Lesser Known Facts About The Common C-ducer
Miking applications for this novel transducer that will turn your electric bass 'acoustic'!
No, this is not a trip into the world of ornithology, but a closer look into a few unnatural applications of the C-ducer contact microphone. Would you believe picking up solid electric guitar from its body or miking speakers directly from their cabinets? How about special underwater effects? Read on...
The C-ducer's excellent response to all modes of vibration and its large sensing area make it highly effective for a variety of uses which will be examined later, uses which have little to do with the successful role it already plays in amplifying conventional acoustic instruments.
A brief description should first be given on how this novel transducer operates.
The transducer itself is less than 1mm thick, flexible and about ¾" wide. It comes in 3" or 8" lengths and the signal taken to a variety of preamp configurations via a thin screened cable which emerges from the plastic block at one end. In normal use the transducer simply adheres to stringed instrument sounding boards including the inside of drum shells. Now the construction within reveals a radically different approach to other known methods of sensing vibration by contact.
Inside the thin PVC sheath lurks the 'element' which is made up of a pair of coaxial foils acting as a sensing capacitor (not unlike the diaphragm of a capacitor microphone). It is essentially the combination of the vibration of this capacitive 'sandwich' with some piezoelectric enhancement from the polymer materials used in insulating the foils (see Figure 1) that gives the C-ducer its remarkably flat response throughout the audio frequency spectrum and exceptional transient headroom. So much for the animal. Now to some interesting applications.
Fundamentally, solid electric guitars only come to life when plugged into amplifiers or DI'd into mixing consoles. The signal is normally created at the points where string vibrations are sensed by built-in magnetic pickups. Any sympathetic vibrations of the instrument body play little part in shaping the final sound. Situating single coil or humbucking pickups at chosen points under the strings then switching between them, adding and reverse phasing, provide a fairly wide variety of electric sound. What is not always discernible to everyone, however, is how much these solid instruments do vibrate, particularly at the headstock.
Intrinsically, they are not designed to mechanically amplify their own sound, nonetheless many bass guitarists, for example, can readily tune their instruments in noisy surroundings with power off simply by sensing pitch through their clothing, demonstrating a certain acoustic radiation. Previous attempts in 'tapping off this information and amplifying it to usable levels has, with few exceptions, been fraught with problems (as anyone who has tried using piezo-electric 'bugs' or ultra-close miking techniques may recall!).
But why even contemplate alternative ways of making an electric guitar sound? They already sound OK don't they? Well, yes they do; a well made, well set up bass guitar with good pickups will sound just as it should - electric. The answer is simple. They can now sound acoustic, taking on a totally new character and opening up some very interesting possibilities. The sound within the solid itself is quite different to that from a string/magnetic pickup relationship, as it embodies many of the characteristics that go to make up the sound of acoustic guitars. Basic design, materials used, type of strings, fingering technique all interact and respond in a totally acoustic fashion. What is now required is to pick up this sound with sufficient level at a convenient point on the instrument.
Figure 2 shows a 3" C-ducer attached to the headstock of an electric guitar (it merely sticks on). This position has been found ideal for most instruments as a 'collection point' for the overall body sound. Alternative positions could be within the body itself, say inside the recess for tone controls etc. The head-stock position works remarkably well on fretless electric bass for that smooth acoustic bass texture, complete with fingerboard sound and authentic 'growl'. This technique is not expected to instantly turn every solid into an acoustic though - a great deal will depend on the richness of sound produced within the body and neck of differently constructed instruments. Nevertheless, here's a unique method of fooling a solid into thinking it's hollow!
With two discrete sound sources now available from a single instrument, a very real stereo effect can be created using the output from the C-ducer and that of the magnetic pickups. The imagery is excellent, both signals being distinctly individual; one side contains much 'acoustic' information that the other side lacks but makes up for in 'electric' information.
The arrangement works well on stage using a pair of amplifiers. 'Heavy Metal' volumes are out of the question though, because the C-ducer will eventually feed back. Available levels for electric playing are still surprisingly high, the stiffness of the guitar neck delaying the onset of sympathetic feedback. A word of caution: solid instruments with C-ducer attached need gentle handling. Any shocks or bumps received will be faithfully reproduced with the music!
The purpose of miking up amplifier speaker cabinets is to include their individual character in the mix rather than simply DI'ing electric and electronic instruments. This is traditionally achieved by simply mounting a microphone facing the speaker aperture. Figure 3 shows an 8" C-ducer stuck diagonally across a speaker grille; another method of doing the same job but having some major advantages.
Although designed essentially as a contact transducer, the C-ducer also works remarkably well as a close miking device. The derived signal from the cabinet is smooth, undistorted and ambience-free. It is much less prone to feedback in live use and presents a neat way of reducing cable clutter whilst freeing conventional mics to be used elsewhere. Several major bands have built C-ducers into their backline amps by mounting the transducer behind the speaker grille and the preamp inside the cabinet. The output from the preamp is then hardwired to a jack or XLR socket on the cabinet. Hey presto, plug-in enclosure sound!
As the transducer provides such a ready means of tapping off good acoustic sound with extraneous unwanted information leaking in, samples can be easily dumped into digital memory. One Fairlight user went as far as storing scales of Chinese gong, creaking door and flushing loo!
Another fairly obvious use of the C-ducer is to trigger microphone gates, an especially useful technique on the drum kit to give almost total separation between toms, bass and hi-hat. The transducers are mounted on the heads themselves and Gaffa tape added to achieve the required damping. Results can be quite stunning since a complete freedom of processing is allowed in a way that is usually restricted purely to electronic drums.
Without doubt the C-ducer is becoming a very useful tool to recordists and sound engineers, and doing a fine job on acoustic instruments in general (the Company's user list it is reported is taking on the appearance of a Who's Who in the music industry). But the system also invites many more applications than first envisaged, some rather bizarre. A while ago a BBC outside broadcast unit called up for urgent replacements as their underwater recording of dolphins had gone horribly wrong. Apparently, dangling C-ducers are irresistable to these creatures as all were promptly eaten! Ah, well, nothing's perfect...