Acoustic Instrument Equalization

We took a radical design approach when it came to implementing equalization into our acoustic instrument preamps. The industry is full of the same nasty and noisy cheap equalizer circuits that are for the most part built and tuned exactly like a dollar store boom box. You can recognize these quickly when playing by simply turning the knobs. If you hear hiss changing frequencies and smearing the phase as you rotate the knobs, it is a complete design failure combined with cheap components. The tuning of the equalizer frequency bands on many preamps are the common 100Hz, 1kHz, and 10kHz. This works fine for playing a mp3 but is pretty useless for acoustic instruments.

This article will give a background on the four primary Archangel Eq types. Some heavy technical information is included, to give a deeper understanding of how we approach Eq design and instrument resonance as radical departure from the accepted industry standards. Several customers have asked specifics of how the Eq is built, so here it is without any of the marketing language or artificial hype you will encounter from many other brands.

Subtractive Shelving Eq

Our simplest eq also has the lowest noise of any filter in the industry. Our Subtractive Eq is featured in our entry level performance series preamps, and also in the channel strips of the Master's series preamps. They are passive filters wrapped around a buffer that give control over low frequencies at 125 Hz and high frequencies at 5kHz. The filters are subtractive, meaning that they work as a cut only filter. The normal knob position is fully clockwise for no reduction. Fully anti-clockwise gives a -24dB reduction. This is a great feature to reduce string squeak and for eliminating any hum induced to the instrument from 50/60Hz line noise or 100Hz flourescent ballast hum.

On our dual input preamp models, the Subtractive Eq is available on each channel. Some of our pro artists use eq in a unique way when tracking their instrument using multi-source inputs. They will roll off the lows on a mini condenser mic on one input channel, and roll off the highs on a set of soundboard transducers on the other channel. The result is clarity and separation that shows off the instruments natural harmonics and resonance without phase cancelation, comb filtering, or phase addition causing low frequency build up or the muddiness that can be encountered with multi-source systems.

The low frequency filter in our Subtractive Eq also does a great job of reducing the resonant feedback that is sometimes encountered in acoustic guitars when using soundboard transducers in front of hot monitors or very loud stage volume. It is a simple effective way to quickly tune your instrument to the room.

Archangel's Exclusive Isometric Eq
When it was time to design an active eq for our 6th generation of preamps we were very demanding of low noise and high performance. We built, modeled, and tuned over a dozen different eq circuits that covered the typical types used as audio filters. We rejected every one until we tuned the unique filter design that would become the Archangel Isometric Eq which continues on as a integral feature in many Generation 7 models. This filter design is not used in any other current products offered by any other manufacturer in the business, so naturally it behaves a little differently that what you may be used to.

The Isometric Eq is among the lowest noise designs ever offered in the industry. It does not phase shift or add a wall of hiss like many conventional Eq circuits do. It is highly regarded as the cleanest and most transparent eq in any acoustic preamp or processor available. This was not by accident, but by months of detailed design and analysis that was focused exclusively on building the finest equalizer available anywhere.

We used the word Isometric to describe the eq. The Isometric name has dual meanings. Isometric by definition is a 2D schematic representation of a 3D space. It also eludes to the Eq topology as an isolated parametric active eq. (By definition, Para means multi and Metric means parameters.) It's filters are both band-pass and band reject. They are cascaded together with each filter independently buffered and tuned for minimal interaction between bands, ultra low noise, and precise tuning.

The frequency adjustments used in the Isometric Eq are implemented as presets. The different tunings are available on switches that allow the filters to be tuned to multiple settings. This allows simple three knob operation for users familiar with conventional active Eq, and does not complicate the preamp with extra controls for Q or frequency. With Generation 7, the switches are now located on the sides of the enclosure.

The bass filter is tuned to 60 Hz to adjust the subharmonic an octave below the fundamental resonant frequency of acoustic guitars. The filter has a wide enough bandwidth to cover the subharmonic of many guitar types, but narrow enough to not boost infra-sonic frequencies below 20Hz. The 60 Hz frequency was also chosen so that adjusting the bass filter does not reduce the frequencies of the low E string notes like typical 100Hz tuned equalizers do. The 60Hz attenuation is tuned roughly to the low B (61.7 Hz) of a seven string or baritone guitar. It is very effective for equalizing bass instruments or baritone guitars. It also gives a powerful low end punch for the percussive hits and taps used in contemporary fingerstyle techniques. The bass filter is the only band that has a fixed frequency.

The treble filter has two tunings that are selectable with the bright switch. The bright setting is tuned to 6kHz. This captures more high midrange voicing into the treble filter. The sweet setting (with the bright switch off) is tuned an octave higher to 12kHz. The treble filter is tuned to compliment the subtle complex harmonics of acoustic instruments perfectly. You will notice slightly more effect from the treble filter when the midrange filter is set to the 1.5kHz setting detailed below.

The midrange filter also has dual tunings for a very unique solution to a common problem with SBT type pickups. The HFE setting is our proprietary Harmonic Feedback Elimination. It is designed to reduce the level of the 2nd harmonic (an octave over the fundamental resonant frequency node). The HFE tuning is at 237Hz to fall just on the high side, yet still capture the 2nd harmonic of many guitar types from 000 or parlor sizes up to grand auditorium and jumbo models. The HFE tuning also covers beautifully the frequency ranges of the wolf tone resonance of a violin (typically 204Hz) and the air prime resonance of a mandolin.

Most systems attack the fundamental frequency (typically a low G#). In our testing the solution to controlling feedback was an octave higher. When we looked closely at acoustic guitar feedback using SBT pickups we discovered that by lowering the 2nd harmonic (an octave above the fundamental) it eliminated the feedback without touching the notes on the low E string. It instead lowers the frequencies near the G string range, which is usually very loud compared to the treble strings anyway. When we balance the response of the strings this way, it eliminates the resonance completely and the feedback simply disappears.

The Midrange filter also has a setting for a more normal midrange control that is tuned to 1.5kHz. This is very useful to reduce the frequencies that compete with vocals in the mix. Many mandolin and violin players find this to be a natural tuning for their instrument's higher voicing. It is also very practical with acoustic and electric basses to balance the punch of the bottom end with the high frequency finesse and detail and to mix well with other instruments.

You will notice on our Isometric Eq that the zero position of the knob on our Generation 7 models is shifted to the 2 o'clock position. This is an accurate representation of where the filter zeros to a flat position. This accounts for the insertion losses from cascading the filters, which we don't use make-up gain to recover. To do so would add noise and require three more amplifier stages in the Eq alone. The 6dB drop between filters is aslo accounted for in the extreme clockwise position markers as +24dB, +18dB, and +12dB as each filter is added to the circuit. Although the filters at least on paper are modeled as -15dB to +24dB, we label the knobs to more accurately reflect the way that the filters actually behave in the circuit.

The Archangel Parametric Midrange Filter
Starting with Generation 7, several models will include our new parametric sweep midrange. We have developed a filter that is dead quiet, does not smear the phase, and meets the power requirements of phantom power's low current limitations. The Parametric Midrange has a level control for 15dB of both cut and boos.t There is also frequency sweep control that adjusts the range from 150Hz up to 2.4kHz. This will allow for some serious tone sculpting over a wide range that covers from mid-bass to low treble.

The Archangel Notch Filter
Our new Notch Filter is a welcome addition for many customers. We refused several conventional designs before dialing in the ideal combination of simplicity and outstanding tone. Several other notch filters designs on the market are difficult to tune, or have a bandwidth so narrow that when you get it tuned and the instrument moves just a few feet, the feedback returns. Many are terribly noisy and some even hiss, whistle, or whine at a high frequency with some settings.

The Archangel Notch Filter is marked in notes from below the low E string up to just below the open G string, in half steps. It has a super simple one knob tuning adjustment or it can be bypassed completely. The circuit is virtually noiseless and wont flip the phase. It works great if you have a string or a note that is a little hotter that the others. Between the Subtractive filters, the Isometric Eq's HFE setting, and the Archangel Notch Filter, you have a triple layer of feedback and resonance control at your fingertips.

Use wisdom before tweaking
Like with all equalizers, less is more. It is always more beneficial when possible, to cut an offending frequency than to boost those around it. Our equalizers have been designed from the ground up to have super low noise, and tuned to perfectly compliment acoustic instruments. It is however possible to get your tone all jacked up and even clip the outputs if equalization is not used with discretion. The same controls that eliminate resonance can also boost them into feedback. If you find that you are using extreme settings in Eq, there may be another component in your gear that is not working as well as it should.


The resonance of an acoustic instrument is a compounded and complex system that is a result of several elements working together including the string vibration, the resonant frequency of the top, the resonant frequency of the back, the frequency of the air column, the harmonics of the instrument derived from the fifths and octaves in the upper bouts, multiple compound resonance nodes, and the relationship of phase as the top and back breathe in opposing directions, or in phase with each other. The following are clips from a couple of articles that expound on the resonance of acoustic instruments, which is also the basis of the theory behind out equalizer tunings. Ours are certainly not your average boom box equalizers.



Some additional technical references:

Here is an excerpt for an article on the physics of acoustic guitars: Link to article is here

One of the overlooked mechanics when examining the physics of instruments is the effects of the air cavity on the sound. Just as the strings, face plate, and back plate all have different modes which help to bring out individual tones, so does the air inside the guitar itself. It is from this cavity that the Helmhotlz resonance comes, as it is the lowest frequency resonance of the air inside the guitar and is mostly dependent upon the volume of air, and the shape and size of the sound hole. From this resonance other internal resonances are compared. Still, with a guitar it is hard to just separate each individual part to understand it. The Heisenberg principle could even be considered to apply when looking at the mechanics of this instrument. Each time you try to isolate a single part of the instrument to study itís principals, you change the system. If you look just at the resonance of the cavity inside the guitar, it means stopping resonance of the back and face plate, which in turn can change the volume of the cavity, affecting it just as it affects them. This holds true for all aspects of the guitar. When these aspects are taken into affect, there are usually three strong resonance frequencies formed from patterns of simple harmonic motion of the face and back plates.

The following are excerpts from an article published in Electronic Musician, called Resonance and Radiation:
Link to the entire article is here

Like all string instruments, the acoustic guitar consists of a body, neck, and tensioned strings. Plucking the strings produces sound, but very little of this sound actually comes from the strings themselves. Instead, the vibrating strings transfer energy to the bridge and top plate of the guitar body, which in turn excites the air cavity within the instrument (as well as the sides and back). Most of the sound is radiated by the vibrating body and air moving through the sound hole.

Any enclosed space exhibits resonance, which is the tendency to vibrate easily at certain frequencies, and the guitar body and air cavity are no exceptions. Most acoustic guitars have three strong resonances in the 100 Hz to 200 Hz range. The lowest resonance results from the soundboard (top) and back plate (bottom) vibrating in opposite directions, causing the guitar to "breathe" air in and out of the sound hole. On a Martin D-28 used in experimental measurements, this resonance occurs at 102 Hz.

The other two resonances occur slightly above and below 200 Hz. On the D-28, the resonance at 193 Hz results from the soundboard and back plate vibrating in the same direction, while the resonance at 204 Hz occurs as they vibrate in opposite directions.

Higher resonances occur as the soundboard and back plate vibrate in more complicated patterns. For example, they can vibrate in halves—that is, when the right half is bending outward, the left half is bending inward. This way, the soundboard and back plate can vibrate in-phase or out-of-phase with each other, resulting in a resonance around 300 Hz in classical guitars and around 400 Hz in folk guitars.

Although it's difficult to visualize, all these vibrations and resonances occur simultaneously and radiate from the instrument in different ways (see Fig. 1). The two lowest resonances radiate in a circular pattern, while the third resonance radiates in a figure-eight pattern. The highest resonance exhibits a more complex radiation pattern. Knowing where these patterns occur can be very useful when placing one or more mics to emphasize or reduce the higher resonances in the total sound.

Strings such as the violin, viola, cello, and contrabass can be plucked like a guitar (a performance technique called pizzicato), but most of the time, they are bowed. Most of the radiated sound comes from the body, rather than the strings.

These instruments also consist of top and bottom plates and side walls. Instead of the extensive internal bracing of acoustic guitars, however, strings have a single piece of wood (called the bass bar) attached to the top plate, parallel to the lowest string, in addition to a wooden peg (called the sound post) in the middle of the body. The bass bar and sound post help control the resonances of the instrument.

Unfortunately, the specific resonant frequencies vary widely from one instrument to the next, so it's difficult to provide hard numbers. The lowest resonance of importance is called the air resonance, which results from the vibration of the air within the instrument. In a violin, the best possible tone is obtained when the air resonance is about the same frequency as the D string (approximately 204 Hz).

The lowest resonance of the body itself is called the wood resonance. In a violin, the best tone is produced when the wood resonance coincides with the frequency of the A string (440 Hz). A second wood resonance, called wood prime, occurs one octave below the main wood resonance because of the interaction of the main wood resonance and the second harmonic of the G string. All of these resonances are transposed accordingly for the other members of the bowed string family.

A particularly troublesome acoustical phenomenon in bowed string instruments — especially the cello — is called the wolf tone. In all stringed instruments, each string and the body are separate, but connected, resonant systems. If any string and the body have the same resonant frequency when you bow the string, two frequencies appear, one above the normal resonance and the other below. These frequencies interfere with each other, causing an ugly beating sound, which ancient musicians likened to the howling of wolves. The wolf tone occurs when a string vibrates at the same frequency as the main wood resonance. If this frequency lies between two half-steps in the scale, it shouldn't cause a problem. If it lies close to a note in the scale, however, it can spell disaster. There is nothing much an engineer can do, except hope that the instruments being recorded don't howl when important notes are played. Interestingly, the radiation pattern from the bowed strings depends on the frequency being played.

When I was studying physics in college, I found that my ability to play trombone was enhanced by learning about the acoustics of the instrument. I wasn't necessarily thinking about formant frequencies and radiation patterns as I played, but that background knowledge seemed to make its way to my lungs, lips, and hands in an unconscious way, and my playing improved. I believe the same holds true for recording engineers who have learned how to place microphones and apply EQ to different instruments. Empirical knowledge works well, but it can be greatly enhanced with some theoretical information to support it. With this in mind, I hope your skills improve as you learn more about the fundamental acoustics of the instruments you record.