Lesson 2: Signal Sources

Function of a Transducer

Sound comes to us in the form of vibrations. An object at rest can be considered to be at the "zero" position. If the object (eg, piano string, drum head) is struck it will move away from the zero position eventually reaching a maximum (lets say in the positive direction) and returning to the zero position. As we all know, it doesn't stop there. It's momentum carries it to almost the maximum in the opposite direction, and back to the zero, and so the cycle continues.

This vibration is then converted into an electrical signal by a "transducer"(eg microphone, guitar pickup) and that signal can now be amplified, recorded, processed, transmitted etc. This lesson we will be looking at the different types of transducers and their characteristics. We will categorise them by method of operation.

There are 4 main categories of transducers used in the industry today: dynamic, condenser, crystal, and electronic. We will now take a look at the first one.

To understand Dynamic transducers you need to know about magnets and their influence on coils of wire. We are all familiar from primary school science about the lines of force that surround magnets. If the magnet is moved near a coil of wire an electric current begins to flow in that wire, but only as the magnetic field is changing. Once the magnet stops moving, the electric current also stops. If the magnet is then moved away from the coil a current then begins to flow in the opposite direction until the magnet is far enough away that the magnetic field no longer has influence on the coil.

This process works the same if the magnet moves (magnetic cartridge for playing vinyl records), or if the coil moves (dynamic microphone), or if a third element (eg, a steel guitar string) is introduced into the field while the magnet and coil are in close proximity. Studio Mics

The second type of transducer we will look at is called a "Condenser", and it works on static electric charges. You need to read the section about "capacitors" first to help you understand this section. When a capacitor is connected across a battery, electricity flows quickly to charge it up. At first it is a sudden high current, but gradually as the static charge builds up in the capacitor the flow gradually decreases. If the plates are moved closer together the capacity is increased so more electrons rush in to create the higher charge. If the plates are moved apart the capacity for static charge reduces so the excess electrons flow away from the plate. This in fact is the principle of the condenser microphone. One plate is stationary and the other forms the soft diaphram, which moves back and forward under the influence of the sound vibrations.As the diaphram moves in and out influenced by the sound wave, the electric current produced is a direct copy of the shape of the sound wave. This is again explained as an analogue signal in our Analogue and Digital Audio section. Most big studio microphones are condensers.

The third type of transducer is based on a crystal of quartz or Rochelle salt. The principle is given the name the piezoelectric effect. These crystals emit electric current as a direct result of varying pressure (which might be vibrations from an acoustic guitar string) applied to it. Some specialist microphones also use this principle.

Chrystal Transducer

The fouth type of transducer differs from the others in that it doesn't sense physical vibrations and convert them to electrical signals, instead it creates the electronic signal itself under the influence of a predetermined set of conditions. These sources are simply described as "Electronic", and the predetermined conditions could be computer programming (eg, mp3 player), grooves in a vinyl record, magnetic fluctuations on a tape, reflective indents in a laser disc (CD, DVD), or even a physical combination of resisters under an analogue synthesizer keyboard.

The question is, "Why do we need to know about all these transducers", and the answer is, "because they all have different characteristics, and need different treatments in a sound system". Below is a chart of 4 transducer types and their characteristics. These figures are approximate and can vary depending on individual models, and the strength of the vibration thats activating it.

Transducer TypeCommon UsesOutout VoltageOutput Impedance (See note below)
DynamicVocal/Instrument Microphoneless than 25mVBalancad:200-600k ohm Unbalanced:10-50kOhm
Guitar/Bass Pickup50-100mV10-50kOhm
Magnetic Record Cartridgeless than 25mV10-50kOhm
CondenserMicrophoneless than 25mVBalancad:200-600k ohm Unbalanced:10-50kOhm
CrystalGuitar Pickup100mV10-50kOhm
Electronic
CD/DVD, Wireless Recievers, (in other words, small home stereo type equipment)
150 - 500mVUnbalanced:10-50kOhm
Professional quality Mixers, Preamplifiers, etc500-1000mVBalancad:200-600k ohm
Keyboards & Synthesizers50/150/500mV (switchable)Balancad:200-600k ohm Unbalanced:10-50kOhm

Having said all that, I should point out that most high quality equipment operates at one of 3 levels (mic level, mic+pad, and line level), and nearly always operates balanced to reduce noise interferance, and improve impedance matching. To this end, many sources will contain preamplifiers, tone controls and transformers to prepare the sources output conditions to match the standardized inputs on the professional mixers etc.

I hope you enjoyed this lesson, and I look forward to seeing you in the next. Click here to go to the lesson about Mixers, or Click here to return to the Stage & Studio Equipment index page.