Ken Ellis Luthier
© Whippoorwill Acoustics LLC, 2015-2020
Choosing a Pickup for Your Autoharp Ken Ellis In this article we'll compare the two types of pickups and look at how the placement of a pickup on an autoharp affects the sound you get. Choosing a Pickup There are two types of pickups, magnetic and piezo-electric. The magnetic pickup consists of a wire coil and a magnet. When a steel string moves through the magnetic field of the magnet, it causes the field to change. This changing field generates a current in the wire coil, a phenomenon known as Faraday's Law. When the steel string is oscillating, the current in the wire oscillates at the same frequency as the string, and can be amplified and sent to a speaker. The advantages of using a magnetic pickup are: 1) When properly installed, they can eliminate many of those annoying string harmonics in the bass octaves. I'll explain how this works below. 2) You can plug in a tuner and tune in even the noisiest environments, since it is insensitive to vibrations of the box (thanks to Oscar Harris for this tip). The disadvantages of a magnetic pickup are: 1) Because the pickup is insensitive to box vibrations, it does not reproduce the acoustic sound of the autoharp. So you lose much of the characteristic autoharp sound. 2) Magnetic pickups are real good at picking up a 60Hz hum from nearby fluorescent lights and are susceptible to interference from other sources of electromagnetic radiation. The piezo-electric pickup is made from a crystal that generates an electrical signal when it is mechanically disturbed. Usually attached to the top or back of an instrument, it produces a signal proportional to the vibrations of the attached surface. Thus it is ideal for capturing the vibrations of the autoharp box. Note that neither piezo nor magnetic pickups measure the vibrations of the plug of air in the sound hole. However, some top or back vibrations will correlate with the air plug vibrations, since the back and/or top drive them. The advantages of piezo-electric pickups are: 1) When properly installed, they can accurately reproduce the acoustical sound of an autoharp. 2) They are insensitive to electromagnetic interference and the 60 Hz hum. The disadvantages are: 1) Finding the right place to install the pickups can be difficult. Poor placement will give poor sound reproduction. However, I have solved this problem. 2) Piezos require a high-impedance amplifier input in order for the full sound to come through. If your amplifier doesn't have a high impedance input, you will need a preamplifier that can cost several hundred dollars. Piezo-electric pickups are known for introducing a type of distortion called “quack”. Tech support at K & K Sound, a manufacturer of high quality piezo pickups, assures me that only under-the-saddle guitar pickups will “quack”, not those which would be used on autoharps. Which type of pickup is best for you? Well, it all depends on your priorities. Magnetic Pickups A magnetic pickup primarily captures the vibrations of the autoharp string, not the vibrations of the autoharp box. A little of the box vibrations can be captured, however, as follows. Newton's Third Law tells us that for every action there is an equal and opposite reaction. Applied to autoharp strings, this means that if the strings make the box vibrate, the box vibrations will also make the strings vibrate. So some of the sound from the box will feedback into the strings, allowing it to be picked up by the pickup. However, this is only a small fraction of the total sound, so with magnetic pickups you mostly just hear the strings and not so much of the characteristic autoharp sound. One of the things that strings do when plucked is generate harmonics. These harmonics can ring out even when a string is damped by the felt, causing dissonance. As I said before, if a magnetic pickup is carefully placed, it can minimize if not eliminate these undesirable harmonics. Allow me to explain. When a string vibrates, it vibrates at many frequencies simultaneously. The lowest of these frequencies is the “fundamental” and is the frequency of the note to which the string is tuned. The higher frequencies are “harmonics”, or “partials”. For an ideal string, the string length is equal to one-half of a wavelength of the fundamental string frequency. The string length is equal to an integer number of half-wavelengths of the harmonic string frequencies. At half-wavelength intervals from the end of the string, the harmonics have what we call a “node”. This is a point on the string that does not vibrate at all at the harmonic frequency. Figure 1 shows the nominal node locations for the first three harmonics on an autoharp. You can easily hear one of these harmonics by lightly touching a string at a node with one finger and plucking with another finger. By touching the string at a node, you are damping out the fundamental and all harmonics except those which have a node at that point. You get a clear, bell-like tone, not as loud as the fundamental would be, but loud enough to easily hear it. The first problem with harmonics is this: if a felt lands on a string node, it will not damp out the harmonics associated with that node. So you hear a wrong note, perhaps an octave or an octave and a fifth above the fundamental of that string, but a note outside of the desired chord. The second problem with harmonics is this: no matter how carefully you design an autoharp, you can't avoid landing a felt on all of them. Figure 1. Approximate string node locations for the first (green), second (blue), and third (red) harmonics. Most chord bar assemblies are positioned so that the first harmonic nodes of the bass strings are above the chord bars, avoiding them. The second and third harmonic nodes, however, will fall under the chord bars, as will the treble string nodes. Fortunately, the second and third harmonics are not as loud as the first harmonics. But the second harmonics can still be problematic. Now here's the neat thing about a magnetic pickup: if it is placed under a node, it will get no signal from the associated harmonic, because the string is not vibrating at that harmonic's frequency at the node! On the other hand, if the pickup is placed halfway between two string nodes, it will accentuate the associated harmonic. So you want to be careful. You might well ask, “Ken, that all sounds very hypothetical to me, but does it really work in practice?” A good question. To the Acoustics Lab! So I just happened to be installing a pickup while writing this article, and had a chance to test this hypothesis. With the pickup under the second harmonic nodes of the bass strings, I plucked a string while lightly touching the node over the pickup. First results were confusing; the harmonic still sounded through the amplifier. However, I soon realized that I was not amplifying the harmonic of the plucked string, but was hearing sympathetic vibrations from other strings that were stimulated by the harmonic. By damping all the strings but the one being plucked, the harmonic no longer came through the amplifier, as expected. Check this out: the strings that ring in sympathy with a harmonic are strings that will be damped if the harmonic is outside of the desired chord. If these strings are not damped, then the harmonic tone is one of the chord notes and you won't care that it rings. Either way, it's not a problem. Piezo-electric Pickups The trick with piezos is finding the right place to put them. With a large instrument like a guitar, you have easy access to the inside and so can play the instrument with the pickups taped in different locations and then glue them inside when the best spot(s) are found. Also, lots of people have installed piezos on guitars, so they can tell you where to start looking for that “sweet spot”. With an autoharp there just isn't enough room to reach inside the box. So piezos generally appear on the outside of an autoharp, along with wires and a jack. Not a pretty sight, and something might get snagged on something. There is a nice alternative. Rather than guessing and getting lucky, it is possible to get an amplified sound that sounds just like the acoustical sound using a systematic process that maps out the vibrational modes of the back of an autoharp. By placing one sensor of a twin piezo pickup so that it measures mostly low frequency modes and the other sensor so that it measures mostly high frequency modes, you can combine the signal from the two sensors in varying amounts to dial in a specific sound that you want, including the live acoustic sound. Tests show that when only one sensor is used, the amplified sound is very mellow with a lot of bass, similar to what you hear from the neck pickup of an electric guitar. When only the other sensor was used, the sound is very bright and trebly, similar to what you hear from the bridge pickup of an electric guitar. This suggests the possibility of wiring autoharp piezo pickups like that of an electric guitar in order to obtain a similar variety of sounds. Other than the fact that this process does indeed do an excellent job of capturing the acoustic sound, there is something very interesting about it. Neither sensor by itself produced a sound that you would think came from an autoharp. It was only when I combined the two signals in equal amounts that the characteristic autoharp sound came out!