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1. Definitions

This section contains background information which defines some of the acronyms and terminology commonly used in the car audio world. Understanding these definitions is important in order to understand the other sections of this document.

1.1 What do all of those acronyms mean? [JSC,MZ]  
1.2 What is meant by "frequency response?" [JSC]  
1.3 What is a "soundstage?" What is an "image?" [CD]  
1.4 What is meant by "anechoic?" [JSC]  


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1.1 What do all of those acronyms mean? [JSC,MZ]

`A' is for amperes, which is a measurement of current equal to one coulomb of charge per second. You usually speak of positive current - current which flows from the more positive potential to the more negative potential, with respect to some reference point (usually ground, which is designated as zero potential). The electrons in a circuit flow in the opposite direction as the current itself. Ampere is commonly abbreviated as "amp", not to be confused with amplifiers, of course, which are also commonly abbreviated "amp". In computation, the abbreviation for amps is commonly "I".

`V' is for volts, which is a measurement of electric potential. Voltages don't "go" or "move", they simply exist as a measurement (like saying that there is one mile between you and some other point).

`DC' is for direct current, which is a type of circuit. In a DC circuit, all of the current always flows in one direction, and so it is important to understand which points are at a high potential and which points are at a low potential. For example, cars are typically 12VDC (twelve volts direct current) systems, and it is important to keep track of which wires in a circuit are attached to the +12V (positive twelve volts) lead of the battery, and which wires are attached to the ground (or "negative") lead of the battery. In reality, car batteries tend to have a potential difference of slightly higher than 12V, and the charging system can produce upwards of 14.5V when the engine is running.

`AC' is for alternating current, which is a type of circuit in which the voltage potential fluctuates so that current can flow in either direction through the circuit. In an AC circuit, it is typically not as important to keep track of which lead is which, which is why you can plug household appliances into an outlet the "wrong way" and still have a functioning device. The speaker portions of an audio system comprise an AC circuit. In certain situations, it is indeed important to understand which lead is "positive" and which lead is "negative" (although these are just reference terms and not technically correct). See below for examples. The voltage of an AC circuit is usually given as the RMS (root mean square) voltage, which, for sinusoidal waves, is simply the peak voltage divided by the square root of two.

`W' is for watts, a measurement of electrical power. One watt is equal to one volt times one amp, or one joule of energy per second. In a DC circuit, the power is calculated as the voltage times the current (P=V x I). In an AC circuit, the average power is calculated as the RMS voltage times the RMS current (Prms=Vrms x Irms).

`Hz' is for hertz, a measurement of frequency. One hertz is equal to one inverse second (1/s); that is, one cycle per second, where a cycle is the duration between similar portions of a wave (between two peaks, for instance). Frequency can describe both electrical circuits and sound waves, and sometimes both. For example, if an electrical signal in a speaker circuit is going through one thousand cycles per second (1000Hz, or 1kHz), the speaker will resonate at 1kHz, producing a 1kHz sound wave. The standard range of human hearing is "twenty to twenty", or 20Hz-20kHz, which is three decades (three tenfold changes in frequency) or a little under ten octaves (ten twofold changes in frequency).

`dB' is for decibel, and is a measurement for power ratios. To measure dB, you must always measure with respect to something else. The formula for determining these ratios is P=10^(dB/10), which can be rewritten as dB=10log(P). For example, to gain 3dB of output compared to your current output, you must change your current power by a factor of 10^(3/10) = 10^0.3 = 2.00 (that is, double your power). The other way around, if you triple your power (say, from 20W to 60W) and want to know the corresponding change in dB, it is dB=10log(60/20)=4.77 (that is, an increase of 4.77dB). If you know your logarithms, you know that a negative number simply inverts your answer, so that 3dB corresponding to double power is the same as -3dB corresponding to half power. There are several other dB formulas; for instance, the voltage measurement is dB=20log(V). For example, a doubling of voltage produces 20log2 = 6.0dB more output, which makes sense since power is proportional to the square of voltage, so a doubling in voltage produces a quadrupling in power.

`SPL' is for sound pressure level and is similar to dB. SPL measurements are also ratios, but are always measured relative to a constant. This constant is 0dB which is defined as the smallest level of sound pressure that the human ear can detect. 0dB is equal to 10^-12 (ten to the negative twelfth power) W/m^2 (watts per square meter). As such, when a speaker is rated to produce 92dB at 1m when given 1W (92dB/Wm), you know that they mean that it is 92dB louder than 10^-12W/m^2. You also know than if you double the power (from 1W to 2W), you add 3dB, so it will produce 95dB at 1m with 2W, 98dB at 1m with 4W, 101dB at 1m with 8W, etc.

`THD' is for total harmonic distortion, and is a measure of the how much a certain device may distort a signal. These figures are usually given as percentages. It is believed that THD figures below approximately 0.1% are inaudible. However, it should be realized that distortion adds, so that if a head unit, equalizer, signal processor, crossover, amplifier and speaker are all rated at "no greater than 0.1%THD", together, they could produce 0.6%THD, which could be noticeable in the output.

An Ohm is a measure of resistance and impedance, which tells you how much a device will resist the flow of current in a circuit. For example, if the same signal at the same voltage is sent into two speakers - one of which is nominally rated at 4 ohms of impedance, the other at 8 ohms impedance - twice as much current will flow through the 4 ohm speaker as the 8 ohm speaker, which requires twice as much power, since power is proportional to current.

`PSRR' is the Power supply rejection ratio. This is a spec sometimes provided with amplifiers, but is not exclusive to amplifiers. It refers to the propensity for an AC signal present at the output of the power supply to appear somewhere in the signal path. A poor PSRR often leads to an increase in noise, distortion, and crosstalk.

`BJT' is short for Bipolar junction transistor. It is a very common type of transistor that is found in a multitude of circuits. Often times, amplifier manufacturers will specify that a certain amplifier utilizes a BJT output stage. This simply means that the major current-carrying output devices (the output transistors) are of the BJT variety rather than FET. Regardless of output device type, most discrete amplifiers will incorporate many BJTs throughout the entire amplifier.

Many people do not realize that `MOSFET' is an acronym, but it stands for Metal oxide semiconductor field-effect transistor. It is another common type of transistor, but of the FET class. This type of transistor operates in a different manner than BJTs. There's much debate about which type of transistor is more suitable for car audio amplifiers, but in the end it becomes a matter of personal preference for the designer. It's important to note that neither design has an inherent benefit in terms of sound quality. However, properly constructed MOSFET output stages are sometimes more durable and immune to damage produced by device failure or poor output protection circuitry, whereas BJT output stages can sometimes be slightly more efficient and cheaper. But both of these statements depend even moreso on the circuit design and the transistor used.

`RMS' is the abbreviation for root mean-square. It is usually associated with power measurements, and refers to a calculation that consists of the following procedure: 1) square the waveform; 2) take the mean of the result; 3) take the square root of this number. If the waveform is a sine wave, the RMS value is .707 times the peak value. If the waveform is a square wave, the RMS value is equal to the peak value. Importantly, the RMS voltage of a signal multiplied by the RMS current of a signal will yield the signal's average power. Sometimes, average power is erroneously referred to by manufacturers and hobbyists alike as "RMS power". "RMS power" would require calculating the root mean-square value of a power waveform, which ends up yielding a different result than average power, but it's become almost an industry standard.

`DCR' is the DC resistance of a speaker driver (it is sometimes referred to as `RE'). This number is typically lower than the nominal impedance provided by the manufacturer. It specifies the resistive component of the speaker's impedance characteristic at a given temperature. Sometimes it's useful to estimate a speaker's impedance by measuring its DCR value with a simple ohmmeter. Typical DCR values for 4 ohm speakers are on the order of 3.2 to 3.5 ohms, and for 8 ohm speakers, usually over 5.5 ohms.

`Q' is typically known as quality factor, and tends to refer to the rolloff behavior of a filter or group of filters. Values of Q are intimately related to the breadth of bandpass rolloff (eg. in equalizers), degree of overlap between filters, filter alignment, or loudspeaker/enclosure interaction. In general, the higher the Q, the narrower the passband, greater the overlap, or steeper the slope (depending on what the Q is referring to).


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1.2 What is meant by "frequency response?" [JSC]

The frequency response of a device is the range of frequencies over which that device can perform in some fashion. The action is specific to the device in question. For example, the frequency response of the human ear is around 20Hz-20kHz, which is the range of frequencies which can be resolved by the eardrum. The frequency response of an amplifier may be 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz. In the car audio world, frequency responses should usually be given with a power ratio range as well, such as (in the case of the speaker) 120Hz-17kHz +/-3dB. What this means is that given an input signal anywhere from 120Hz to 17kHz, the output signal is guaranteed to be within an "envelope" that is 6dB tall. Typically the extreme ends of the frequency range are the hardest to reproduce, so in this example, the 120Hz and 17kHz points may be referred to as the "-3dB points" of the amplifier. When no dB range is given with a frequency response specification, it can sometimes be assumed to be +/-3dB.


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1.3 What is a "soundstage?" What is an "image?" [CD]

The soundstage is the position (front/back and high/low) that the music appears to be coming from, as well as the depth of the stage. A car with speakers only in the front will likely have a forward soundstage, but may not have enough rear fill to make the music seem live. A car with both front and rear speakers may have anything from a forward to a rear soundstage, with an accompanying fill from the softer drivers depending on the relative power levels and the frequencies reproduced. The high/low position of the soundstage is generally only obvious in a car with a forward soundstage. The music may seem to be originating in the footwells, the dash, or out on the hood, depending on how the drivers interact with the environment.

The stereo image is the width and definition of the soundstage. Instruments should appear to be coming from their correct positions, relative to the recording. The position of the instruments should be solid and easily identifiable, not changing with varying frequencies. A car can image perfectly with only a center-mounted mono speaker, but the stereo placement of the music will be absent.


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1.4 What is meant by "anechoic?" [JSC]

Anechoic means not echoing. It usually refers to a style of measuring a speaker's output which attempts to eliminate echoes (or "reflections") of the speaker's output back to the measurement area, which could alter the measurement (positively or negatively).


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This document was generated by Ian D. Bjorhovde on February, 7 2006 using texi2html