AC / DC: Referring back to our water hose, the flow of water can be considered to be Direct Current (DC), meaning that current always flows in the same direction. If the valve that controls the volume of water flow could also change the direction of the flow, it would be called Alternating Current (AC). In general, audio signals are AC, and Battery Voltage is DC.

BL: (measured in Tesla meters) The product of the driver’s gap flux density and the length of the voice coil conductor in the gap.

Capacitor: 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.

For noise suppression, a capacitor is placed between the alternator, or component power lead, and ground. This will present an open circuit to the 12 volts DC once the cap is charged, but will provide a short circuit for the small AC “ripple”. Polarized electrolytics are most commonly used. They are much cheaper than the other types, especially in larger values. A minimum of 25 DCWV is needed for safe operation in a 12 volt system.

Another use for the capacitor is in speaker crossovers. Although they pass AC, their impedance is inversely proportional with frequency. Placed in series with a speaker, it forms a 6dB / octave high-pass filter. A nonpolarized capacitor must be used.

Electrolytic caps are the cheapest for their size. If using electrolytics for crossovers be sure that the DCWV is at least 100 volts for amplified systems. Using a cap with too small of a working voltage will usually cause the cap to explode. Mylar caps are a bit more expensive than electrolytics of the same value, but offer higher reliability and sonic improvement. Metalized film poly caps are very expensive, but offer the highest sonic quality in critical listening applications.

Current (Amperage): The movement of free electrons from one atom to another is called current flow. Current always flows from negative to positive. A good analogy can be made with a water hose. The hose may be considered a conductor, and the water moving electrons. The current, measured in amperes, would be the volume of water passing by a point along the hose in a given amount of time. The higher the volume, the more current.

Decibel (dB): The basic unit of measurement in electronic and acoustic work. It is a logarithmic scale to express the difference between two values. For simplicity, it can be regarded as a measure of relative loudness; for example, in frequency response measurements.

DC Resistance (Re): (measured in Ohms) The actual DC resistance of a speaker’s voice coil as measured with a standard volt/ohm meter. The reading will be lower than the speaker’s nominal impedance. A 4 ohm speaker will typically measure at 3.2 ohms, while an 8 ohm speaker will be about 6.4 ohms. Measuring resistance can be helpful when you are not sure of the speaker’s impedance.

Damping Factor: The ability of an amplifier to control the load without overhang. This applies to how tight the bass is. Usually the higher the number, the better, although it is debatable if anything over 50 is audible. Damping factor is calculated by dividing the load (speaker) impedance by the output impedance of the amplifier. Thus, a given amplifier’s damping factor will decrease as the speaker’s impedance decreases. This means an amp running at 4 ohms will provide tighter bass than at 2 ohms.

Driver Volume: The amount of enclosure airspace that is displaced by the speaker itself.

Effective Piston Area (Sd): (measured in square inches or meters) The active radiating area of a speaker cone, including the part of the surround which moves to produce acoustic output.

Electrons: Atoms are made up of three smaller particles; protons, electrons, and neutrons. Electrons have a unit negative charge, protons have a unit positive charge, and neutrons have no charge. Usually there are an equal number of protons and electrons in an atom yielding a net zero charge. When some kind of energy (electromagnetic, chemical, etc.) is applied, some of the electrons may be broken free, thus leaving an imbalance of protons and electrons in the atom. This will result in the atom having a net positive charge.

Enclosure Volume: The total amount of internal airspace of an enclosure. This includes the net, driver and vent volumes.

Speaker enclosures are designed with a size that optimizes the sound by providing the optimum volume for reproducing sound waves accurately and/or with the most SPL for a given driver / woofer.

Fb: (measured in Hz) The resonance frequency of the air in a port and the stiffness of the air in a ported system. Also known as the tuning frequency of a vented enclosure.

Fs or Fo: (measured in Hz) The frequency at which a speaker naturally resonates in free air.

F3: (measured in Hz) The frequency at which the acoustic power output from a system has fallen to one-half its reference value. Known as the systems 3dB down point.

Hertz (Hz): The unit of measurement for frequency. 1 Hz is equal to one cycle per second, or the complete cycle of an alternating wave form per second. Impedance: (measured in Ohms) The total opposition to the flow of alternating current in an electrical circuit. The measure of the magnitude of an electrical load when using alternating currents, such as in audio. It describes the combined effect of resistance, capacitance and inductance.

Imaging: 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.

It becomes increasingly difficult to localize sound sources as the frequency decreases. This is due to the wavelength being so much larger than our heads, that it becomes difficult to differentiate between the arrival times at our ears. This effect keeps us from localizing pure tones below around 100 Hz. If the sound source is not pure, however, and there are harmonics associated with the source, we can localize these sounds and therefore localize the source.

Impedance: (measured in Ohms) The total opposition to the flow of alternating current in an electrical circuit. The measure of the magnitude of an electrical load when using alternating currents, such as in audio. It describes the combined effect of resistance, capacitance and inductance.

Inductor: 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.

An inductor’s impedance is proportional to frequency. They are used in noise suppression and speaker crossovers. Placed in series with a speaker, it forms a 6 dB / octave low pass filter. Placed in series with a components B+ lead, they will suppress noise.

Le: (Measured in millihenries, mH) The electrical inductance of a speaker’s voice coil.

Net Volume (speaker enclosure): The amount of airspace that is enclosed within the enclosure. This does not include the airspace taken up by bracing, vents, or the speaker itself.

Octaves and Harmonics: An octave is a doubling or halving of frequency. One octave up from 100 Hz is 200 Hz, where one octave down from 100 Hz is 50 Hz. A harmonic is a doubling (2nd harmonic), tripling (3rd harmonic), quadrupling (4th Harmonic... etc) of a fundamental frequency. Musical instruments (with the exception of synthesizers) do not create pure tones. The fundamental (main frequency) is combined with its harmonics at various levels to create the sonic signature of that instrument.

Potential: 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.

It also can be shown that a larger hose can move a greater volume of water (more current) than a smaller hose. This would correspond to the size of the conductor. A wire conductor’s size is called its gauge. The smaller the number, the heavier the gauge, and the larger the wire. Heavier gauge wire can conduct more current than a smaller gauge.

Power: Power is the amount of work done in a specified amount of time. The electrical unit of power is the Watt. Power = Voltage * Current.

Power Handling: (measured in watts RMS) It is the continuous sine wave power that can be dissipated by the voice coil/magnet assembly without failure. Most speakers fail because the amplifier is driven into hard clipping. Hard clipping produces square wave distortion which can be considered DC. Voice coils are not designed for DC because music is AC. Hard clipping is difficult to identify in subwoofer systems because it is very hard to hear the results of square wave distortion at lower frequencies.

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.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.

Q: The ratio of reactance to resistance in a series circuit, or the ratio of resistance to reactance in a parallel circuit.

Qes: The Q of a driver at its free air resonance considering only its electrical losses.

Qms: The Q of a driver at its free air resonance considering only its mechanical losses

Qtc: The total Q of a woofer and sealed enclosure at the system’s resonance frequency, considering all resistive losses.

Qts: The total Q of a woofer at Fs, considering all driver resistances.

Resistance: Most all conductors exhibit a property called resistance. Resistance impedes the flow of current. It is measured in units called Ohms. With the water hose, resistance could be regarded as friction between the water and the hose. A larger hose would create less friction and have a lower resistance than a smaller hose.

Resistors: The resistor is a device that resists, or limits current flow. Their value is specified in ohms. A resistance to current flow creates heat, and as such, these devices have a maximum power dissipation, rated in watts. Placed in series with a voltage source, they limit current to a device. In parallel with a voltage source they make up a voltage divider. A potentiometer is a special type of resistor which, when the shaft is rotated, changes resistance via a sliding contact arm. They are used as volume controls, gain matching controls, etc.

S Factor: A description of a bandpass enclosure’s frequency response in the pass band. It is a general indicator of the bandwidth.

Sealed Volume (speaker enclosure): The amount of enclosure airspace on one side of a woofer in a bandpass system that acts as a sealed enclosure.

Semiconductors: 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.

There are other parameters which are of importance when selecting a rectifier. The peak inverse voltage (PIV) is the highest voltage which can be present while the rectifier is reverse biased.

Transistors are semiconductor devices which either act as current amplifiers, voltage amplifiers, or switches. The two most often used in mobile electronics are the bipolar and the metal oxide semiconductor field effect transistor (Mosfet). They act as “valves” where a small change in control current or voltage on the input yields a great change in current or voltage at the output.

The chief disadvantage to using bipolar transistors is “thermal runaway”. Thermal runaway is characteristic of bipolar transistors which causes more current to flow when the transistor gets hot. As more current flows, more heat is dissipated allowing even more current to flow. This cycle continues until the transistor destroys itself.

Mosfets are more similar to vacuum tubes in operation than bipolar transistors. They are transconductance devices, which means that the input works on voltage only without the need for current. They have extremely high input impedances. They act primarily as voltage amplifiers. Mosfets do not exhibit thermal runaway like bipolars do, as when Mosfets get hot, they actually allow less current to flow.

Sound Pressure Level (SPL): (measured in dB) An acoustic measurement of sound energy. One dB SPL is the smallest audible difference in sound level. Theoretically, 0 dB SPL is the threshold of human hearing at 1 kHz, while 120dB is the threshold of pain.

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.

A decibel is 1/10th of a Bel. A doubling of sound pressure level (SPL) is a 10 dB increase, where a halving of SPL is a 10dB decrease. The human ear can detect differences as small as 1 dB, however 3dB is commonly referred to as a level where a change is readily apparent.

A doubling of power input into a speaker will produce a 3dB change in output, a doubling of cone area will result in a 3dB increase over that of a single driver.

A doubling of distance from a sound source reduces SPL by 6dB.

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

When a speaker moves forward, backward and back to its original position, this is called a cycle. Frequency is determined as to the number of cycles per second. It is labeled Hertz (Hz) after an early scientist in the field of acoustics. Diagram 1 is a graphical representation of a sound wave. The top half of the curve is where pressure increases, the bottom half is the decreasing of the pressure.

It has been determined that sound travels through air at sea level at a speed of 1128 feet per second. The wavelength is the distance a sound travels in one cycle. This distance can be determined by dividing 1128 by frequency. With this formula we determine that a 20 Hz note is 56.4 feet long. On the other hand a 20kHz (20,000 Hz) is .056 feet which is about 5/8 of an inch. Understanding this relationship between distance and soundwaves will be important in understanding how we hear, and therefore how to properly set up a sound system.

Yet another description of the soundwave is phase. Phase refers to the pressure change of the soundwave at a certain time and place. Once again, refer to diagram 1, and see how the first part of the curve is above the zero line. This is known as positive phase (increase of pressure). When the curve is below the zero line, it is known as negative phase (decrease of pressure). When Two or more waves combine the combination can be referred to as being in phase or out of phase. In phase means that the soundwaves are combining to form a wave that is larger than either of the two. When the waves are out of phase, meaning the phase of one wave is positive and the other is negative, they will subtract from each other, therefore reducing the pressure level. Soundwaves are constantly interacting with one another and creating what is known as a complex wave. All musical instruments create complex sound waves. The instruments combine waves to create the sound that makes them unique.

SPL: (measured in dB) The speaker’s reference efficiency measured with 1 watt input at a distance of 1 meter from the center of the cone.

Thiele / 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.

Vas: (measured in cubic feet or liters) Volume Acoustic Suspension. It is the volume of air having the same stiffness as the speaker’s suspension.

Vb: (measured in cubic inches, cubic feet or liters) The internal volume of air of a given enclosure.

Vd: The volume of air displaced by the speaker’s cone during an Xmax displacement.

Vented Volume (speaker enclosure): The amount of enclosure airspace on one side of a woofer in a bandpass system that acts as a vented enclosure.

Volume: (Enclosure, Internal - measured in Cubic Inches, Feet or Meters) The result of the product of three linear measurements. Length times width times depth or height, (LXWXH). Volume describes a loudspeaker enclosure size.

Wiring: (Series or Parallel) How two or more speakers are wired together to form a mono system. The wiring of two or more drivers can affect impedance for the whole system. Care must be taken to assure that the resulting impedance does not exceed the amplifier’s requirements.

Xmax: (measured in inches or millimeters) It is the measure of a speaker cone’s maximum excursion in one direction while maintaining a linear behavior.