Measure loudspeaker Thiele-Small parameters from impedance curves. Free-air and added-mass methods with automatic peak detection and enclosure design recommendations.
1. Suspend the driver freely (do not mount in a baffle or box).
2. Connect the measurement signal to the driver through a known series resistor.
3. Place the microphone in free-field conditions.
4. Click "Measure" to run the impedance sweep.
Thiele-Small (T/S) parameters are the fundamental electromechanical properties of a loudspeaker driver that describe its behavior in an enclosure. Originally developed by A.N. Thiele in 1961 and refined by Richard Small in 1972, these parameters allow engineers to predict the low-frequency acoustic output of a driver in various enclosure alignments before building a single prototype. The parameters are derived from the driver's impedance curve measured in free-air and with a known perturbation (added mass or known box volume), making them accessible to both professionals and hobbyists with basic measurement equipment.
A loudspeaker's electrical impedance varies with frequency, exhibiting a prominent peak at the free-air resonant frequency (Fs) where the mechanical and acoustic reactances cancel. Below Fs, the impedance is dominated by the compliance of the suspension (spider and surround). Above Fs, the moving mass dominates, and at higher frequencies, the voice coil inductance causes the impedance to rise progressively. The height and width of the resonance peak are characterized by the quality factors: Qms (mechanical losses from suspension friction and air resistance), Qes (electrical damping from the motor's back-EMF), and Qts (the parallel combination of Qms and Qes).
To determine the moving mass (Mms), a known mass is attached to the driver cone and the impedance is re-measured. The resonant frequency shifts downward because Fs is inversely proportional to the square root of the moving mass. By comparing the original and shifted resonance frequencies, Mms can be calculated. From Mms and Fs, the compliance (Cms) is derived. The alternative known-box method seals the driver in a box of known volume and measures the shift in resonance caused by the added air stiffness, which yields the equivalent compliance volume (Vas) directly.
The T/S parameters determine which enclosure type best suits a given driver. Drivers with Qts between 0.3 and 0.5 typically work well in vented (bass reflex) enclosures, offering extended bass with higher efficiency. Drivers with Qts between 0.5 and 1.2 are better suited for sealed (acoustic suspension) enclosures, which provide tighter transient response and gentler roll-off at the expense of bass extension. The Vas parameter determines the enclosure volume: high-Vas drivers require large boxes, while low-Vas drivers can work in compact enclosures. SonaVyx calculates standard alignments (Butterworth B2 for sealed, QB3 for vented) and provides port dimensions for the vented design.
The reference efficiency (eta0) represents the percentage of electrical input power converted to acoustic output power. Most loudspeaker drivers have efficiencies between 0.1% and 5%, with professional drivers achieving higher values through stronger motors and lighter moving assemblies. The sensitivity rating (SPL at 1W/1m) is derived from efficiency and is the primary specification for comparing drivers in system design. A 3 dB increase in sensitivity is equivalent to doubling the amplifier power, making efficiency a crucial parameter in professional sound reinforcement.
The added-mass method attaches a known mass to the cone and measures the frequency shift to determine Mms. It is simpler but requires accurate mass measurement (0.1 g precision). The known-box method mounts the driver in a sealed enclosure of precisely known volume and measures the resonance shift caused by the enclosed air spring. It directly yields Vas but requires building a test enclosure. Both methods produce equivalent final parameters.
The added mass should shift the resonant frequency by at least 20-30% (a factor of about 1.3-1.5 downward). As a rule of thumb, use a mass equal to the expected Mms of the driver. For a typical 6.5-inch woofer, 10-20 grams is appropriate. For a 12-inch subwoofer, 50-100 grams may be needed. Use modelling clay, Blu-Tack, or stacked coins, weighed to 0.1 g accuracy.
For sealed enclosures, Qts between 0.5 and 1.2 is ideal. Higher Qts yields a smaller box but with a higher F3 (-3 dB point). For vented enclosures, Qts between 0.2 and 0.5 gives the best results. A Qts around 0.35-0.40 is the sweet spot for most vented alignments. Drivers with very low Qts (under 0.25) indicate overdamped systems that benefit from horn loading.
Manufacturer specs are typically measured on production samples with break-in (10-50 hours of exercising the driver). New drivers out of the box may have stiffer suspensions, resulting in higher Fs and lower Vas. Temperature affects Re (copper resistance increases ~0.4%/C). Measurement conditions (free-air vs. baffle-mounted, room reflections) also contribute to differences. Break in the driver for at least 10 hours before final T/S measurements.
Yes. The traditional method uses a signal generator, a known series resistor (e.g., 100 ohms), and a multimeter to measure the voltage across the driver at various frequencies. This manual method is accurate but time-consuming. SonaVyx automates the process using a log frequency sweep played through the audio output and captured through the microphone input, processing the impedance curve computationally for faster and more detailed results.