both the ribbon velocity and magnetic field direction. As the sound wave causes the ribbon to move, the induced current in the ribbon is proportional to the particle velocity in the sound wave. The voltage output of older ribbon microphones is typically quite low compared to a dynamic moving coil microphone and a step-up transformer is used to increase the voltage output and increase the output impedance. Modern ribbon microphones do not suffer from this problem due to improved magnets and more efficient transformers, and have output levels that can exceed typical stage dynamic microphones.

Ribbon microphones have usually been delicate and expensive, but modern materials make present-day ribbon microphones durable enough for loud rock music and stage use. They are prized for their ability to capture high-frequency detail, comparing very favorably with condenser microphones, which can often sound subjectively "aggressive" or "brittle" in the high end of the frequency spectrum. Due to their bidirectional pickup pattern, ribbon microphones are often used in pairs to produce the Blumlein Pair recording array. In addition to the standard bidirectional pickup pattern, ribbon microphones can also be configured to have cardioid, hypercardioid, omnidirectional, and variable polar patterns.

One of the first ribbon microphones was the RCA PB-31. Produced in 1931, it was a breakthrough technology in sound, and revolutionized the recording and broadcast industry, setting a new standard in frequency response. The clarity and realism were unmatched by any of the condenser microphones of its day. Also of note is the ST&C 4038 (or PGS - pressure gradient single) designed by the BBC in 1954 and still used for some applications to this day. Its uses varied from talks to symphony concerts and is regarded as a fine microphone.

As many mixers are equipped with Phantom Power in order to enable the use of condenser microphones, care should be taken when using condenser and ribbon microphones at the same time. If the ribbon microphone is improperly wired, which is not unheard of with older microphones, this capacity can damage or destroy a ribbon microphone.

Around 2002, Chinese-manufactured ribbon microphones inspired by the RCA-44 (and in one case, copied from the AEA-R84) became available. Branded as Apex, Nady, Stellar, and T. Bone these mics are relatively inexpensive ($80 - $200). They are often the basis of microphone modification upgrades that rival classic ribbon mics. Listen to: "Ribbon Madness...Some Comparisons"for several examples.

Because the Ribbon Microphone is a very simple design with no active circuitry, some people have attempted to build them at home (with varying degrees of success) with basic tools and materials.

Some very good DIY project pages are available online:

• A complete step-by-step tutorial for building the easy and popular AUSTIN Ribbon Mic
• A wordy project page about the Spike ribbon mic
• A nice page on how to fix a broken ribbon microphone
• A version of a home-made, yet scary-looking ribbon mic
• How to evaluate a mic for "ribbon sag" and repair it

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