What do the Thiele–Small parameters mean?

Fs is the driver’s free-air resonance; Qes and Qms are the Q values from electrical and mechanical damping, combining into Qts; Vas is the volume of air with the same compliance as the suspension; Re is the voice-coil DC resistance, Sd the effective cone area and Xmax the linear one-way excursion. Together they describe the driver as a mass–spring–damper, which is all enclosure design and response prediction need.

Sealed or ported: how do I choose?

A common guide is EBP = Fs/Qes: below about 50 suits a sealed box, above about 100 a ported box, and in between either works. Sealed boxes roll off gently (12 dB/octave), have good transient response and are compact. Ported boxes go deeper with the same driver and have less excursion near fb, but below fb they fall at 24 dB/octave and the cone loses its air load, so excursion soars and a subsonic filter is usually wise.

How is port length calculated, and why does air speed matter?

The air mass in the port resonates with the box’s air spring at fb. The effective length is L = c²S/((2πfb)²Vb); subtract an end correction of about 1.46 times the port radius to get the physical length. A narrower port is shorter, but at high levels the air speed gets too high (above roughly 17 m/s as a rule of thumb) and you hear chuffing and compression, so the port must be wide enough, even if that means a longer port or several ports.

Why is the tweeter sometimes wired in reverse polarity?

At the crossover point, second-order low-pass and high-pass outputs are 180° apart. With LR2 (Q = 0.5) each branch is −6 dB there, and inverting the tweeter brings them in phase for an exactly flat sum. With Butterworth (Q = 0.707) each is −3 dB: normal polarity gives a deep notch and inverted gives a +3 dB bump. In practice the drivers’ own phase and acoustic centres matter too.

Why do noise-cancelling headphones work on low rumble but not voices?

ANC cancels noise with an inverted wave, leaving a residual of |1 − ae^{jφ}|; a phase error of just 6° limits the reduction to about 20 dB. Tens of microseconds of processing delay is under 1° at 100 Hz but tens of degrees at 3 kHz, where cancellation basically fails. The quiet zone is also only about a tenth of a wavelength across, which short high-frequency wavelengths can hardly cover around the ear. So ANC targets low-frequency rumble, while mid and high frequencies are handled by the passive isolation of the ear cups.

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🔊Loudspeaker & Speaker Box Calculator - Thiele–Small, Sealed, Ported, ANC

Enter a driver’s Thiele–Small parameters, or pick a generic preset, and the tool derives BL, moving mass, compliance and sensitivity. It then solves the full electrical–mechanical–acoustic equivalent circuit frequency by frequency, plotting SPL response, input impedance, cone excursion and port air speed for an infinite baffle, sealed box or ported box. It also works out SPL at any power and distance, component values for a second-order passive crossover, and how much active noise cancelling achieves with a given latency and amplitude error.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Loudspeakers, enclosures & noise cancelling

1On Driver, pick a preset or enter Fs, Qes, Qms, Vas, Re, Sd and Xmax, and check the derived values and cone animation
2On Enclosure design, choose sealed or ported and set the volume and tuning from a target Qtc or the rule-of-thumb alignment
3Check the response, F3, excursion vs Xmax, port length and air speed, and adjust as needed
4Use the other tabs for SPL, crossover component values, or noise-cancelling performance vs latency

FAQ

What do the Thiele–Small parameters mean?: Fs is the driver’s free-air resonance; Qes and Qms are the Q values from electrical and mechanical damping, combining into Qts; Vas is the volume of air with the same compliance as the suspension; Re is the voice-coil DC resistance, Sd the effective cone area and Xmax the linear one-way excursion. Together they describe the driver as a mass–spring–damper, which is all enclosure design and response prediction need.
Sealed or ported: how do I choose?: A common guide is EBP = Fs/Qes: below about 50 suits a sealed box, above about 100 a ported box, and in between either works. Sealed boxes roll off gently (12 dB/octave), have good transient response and are compact. Ported boxes go deeper with the same driver and have less excursion near fb, but below fb they fall at 24 dB/octave and the cone loses its air load, so excursion soars and a subsonic filter is usually wise.
How is port length calculated, and why does air speed matter?: The air mass in the port resonates with the box’s air spring at fb. The effective length is L = c²S/((2πfb)²Vb); subtract an end correction of about 1.46 times the port radius to get the physical length. A narrower port is shorter, but at high levels the air speed gets too high (above roughly 17 m/s as a rule of thumb) and you hear chuffing and compression, so the port must be wide enough, even if that means a longer port or several ports.
Why is the tweeter sometimes wired in reverse polarity?: At the crossover point, second-order low-pass and high-pass outputs are 180° apart. With LR2 (Q = 0.5) each branch is −6 dB there, and inverting the tweeter brings them in phase for an exactly flat sum. With Butterworth (Q = 0.707) each is −3 dB: normal polarity gives a deep notch and inverted gives a +3 dB bump. In practice the drivers’ own phase and acoustic centres matter too.
Why do noise-cancelling headphones work on low rumble but not voices?: ANC cancels noise with an inverted wave, leaving a residual of |1 − ae^{jφ}|; a phase error of just 6° limits the reduction to about 20 dB. Tens of microseconds of processing delay is under 1° at 100 Hz but tens of degrees at 3 kHz, where cancellation basically fails. The quiet zone is also only about a tenth of a wavelength across, which short high-frequency wavelengths can hardly cover around the ear. So ANC targets low-frequency rumble, while mid and high frequencies are handled by the passive isolation of the ear cups.

Features

Derives BL, Mms, Cms, Rms, reference efficiency and 1 W/1 m sensitivity from T/S parameters, with an EBP box-type hint
Full equivalent-circuit solution: SPL, input impedance, cone excursion, port air speed and Xmax-limited maximum power
Sealed box volume from a target Qtc; ported rule-of-thumb alignment; port length with end correction
Overlay infinite baffle, sealed and ported responses
Second-order Butterworth / LR2 crossover values, L-pad and summed response
Active noise cancelling wave animation and reduction vs phase error, latency and frequency

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