JL Audio 13W3v3-4 - Subwoofer Test Report

In the Lab

Part 1 of this objective measurement section consists of large signal analysis using the rather expensive (about $50,000 including all the software) but enormously effective Klippel analyzer followed by the LEAP 5 parameter and box analysis. Working with the Klippel analyzer (on loan from Klippel GmbH), Pat Turmire, CA&E reviewer and CEO of Redrock Acoustics, ran the large signal analysis on the 13W3v3-4 subwoofer and provided the Bl (x) curve. The black curve is the Bl curve and shows the motor strength of the woofer as it moves in both directions outward from the center rest position. The lighter curve is a type of displacement curve and if both curves were identical, the motor systems motion in and out of the frame would be perfectly symmetrical. When a sub is completely linear ("linear" indicates that the woofer motion exactly tracks the input signal perfectly with absolutely no distortion), the Bl curve should be centered on the 0mm "rest" position (where the cone is positioned when there is no signal) and symmetrically decrease Bl (as the number of turns in the gap decreases with outward motion) with the same slope in either direction of voice coil travel. When a woofer exhibits a forward or rearward offset, it may indicate the magnetic and mechanical systems are not absolutely optimal. If the motor strength decreases more rapidly in one direction (usually the outward direction) than in the other, the result is increased levels of distortion at high operating levels.

The 13W3v3 Bl(x) curve shows the woofer voice coil is offset by a rather inconsequential 1.75mm forward (outward) from its rest position. This Bl curve has a reasonable degree of symmetry with a somewhat shallower slope in the rearward direction. The displacement at operating SPL near Xmax drops to an even less significant 1mm. Bl can decrease to approximately 70 percent of its small signal value and the driver will still function in a satisfactory manner, only with an elevated level of distortion (about 20 percent). The 70 percent of maximum Bl displacement limit for the W3 is greater than 15.5mm, which is the same as the woofers physical Xmax.

This subwoofer's Kms(x) or stiffness of suspension curve also exhibits reasonably good symmetry in both directions of travel, but does have a fairly large 4.9mm forward offset, which decreases to 3.9mm at the physical Xmax of this woofer (remember, symmetry is more important than the size of the offsets). The compliance limit for the suspension when it drops to 50 percent of its rest value is greater than 22.4mm. Both "limit" numbers, Bl and compliance, represent the level at which distortion climbs to 20 percent, which is a realistic criteria for subwoofers given the ear's lack of sensitivity to distortion at low frequencies. In the case of the 13W3v3, Bl is the more limiting factor in terms of reaching the 20 percent distortion number.

Next I generated the T/S (Thiele/Small) parameters. Employing my normal speaker characterization protocol, I used a LinearX LMS (Loudspeaker Measurement System) analyzer and VIBox (VI for voltage/current) for measuring dynamic impedance (impedance at different voltage levels). Testing is accomplished by performing a series of voltage and current sweeps that are later converted to multiple voltage impedance curves (basic Ohm's Law, divide voltage by current and you get resistance, AC impedance in this case). After clamping the driver to a rigid test stand, measurements were made at 1V, 3V, 6V, 10V, 20V and 30V (the 30V curves were later discarded as too non-linear for LEAP 5 to curve fit properly). Instead of using the standard added mass (attaching a measured chunk of clay to the cone) or test box method to find the Vas (volume of air equal to the driver compliance) of this driver, the measured weight of the cone body (with 50 percent of the surround and 50 percent of the spider removed) was employed, as this is significantly more accurate. The group of multi-voltage impedance curves was then copied from LMS and pasted into the LEAP 5 CAD program and the Transducer Model Derivation utility used to create the T/S parameters shown in the Data Chart. The dynamically calculated driver parameters were then used to create the computer enclosure simulations, with the results also listed in the Data Chart.

DATA CHART

Mechanical Specifications

Measured T/S Parameters

Power and Excursion Data

Computer Simulation Data
Enclosure size for simulation (cubic feet)

-3dB (F3) at 2.83V

Sealed (Qtc=0.84)	40Hz
Vented	33Hz

Voltage to achieve Xmax + 15%

Sealed	53V
Vented	60V
SPL at Xmax + 15%	(17.8mm)
Sealed	111.5dB
Vented	114.2dB