Subwoofer Testing Glossary

A signal processing average over one sixth of an octave applied to frequency response or distortion data to reduce fine ripple while preserving trends. It softens the jagged lines of a frequency response or distortion graph so you can see the overall shape without “noise” while still keeping important detail.

A room with highly absorbent boundaries that approximates free field by eliminating reflections. A room that kills echoes so the mic hears only the loudspeaker under test.

A condition where the loudspeaker cone and suspension behave differently for inward versus outward motion, producing stronger even order harmonics. The cone moves differently pushing out than pulling in, which adds an octave-up copy of the note.

A flat mounting surface that separates front and rear radiation and provides a stable reference for measurement. The board or plate you mount the driver to so measurements are consistent.

A static offset in the suspension that makes one direction of travel stiffer or looser than the other. The spring inside the driver favors one side, nudging the cone off center at level.

The force factor of the motor in newtons per ampere, equal to magnetic flux density times effective coil length, linking current to force on the voice coil. It is the driver’s motor strength that pushes the cone when you send current through the voice coil of a speaker.

An industry-accepted test reference where excursion (Xmax) is taken to the displacement at which BL has fallen to about 70 percent of its peak value. An industry-wide accepted reference for a “push-it-hard” point that is near the clean limit without going past it.

A Klippel plot that shows offset of the BL curve versus excursion, indicating motor centering or bias. A line that tells you if the motor pulls harder in one direction than the other.

Force factor as a function of cone displacement from rest. A map of motor strength at each position of the cone’s travel.

Spread across a wide frequency range rather than concentrated at a narrow band. Changes that cover a chunk of the spectrum instead of a skinny spike.

Verification and adjustment of instruments to meet stated accuracy. Making sure the mic and sensors are reading correctly before testing.

Suspension compliance in meters per newton, the inverse of stiffness Kms. How soft the driver’s spring is.

The mechanical flexibility of the suspension near rest, inverse of stiffness. How easily the cone can start moving.

Deviation from rigid piston motion where parts of the cone bend, flex, or break up during use. The cone surface ripples like a drum head instead of moving as one piece, adding distortion.

A logarithmic unit for level, 20 log10 for voltage ratios and 10 log10 for power ratios. The standard scale for loudness and levels where +3 dB is twice the power, and +10 dB feels roughly twice as loud to our ear.

The set of electrical and mechanical loss mechanisms that dissipate energy and control oscillation. The “brakes” that stop the cone from ringing.

A level-dependent change in average cone position due to asymmetry in BL or Kms. The cone’s resting center slides forward or backward when you play loud.

The amplitude of the test signal, often specified in volts RMS. How much power is applied to the speaker.

The dome that covers the voice coil former opening and stiffens the cone center. The small cap in the middle of the speaker cone.

A laboratory measurement microphone with flat response and tight tolerances. A very accurate mic used for these measurements.

The effective center point of motor force application in the electrodynamic model. The center of the magnetic force that the motor “pushes” from in the math model.

Losses associated with resistance and eddy currents that convert electrical energy to heat. Power wasted as heat in the coil and metal parts.

Instantaneous displacement of the cone from its rest position, usually measured in millimeters. The in and out movement of the cone, and sometimes in reference to how far the cone can move in and out cleanly.

Another name for BL, the proportionality constant between current and force. The motor’s push per amp of current.

Measuring a driver without an enclosure so results reflect the driver itself. Testing the bare driver so the box does not change the result.

A sound field without reflections, approximating an infinite space. A space with no room sound or reflections added.

Output magnitude versus frequency for a specified input. How loud the driver is at each note across the range.

Low level verification of basic operation before formal measurements. A quick listen and scan to make sure the driver is healthy.

A heavy, inert mass of granite countertop used under the fixture to reduce vibration and movement. A big rock slab that keeps the stand from shaking.

Second harmonic component at twice the input frequency. An octave above the frequency that you played, which can sound thicker or boomier in bass frequencies if it gets high.

Third harmonic component at three times the input frequency. A harmonic that is 3x the input signal frequency, often heard as rough or buzzy when elevated.

An assumption that radiation is into a hemisphere rather than a full sphere. Treating the speaker as if it plays into half of space like it is mounted on a big wall.

A measurement condition that approximates free field with radiation into half space and minimal reflections. A setup where the room influence is small and the speaker radiates into a half room.

Output at integer multiples of the input frequency caused by nonlinearities. Extra higher notes the speaker creates that were not in the music.

Measurement of complex impedance versus frequency used to derive small signal parameters and detect faults. A graph that shows the resonant frequency in the form of impedance and helps find issues like rubbing or open coils.

In the context of a speaker, the voice coil and motor inductance, usually in milli-henries, that opposes rapid current change and can vary with position and current. Practically, the motor’s tendency to resist fast changes that can shift tone as the cone moves or volume rises, and causes upper frequency roll off, as it acts like a low pass crossover.

Pretest inspection that includes physical exam, impedance check, and basic function tests. The intake checklist to confirm the sample is good before running full tests.

Distortion products at sum and difference frequencies when two or more tones drive the system. Play two frequencies and you get extra wrong frequencies in between.

A high precision laser displacement sensor that is used to measure cone motion as part of a Klippel measurement setup. A laser that tracks exactly how the cone moves.

A measurement hardware and software platform with tools and methods for identifying loudspeaker parameters and nonlinearities. The industry standard test system that generates the BL, Kms, Le, response, distortion plots, and more.

A rigid mounting fixture supplied by Klippel for standardized free-air tests. The official stand that holds the driver firmly during testing.

Suspension stiffness in newtons per meter, the inverse of compliance. How stiff the driver’s spring is.

A Klippel plot that shows offset of Kms versus excursion, indicating suspension centering or bias. A line that tells you if the spring is tighter on one side.

Suspension stiffness as a function of cone displacement. How quickly the spring stiffens as the cone moves farther.

A Klippel module that identifies large signal parameters and nonlinear characteristics such as BL(x), Kms(x), and Le variations. The part that shows how the driver behaves when pushed hard.

Voice-coil inductance as a function of current (i) at a fixed coil position x, usually at rest. This captures current-dependent effects like magnetic saturation and eddy currents. It tells you whether the coil’s inductance changes as you turn it up, which can shift response and add distortion.

Voice-coil inductance as a function of cone position (x) at negligible current (i≈0). Whether the coil’s inductance changes as the cone moves.

Voice-coil inductance as a function of current (i) with the cone centered at rest (x=0). How inductance changes with drive level when the cone is centered.

Voice-coil inductance as a function of position (x) at negligible current. How inductance changes only with position at small signal.

A frequency sweep whose rate is proportional to frequency, providing equal time per octave. A smooth whoop from low to high that spends the same time in each octave.

The annular region in the motor where the voice coil moves within a concentrated magnetic field. The narrow slot the coil slides through while the magnet pulls.

The equilibrium position of moving parts where restoring forces balance at rest. The cone’s true center when no input signal plays.

Structural separation that prevents vibration transfer between the device under test and sensors. Putting the laser and mic on separate stands so one does not shake the other.

Energy dissipated by friction, air drag, and material hysteresis in the moving system. Motion turned into heat and drag in the suspension and air.

A visualization where a measured curve is mirrored about zero to reveal asymmetry. Flip the curve for an easy visual to compare and see if forward and backward travel match.

The effective mass driven by the motor including cone, voice coil, former, air load, and parts of the suspension. The weight the motor has to start and stop every cycle, including the cone, voice coil former, voice coil winding, etc.

A localized spike in response or distortion confined to a small frequency span. A skinny spike that points to a specific resonance or mechanism.

A high level acoustic sweep taken at a drive level close to the excursion limit chosen for the test, just below the BL 70 percent point. A loud test near the industry accepted standard definition of clean and linear travel limit.

Microphone placement very close to the cone to minimize room influence and approximate free field. Put the mic a couple inches from the dust cap so the room mostly disappears.

Microphone oriented perpendicular to the cone surface at the measurement point. Aim the mic straight at the center of the cone.

A frequency ratio of two to one. Doubling or halving of frequency, like 50 Hz to 100 Hz, which is how many graphs and analyses are organized.

A reduction in output growth with increased input due to nonlinear force, heating, or limits. You turn it up and it stops getting proportionally louder.

The frequency range under evaluation for performance. For subs judge mainly 10 to 120 Hz.

The total quality factor that combines electrical and mechanical damping near resonance and is dimensionless. A number that hints how tight or loose the driver is around its natural bounce, where lower often means tighter control and higher can lean boomy if the box is not chosen well.

The total loss factor expressed as a function of displacement, showing how damping changes with excursion. Whether that tightness stays the same as the cone swings farther from center.

The degree to which repeated measurements under unchanged conditions yield the same result. Running the same test three times and getting the same result.

The frequency in free air where the driver’s moving mass and suspension compliance exchange energy and their reactances cancel, producing a clear peak in electrical impedance, box loading will shift this to a different system resonance. The driver’s natural spring and mass frequency, where it moves most easily.

The cone position at mechanical equilibrium with no input signal. Where the cone sits when silent.

Decaying oscillation after an excitation due to insufficient damping. When a speakers cone “wobbles” on after the initial signal is complete.

The square root of the mean of the squared instantaneous voltage over a cycle, equal to the DC voltage that would deliver the same average power into a resistor, for a pure sine wave Vrms = Vpeak/√2. The steady value that matches a “wavy” AC signal for power comparisons.

The inner suspension element that centers the voice coil and provides restoring force. The corrugated fabric ring under the cone connects the voice coil former on the inside, and to the basket of the speaker on the outside that keeps things aligned and acts like a spring.

Predefined conditions that terminate a test to prevent damage or invalid data. Rules like stop if excursion or temperature goes too high.

The outer suspension element that seals the cone edge and allows excursion.

The combination of surround and spider that provides centering and restoring force. The spring system of a speaker that lets the cone move but recenters it.

A nonlinear behavior that is similar for both directions of motion, and causes odd order harmonic distortion. The cone behaves in a non-linear fashion in and out in the same manner, which mostly adds H3 and other odd order harmonics when pushed.

A Klippel derived plot that shows offset from ideal symmetry across excursion. A zero-line reference that reveals if things stay centered as the cone moves.

The mechanical setup that holds the driver and sensors in a controlled geometry. The stand and mounts that keep everything aligned.

Increase in temperature of the voice coil and motor parts during operation. The voice coil heating up as you play longer and/or louder.

Small signal electromechanical parameters derived from impedance, such as Fs, Qts, and Vas. The gentler level spec sheet that helps determine speaker behavior and choose an enclosure type.

The sum of all harmonic components relative to the fundamental, usually expressed as a percentage or in dB. One number that tells you how much extra harmonic content the driver adds.

A Klippel module that measures frequency response and separates harmonic content using a swept excitation. The tool that generates the response and distortion plots.

Differences between samples of the same model caused by normal manufacturing tolerances. Two of the same woofers may not be exactly identical.

The cylindrical substrate that supports the voice coil and bonds to the cone apex. The tube the wire is wrapped on that connects to the cone.

In the context of an LSI measurement, this indicates that the parameter is evaluated across cone position over its stroke. In simple terms, it shows how the measured value changes as the speaker moves in and out from its center rest position.

The rated one way linear excursion limit per a chosen criterion such as 70 percent BL or 50 percent Kms. How far the cone can swing cleanly before distortion rises fast.