A Sound wave can be described by two basic characteristics, the speed of the vibration (Frequency), and the extent of the vibration (Amplitude).

Our ears have a tremendous ability to evaluate sounds that range from very soft to very loud. The decibel (dB), named after Alexander Graham Bell, is a method of describing acoustic pressure, without having to deal with the billion-fold range of sound pressures to which our ears are sensitive. A Bel is the difference in loudness produced by a ten fold increase in power.

By comparing arrival times at both ears, our brain can determine the direction from which a sound came from. A sound arriving at both ears simultaneously tells our brain that the sound is centered either in front, above, below, or behind us. The brain then looks at the frequencies received, and compares these frequencies with sounds that we have heard before. When the brain remembers a sound like the sound it just heard, it looks at the late soundwaves that arrived at the eardrum that were reflected off the outer ear. This process allows us to perceive height. When a soundwave comes from behind, the soundwave must pass through the pinna. This causes a filtering of high frequencies in comparison with reflections off of walls, ceilings, and floors. This is what allows us to localize sound sources behind us.

For current to flow, there must be a potential difference between two objects. This potential difference is created from an excess of electrons at the source, and a lack of electrons, or positive charge, at the destination. Using our hose analogy, the potential difference, measured in volts, would be the pressure of the water within the hose. The higher the pressure, the more potential difference, and the higher the voltage.

A capacitor is a device that resists change in voltage. It stores a charge. For our uses it will pass AC but not DC (while it is charging it will pass DC). Their value is expressed in Farads (or more commonly in microfarads). They have a working voltage (called DCWV) that must never be exceeded. They can be found in both polarized and nonpolarized versions. The most commonly used types are the electrolytic, mylar and poly (polypropylene, polystyrene, polyester). They are used primarily for filtration (in power supplies), noise suppression, and speaker crossovers.

An inductor is a device which resists change in current flow. Usually it has a low DC resistance but a high AC impedance. This means that it passes DC well, but does not pass AC easily. It is electrically opposite to the capacitor. Its value is expressed in Henries (more commonly milliHenries). There are two major types of inductors, air core and iron core. The other important factor is the gauge of wire that is used to make the inductor. The heavier the gauge, the less signal loss and the higher the current capability. For a given value and wire gauge, air core inductors are more expensive, but they do offer some benefits. The greater the current flowing through an inductor, the larger the magnetic field produced. With iron core inductors, there is a point where the core saturates and no greater field may be produced. At this point, severe distortion sets in. Air core inductors for the most part do not exhibit saturation.

A diode (or rectifier) is a semiconductor which allows current to flow in only one direction. It can be used to change (rectify) AC into DC. The two leads of a diode are labeled anode and cathode. A diode is said to be forward biased when its cathode is at least .7 volt more negative than its anode. A diode is reverse biased when its cathode is not at least .7 volt more negative than its anode. Forward biasing a diode causes current to flow, and reverse biasing causes no current to flow. Although there are numerous types of diodes, the ones of interest to the mobile electronics installer are power diodes, or rectifiers. A rectifier’s value is expressed in amps, and is the maximum forward current which the device can safely handle.

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.

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.

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.