Understanding Driver Specifications

A discussion of dynamic drivers and Theil-Small parameters

Dynamic Drivers This is the oldest type of driver, utilizing technology 80 to 90 years old. Although it has improved over the years, its principle is still the same. A typical Dynamic Driver employs a conical diaphragm via the interaction of a time-varying magnetic field generated by an electromagnet and a static field set up by a permanent magnet. The time-varying field is setup by a voice coil, an electromagnet driven by the output of the power amplifier. The magnetic field setup by the voice coil varies in step both in amplitude and polarity with the audio-frequency current supplied by the power amplifier. Alternate repulsion and attraction between the two magnets cause the cone ( commonly made of polypropylene or paper), which is attached to the voice coil and supporting structure, to compress and rarefy the air depending on its motion relative to the internal and external air masses.
When the cone moves forward, it compresses the air in front of it and rarefies the air behind it and vice-versa. It therefore can be seen, that the driver is a dipole radiator, i.e. generating two out-of-phase acoustic signal at the same time. At low frequencies, these two signals will meet while still in an out-of-phase condition and cancel each other out. To prevent this destructive interaction through design, either the "front wave" or the "back wave" will have to be phase-shifted before it meets up with its counter-part. In another approach, one wave front must be attenuated or otherwise prevented from reaching the other.
Whichever alternative is chosen, this is usually done by the design of the speaker enclosure which is used to isolate/attenuate the "back wave" from the "front wave"...these will be discussed later.
Ideally, the dynamic driver behaves like a rigid piston over its entire operating frequency range. In practice, a dynamic driver cannot provide this ideal over the entire audible spectrum. Its useful bandwidth is only limited to those frequencies whose wavelengths are large compared to the diaphragm's physical dimensions. Above these frequencies, the driver begins to "beam" (becomes directional) due to diffraction effects.... i.e. above a certain frequency, standing waves begin to appear on the cone's surface, other air pressure anomolys such as severe cone breakup occurs, and the cone's surface is covered with loops and nodes and only a little sound is radiated. Not only do these effects make the driver more directional as the operating frequency is increased, but they also cause fluctuations in its frequency response. If the dimensions of the driver are kept small to enhance the mids and highs, it will not be able to move enough air to provide substantial acoustic output at the low frequencies required for HIFI (High Fidelity) reproduction.
No conventional dynamic driver can single-handedly cover the full range of audible frequencies. This has led to the development of specialized dynamic drivers, each designed to handle a given portion of the audio spectrum. A typical speaker system will thus consist of all dynamic drivers like the woofer, a mid-range driver and a tweeter. Some systems even employ four or more drivers, including a super-tweeter handling the extreme highs and/or a sub-woofer reproducing the deepest bass notes. For design purposes, it is often desirable to employ drivers with useful frequencies as large as possible.

The Dynamic Woofer To improve the performance of the dynamic woofer, development of new and better materials for the cone has begun to replace the paper cone traditionally used. The ideal material should be light-weight (for efficiency and good transient response) and very stiff (for good/extended frequency response). Usual materials used today are paper, special plastics, aluminum and even paper doped in special material coating. High compliance suspension systems also allow more efficient bass reproduction.

The Dynamic Midrange This is a driver that can utilize either a 1.5 to 3"dome or a 2 to 5" cone to acheive it's purpose. The frame in which the radiator is mounted is usually of a closed back design to prevent interaction with the bass wave inside the enclosure in which it is usually mounted. Excursion of the lightweight cone or dome is limited by a relatively stiff suspension because the production of midrange sound in the area between 400 to 5000 Hertz, does not require the long wavelengths associated with the lower fequencies produced by the woofer. Power handling is usually rated lower as well because an isolating crossover is used to couple the speaker to the amplifier. This permits higher power to be sent to the woofer, while the midrange and tweeter, being isolated from the direct ouput, may use drivers rated much lower in RMS power capability.

The Dynamic Dome Tweeter This is a high-frequency driver with a tightly suspended dome-shaped diaphragm. This type of driver is very popular. The 1" to 1/2 " dome is made of light-weight material allowing it to be an efficient radiator. Materials used for the dome are usually of mylar-type plastics, polystyrenes and treated fabrics like beryllium. Most of these domes radiate or disperse their sound over a fairly wide area, enabling excellant sound stage chactersitics. Nearly all can effectively radiate sounds reaching to the upper frequency limit of human sensitivity.