FIG. 1: A theoretically perfect transducer would have uniform response across all -frequencies at all amplitudes. The equal -loudness curves show how our personal -transducers, our ears, deviate from that ideal.
As the term implies, an electronic musician depends on electrical devices to produce, capture, manipulate, and deliver musical ideas. At some point, the electric signals that electronic musicians create must be transformed into acoustic energy that then excites, tickles, or assaults the ears of listeners — hopefully producing the desired emotional effects. Similarly, if you want to record an acoustic instrument or voice, you need to transform acoustic energy into electric energy.
Transforming between acoustic and electric energy is the job of a transducer. Several types of transducers are used in producing any given musical work. In this article, I'll look at transducers, their various types, their functions in the musical process, and the factors that determine their quality.
As noted, a transducer converts energy from one form to another. Think of the various forms of energy that musicians deal with — electricity, acoustics, mechanics, light, magnetism — and you will see the potential uses of a transducer.
Just making a field recording might require three types of transducers. A microphone converts acoustic energy into electricity, a tape head converts electricity into magnetism, and headphones convert electricity to acoustic energy so we can monitor the recording. Playing a CD requires a laser pickup to convert the light energy of the laser reflecting off the pits and lands into electricity. And your inner ear has specialized hair cells called cilia, which convert the mechanical energy of the vibrating eardrum into electrical energy that is transmitted to your brain via the auditory nerve.
Truth or Consequences
The ideal transducer would produce an output signal that has characteristics identical to those of the input signal. In other words, the result should be a perfect analog of the impetus. A transducer is judged by how near it comes to that ideal.
A transducer must be sensitive to a wide range of intensities and frequencies. A perfect transducer would have a flat response, producing the same output-to-input ratio at all frequencies and at all input levels. Our ears, by contrast, are much more sensitive to frequencies between 1 and 6 kHz, and their frequency response changes with volume (see Fig. 1).
A perfect transducer adds no noise and does not distort the signal. A transducer's response is said to be linear (undistorted) when its output level is directly proportional to the input level so that, for example, a doubling of input level results in a doubling of output level.
The speed of a transducer's response to a stimulus is important in preserving the attack and release of a sound. A large mechanical transducer such as a loudspeaker must therefore be sufficiently powered to give it quick transient response and properly damped to counteract its momentum.
Always having perfect transducers, however, is not only unrealistic but often undesirable. That pretty presence peak in your favorite vocal microphone is a technical imperfection, but it may be the perfect choice artistically. Similarly, the revered warmth of analog tape is actually a type of distortion.
Many of the transducers that we depend on function as a result of the interaction of three properties: electricity, magnetism, and motion. Current flowing through a wire, especially a coiled wire, creates a magnetic field. Conversely, changing the magnetic field around a wire produces electric current in the wire. Two magnets physically repel or attract each other depending on the relative orientation of their poles. Therefore, if a wire and a magnet are brought into close proximity, a change in any one of those properties while a second remains constant will induce a proportional change in the third.
FIG. 2: When a wire is moved within a magnetic field, current is produced in the wire that corresponds to the direction and magnitude of the motion.
Fig. 2 illustrates that principle. A wire is positioned between the poles of a magnet. If the wire (or the magnet) is moved, electric current flows through the wire relative to the magnitude and direction of the motion. If instead a current is introduced into the wire, a magnetic force is created that pushes and pulls against the magnet, causing either the wire or the magnet to move. Varying the properties of the magnet will induce either current or motion in the wire.
With dynamic microphones, a coil of wire is attached to a thin membrane that is called a diaphragm and suspended between the poles of a magnet. A singer shouts into the microphone, creating changes in air pressure that move the diaphragm back and forth, causing the coil to oscillate within the magnetic field. A corresponding voltage is created in the wire and subsequently amplified through the sound system to the delight of screaming fans.
A ribbon microphone suspends a thin strip of metal in a magnetic field. A cello, for example, creates fluctuations in air pressure that move the ribbon within the field, inducing current in the ribbon itself. That signal is then carried along a wire to a recording device that captures the performance for posterity.
Speakers depend on the same principles as microphones, but the flow of energy is reversed. Replace the diaphragm of a dynamic microphone with a speaker cone, apply voltage to the coil, and the current flowing through the wire within the magnetic field causes the coil-cone assembly to move, pushing on the surrounding air to create acoustic energy. Similarly, ribbon speakers reverse the behavior of a ribbon microphone.
With tape recorders, electricity flowing through a wire wrapped around a metal core creates an oscillating magnetic field that realigns the magnetic domains of the tape's emulsion. In playback, the variations in orientation of the domains create corresponding variations in the magnetization of the metal core, which induces a corresponding current in the wire.
Try This Instead
Condenser, or capacitor, microphones have a diaphragm that is suspended close to a stationary backplate. Connecting the diaphragm and the backplate to an opposite lead of a voltage source (either a battery or phantom power from the mic preamp) turns the assembly into a capacitor. The vibration of the diaphragm in response to acoustic energy changes its spacing relative to the backplate, changing the assembly's capacitance and thereby creating a voltage proportional to the vibration.
Piezoelectric pickups depend on the characteristic of certain crystals and ceramics to produce voltage in response to physical pressure. The deformation of the crystal caused by acoustic energy disrupts the internal electrical balance of the material, and voltage results in proportion to the deformation. Conversely, electricity applied to piezoelectric crystal causes physical deformation — the speaker in your watch or phone may take advantage of that to vibrate the surrounding air. Phonograph cartridges were originally piezoelectric devices (ceramic cartridges), but nowadays they are ordinarily electro-mechanical transducers (magnetic cartridges), functioning like a dynamic microphone that tracks the undulating record groove instead of air pressure.
The light-emitting diode (LED), which converts electricity into light, is a transducer with surprising utility for musicians. With a classic opto-compressor, the detection circuit is a light source (the LED) whose intensity follows the signal voltage. The light controls the behavior of a light-dependent resistor (LDR), which attenuates the output voltage. The slew rate, or response time, of the LED and LDR determine the attack and release characteristics of the compressor. Such compressors are sought after for the unique character of their response.
Similar use of light to control musical signals is found in the vibrato circuit of a Rhodes Suitcase piano and in some oscillator and filter designs. It is even the basis for the optical isolation of a MIDI In jack, which prevents ground loops in a daisy chain.
Musical creativity is all about transforming an abstract idea into an audible work. Until we can make that transformation directly, we will continue to depend on all sorts of transducers to help us with the process.
Brian Smithers is Course Director of Audio Workstations at Full Sail Real World Education and the author of Sonar 5 Ignite! (Thomson Learning, 2005).