A subwoofer is a loudspeaker that reproduces low end sound frequencies between 20 to 200 Hz. It consists of one or more woofers mounted in a sealed or ported wooden or MDF (medium density fiberboard) subwoofer enclosure. It can have a built-in amplifier or be driven by an external one. A typical subwoofer is an active speaker with one or more 8 to 15 inch drivers. It has an electronic crossover with a low-pass filter to screen out high and midrange frequencies. The crossover might also have a high-pass filter to prevent low frequencies from entering the main speakers which are not designed to handle bass. Passive subs are a little less common.
In this article we are going to talk about a specific kind of subwoofer, the compact subwoofer. It is a shallow-height device used mostly in automobile applications. It has a slim space-optimized housing and easily slides under car seats for discrete installation.
Normally, to produce bass a speaker has to move a lot of air and that is made possible by the large sizes of both the woofer and the enclosure. Compact subwoofers such as underseat subwoofers are smaller and lack the advantage of extra space inside the enclosure. So how do they generate room-filling bass?
Let’s find out.
How does a compact under-seat subwoofer work?
The concept of a compact subwoofer was popularized by audio maker, Sunfire. It has a wide selection of subwoofers with 9-, 11- and 13-inch drivers that deliver high-output bass from compact housings. One of its pioneer compact subs, the True Subwoofer, consists of a high-excursion long-throw driver inside a diminutive enclosure. The terms high-excursion and long-throw indicate the distance a speaker cone moves in and out when pumping out bass. The device, developed by Sunfire founder and legendary sound expert, Bob Carver, is fitted with a 10-inch woofer with an extra-long 2.5-inch excursion (which is about five times that of a standard 15-inch woofer), and a passive bass radiator which is similarly configured.
Due to the high cone movement, the company had to design new surrounds, voice coils and suspensions that would better support the active driver. Even the magnet structure is bigger and weighs 14 lbs. The passive radiator of the device is not connected to the amp. It does not have a voice coil and magnet. Instead, its diaphragm is tied to a 1.8 lbs weight and moves in response to the back-pressure generated by the active driver’s movement. The passive radiator functions like a vent in a bass-reflex enclosure, but provides comparatively higher tuning range.
The True Subwoofer is equipped with an active and passive crossover for channeling low frequencies into the sub. It has a control panel for quick adjustments. It has a high-quality MDF enclosure that is smaller than regular sub enclosures. Now, the problem with a small enclosure is that it creates a lot of air pressure inside it which can be hard to overcome. But in order to stay compact and fit narrow spaces, the company could not increase the size of the box. To resolve this issue, Carver developed a hyper-efficient amplifier built around a Tracking Downconverter which is basically an amplifier that monitors transient power requirements. The high-power amp is mounted to the back of the control panel inside the box. It delivers exceptional power into low-impedance loads without overheating. For example, it can generate 2700 watts for a 4 ohm load quite comfortably. For a conventional subwoofer, 2700 watts is a lot and can cause serious damage to the speaker. But an under-seat sub can handle it without any repercussions.
A compact sub is essentially an all-in-one system with one or more woofers, an integrated high-power amplifier and crossovers, all mounted and wired inside a single speaker cabinet. It contains a specially developed high-excursion long-throw woofer cone for better bass reproduction. It has custom-made surrounds, suspensions, magnets and voice coils.
An under-seat sub is best suited for vehicles that do not have much truck space or rear area behind the backseat. It is easiest to mount under seats due to its shallow profile. For instance, the JBL BassPro, a leading car under-seat subwoofer, is less than 3 inches tall and comfortably slides under the driver or passenger seat.