For this week’s Harmonica Microphones post we asked Greg Heumann to give us the low-down on the bewildering universe of mic elements. In the first of a double post, he describes two kinds of elements used for amplified blues harp and… well, it gets technical after that.
There are many kinds of elements in all of microphone-dom, including ribbon mics, condensers, electret, crystal and dynamic. Acoustic players may well use any of these. However, except in the recording studio, amplified players will only be concerned with dynamic and crystal elements.
Fact 1: There is no such thing as a mic element designed for harmonica players.
Every microphone uses an element that was designed for more general purposes. As a rule, the more expensive a mic was when new, relative to other mics of its vintage, the better it performs as a general purpose mic. To engineers, this means it has better frequency response (able to “hear” higher and lower sounds), flatter frequency response (no particular frequencies are made significantly louder or softer), and/or more “headroom” (the ability to tolerate higher sound pressure levels without distorting).
In this case “better” is usually means better for acoustic players. But…
Fact 2: For amplified, bluesy ballsy tone, “better” is worse.
Although low-end frequency response is good, harps simply don’t make any fundamental notes that are way down in the bass range. All the bass is added by the amplifier through harmonic distortion. And too much high frequency response can sound harsh, even hurt people’s ears. The reason vintage bullets are so popular is that their frequency response typically falls off at as low as 3000 Hz – even though some humans can hear as high as 20,000 Hz.
Fact 3: High headroom also works against us.
It prevents us from overdriving the microphone. The Audix Fireball, marketed as a good harp mic, has the most headroom of any mic I’ve played – 140dB according the spec. And it is a good harp mic for playing acoustically, or for avoiding a distorted input signal for other reasons such as playing through an amp modeling pedal. But the difference in tone between cupped and uncupped with this mic is far less than it is with an “older tech” microphone. In the case of amplified tone, older and cheaper is often better.
The venerable Shure Green Bullet and the Astatic JT30 are prized by harp players – because they give us that “old-school” sound. They are, in many cases, what our harmonica heroes used. Do you know why they chose them in the first place? Because they were cheap, even when they were new! The JT30 was the entry-level microphone in Astatic’s line, and came on the market at a price of about $6.
Dynamic and crystal: what’s the difference?
In my next post I’ll be talking more about vintage dynamic elements made by Shure: “Controlled Reluctance” and “Controlled Magnetic” elements (“CR” and “CM”). Both crystals and CRs/CMs have a great tone for amplified blues. It is hard to describe the difference, but to me a good crystal has a slightly nasal “honk” to it that the CR or CM doesn’t have. CR/CM elements have more bottom-end and can sound a little fatter and richer. The only way to know is to try good examples of each.
However, there are some other important factors to consider. Crystals have a lot going against them. The good ones were made 70 years ago and are at the end of their life. Many have died, most are dying, and a single drop can kill a good one. They are also extremely high-impedance elements. This means a low-impedance connection will dramatically reduce their tone. Just as turning on all the electric stuff in your car makes the engine work a little harder to keep turning the alternator, lowering the resistance across the mic’s terminals is like dragging your foot on the brake. Not only does the total output drop, but the frequency response changes too.
For this reason some custom harp amps are made with a 5 megohm input specifically for crystals. An amp with 50K input impedance will suck the tone right out of the best crystal. Anything you add between the mic and the amp, such as a volume control or a long cable, can lower the impedance as well. A good amp and good volume control will still let you get good tone, but you have to be aware that this is an issue and manage it. Finally, if your amp hums when you connect a cable that has no mic at the end, you can expect hum when you connect a crystal-element mic. A dynamic element may well reduce or even mute the hum.
Dynamic elements (including vintage ones) are practically bulletproof. They have lasted well all these years, so they are less expensive and more plentiful. Their impedance is lower so they stand up well to volume controls, pedals, splitters, etc. (Ultimately though, if the input impedance drops too far you will suck tone from any element.) And you can drop them (protected, of course, by a proper gasket inside a microphone shell) and they don’t break!
How do they actually work?
A dynamic element uses the electromagnetic principle to convert sound into electrical energy. A diaphragm is an extremely thin membrane that vibrates in response to sound waves. The diaphragm in a dynamic mic either directly moves a coil around a fixed magnet, or moves a pin which in turn moves inside a coil. Either way, the movement generates an alternating electrical current. A dynamic microphone and a speaker are almost identical in principle. With the speaker, you take electric energy and convert it into sound by sending the current through a coil, which then wants to move relative to a fixed magnet. The speaker cone is attached to the coil. Voila – sound! A microphone is the reverse – you take sound energy and turn it into electrical energy by moving a coil past a magnet. A speaker can actually be used as a microphone, and a dynamic microphone can be used as a speaker (think headphone speaker.) Please don’t experiment with your good harp mic elements to see how well they work as speakers. They don’t work well, and the risk of your damaging them is very high.
A crystal element relies on an entirely different principle called the “piezoelectric” principle – the diaphragm is connected to a crystal of “rochelle salt” or a manmade ceramic with piezoelectric properties. Mechanically deflecting the crystal generates an electric current. The lighter on your gas barbecue uses a crystal too. You press down on a button which mechanically bends and “snaps” a crystal (without breaking it) and with no battery required, a spark is generated. Crystal elements are rarely made these days, although they can be made incredibly cheaply so they still exist for entry level applications and are found in some production microphones. The new ones, unfortunately, don’t have the tone of the older ones – and the older ones are becoming very expensive.
Keep an eye open for the second half of Greg’s post on elements, where he names some of the most sought-after crystal and dynamic elements, and explains how to shop around for one.