Glossary

Short definitions of the less familiar terms that turn up in the Learn articles. Anywhere one of these is underlined, click it to see its definition without leaving the page.

Anechoic: "Without echoes." An anechoic measurement, or a simulation like Baffle's, shows the speaker on its own, with no reflections from walls, floor or ceiling. A real room adds those reflections and, in the bass, resonances of its own, so what you hear in a room differs from the anechoic curve, most of all at low frequencies.
B4: The fourth-order Butterworth alignment for a vented box: the flattest response a vented box can have, with no peak. In the lossless textbook case it needs a driver with a Qts of about 0.383, in a box of Vas/√2 tuned to fs. "Suggest an enclosure" in the workspace solves it for your driver.
Bessel alignment: A response shaped for the flattest possible group delay, so every frequency in the passband is delayed by nearly the same amount. It pays for that with a gentler, earlier rolloff than . In a sealed box it's a Qtc of about 0.577.
Butterworth alignment: A response shaped to be as flat as possible across the passband, with no peak. In a sealed box it's a Qtc of 1/√2, about 0.707, which is exactly 3.01 dB down at the system resonance. The vented-box version is .
Compliance: The reciprocal of stiffness: how far something moves for a given force. A driver's suspension compliance, Cms, is in meters per newton, and a softer suspension has a larger one. The air sealed in a box is a spring too, which is why Vas can express the suspension's compliance as a volume of air.
Compliance ratio (α): α = Vas/Vb: how much stiffer the air in the box is than the driver's own suspension. In a sealed box both the resonance and the Q rise by a factor of √(1 + α), so a box of a tenth of Vas raises them by more than three times.
Critical damping: The amount of damping at which a resonance comes to rest fastest without overshooting. With less damping it overshoots and rings; with more it creeps back more slowly. For a second-order system like a sealed box, it's a Q of 0.5.
Diffraction: Sound bending around an obstacle. At a cabinet's edges it makes each edge act like a second, delayed source, and those contributions ripple the response. Lower down, where the whole cabinet is small next to the wavelength, the sound wraps all the way around it, which is the baffle step.
Directivity: How a speaker's output changes with angle. A driver radiates nearly equally in every direction while the wavelength is much larger than its cone, and beams increasingly forward as the wavelength shrinks toward the cone's size. Baffle doesn't model it, which is one reason its curves are best trusted in the bass.
Half space and full space: Half space is the hemisphere in front of a driver set into an endless wall: 2π . Full space is the whole sphere around a driver hanging in the open: 4π steradians. For the same cone motion, the level in half space is 6.02 dB higher. Box simulations are conventionally worked out in half space; the baffle step is what a real cabinet does instead.
Helmholtz resonator: A volume of air with a narrow opening, like a bottle. The air in the neck is a mass that bounces on the springiness of the air inside, so it resonates at one frequency: the note you get by blowing across a bottle. A vented box is one, and its tuning, fb, is that resonance.
Infinite baffle: A driver mounted so that the sound from its back never reaches the front: in theory an endless wall, in practice a wall or ceiling with the rear opening into a space so much larger than Vas that the air behind barely stiffens the suspension. It's also the name of the workspace's default cabinet setting, where the response is worked out in with no cabinet edges.
Isothermal: At constant temperature. Air squeezed by a moving cone warms up as it's compressed, which makes it stiffer. Fibrous filling soaks up that heat and moves the process toward isothermal, so the air becomes a softer spring and the box behaves as if it were larger. In principle that could be up to 1.4 times, the ratio of air's specific heats; in practice filling gets much less, and Baffle caps the effect at 1.15.
Lumped-element model: A model that treats each part of the system (the cone's mass, the suspension, the air in the box, the air in the port) as a single ideal element, as if it sat at one point, and solves them together as an equivalent electrical circuit. It holds while every part is small compared with the wavelength, which for a loudspeaker means the bass. Thiele and Small's analysis is built on it, and it's the model Baffle solves.
Minimum phase: A system whose phase response is fixed by its amplitude response, with no extra delay beyond what that requires. Sealed and vented boxes behave this way in the lumped-element model, so a minimum-phase equalizer that corrects their amplitude corrects their phase and ringing along with it.
Push-pull: Two drivers arranged so that when one cone moves out of its motor, the other moves in. Nonlinearities that depend on which way the cone is moving, the source of even-order distortion, then act in opposite directions in the two and partly cancel. An isobaric pair mounted cone to cone, one wired in reverse, is push-pull.
Rigid piston: The idealization that a driver's cone moves as one solid piece, every point together, like a piston in a cylinder. It holds at low frequencies. Higher up the cone flexes and different parts move differently, which is called breakup, and neither the model nor Baffle includes it.
Spider: The corrugated fabric ring behind the cone, joining it to the frame near the voice coil. It keeps the coil centered in the magnet gap and, together with the surround at the cone's outer edge, makes up the suspension whose stiffness helps set the driver's resonance.
Steradian: The unit of solid angle, the three-dimensional version of the radian. On a sphere of radius r, a patch with an area of r² takes up one steradian. A whole sphere is 4π steradians, about 12.6, and a hemisphere is 2π.

Know a term better than we do? Tell us what to correct or add.