Drivers ListFi Car Audio DriversSP4 18D1 (HT option)

ManufactureFi Car Audio
Motor TypeOverhung
Driver Size18 "
Driver Weight70 lbs
Coil Diameter4 "
Coil MaterialAL
Manufacture's Xmax33 mm

TS Parameters (Measured by

Fs21.2 Hz
Res1.3 Ω
Le 1khz2.61 mH
Sd0.117 m2
Vas206.3 liters
Mms521.5 grams
Cms108 μm/N
BL16.59 Tm
BL2/Res211.8 N2/W
L/R Time Constant2.01 milliseconds
SplSens89.62 dB


This is a Fi Audio SP4 18” driver which has had the suspension softened up a bit for home theater or home audio use. This is the third Fi branded 18” driver that has been through Data-Bass and there have been a couple of other drivers which are also out of the Fi build house. This is the heaviest and largest of the drivers so far. The street price on this driver is roughly $550 or so after options and shipping but it will vary a bit on what options are chosen and where the driver has to be shipped to. As with the other Fi drivers I was not blown away with the quality of the packaging, but with the SP4 a plywood baffle board has been added to help stabilize and protect the woofer at least, so it is a cut above what was seen with the SSD and Q series drivers. Despite how the spartan the packaging looks all three of those drivers were shipped multiple times without be damaged so it does seem to be adequate.

The SP4 is a heavy duty subwoofer described as a high linearity design made for extreme power handling and ruggedness, with the primary intended use said to be high power “daily”, or “ground pounder” type car audio installs. The driver is built on the cast aluminum “Ti” frame design popularized by TC Sounds and Eclipse. This frame offers a larger spider landing and clearances than the more common 12 spoke, or 4 (6 on the spoke  18”) frames, but it is also a bit heavier and deeper. It comes with the common TC style rubber gasket as well. The SP4 series all share a kraft pulp fiber reinforced (paper) cone with a wet look, weatherized front and a large, single layer foam half roll surround. The voice coil arrangement is either a dual 1ohm or dual 2ohm nominal impedance and a full 4” diameter. This driver is a D1 arrangement. The winding is 4 total layers with a very long winding length for high excursion and utilizes flat wire that appears to be aluminum or possibly CCA.  The progressive rate spiders are a large 10” diameter to support big coil excursions with linearity and would usually be a pack of 5 on the standard SP4 configuration, but in this case since the driver was optimized for home theater, it only carries a pair of spaced spiders in order to drop the Fs and Qts down a bit. The coil leads are listed as Fi’s FOAD bolted lead design and are listed at 280 count tinsel leads, but being unfamiliar with that type of lead design I'm unsure whether this driver had this design, or was using a standard sewn in set of leads. The terminals are the commonly used spring loaded type. The motor consists of a pair of thick ceramic magnets that are about 8.5” or 9” in diameter and a heavily extended, cone shaped  back plate similar to that seen on most of the current Fi lineup. The pole is solid and extended past the gap, but does not have any venting built into the pole or under the gap. There is venting under the frame by the gap but there is no air venting in the motor as is typical with most modern high power drivers. The top plate has a shaped and extended profile that may make the effective gap a bit taller than it would appear just from glancing at the top plate. The mathematical coil overhang was not measured exactly but it is at least an inch if not more.  Fi rates the Xmax at 33mm one way on this driver but there is no mention if this is measured, or how, or whether it is a calculation or what that calculation is. Either way the large top plate, very long coil and big surround indicate the driver will certainly be able to move the cone assembly. The motor design is mentioned developed with the aid of FEA analysis. The standard options list for this driver is short and includes the dust cap, which can be upgraded to carbon fiber for about $20, the color of the logo on the dust-cap and a cooling option which is not explained very well. It may be an aluminum heat sink on the pole or it may be extra machining to the pole piece. An option not listed is the number of spiders used in the suspension, but Fi can modify the spiders used to suit the application a bit better, as done in this case. This particular driver has the cooling option, a standard black dust cap with no logo and the pair of spaced spiders previously mentioned. It’s a big  heavy driver no doubt and it tips the scales at 70lbs. As always Fi assembles this driver and machines all of the steel parts in house here in the USA.

The driver on hand has been thoroughly broken in by its owner previously and out of the box I noticed how compliant the suspension was. It is the softest suspension I’ve encountered out of the Fi drivers so far which are usually quite stiff. Even though it was already well broken in I still ran a 20Hz sine wave through it and checked out the amount of air and suspension noise. As I’ve noticed with fi drivers previously there is a bit of air noise in the motor even at lower excursion levels simply due to the fact that the air has to be forced up around the coil through the gap. However the suspension was very quiet up until driven to around 2” peak to peak where things were starting to work a bit. At that level there was obvious air noise from the gap as the big 4” coil moved back and forth. The driver was quite well behaved to around 2.5” peak to peak. I pushed it to around 3” peak to peak if not a bit further but it was starting to make a lot of mechanical noise, the surround was getting stretched tight and some distortion was evident by that point so I did not push further. There seemed to be plenty of clearance left in the motor and spiders but I’m not sure how much so I did not chance going further.

After the20Hz noise test, the driver impedance was measured and a set of small signal parameters were pulled from it. The impedance curve is attached in with the pictures of the driver as usual. The RE of the driver with the coils wired in series was almost dead on with Fi’s rating of 1.3ohms. The SD of the driver measured to 1090cm using the 1/3rd surround area method. Fi lists 1210cm for SD which is undoubtedly using the ½ surround area method as most manufacturer’s now do so that explains the differences there. It can be seen that there is a marked up turn in the resistance above the minimum impedance above the driver Fs. This indicates a lot of driver inductance. With a very large coil mass and no mention of any measures to control inductance or distortion in the motor this is not a big surprise. Fi lists no inductance parameters for their drivers but the static inductance at 1kHz was 2.61mH which is quite high for a 1.3ohm driver. The Fs came in at about 21.2Hz,  lower than the factory spec of roughly 25Hz, but this is expected since this driver has less spiders and is well broken in. The MMS measured a bit lighter as well at 521g versus 555g for the factory but overall this is pretty close especially considering that the missing spiders will account for a few grams themselves. The suspension compliance was indeed softer than factory at 1.08mm/N, which again is expected due to the change in spiders. The real surprise was the BL measurement which came in at 16.6Tm, quite a bit higher than the factory rating of 13.7Tm.As a result the Qes and Qts are both dramatically lower than the factory specs. Qts measured as 0.300 which is much below the factory 0.55 rating. A lower Fs due to a looser suspension does affect this but not by such a dramatic amount. Perhaps there are some changes to the coil for this particular driver that I’m not aware of. A copper coil would increase BL but would also increase the MMS dramatically as well.  It’s an unknown in this case but all things equal I’d take the extra motor strength as a plus.

Fi Car Audio 18" SP4 18D1 (HT option) Systems