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Glossary Of Car Audio / Visual Terms

  • Alternating Current (AC) – An electric current that reverses direction at regular intervals. Measured in Volts AC at Hertz, example: 110 volts AC 60 Hz.
  • Amplifier – 1 A device which increases the level of a signal by increasing the current or voltage. 2 May also be used to isolate or control a signal and even decrease the level as in a line output converter.
  • Bandpass – A two-part filter that cuts both high and low frequencies allowing the band of frequencies between these two points to pass.
  • Bandwidth – The range of frequency response between lower and upper frequencies points which audio signals pass through an electrical device or conductor where the signal has rolled off by three decibels.
  • Capacitor – 1 (polarized) An electrical circuit element used to store charge temporarily, consisting in general of two metallic plates separated by a dielectric. 2 (non-polarized) A crossover component used to filter out lower frequencies and allow higher frequencies to pass.
  • Circuit Breaker – A device that protects electric circuits by interrupting power in a circuit when an overload occurs. Unlike a fuse a circuit breaker is resetable. Rated in amperes (amps).
  • Clipping – Audible distortion that occurs when continuous power-to-peak power capabilities (headroom) are exceeded. “Turn it down!”
  • Closed Circuit – A continuous unbroken circuit in which current can flow without interruption.
  • Coaxial – A speaker composed of larger cone for low range frequencies and a smaller cone or tweeter for higher frequencies aligned on the same axis. A crossover network is necessary to route the proper signals to each driver. These may be passive (usually included). If the speakers are bi-amplified, an active crossover will be used to route the proper range of frequencies to the respective amplifier channels.
  • Decibel (dB) – A unit of measurement for the ratio of loudness. The threshold of hearing is 0 dB. One dB SPL is the smallest audible difference in sound level.
  • DC/DC Converter – A group of components within an amplifier that converts battery voltage (DC) into AC so that it can be increased by the switching devices and transformer and converted back to DC (rectified) to provide higher voltage to drive the amplification stage.
  • Digital Output – An output where the signal is in digital form to allow external processing before being converted to an analog signal.
  • DSP – Digital Signal Processing (or Processor). A type of processing accomplished by a microcomputer chip specifically designed for signal manipulation, or a component using such processing. The term is often misused as a synonym for ambience synthesizer; however, DSP can do much more than sound field creation.
  • Driver – Synonymous with loudspeaker. The term also refers to a loudspeaker being coupled to a horn for acoustic coupling and controlled dispersion of sound.
  • Dynamic Range – The difference between the softest and loudest portions of sound that an amplifier or recorder can reproduce within an acceptable range of distortion. Expressed in decibels, the higher the number the better.
  • Efficiency – The ratio of energy output to total energy input, expressed as a percentage. With speakers, this refers to the ratio of total acoustic watts radiated to total electrical watts input.
  • Farad – The basic unit of capacitance. A capacitor has a value of one farad when it can store one coulomb of charge with one volt across it.
  • Frequency – The number of wavelengths which pass a specific point in a specific time period, measured in Hertz (Hz). Cycles per second.
  • Fuse – A device that protects electric circuits by interrupting power in a circuit when an overload occurs. Rated in amperes (amps).
  • Gain – The amount of amplification used in an electrical circuit.
  • Gauge (wire) – The diameter of a wire. The higher the number, the thinner the wire.
  • Ground – An electrical line with the same electrical potential as the chassis of the vehicle, most commonly negative 12 volts DC.
  • Hertz (Hz) – The unit of measurement for frequency. 1 Hz is equal to 1 cycle per second.
  • High Level Input – An input configured to accept speaker level signals.
  • High Pass Filter – A network of elements used to attenuate all frequencies below a predetermined frequency. Frequencies above the cutoff point pass without any effect
  • Impedance – The opposition to the flow of alternating current (AC) in a circuit. Measured in ohmsYour browser may not support display of this image
  • LCD – See Liquid Crystal Display.
  • LED – Light Emitting Diode.
  • Low Pass Filter – A network of elements used to attenuate all frequencies above a predetermined frequency. Frequencies below the cutoff point pass without any effect
  • Liquid Crystal Display (LCD) – A type of digital display made of a material that changes reflectance or transmittance when an electrical field is applied to it
  • Magnet – A device which has the ability to attract or repel pieces of iron or other magnetic material. Speaker magnets provide a stationary magnetic field so that when the coil produces magnetic energy, it is either repelled or attracted by the stationary magnet.
  • Midrange Driver – A loudspeaker specifically designed to reproduce the frequencies in the middle of the audible bandwidth. Usually between three and eight inches in diameter. Commonly referred to as the low frequency driver in a set of separates.
  • MOSFET – Metal Oxide Semiconductor Field Effect Transistor. A form of field-effect transistor controlled by voltage rather than current, like a bipolar transistor. MOSFETs have a significantly higher switching speed than bipolar transistors. They generate almost no loss (little heat generation), which lends the power supply fast response, excellent linearity, and high efficiency. Typically used in high power output amplifiers.
  • NTSC – National Television System Committee. Refers to the standards used for video broadcast and playback signal parameters.
  • Ohm’s Law – Current in a circuit is directly proportional to the voltage, and inversely proportional to resistance.It also includes the relationships of watts to amps, volts and ohms.
  • Polarity – The electrical quality of having two opposite poles, one positive and one negative. Polarity determines the direction in which a current tends to flow
  • Pre-amplifier (pre-amp) – The circuit which takes a small signal and amplifies it to be fed into the power amplifier for further amplification. Contains controls for volume, regulating tone, and channel balance.
  • Sensitivity (loudspeaker sensitivity) – The sound pressure level a speaker produces when fed by a given input power, measured at a specific distance on axis directly in front of the speaker. Typically specified in dB SPL at 1 meter with 1 watt of input signal.
  • Sub-woofer – A loudspeaker made to reproduce the lowest of audio frequencies, approx. 25 Hz to 125 Hz
  • Tweeter – A high frequency driver specifically designed to reproduce only the high frequencies (treble) of the audible spectrum
  • Total Harmonic Distortion (THD) – The noise referenced to signal in decibels (dB) as a percentage.
  • Voice Coil – Coil of wire wrapped around a tube and attached to the speaker cone or driver diaphragm. Becomes an electromagnet when an audio signal is applied and interacts with a permanent magnet which causes the cone or diaphragm to vibrate
  • Voltage – he difference in electrical potential between two points in a circuit. It’s the push or pressure behind current flow through a circuit.
  • Woofer – A loudspeaker made to reproduce the lower range of the audio spectrum (bass), in a 2-way or more complex speaker systems.

Thiele-Small Parameters

  • B – Magnetic flux density in gap, in Tesla-meters (TM)
  • BL – The magnetic strength of the motor structure. “Expressed in Tesla meters, this is a measurement of the motor strength of a speaker. Think of this as how good a weightlifter the transducer is. A measured mass is applied to the cone forcing it back while the current required for the motor to force the mass back is measured. The formula is mass in grams divided by the current in amperes. A high BL figure indicates a very strong transducer that moves the cone with authority!”
  • C – Propagation velocity of sound at STP, approx. 342 m/s
  • Cas – Acoustical equivalent of Cms
  • Cmes – The electrical capacitive equivalent of Mms, in farads
  • Cms – The driver’s mechanical compliance (reciprocal of stiffness), in m/N
  • D – Effective diameter of driver, in meters
  • F3 – -3 dB cutoff frequency, in Hz
  • Fb – Enclosure resonance (usually for bass reflex systems), in Hz
  • Fc – System resonance (usually for sealed box systems), in Hz
  • Fs – Driver free air resonance, in Hz. This is the point at which driver impedance is maximum. “This parameter is the free-air resonant frequency of a speaker. Simply stated, it is the point at which the weight of the moving parts of the speaker becomes balanced with the force of the speaker suspension when in motion. If you’ve ever seen a piece of string start humming uncontrollably in the wind, you have seen the effect of reaching a resonant frequency. It is important to know this information so that you can prevent your enclosure from ‘ringing’. With a loudspeaker, the mass of the moving parts, and the stiffness of the suspension (surround and spider) are the key elements that affect the resonant frequency. As a general rule of thumb, a lower Fs indicates a woofer that would be better for low-frequency reproduction than a woofer with a higher Fs. This is not always the case though, because other parameters affect the ultimate performance as well.”
  • L – Length of wire immersed in magnetic field, in meters
  • Lces – The electrical inductive equivalent of Cms, in henries
  • Le – “This is the voice coil inductance measured in millihenries (mH). The industry standard is to measure inductance at 1,000 Hz. As frequencies get higher there will be a rise in impedance above Re. This is because the voice coil is acting as an inductor. Consequently, the impedance of a speaker is not a fixed resistance, but can be represented as a curve that changes as the input frequency changes. Maximum impedance (Zmax) occurs at Fs. “
  • Ms – The total moving mass of the loudspeaker cone.
  • Mmd – Diaphram mass, in grams
  • Mms – The driver’s effective mechanical mass (including air load), in kg. “This parameter is the combination of the weight of the cone assembly plus the ‘driver radiation mass load’. The weight of the cone assembly is easy: it’s just the sum of the weight of the cone assembly components. The driver radiation mass load is the confusing part. In simple terminology, it is the weight of the air (the amount calculated in Vd) that the cone will have to push.”
  • n0 – The reference efficiency of the system (eta sub 0) dimensionless, usually expressed as %
  • p – (rho) Density of air at STP 1.18 kg/m^3
  • Pa – Acoustical power
  • Pe – Electrical power
  • Q – The relative damping of a loudspeaker
  • Q Parameters – “Qms, Qes, and Qts are measurements related to the control of a transducer’s suspension when it reaches the resonant frequency (Fs). The suspension must prevent any lateral motion that might allow the voice coil and pole to touch (this would destroy the loudspeaker). The suspension must also act like a shock absorber. Qms is a measurement of the control coming from the speaker’s mechanical suspension system (the surround and spider). View these components like springs. Qes is a measurement of the control coming from the speaker’s electrical suspension system (the voice coil and magnet). Opposing forces from the mechanical and electrical suspensions act to absorb shock. Qts is called the ‘Total Q’ of the driver and is derived from an equation where Qes is multiplied by Qms and the result is divided by the sum of the same.
  • As a general guideline, Qts of 0.4 or below indicates a transducer well suited to a vented enclosure. Qts between 0.4 and 0.7 indicates suitability for a sealed enclosure. Qts of 0.7 or above indicates suitability for free-air or infinite baffle applications. However, there are exceptions! The Eminence Kilomax 18 has a Qts of 0.56. This suggests a sealed enclosure, but in reality it works extremely well in a ported enclosure. Please consider all the parameters when selecting loudspeakers. If you are in any doubt, contact your Eminence representative for technical assistance.”
  • Qa – The system’s Q at Fb, due to absorption losses; dimensionless
  • Qec – The system’s Q at resonance (Fc), due to electrical losses; dimensionless
  • Qes – The driver’s Q at resonance (Fs), due to electrical losses; dimensionless. “A measurement of the control coming from the speaker’s electrical suspension system (the voice coil and magnet). Opposing forces from the mechanical and electrical suspensions act to absorb shock.”
  • Ql – The system’s Q at Fb, due to leakage losses; dimensionless
  • Qmc – The system’s Q at resonance (Fc), due to mechanical losses; dimensionless
  • Qms – The driver’s Q at resonance (Fs), due to mechanical losses; dimensionless. “A measurement of the control coming from the speaker’s mechanical suspension system (the – surround and spider). View these components like springs.”
  • Qp – The system’s Q at Fb, due to port losses (turbulence, viscousity, etc.); dimensionless
  • Qtc – The system’s Q at resonance (Fc), due to all losses; dimensionless
  • Qts – The driver’s Q at resonance (Fs), due to all losses; dimensionless. “The ‘Total Q’ of the driver and is derived from an equation where Qes is multiplied by Qms and the result is divided by the sum of the same.”
  • R – Ripple, in dB
  • Re – “This is the DC resistance of the driver measured with an ohm meter and it is often referred to as the ‘DCR’. This measurement will almost always be less than the driver’s nominal impedance. Consumers sometimes get concerned the Re is less than the published impedance and fear that amplifiers will be overloaded. Due to the fact that the inductance of a speaker rises with a rise in frequency, it is unlikely that the amplifier will often see the DC resistance as its load.”
  • Ras – Acoustical equivalent of Rms
  • Res – The electrical resistive equivalent of Rms, in ohms
  • Rms – “This parameter represents the mechanical resistance of a driver’s suspension losses. It is a measurement of the absorption qualities of the speaker suspension and is stated in N*sec/m.”
  • Revc – DC voice coil resistance, in ohms
  • Rg – Amplifier source resistance (includes leads, crossover, etc.), in ohms
  • Rms – The driver’s mechanical losses, in kg/s
  • Sd – Effective piston radiating area of driver, in square centimeters. “This is the actual surface area of the cone, normally given in square cm.”
  • SPLo – Sound Pressure Level, usually measured at 1 watt, at 1 meter in front of the loudspeaker
  • Vas/Cms – “Equivalent volume of compliance”, this is a volume of air whose compliance is the same as a driver’s acoustical compliance Cms (q.v.), in cubic meters. “Vas represents the volume of air that when compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker. Vas is one of the trickiest parameters to measure because air pressure changes relative to humidity and temperature — a precisely controlled lab environment is essential. Cms is measured in meters per Newton. Cms is the force exerted by the mechanical suspension of the speaker. It is simply a measurement of its stiffness. Considering stiffness (Cms), in conjunction with the Q parameters gives rise to the kind of subjective decisions made by car manufacturers when tuning cars between comfort to carry the president and precision to go racing. Think of the peaks and valleys of audio signals like a road surface then consider that the ideal speaker suspension is like car suspension that can traverse the rockiest terrain with race-car precision and sensitivity at the speed of a fighter plane. It’s quite a challenge because focusing on any one discipline tends to have a detrimental effect on the others. “
  • Vd – Maximum linear volume of displacement of the driver (product of Sd times Xmax), in cubic meters. “This parameter is the Peak Diaphragm Displacement Volume — in other words the volume of air the cone will move. It is calculated by multipying Xmax (Voice Coil Overhang of the driver) by Sd (Surface area of the cone). Vd is noted in cc. The highest Vd figure is desirable for a sub-bass transducer.”
  • Xmax/Xmech – Maximum peak linear excursion of driver, in meters. “Short for Maximum Linear Excursion. Speaker output becomes non-linear when the voice coil begins to leave the magnetic gap. Although suspensions can create non-linearity in output, the point at which the number of turns in the gap (see BL) begins to decrease is when distortion starts to increase. Eminence has historically been very conservative with this measurement and indicated only the voice coil overhang (Xmax: Voice coil height minus top plate thickness, divided by 2). Xmech is expressed by Eminence as the lowest of four potential failure condition measurements times 2: Spider crashing on top plate; Voice coil bottoming on back plate; Voice coil coming out of gap above core; Physical limitation of cone. Take the lowest of these measurements then multiply it by two. This gives a distance that describes the maximum mechanical movement of the cone.”
  • Zmax – “This parameter represents the speaker’s impedance at resonance.”

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