Microphone considerations for the bodhrán

Mii - Finland (c)2006

Reproducing the sound of a bodhrán can be tricky because of the drum's wide tonal range. In addition, there are many considerations along the signal path from the mic to the loudspeaker.

The first of them is choosing a suitable microphone (or two) for the job. All microphones sound different and behave differently.

I would like a bodhrán mic to have:

• good transient response; some mics are better than others in reproducing sounds with sharp attacks
• excellent frequency response throughout the audible frequency spectrum; there's lots of tonal variance with a bodhrán and you want to capture both the low thumping and the brittle popping
• suitable overall sound; one microphone's frequency response may suit better to my drum and my playing than another's

It's not enough just to have a good mic. You also need to have it in a good place to capture the full sound of a bodhrán. Little changes in the microphone's position can make a huge change to the sound. You can get extremely varying tonal results by moving the mic a few feet around the player. If you use static mics on stands, you can use pretty much any mic you want. If the player wants to be able to move during the gig, your options are limited to clip-on mics.

The circumstances of a live gig may limit the choices of microphones and microphone positioning. For example, if the band is loud and the drummer is playing next to a screaming guitar amp, you have to put the drum mic very close to the drumskin. Otherwise the guitar sound will leak in the drum mic and ruin the drum sound. The drummer may also want to dance and run around while performing. Then you can choose a clip-on mic so the drummer can move about without affecting mic positioning. What is the perfect mic for the job depends much on the instrument, the player, the situation and the type of music played. There is no "one perfect bodhrán mic for all situations". You will have to use your ears, evaluate the characteristics of your drum and your playing and choose a mic that works best for you. Sometimes convenience rates over sound quality and you go with just one clip-on mic. (No problem with that. Some of them are very good.)

Mic types............ See page relating to users' experiences with different mics

Microphones are commonly divided into two basic types defined by their working principle.

I won't delve into electronics in this article, but will give a practical outline of typical characteristics of the two most common mic types, the dynamic and the condenser microphone. (Piezo-electric mics are also common today but they're not relevant in this article. They're not much use with drums.)

Dynamic microphone

The dynamic microphone is the most common type of microphone. They're relatively inexpensive and rugged, they can take a lot of sound pressure and come in all shapes and sizes. Dynamic mics are typically not very sensitive and can't reproduce the highest end of the frequency spectrum well. They can take great sound pressure levels without distorting, and therefore are often preferred for drums. In comparison, most condenser mics won't perform well inside a heavy metal drummer's bass drum. They're too sensitive for that and will distort.

Because of the dynamic microphones' relative insensitivity, they're also preferred in all kinds of loud/noisy situations. A rock band has mostly dynamic mics on stage. Also, dynamic mics do not require power (like condenser mics do).

A well-known example of a dynamic mic would be the Shure SM58 (pictured) which is probably the most popular vocal mic in the world. If you ask a 4-year old to draw a microphone, he/she will probably draw something that resembles an SM58. (The SM57 is the same mic without the metal grill; it's probably the most popular instrument mic in the world.)

Condenser microphone

The condenser mic is the other common type of microphone, they are very sensitive and have a wide frequency spectrum. Because of these qualities, they are widely used in recording studios, radio / TV studios and such. A condenser mic can typically reproduce much higher frequencies than a dynamic. They're also more expensive than dynamic mics.

Condenser mics require power. It is usually supplied from a mixing desk through the microphone cable in the form of "phantom power". If your mixing desk doesn't supply phantom power, you will need a separate phantom supply for your mic or a mic with a built in battery.

The AKG C414 (pictured) is a well-known example of a condenser mic. It's widely used in recording studios all over the world.

Diaphragm size

Diaphragm size affects the overall 'colour' or timbre of the mic's sound. Large-diaphragm microphones' tonal characteristics are commonly described as "full" and "warm". These mics excel in the low frequency range. If your instrument operates in the bass domain, you may want to choose a large-diaphragm mic. There are mics specially designed for bass instruments, such as the dynamic AKG D112, which is very commonly used with bass drums and bass guitar cabinets. A good large-diaphragm condenser can reproduce the whole audio spectrum from the lowest lows to the highest highs. On the other hand, they don't always have a good transient response due to the large physical size of the diaphragm.

Most engineers, including me, use large-diaphragm condensers for vocal recordings. The AKG C414 mentioned above is a good example. Take a look at the frequency response graphic more.... You can see that it's practically flat from the bottom 30 Hz (extremely low bass) up to 5000 Hz. There's a slight emphasis at 7000 Hz and another at 14 000 Hz. Would I use this mic for an upright bass? Sure thing!

A mic with a small diaphragm can typically articulate the highest frequencies excellently. Small-diaphragm mics are often said to sound "crisp" or "bright". They typically have excellent transient response due to the diaphragm's dimensions and lightness. Small-diaphragm condensers are used for f.ex. drum overheads, hand percussion, acoustic guitars etc. Here, take a look at the graphical data of the AKG C451, a small-diaphragm condenser:This mic's response is practically flat from 200 Hz to 5000 Hz, but frequencies below 200 Hz are gently rolled off and frequencies above 5000 Hz are gently emphasized. Would this be a good mic for bass? No, not really.

But then, what's large and what's small? Well, they're just words to give you a general direction. Most microphones' data sheets don't mention the diaphragm's exact dimensions, but often the manufacturers label their mics as "large" or "small" diaphragm. Some mics can be said to have a "mid-size diaphragm", like for instance the Sennheiser MD421, which is one of the nicest all-purpose dynamic mics and a great drum mic.

Directivity - polar patterns

Most microphones are directional. It means that they're designed to pick up sound only from a specified direction (or two). The directional characteristics of a mic are usually presented as a polar pattern. The images shown demonstrate the microphones' polar patterns with the sound object being at 0 degrees.

Some mics (like C414) offer several polar patterns that you can select from a switch. Notice that the frequency response varies a little with different polar patterns of the same mic.

The most common microphone polar pattern is cardioid. A cardioid mic picks up sound mostly from its direct front, a little from the sides and almost nothing from its back. If the cardioid's directivity is very narrow, it is called hyper-cardioid. Other common polar patterns include omni-directional (no directivity, picks up sound evenly from all directions) and figure-of-eight (picks up sound only from direct front and direct back, but nothing from sides). On live gigs it's practical to use cardioid mics to separate sounds from each other. You don't want that fiddle sound leaking in your bodhrán mic.

If we want clear separation of sounds from each other, wouldn't it be a good idea to use the tightest hyper-cardioid polar patterns on the market? Not necessarily. Directivity is also linked with another issue called "proximity effect". Proximity effect means that the closer the sound source is to the mic, the more the bass frequencies are emphasized. This happens with all directional mics. Pick a vocal mic and sing to it; move it slowly towards your mouth and notice how your voice
gets boomy. The tighter the polar pattern is, the more extreme the effect is.

There's also another directivity-related issue called "off-axis response". All directional mics give their best frequency response when the sound comes to the mic directly from the front (on-axis). Sounds that come from other directions (off-axis) don't get reproduced as well. Mics with very tight polar patterns often have worse off-axis response than mics with more open polar patterns.

Two mics on one drum

It may be difficult to find one single mic that will reproduce both the deep low end and the high popping of a bodhrán. A further option is to use two mics. You could have a bass drum mic (such as AKG D112 or Shure Beta52) to pick up the low frequencies and a small-diaphragm mic (such as AKG C451 or Shure SM81) to get clear and open mids and highs. Read player opinions on dual micing systems

Technical note on specific issues with using two mics on one sound source

When you use more than one mic on the same sound source, particularly in the studio environment, you may experience phase cancellation. more...

Maximum phase cancellation occurs as a result of a relationship in time when two or more waveforms with the same frequency and amplitude (volume) reach multiple mics more.... Depending on the distance of the mics from each other and the source, some frequencies may cancel each other out, leaving the sound undefined and fuzzy. This is often described as a 'hollow' sound. The effect can be variable particularly with a close-miced drum like the bodhran.

Here are a few tips on how to avoid phase problems:

• Keep the mics close together. If the diaphragms are side by side, sounds will reach them at about the same time.
  
• If they can't be side by side, a rule of thumb is to position the second mic at least three times as far from the first mic as the first mic is from the sound source. If the first mic is 4" away from the drum, the second mic must be at least a foot away from the first mic.

Some mixing desks have phase reverse switches on all channels. If you experience cancellation reverse one of the two signals and the problem is solved. If you don't have phase reverse switches, it's safer to keep the mics on the same side of the drum.

Mii

Editorial Note

Following prolonged discussion within the webteam, the information on phase relationships reflects the input of more than one individual. Thanks to Cat Lake for her input and suggestions.

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