Systems ListRythmik AudioF18

ManufactureRythmik Audio
System typeSealed
Driver Size18 "
Number of drivers1
Internal volume4 ft3
External volume5.59 ft3
Dimentions (Width Height Depth)20.5 " 20.5 " 23 "
Weight115 lbs
Power900 W
Aprox. Price$1530 USD


The F18 subwoofer is currently one of Rythmik Audio’s most powerful sealed subwoofers, sitting just below the even larger and more powerful F25HP. The F18 is a sealed design based around an 18 inch forward firing driver with the companies patented servo control system. I will not go into the operation of the servo system here. For more information on that I recommend reading the documentation available at the Rythmik website here, or for an even better understanding, a perusal of the patent documentation. The F18 is compact for an 18 based unit with dimensions of just 20.5x20.5x23 inches including the grille and amplifier heatsink. The unit does weigh in at a chunky 115lbs. Available finishes include: Matte black, black oak and gloss black. Additionally most of the Rythmik subwoofers are available with a silver driver cone, which is the finish of the raw spun aluminum. The F18 sells for $1530 shipped to the continental 48 US states. The gloss black finish carries an upcharge of $200 and the raw aluminum look cone adds an additional $100. The standard Rythmik warranty is 5 years on the cabinet and driver and 3 years on the electronics.

The driver inside of the F18 and FV18 subwoofers is Rythmik’s proprietary DS1820 driver. The driver features a spun aluminum cone, rubber half roll surround with the Rythmik logo molded into the mounting flange area and the familiar 12 spoke frame. Interestingly the frame has a 12 bolt pattern instead of the usual 8 bolts. The spider system appears to be a single 8 inch diameter high excursion unit. The motor system consists of a large double stack of ferrite magnets that are about 8.5 inches in diameter and 1 inch thick. The back plate features a generous bump out to clear high incursion levels and a large pole vent for cooling. The top plate appears to be about 12mm thick. The frame also features large under spider vents to help facilitate cooling. The motor design incorporates multiple shorting rings to combat inductive distortions. The voice coil is nominally a 3 inch diameter unit with a secondary smaller coil for the servo circuit. The coils are labeled and also keyed by manner of smaller spade terminals and lead gauge for the servo coil connection. Xmax for the unit is said to be 20mm one way. The DS1820 is a weighty and substantial driver.

The amplifier for the F18 and FV18 subwoofers is Rythmik’s HX1000XLR3, which is rated as a 900w rated amplifier. In keeping with Rythmik tradition it has a veritable buffet of control knobs and toggles on the face plate. There are stereo balanced XLR inputs which are switchable via a toggle to operate as a matched pair or with the R source in XLR operating as an unfiltered LFE input. There are also 2 sets of unbalanced RCA style inputs. One set for LFE and one set for use in systems without a dedicated external low pass filter. The power switch is able to be set for Auto/On/Off. There is a 12v trigger available as well as a switch to enable the amplifier to be connected to either 120v or 240v AC. The usual volume, phase and on board low pass filter controls are included as well as a single band parametric EQ, which can be enabled or bypassed via a toggle switch. Towards the bottom of the amplifier there is a series of 4 toggle switches used for tailoring the subwoofers response shape and behavior. The left most switch enables a fixed low pass filter at 50, 80 or 100Hz when not using the LFE input. Of note is that this toggle switch also interacts with the typical low pass filter knob provided, which further increases the range of adjustment of the internal low pass filter. Next to the low pass filter toggle there is a high pass filter toggle labeled Rumble Filter. This is toggle is an On/Off switch for a high pass filter that rolls off the extreme deep bass frequencies for added protection of the subwoofer. The right 2 toggle switches at the bottom of the amplifier are for tailoring the deep bass extension of the subwoofer and the amount of damping provided by the servo system between Low/Mid/High. There is even a switch to turn on or off the amplifier limiter. Rythmik is the only company I am aware of that provides an option to defeat the limiter. Needless to say the amount of controls provided, what each does, how they interact and when to use each could be overwhelming to some users even after reading the guides provided by Rythmik. Removing the amplifier from the cabinet revealed that these are not SMPS based or switching amps, but are analog based with a large, old school, transformer. These Rythmik amplifiers are a serious hunk of equipment. The HX1000XLR3 probably weighs 20lbs or more and the HX2000XLR3 is even heavier.

The cabinet of the F18 is constructed of MDF material and is cross braced internally. Additionally there is a generous amount of damping material placed inside of the cabinet. The top and bottom corners are rounded over. The cabinet proved to be quite inert and solidly built during testing.

The sum of the F18 parts is a sub that does a lot right. The available amplifier controls allow the subwoofer response to be tailored to almost any shape needed. The F18 exhibits low distortion behavior at low to moderate playback levels and overloads gracefully. With the limiter on I was unable to get the F18 to make an offensive noise. At its output limits the harmonic distortion below 30Hz increases, but without any vent noises or obvious mechanical sounds from the driver accompanying it, it was only noticed as an increased warmth or heaviness to the sound. This was outdoors. Indoors, where there is often a generous amount of deep bass distortion reduction, it sounded even cleaner. Turning the limiter off would allow the F18 to produce a bit more output but with greatly increased distortion and the possibility of clearly audible overload sounds or clipping if driven hard enough. I would have to imagine that pushing the driver and amplifier that much further also increases the likelihood of damage, though even while maximum output testing with the limiter off, the F18 survived everything thrown at it. The tiny amount of extra output headroom available from operating it with the limiter off doesn’t seem worth the increased distortions and stress on the system. I’d recommend leaving the limiter on. The F18 exhibits good deep bass headroom for a sealed design of this size, which also allows it to be placed into a lot of rooms that truly large subs may not fit well. In general the behavior of the F18 was tightly behaved and predictable in the best way. The slew of F18 measurements back this up and show it to be a well rounded performer in all of the critical metrics.