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Dynamic Microphones – The Live Sound Grunt

Steve LaCerra • November 2020On the Digital Edge • November 5, 2020

The author demonstrates a frequently used — yet not recommended — application for a dynamic microphone.

This month in FRONT of HOUSE, we’re featuring a Buyer’s Guide to handheld dynamic vocal microphones (See
page 16
). The dynamic microphone is like the grunt of the sound reinforcement industry. We beat the daylights out of dynamic mics, leave them out on stage to cook in the heat of summer, freeze during the cold of winter, or to drown in hurricane rains, and even still, they keep working — until they don’t. And then we become upset that the $100 mic we’ve been using for 10 years to close-mic a snare drum suffered an untimely death when a 500-pound drummer cracked it on the head with a 3S drumstick. Condenser microphones get all the glory, but a dynamic mic is like a Ford F-150. Treat it right, and it’ll live longer than you.

‡‡         A Little History

It’s hard to imagine, but the first moving coil microphone was patented way back in 1874 by Ernst Siemens. It didn’t really catch on right away because carbon granule mics were easier to produce at the time and were widely used for telephones. As telephone and radio technology improved, Bell Labs’ Edward Christopher (“E.C.”) Wente created the first condenser mic in 1916. Condenser mics provided better audio performance, but were large and fragile, and required a separate power supply. They also didn’t like being left out in the rain.

Fig. 1: The Western Electric 618A was the first commercial dynamic microphone.

The first commercial dynamic microphone was invented in 1928 at Bell Labs by Wente and Albert Thuras. It was released in 1931 as the Western Electric 618A, shown in Fig. 1. The goal for Wente and Thuras was to create a reliable, more affordable microphone that could stand up to rough treatment in the field. It employed a thin duralumin (a hard, light aluminum alloy) diaphragm and a magnet made from a cobalt and steel alloy — which was big news at the time. The omnidirectional 618A yielded high output and a frequency response that ranged from 35 Hz to around 9.5 kHz(!). The 618A was used for years in P.A., film and broadcast applications, and was a popular choice for FDR’s famed Fireside Chat broadcasts (Fig. 2). It may be hard to believe, but almost 100 years later, you can still find working examples of the 618A.

Fig. 2: The 618A was frequently the choice for president Franklin Delano Roosevelt’s “Fireside Chats.”

The basic approach to the operation of a moving coil mic today is essentially the same as it was when the 618A was invented. A voice coil attached to a lightweight diaphragm is suspended within a magnetic field (see
Fig. 3). When sound waves hit the diaphragm, they cause it (and the voice coil) to move, generating a voltage in the coil. Lo and behold, that voltage is a pretty darn good representation of the original audio signal. Amazing.

Fig. 3: A cutaway diagram of a dynamic microphone capsule. Image courtesy Shure.

‡‡         Lots to Consider

These days, you can pick from hundreds of dynamic mics, many of which were designed for use on stage. Some of these mics have distinct advantages over their glamorous (and often more expensive) condenser cousins. Take, for example, tom mics. A lot of engineers like to use small-diaphragm condensers on toms. They’re easy to place, there are a variety of clip-on mounting options available, and small diaphragms tend to have lower mass, which results in faster transient response when compared to moving coil (or large-diaphragm condenser) microphones. The diaphragm motion of a typical moving coil mic is considerably slower, because the diaphragm is saddled down with that pesky coil — oh yeah, we need that coil to generate the electricity. That’s one of the reasons that moving coil mics tend to be less sensitive to transients than condenser mics, and less sensitive in general.

On the other hand, with their extended bandwidth, quick transient response and high sensitivity, small-diaphragm condenser mics are capable of delivering plenty of low-end as well as plenty of sparkle up top. You know what that means? More leakage from the cymbals. If a drummer places their cymbals close to the toms, you may experience a lot of cymbal leakage into the tom mics — especially if the drummer bashes the cymbals. You might even hear a cymbal get louder when the gate on a tom mic opens. Don’t look to EQ to help you out in such a situation, because all the EQ in the world won’t help reduce that leakage.

What you need is a microphone that’s less sensitive, and maybe has a bit less fidelity than a condenser mic. Those traits often translate into tighter pickup patterns and reduced leakage — important attributes in a live situation. A super- or hypercardioid mic placed on a tom can produce less cymbal leakage if the position of the cymbal falls within one of the nulls of the pickup pattern. Many dynamic instrument mics don’t have the extended HF response of a condenser, so you may need a bit of EQ to brighten things up, but if the mic has lower sensitivity, then leakage from surrounding instruments will be reduced.

You can take a similar approach with vocal microphones. Many lead singers sound great through a condenser mic, but what happens on a loud stage when the vocalist moves away from the microphone? The vocal mic becomes a cymbal microphone. This issue is made worse if there is compression on the lead vocal, and let’s face it — in just about any live situation, you definitely have compression on the vocal. When the vocalist stops singing, the compressor lets go of the gain reduction. Background noise gets louder and changes the balance of your mix, not to mention the phase issues that are created. On a quiet stage with less worry about instruments leaking into the vocal mic, this might not be an issue and you could go for a premium condenser mic — but when you’re fighting leakage on a loud stage, a less sensitive microphone might be a more effective tool. And as a testament to their durability, a frequently used (yet not recommended) application for dynamic mics is shown in Fig. 4.

Fig. 4: The author demonstrates a frequently used — yet not recommended — application for a dynamic microphone.

Don’t think that I’m hitting you with an “I hate condenser mics” tirade. The bottom line — particularly with vocal mics — is that you need to find the tone you’re looking for while also making the talent happy with the way their instrument (their voice) sounds. Sometimes you have to think outside the box to make that happen.

Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.

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