Mics for Acoustic Instruments

The musical performance industry is on a back-to-the-basics trend. After the last few decades of buildup from the simplistic technical setups of the 60’s to the mega-stages of the 80’s, more acts are unplugging their banks of stage amps and going to a simpler acoustic set up. This is great for musical writing and arrangement, since now popular music has broken the mold of guitars/drums/vocals popular since the Beetles and Elvis. Eric Clapton, Crosby, Stills and Nash, among others, have done acoustic albums and tours, lending pop credibility to this oldest of performance formats. Popular acts are beginning to use a wide variety of ethnic and acoustic instruments both in performance and recording, which really broadens the possibilities of musical structure and style. Many studio techniques can be used in the field for live sound reinforcement of acoustic instruments, as long as the basic physics and limitations of “real instruments” are understood by both the musicians and technicians.

What Are “Acoustic” Instruments?

For the purposes of this article, acoustic and ethnic instruments will be defined as those that are designed and built to sound “complete” without any additional amplification, signal processing, or digital sampling. Even though technically all music becomes acoustic when it’s finally heard by a listener, many popular instruments don’t qualify. For instance, I don’t consider a modern rock drum kit to be an acoustic instrument even though there’s no transistors involved. The process of close miking each drum, gating the snare with added reverb and white noise, and processing the kick drum, produces a drum sound that only remotely resembles the actual sound of a real drum kit. If you don’t believe this, mute all the drum mics for a moment on your next sound check and listen to the drum sound. Or better yet, go to a small jazz club where a drum kit is used intentionally without amplification, a good example of what an acoustic drumset really sounds like. You’ll be surprised at the tonal differences between mic’ed and acoustic drums. They’re really two different instruments, as dissimilar as electric and acoustic guitars.

Speaking of electric guitars, Stratocasters and Les Pauls running through Fender or Marshal guitar amplifiers also don’t qualify as acoustic instruments. Yes, the sound eventually does come out in acoustic form from the speakers, but the endless variations in tone and timbre are mostly produced by the nearly infinite combinations of pickups, amplifier settings, speaker enclosures and external processing gear (stomp boxes), rather than any acoustic treatment done in the instrument itself. Yes, it’s a refined and defined sound all its own… it’s just not acoustic.

Real acoustic instruments make noise on their own, and it’s the job of the recording and reinforcement engineer to capture and amplify that sound without adding or subtracting from the basic sonic qualities of the instrument. While it’s pretty easy to mic a Marshal stack and a Strat’ as long as you have sufficient power in the FOH system to overcome the sonic bleed from the stage, getting the real sound of a didgeridoo or hammer dulcimer can be challenging, especially on a live stage for a large audience.

Start By Listening

This may seem obvious, but there are literally thousands of ethnic instruments, each with its own “proper” sound. Many have resonators, jangles, and timbres that will be strange and unfamiliar to you. First, find a quiet corner off the stage and have the musician play a representative tune that spans the full register of the instrument. Now, really listen and try to memorize a sonic snapshot. Some instruments may even sound unbalanced or unpleasant to your ear. Be sure to ask the musician if the sound is correct or if the instrument needs a little help. For instance, clezmer clarinet is very “honky” by design. If you try to “fix it” by reducing the 1KHz band on the channel equalization, you’ll mess up the intended timbre of the instrument and change the whole intended balance of the song. Likewise, penny whistles and fifes have a lot of presence “bite” by design. I once recorded a Civil War re-enactment fifes and drums band, and tried to “fix the fifes” in post production, since their sound seemed to be very shrill to my ears. The producer told me to go back to the original EQ, since it sounded exactly like the real thing. Who am I to argue with hundreds of years of precedence.

Use Good Microphones

Here’s your chance to really experiment with microphones. I like to use condenser mics on stringed instruments such as acoustic guitar and mandolin. My favorites are Neuman U64’s or the new Crown CM-700 [http://www.crown.com], and large diaphragm dynamics on percussion such as congas and Irish Bodrum (Sennheiser 421’s and Electrovoice RE-20’s are good places to start). As a good low-priced alternative to fragile studio mics in the field, Shure Beta 57 microphones [http://www.shure.com] offer an excellent choice for those on a budget, plus are as rugged as the original 57’s. You’ll notice that especially with the complex dispersion patterns of most acoustic instruments, various microphones will sound totally different on the same instrument Polar response patterns play at least as important a role as the basic response of a microphone in capturing the essence of an instrument. Since there’s generally no single “hole” that the sound comes out of with an acoustic instrument, there will be numerous sound paths to the microphone. These alternate sound paths are colored by whatever the frequency response is for that angle of entry into the microphone capsule.

This effect also makes results with omni-directional mic generally superior to those with hyper-cardiod patterns for capturing the overall sound of an instrument This works especially well in the studio, where isolation from other room noises is great, but is also applicable on a live stage. If the stage is not too cluttered with a loud roar of many other instruments, you can often get a better sound from an acoustic instrument by using an omni-directional mic positioned much closer than a cardioid microphone. If you find yourself using multiple cardioid mics on a single instrument to try to capture all the different sounds from various blow holes and resonators, try substituting an omni-pattern mic. You can get closer to the instrument than with a cardioid microphone without the nasty “proximity-boost” effect and off-axis coloration that plagues most directional patterned microphones.

Mics with poor off-axis frequency response will sound inferior to those with a smoother response pattern. Test the mics you plan to use by simply talking into them at 45 and 90 degrees off their center line. If the sound goes nasally or muddy as soon as you get off-center, then much of the sound of the instrument will be filtered this way. This is also important during live performance where a musician without a vocal microphone may talk into the side of their instrument mic to introduce a song. Fiddle players are notorious for this, insisting they don’t need a separate vocal mic, but then using the side of a vertically mounted instrument mic to “call” or talk to the crowd.

Pad It

This may be hard to believe, but some of the simplest acoustic instruments can be as loud as an electric guitar going at full tilt. For instance, shaker eggs don’t look very imposing, but their spectral output goes into the ultrasonic region. This can quickly overload condenser microphones and pre-amps that aren’t properly padded. If you’re recording to analog tape, be sure to monitor the playback whenever possible, especially with the newer, cassette format “porta studios”. Since these small tape formats use a lot of internal “high frequency pre-emphasis” to make up for the slow tape speed, an instrument sound with lots of high frequencies will quickly overload the tape, rustling in a “crunch” when you’re expecting a “swish”. If you don’t have a playback (or confidence) head, then record a short piece with the suspected instrument. Then playback and listen for distortion. If you’re getting ultrasonic overload it will be very obvious. Try reducing the recording gain on the VU meters by 6 db and try again. LED displays are better than the slow action of mechanical VU meters for recording, especially for the digital formats. Sometimes the meters will only be hitting -10 VU before distortion begins.

Compress Gracefully

High frequency saturation is not usually a problem with digital tape, since the peak meter monitoring lets you know when you’ve hit the wall. But since digital tape doesn’t offer the soft compression of a wide/high-speed analog format, you may need to add a limiter or compressor, especially on Latin and African percussion. Although good quality compressors can be useful at times, the most musical way to do this is often with a tube microphone preamp or line level “saturator”. Since a tube’s transfer characteristics can be designed for slow compression while adding 2nd order distortion, the signal can be hotter without overloading the electronics and data storage downstream while raising the average volume in the mix, which is important for lead instruments. The extra even-order harmonics added by the tube distortion also match the overtones of most stringed instruments, making for a sound generally more pleasing to the ear.

Add Pop Filters

Some wind powered instruments, such as wood flutes from South America which resemble really big pan pipes, are played by the musicians blowing across the top of the opening. This makes a wind blast that will cause most mic diaphragms to “bottom out”. In the studio, pop screens are very effective, but in live situations, a foam pop filter is usually the best you can do. Of course, some pop filters cause high frequency roll-off. But this can usually be corrected by 2 or 3 dB boost in the 10 to 15 kHz region. Just use you own ears…..

On-Board Mics

In the last few years, miniature condenser microphones such as Audio-Technica’s ATM15 become very popular with Bluegrass and Celtic musicians. This miniature electret-condensor microphone can be clipped directly to the sound hole of most stringed instruments. Other miniature mics such as the AKG 409 are mounted on a stalk that allows the element to be positioned a few inches above the surface of the instrument. This is a favorite microphone of a Tuba player I regularly engineer. A mic positioned this way has the advantage of always being in the same relative position with respect to the instrument, an advantage when the artist moves around a lot. (Yes, the tuba player does have a wireless rig) But care must be taken in positioning miniature microphones next to the nose and mouth of musicians playing instruments close to their face. Violins mic’ed this way often pickup the huffing and puffing sounds of the musician mixed in equally with the sound of the instrument. This is most noticeable on quiet passages, and will spoil an otherwise good recording.

Built-In Pickups (DI Sends)

Many acoustic guitars are now factory built with internal pickups: some a simple piezo transducer under the bridge, others consisting of multiple internal condenser mics and piezo transducers with on-board mixers and equalizers. Just down the road from my studio, Paul Beard builds his world-class resonator guitars, adding a Crown miniature microphone inside them for ease of use on stage. Even accordions get treated this way, with a line-array of miniature microphones down the front of the reed chamber, and a separate bass mic with a coiled cord to allow operation of the bellows. K&K Sound Systems [http://www.kksound.com] makes a complete system that has 3 mics for the descant side, and 1 mic for the bass side. Included is a volume control for the bass mic to allow you to adjust balance.

Depending on the design and placement of the pickup elements, the sound of these DI sends can be anything from great to dismal. One good method for using them both live and in the studio is the add a front mic in a good position, and route the DI send and mic feeds to separate channels on the mixing board. If you’re recording, send the DI and mic feeds to separate tracks, if possible. If you’re doing live sound, then the DI feed can be used for the send to the stage monitors, giving a more consistent level for the other musicians, while the mic channel can be directed to the FOH mix. You can then use as much or as little of the DI send in the FOH, depending on its sonic quality. This is, in reality, hedging your bet when doing live sound. But unless you’re using a DI system that’s been totally tested on the full sound system, the slight trouble of hanging an extra microphone is well worth eliminating the possibility of bad sound or pickup failure.

Acoustic Guitar: Microphone Selection

For studio recording of an acoustic guitar, start with a good medium cardioid or omni-directional microphone. Start with one mic placed in front of the strings close to where the body meets the neck about 6″ to 12″ away. A second mic positioned on the body of the instrument will allow you to add bass as desired in the mix. Alternatively, you can use a mic positioned behind the player, pointed at the back of the guitar, but be sure to invert the phase of this mic to avoid bass cancellations. Everyone has their favorite mics for studio guitar, but I like Neuman U64’s, AKG 414’s and EletroVoice RE55’s.

guitpat4For live sound, the challenge is to get enough gain before feedback, so close miking is essential. I usually start with a Shure Beta 57 or SM-81, although lately I’ve been using the Crown CM-700 with excellent results. Position the mic 4″ to 6″ away from the strings, and slightly in front of the sound hole.

Mandolin:

This is similar to miking a guitar, except that there’s no real bass output, so don’t be tempted to add any in the mix. They have a rather midrange-y sound to begin with by design, so be careful that microphones with a presense boost don’t over do the midrange. I like to use a good condensor when possible, but have used a Sennheiser 441 on a number of live gigs and liked the results.

Cherango:

This is a South American guitar, with a body about the size of a mandolin, but with a full size guitar neck. It’s normally played with a fast strumming motion, making close microphone placement difficult, so a microphone with good “reach” is required. The Shure Beta 57 has been my first stage choice on several gigs with the midrange presence boost complimenting the basic sound of the instrument.

Piano:

There have been many books published on proper piano miking technique, so I won’t try to embellish the subject here. But there is a contact pickup made by Barcus-Berry that mounts simply with contact tape and provide a great sound with excellent isolation from the room tone without microphones. Called the Piano Pickup, it includes it’s own preamplifier with XLR and phono outputs. This is great for busy festival stages, where moving the piano can send expensive microphones crashing to the floor. I’ve even used it to record a pianist in a noisy street level office in downtown Baltimore with ambulances screaming by a mere 30 feet away, and it sounded very good, plus no siren pickup. Try that with a regular mic. Of course adding a few mics over the piano in a classic X-Y stereo mode will allow you to add a little of each in a live situation.

Lower The Volume

Many former electric artists have a hard time adjusting to the lower stage monitor volumes necessary for acoustic instruments. This is caused both by a preconception that louder is better, and also hearing loss from years of loud stage amps and monitors. If you turn up the monitors to accommodate their wishes, the stage bleed from the monitors makes mixing in a small room next to impossible. The best way to eliminate this stage roar is through the use of in-ear monitors for the musicians. These are becoming more popular as artists are expected to match the sonic qualities of their latest CD. The expense of in-ear monitors, $350 to $700 per pair for good quality wired units, and up to $5,000 per channel for professional wireless units, is comparable to the cost of professional floor monitors and their amplifiers with none of the stage bleed. Great care must be taken, however, to protect the hearing of the musicians, since monitor levels can easily exceed rocket launch levels without the monitor engineer being aware of it. More on this subject in a future article on stage monitoring.

Copyright 1997 – by Mike Sokol
All Rights Reserved

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