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Speakers

The purpose of a loudspeaker is to move air. The most common system to obtain this goal is the electrodynamic driver. All electrodynamic drivers are based on the same concpt; A diaphragm (cone) is set in motion by an electromechanical motor system.


Components

An electrodynamic driver is composed of 4 systems:
  1. Motor - Voice Coil, Pole Piece, Front Plate, Back Plate, Magnet
  2. Diaphragm - Cone, Dustcap
  3. Suspension - Spider, Surround
  4. Basket/Frame
The speaker gets its motion from a motor system which consists of a magnetic circuit and a voice coil. The voice coil gap is created by the small air space between the top edge of the pole piece and the inside edge of the front plate.

Centered within this gap is the voice coil. The voice coil is thin strands of wire wrapped around a voice coil former. The former acts as a support for the voice coil wires, as well as aiding in the thermal transfer from the coil to the pole piece. When a current is passed through the voice coil, the magnetic forces created in the voice coil force the coil to react against the magnetic force in the voice coil gap. The direction of voice coil travel is dependent upon the direction of current flow.

The inside edge of the speaker cone is attached to the voice coil former. The cone is the part of the speaker that physically moves the air. The shape, weight and strength of the cone relate directly to the frequency response of the speaker.

Covering the voice coil assembly and attached to the speaker cone is the dust cap. This cap exists to keep foreign particles from entering the voice coil area and causing a failure. The shape, weight and strength of the dust cap also relate to the frequency response of the speaker.

The spider is attached to the voice coil former and the basket. The spider acts as a centering device and a restoring force for the voice coil.

The surround, which acts as an air seal between the cone and the basket, adds to the restoring force of the spider. Another function of the surround, is to absorb cone flexure waves as they are transferred up the cone.

The basket is usually made of stamped steel or cast aluminum. Although it does not directly affect the sound of the speaker, it does play a critical role in aligning the voice coil and the magnetic circuit. Speakers with large magnet structures sometimes require the use of cast aluminum baskets. Care should be taken when mounting a speaker so that the basket is not bent, or a rubbing of the voice coil may result, causing failure.

Diagram 1 - Speaker Components

Speaker Components



Theile/Small

Research done in the study of loudspeakers has been conducted by a number of individuals. The works of Neville Thiele and Richard Small are considered to have the most impact on the loudspeaker design field.

A method was found, so that one could predict the frequency response performance of a loudspeaker system, based on its physical characteristics.

The Physical Characteristics of a speaker are:
Re: The D.C. resistance of the voice coil measured in Ohms.
Sd: The surface area of the speaker's cone.
BL: The magnetic strength of the motor structure.
Mms: The total moving mass of the speaker including the small amount of air in front of and behind the cone.
Cms: The stiffness of the driver's suspension.
Rms: The losses due to the suspension.

By understanding the relationship of these physical parameters and how to change them, we may alter the response parameters to fit the desired goal.

The Thiele/Small Response parameters are:
Re: The D.C. resistance of the voice coil measured in Ohms.
Sd: The surface area of the speaker.
Fs: The resonant frequency of the speaker.
Qes: The electrical "Q" of the speaker.
Qms: The mechanical "Q" of the speaker.
Qts: The total "Q" of the speaker.
Vas: The volume of air having the same acoustic compliance as the speaker's suspension.

Understanding the response parameters allows us to calculate the predicted frequency response of a given speaker system. The formulas that accomplish this are rather lengthy and complex, and are best left to a computer. There are a number of high quality computer programs on the market that automate the design process of building an enclosure.


Excursion

How loud a speaker can play depends on how much air it can move without overheating. How much air can be moved is determined by the surface area of the cone and the excursion capability of the motor system.

Xmax is defined as the width of the voice coil that extends beyond the front plate (See Diagram 2). This relates to how far the speaker can move in either direction without appreciable distortion.

Diagram 2 - Xmax
Xmax

The amount of power required to move a speaker to its maximum excursion is referred to as the displacement limited power handling. Please note that this number varies with enclosure size and frequency.


Power Handling

Loudspeaker power handling ratings are one of the most commonly quoted, but most poorly understood of specifications given by loudspeaker manufacturers. It seems that every company has its own way of measuring and specifying power handling. That's because Marketing departments are always looking for ways to be able to list higher numbers for power handling in order to impress their customers with the apparent ruggedness of their products. It is sometimes difficult for product users to understand how these specifications relate to real world amplifiers or how they relate to the way they listen to their favorite kinds of music on loudspeaker systems. Loudspeakers fail in one of two ways - mechanically or thermally. Mechanical failures occur when one of the moving parts of the speaker such as the surround, spider, or cone become fatigued, tear, or break from the effects of continued long excursions. Thermal failures occur when the electrical power dissipated in the voice coil as heat causes the adhesives holding the turns of voice coil wire together to break down, or the insulation on the wire to fail, resulting in shorted turns. Also, the wire itself can melt, which means an open voice coil, or the coil support can melt or burn, again meaning failure of the loudspeaker.