The car audio glossary

How much pressure a system produces, expressed in decibels above a fixed reference.

SPL is 20·log₁₀(measured pressure / 20 µPa), where 20 µPa is roughly the quietest 1 kHz tone a healthy young ear detects. Because the scale is logarithmic, doubling amplifier power buys about +3 dB, not twice the number — and the last few decibels of a competition build cost far more than the first thirty.

A logarithmic ratio between two quantities, not an absolute amount.

A decibel only means something once its reference is stated: dB SPL references 20 µPa, dBV references one volt. +3 dB is a doubling of power, +6 dB a doubling of pressure, and roughly +10 dB before most listeners call a broadband sound twice as loud.

A filter that de-emphasises low frequencies to approximate human hearing at moderate levels.

A-weighting attenuates roughly −39 dB at 31.5 Hz and −26 dB at 63 Hz, so it hides most of what a subwoofer does. It is useful for noise exposure work and misleading for bass measurement, which is why competition SPL is measured unweighted.

RMS describes power a device can sustain; peak describes a brief maximum, often under undisclosed conditions.

An honest rating states supply voltage, impedance, a distortion limit and a bandwidth — for example 4 × 75 W at 14.4 V into 4 Ω at 1 % THD, 20 Hz–20 kHz. A bare "1000 W" with none of those conditions is a marketing figure, not a measurement, and is the single most common source of mismatched systems.

The share of an output signal that consists of harmonics the input never contained.

THD is quoted as a percentage or in dB at a stated power and frequency, so 0.05 % at 1 W says little about behaviour at full power. Distortion normally rises steeply as an amplifier approaches its limit, which is why power figures without a distortion limit are unusable.

How level varies with frequency, usually plotted from 20 Hz to 20 kHz with a tolerance window.

A response claim needs its window: "20 Hz–20 kHz" alone is meaningless without something like ±3 dB. In a vehicle the response you measure at the listening position is the product of the driver, the install, the cabin and any correction — not of the driver alone.

The residual level present with no signal playing, setting the quietest detail a system can reveal.

Measurement is only valid where the signal sits well above the noise floor, so protocols require background noise to be verified before a session. In vehicles the practical floor is often set by alternator whine, ground loops or blower noise rather than by the electronics.

How much level a driver produces for a standard input, typically dB at 1 W and 1 m.

Two speakers with the same power rating can differ by 6 dB in sensitivity, which is a four-fold difference in the power needed for the same output. Sensitivity is stated on axis in a controlled setting; a door-mounted driver behind a grille rarely reproduces it.

Referencing an instrument against a known standard so its readings can be trusted and repeated.

Microphones, SPL sensors and analysers drift, so each carries a calibration record that a report cites alongside its results. Without a traceable calibration chain a number is an observation, not a measurement.

The defined time span over which a competition or test level is integrated.

Peak-hold over 100 ms and an average over 30 s describe very different achievements from the same system, so the window is fixed by protocol rather than chosen by the competitor. Thermal compression means a long window almost always reads lower than a short one.

The low-frequency reinforcement a car interior provides because it behaves as a sealed pressure volume.

Below the cabin's first axial mode — roughly the speed of sound divided by twice the longest dimension, often near 55–60 Hz in a sedan — the interior pressurises rather than supporting waves, adding output that rises as frequency falls. This is why small enclosures work in cars and why the same subwoofer measured outdoors looks weak by comparison.

A fixed pattern of loud and quiet regions formed when reflections reinforce and cancel.

Above the first axial mode a cabin holds pressure maxima at boundaries and minima at nodes, and the difference between two points in the same car at the same instant can exceed 10 dB. Moving a microphone 10 cm moves the reading, which is physics rather than meter error — and the reason placement is specified, not chosen.

A node is a pressure minimum in a standing-wave pattern; an antinode is a maximum.

Seating position relative to nodes explains why one seat sounds thin at a given frequency while another sounds heavy. Competition sensor positions are fixed precisely because nodes make the choice of position worth several decibels.

The relative timing of two signals at the same frequency, expressed in degrees.

Two drivers in phase reinforce and can sum to +6 dB; in opposition they cancel, and a subwoofer wired out of phase can lose most of its useful output while measuring healthy in isolation. Phase problems are usually timing or polarity problems, not level problems.

How long a given frequency band is delayed on its way through a system.

Steep crossovers and ported enclosures add group delay near their corner frequencies, which listeners perceive as bass arriving late rather than as a level error. It is measured in milliseconds and matters most where a subwoofer hands over to the front stage.

A regular series of peaks and dips created when the same signal arrives twice, slightly delayed.

In a car the near-side speaker reaches the listener well before the far-side one, and the resulting interference pattern looks like the teeth of a comb on a response plot. Time alignment addresses the cause; equalisation only flattens the symptom at one seat.

The response shape a tuned system aims for, rather than a ruler-flat line.

Most target curves keep the midrange close to flat and allow a gentle low-frequency rise, because a car cabin and typical listening levels make a literally flat measurement sound thin. Judging compares against the published target window, so the curve is documented rather than a matter of taste.

The apparent height, width and depth of the reproduced performance.

Good staging places the soundstage ahead of the listener at roughly dashboard height rather than in the doors, and it survives moving your head. It depends far more on driver placement, aiming and timing than on equalisation.

How precisely individual instruments and voices can be located within the stage.

Sharp imaging means a voice occupies a defined point rather than smearing across the windscreen, and it collapses quickly with polarity errors or mismatched arrival times. Judges score it with familiar material because it depends on knowing where the elements belong.

The frequency at which a driver's moving system naturally wants to resonate.

Fs is set by the cone's mass and the suspension's compliance, and it rises in a sealed enclosure because the trapped air stiffens the system. Fs drifts downward as a suspension breaks in and upward when a voice coil runs hot, which is one reason protocols require a burn-in period before recorded runs.

A driver's opposition to alternating current, which varies with frequency.

A "4 Ω" speaker is a nominal figure: real impedance dips below it and peaks far above it near resonance, so a bench multimeter reading of 3.2 Ω is normal rather than a fault. Amplifiers are rated into nominal loads, so wiring choices that halve nominal impedance roughly double demand on the supply.

Voltage equals current times resistance — the relationship behind every wiring and fusing decision.

In practice it is used to predict voltage drop (V = I·R) and heat in a cable (P = I²R): 100 A through 10 mΩ of cable loses one volt and dissipates 100 W as heat. Those two consequences, not the current figure itself, are what decide gauge and fusing.

The current an alternator can supply beyond the vehicle's own electrical needs.

A resting battery sits near 12.6 V while a healthy charging system holds about 14.4 V, and because power scales with the square of voltage, sagging to 12.6 V under load costs roughly 1.2 dB of output. Adding capacity without checking headroom simply moves the shortfall to the alternator.

The voltage lost along a conductor because of its resistance and the current flowing through it.

Drop scales with length as well as current, so a five-metre run needs more copper than a one-metre run at the same amperage. Excess drop shows up as dimming lights, early protection cycles and amplifiers that fail to make rated power despite a healthy alternator.

A conductor size scale where smaller numbers mean thicker wire and lower resistance.

Gauge must be chosen for current *and* length together, and copper-clad aluminium conducts at roughly 61 % of copper, so it needs about two sizes larger for the same job. Undersized cable is both a performance problem and a fire risk, because the cable becomes the element that heats.

A deliberate weak point that opens before a cable can overheat.

A fuse protects the cable, not the equipment: the main fuse belongs within about 300 mm of the battery terminal so a short anywhere downstream is covered. Fitting a larger fuse to stop nuisance blowing removes the protection the cable was sized around.

Upgrading the alternator-to-battery positive, engine-to-chassis and battery-to-chassis grounds.

These three factory conductors are sized for the vehicle's original load, so they often become the bottleneck long before the alternator does. It is a cheap way to recover voltage under load, and it does nothing to increase the alternator's actual output.

The current a system actually pulls, set by output power, efficiency and supply voltage.

Current is roughly output power divided by efficiency and supply voltage, so a 1,000 W output at 85 % efficiency near 14 V draws on the order of 84 A. Estimating from the fuse rating printed on the chassis overstates real demand and leads to over-specified, expensive installs.

A switching amplifier topology that wastes far less power as heat than linear designs.

Efficiency in the 80–90 % range is why class-D dominates subwoofer duty and compact installs, and modern designs are transparent enough for full-range use. Efficiency reduces the heat and current problem; it does not change the acoustics of a bad enclosure.

What happens when a signal demands more voltage than the amplifier can deliver, flattening its peaks.

Clipping adds harmonics, raises average power sharply and is a far more common cause of dead tweeters than a nominally oversized amplifier. Headroom — deliberately keeping the amplifier below its limit — protects both fidelity and drivers.

The loss of output that occurs as a voice coil heats and its resistance rises.

Copper gains about 0.393 % resistance per °C, so a coil at a few hundred degrees can approach twice its cold resistance and give up roughly 3 dB. It is why long burn windows read lower than short ones, and why regional heat testing is not a formality.

The rigid panel a driver mounts to, separating front and rear radiation.

A flexing or leaking baffle lets the rear wave cancel the front one, costing midbass that no amount of equalisation restores. Door skins are poor baffles by default, which is why adapters and sealing are part of an install assessment.

Sealed boxes trade output for control; ported boxes trade control for output around tuning.

A sealed enclosure rolls off gently and stays predictable, while a port adds several decibels near its tuning frequency at the cost of group delay and a steeper cut-off below it. Neither is better in the abstract — the choice follows the music, the cabin and the class rules.

A number describing how damped a sealed system's response is at resonance.

Around 0.707 gives the flattest response, lower values a drier and more extended roll-off, higher values a peak that reads as punch and rings. Qtc follows from the driver's parameters and the box volume, so it is decided at design time rather than dialled in later.

Adding mass, damping or decoupling to stop panels radiating their own sound.

Constrained-layer damping lowers panel resonance while closed-cell foam and mass-loaded barriers address airborne noise — different products for different problems. Full coverage is rarely necessary: treating the panels that actually resonate is what changes measurements.

Delaying nearer drivers so output from all of them reaches the listener together.

Sound covers about 34 cm per millisecond, so the path difference between the near and far door is worth a couple of milliseconds — enough to pull the whole stage sideways. Alignment is set per seat, which is why competition classes state which seat is judged.

A filter network that sends each driver only the band it can handle.

Slope, corner frequency and polarity all interact, so a crossover that looks correct on paper can still cancel at the overlap. Tweeters usually fail from being asked to reproduce content below their crossover point rather than from raw power.

A processor that applies crossovers, delay and equalisation in the digital domain.

A DSP makes correction measurable and repeatable, which is what allows a tuned system to be verified against a target window. It cannot fix cancellation caused by placement or a leaking baffle — it can only shape what physics allows.

The return path for current, and the usual culprit behind system noise.

A ground must reach bare metal with paint removed and be as short as practical; sharing a ground point between source units and amplifiers invites loops that show up as alternator whine. Chasing noise with filters before fixing grounds treats the symptom.

Interfacing with the vehicle's data network instead of cutting into it.

Modern vehicles carry audio, chimes, amplifier wake-up and diagnostics over the bus, so splicing blind can disable features or set faults. Integration also has to respect airbag and restraint wiring, which is a safety boundary rather than a preference.

A judged discipline scoring tonal accuracy, staging and imaging against a written rubric.

SQ is judged in controlled listening sessions with defined material, and the rubric is published so a score can be explained rather than asserted. Measured response and perceived quality overlap but are not the same thing, which is why both a plot and a listening session appear in a result.

A measured discipline scoring maximum sound pressure under fixed conditions.

Sensor position, vehicle class and burn window are all fixed by protocol, because each is worth decibels on its own. A dashboard app number is not comparable: it lacks a calibrated sensor, a defined position and a defined window.

A combined class rewarding systems that score on quality while still delivering real output.

SQL exists because pure SQ and pure SPL builds optimise in opposite directions, and most enthusiast systems live in between. Scoring weights both, so neither a quiet, immaculate stage nor a loud, unlistenable one wins.

A class for upgraded factory systems rather than full custom builds.

It rewards integration skill: what can be achieved while keeping factory locations, trim and often the factory head unit. The class exists because most of the market upgrades rather than rebuilds.

The versioned document defining classes, scoring and measurement conditions for an event.

A rulebook is published before a season and cited by version in results, so a score always traces to the rules it was earned under. Judges are trained against it, which is what makes two events comparable.

The scored form a judge completes, breaking a result into defined criteria.

Because each criterion carries its own scale and comment field, a competitor receives a diagnosis rather than a verdict. Sheets are archived with results so a score can be reviewed after the event.

Laboratory confirmation that a product meets its published specifications under a stated protocol.

The certificate is tied to a protocol version, a test record and an expiry, so it says what was measured, how, and when. It is a verification of the manufacturer's claims, not a regulatory approval or an endorsement.

Individual certification earned through practical assessment of installation work.

Assessment covers electrical safety, mechanical quality and system tuning against published criteria, and it certifies a person rather than a shop. It exists so a customer can distinguish demonstrated skill from a well-photographed portfolio.

Facility-level recognition covering equipment, processes, safety and certified staff.

Where installer certification follows the individual, accreditation asks whether the premises can reliably deliver that standard of work. Both carry a validity period and appear in the public directory.

The numbered revision of a test method that a given result was produced under.

Methods are reviewed on a fixed cycle, and every report cites the version used so old and new results are never silently compared. Existing results stay valid for their version; retesting is only needed to claim compliance with a newer one.

The structured document recording method, conditions, instruments, raw data and outcome.

A report is what makes a claim checkable: another lab following the same protocol should reach the same result within stated tolerances. It records conditions such as temperature and supply voltage precisely because those change the numbers.

The fusing, cabling, mounting and vehicle-integrity requirements every certified install must meet.

Compliance covers fuse placement and rating, conductor sizing, secure mounting and respect for airbag and restraint systems. It is assessed pass/fail rather than scored, because a system that sounds excellent and is unsafe does not pass.