Bandpass Enclosures: How to select Right Box For Subs

The cabinet or structure of a speaker into which the various speaker elements (the drivers, the crossover, the binding posts, etc.) are placed and attached.

Speaker enclosures can be essentially any shape and any size made up of almost any material. Most speaker enclosures are made of MDF (medium density fiberboard) and then covered in a thin wood veneer or some other coating. Some cabinets are made of plastic or metal and some even use concrete, Corian, or other highly rigid materials.

Enclosures generally should be very well braced with internal strengthening members and fairly thick cabinet walls. Enclosures should be inert so as not to color the sound by vibrating with the speaker drivers. They must also be designed to function properly with the speaker drivers placed in them (for example a large woofer for a subwoofer will not perform optimally in too small an enclosure). There are as many variations on speaker enclosures as there are speakers.

There are generally no hard and fast rules for what makes a good enclosure other than the resulting speaker sound good (in order to accomplish this there are certain mathematical formulas for matching a speaker driver to the proper volume enclosure). When purchasing speakers, however, look for enclosures that are solid and do not sound hollow when you tap them with your knuckles. Enclosures should be sturdy and well built. A heavy enclosure does not ensure a good enclosure, but often a heavier enclosure is more solid and more inert and so may have some bearing on your decibels.

A type of speaker enclosure used with subwoofers to produce large amounts of bass with minimal power input by utilizing essentially a sealed subwoofer enclosure mated to a ported box. A bandpass enclosure first uses a sealed subwoofer enclosure with an outward firing bass driver. This enclosure is sealed to a second enclosure with the bass driver firing into the second enclosure. The second enclosure does not have its own driver only a port. The audio output developed by the driver is reinforced in the second ported enclosure and then emerges through the port. In a bandpass enclosure the bass driver is not visible from outside the complete enclosure, and all the sound output comes through the port or tube.

Bandpass enclosures maximize the output of a speaker driver over a narrow range of low frequencies. The enclosure must be carefully designed and built to strict tolerances in order to operate properly. Bandpass enclosures are typically not used for high-end, high-quality subwoofers. They are limited in the frequency band they can reproduce and their sound quality is generally inferior to well-designed traditional subwoofer enclosures using ported or sealed designs.

A type of speaker enclosure used for subwoofers and bass drivers which uses a small, sealed enclosure with two bass drivers facing each other (one inside the box facing out and the other outside the box facing in at its counterpart) and wired out of phase. The primary advantage of this type of configuration is that the enclosure is small – about half the size of a sealed enclosure for the same output. The enclosure must be sealed and the two drivers must be sealed together. Isobaric enclosures leave one driver essentially hanging out in the open air making it a somewhat challenging configuration to achieve due to aesthetics and the need to protect the driver outside the box. This type of configuration is sometimes found in car audio but is rare in home theater and other home audio systems.

Tube of a specified length and diameter (length and diameter dependent on specific application) with one end open to the outside of a speaker enclosure through a round hole and the other open to the inside of the speaker enclosure. Ports are used primarily for subwoofers and other bass speaker drivers. They allow the air movement inside the speaker enclosure caused by the back of the speaker driver to vent or move to the outside.

Using a port enables a speaker driver to play at a louder level than an equivalent speaker driver in a sealed enclosure when driven with the same power. Ported enclosures, also known as bass reflex enclosures, enhance the efficiency of a speaker driver, but they result in some decrease in total low frequency extension. As a compromise between ports and completely sealed enclosures, some speaker enclosures feature passive radiators allowing the cabinet to remain sealed while taking advantage of some energy generated inside the enclosure.

Mechanical device used to reproduce sound waves when a power signal is applied representing those sound waves by vibrating some material that in turn creates vibrations in the air thus generating sound.

Speakers come in many varieties and many versions suited for a variety of tasks. The majority of speakers are dynamic designs using cones or domes vibrated by the movement of a coil of wire near a stationary magnet (the coil of wire generates a magnetic field that attracts it to and repels it from the magnet thus generating motion). Electrostatic and planar-magnetic speakers are also used along with a number of variations and other sound reproduction methods.

Most speakers use a crossover to split up the audible frequency into a number of bands or sections. The frequency bands are then reproduced by different speaker drivers most capable of reproducing those sounds. Low-frequencies use woofers that are large in size. The middle frequencies are often reproduced with midrange drivers and the highs are handled by tweeters. Subwoofers are sometimes used also. Subwoofers are specially designed speakers whose purpose is to reproduce low frequency sound waves only.

Small speakers are typically known as satellites or bookshelf speakers while larger speakers are known as floor-standing speakers. There are center channel speakers designed to sit on top of or below a video display to reproduce movie and television dialog and other effects. Surround channel speakers are mounted high up on the walls near or behind the listening position to create a three-dimensional, fully enveloping sound field. Most surround speakers are dipole or bipole designs.

Dipole speakers feature speaker drivers on two opposite sides of a speaker enclosure. Their drivers operate out of phase so that as one driver moves in the other driver on the opposite side movies out. Dipolar speakers create a broad sound on either side of the speaker and eliminate most sound from the sides of the speaker.

Bipolar speakers also emanate sound from two opposite sides of a speaker enclosure, but their speaker drivers operate in phase meaning that they both go in and out at the same time. Bipolar speakers create a wide soundstage with sound primarily at the front and back of the speaker but also propagating to the sides.

Direct radiating speakers emit sound from one side with a single set of speaker drivers. Direct radiators are easier to locate sonically. They also provide the best imaging and space delineation, but their sweet spot (optimal listening position) is fairly small and their soundstage is not as wide as that of a bipolar speaker.

Speaker drivers of any type are built into some form of cabinet to hold them. Most speaker enclosures are built of sturdy materials such as MDF and are braced internally with crossbeams and other such pieces to ensure they are strong and do not vibrate too much with the drivers (this is called resonance and creates a sonic distortion). The enclosures are designed in different ways to affect the sound of the drivers mounted within them, especially in regard to low frequency output. The most common types of enclosures are sealed or acoustic suspension enclosures, ported or bass reflex enclosures, bandpass enclosures, and isobaric enclosures. Of these, the most widely used are the ported and sealed designs.

Speakers feature some type of binding that allows power from an amplifier to be connected with speaker wire. The two primary connections are five-way binding posts and spring connections. Of the two, the five-way binding post is vastly superior.

Speakers all serve essentially the same function, the faithful reproduction of sound. However, there are seemingly as many variations of speaker design as there are sounds to reproduce. The important thing is finding a speaker with a good quality of sound, a good dynamic range and a reasonably flat frequency response that fills the specific needs of an individual.

Special form of speaker used to reproduce only the lower portion of the audible frequency spectrum usually from 80 Hz down to or below 20 Hz. True subwoofers should be able to play useful audible information down close to 20 Hz (the lower limit of human hearing). Most subwoofers feature one or more speaker drivers measuring ten or more inches in diameter (with powerful subwoofers often using drivers of a minimum twelve inches in diameter).

There are two primary types of subwoofer – powered and non-powered. Powered subwoofers feature their own built-in amplifier while non-powered models need an outboard amplifier (or may be connected to the amplifier used for the main stereo speakers using a crossover). Powered subwoofers are by far the most popular and most versatile models.

Most subwoofers contain some type of passive or active crossover network to separate the low bass signals (usually below 80 Hz) from the rest of the frequency spectrum. The subwoofer then reproduces the low frequency signals and sends the remainder on to the other speakers in an audio system.

Within the realm of subwoofers, there are a number of cabinet designs used including bass reflex or ported enclosures, sealed or acoustic suspension enclosures, isobaric enclosures, and bandpass enclosures. Of these, the two most popular for home audio are the sealed and ported enclosures.

Sealed enclosure subwoofers feature the lowest extension (play the lowest into the frequency spectrum) and best sound quality, but they are also the least efficient needing additional power to reach the same output levels (volume) as the other designs. Ported subwoofers, on the other hand, use a port (a tube of a particular length allowing air to pass from the inside of the enclosure to the outside through a round hole) to increase efficiency and add around three decibels more output for a given power input versus a sealed enclosure while giving up some ultimate low frequency extension. Both ported and sealed box subwoofers can provide powerful, engaging sound with excellent quality when properly designed and set up.

Subwoofers need a large amount of power in order to optimally reproduce low frequency sounds. Low frequency sound waves are very large (about 56 feet long for a single cycle at 20 Hz) and thus require large drivers and large amounts of power to faithfully reproduce them. Most subwoofers feature long-excursion drivers meaning that the speaker drivers travel a long distance back and forth to generate sound waves (a long distance, of course, is relative with most long-throw driver moving up to one inch or so). As drivers become larger and they travel farther, more and more power is required to move them and provide adequate control (damping). Most powered subwoofers, thus, feature amplifiers of 200 watts or more, although some subs use amplifiers of 100 watts and others use more than 1,000 watts.

Due to the nature of low frequency sound waves, subwoofers do not need to be used in pairs or located in a specific location relative to the other speakers in a room. Low frequency sound waves are omnidirectional and the human ear cannot locate the origin of sound waves below 80 Hz. This attribute allows subwoofers to be placed anywhere they sound best and allows the use of single subwoofers. Multiple subwoofers provide benefits of increased low frequency power and extension while helping to smooth out frequency response in a given room making two or more subwoofers desirable if possible but certainly not necessary in most cases.

Some subwoofers make use of a servo feedback loop allowing the subwoofer to correct its driver motion and reduce distortion. The servo circuit monitors the movement of a subwoofer’s speaker driver and compares it to the input signal (what the subwoofer is supposed to output). If the two do not match, there is distortion. The servo circuit watches for such distortions and then adjusts the amplifier’s output to correct the speaker driver’s motion resulting in reduced distortion.

Subwoofers are becoming increasingly important and useful in home audio and home theater. They produce the lowest frequencies with weight and accuracy allowing the other speakers to concentrate on frequencies above 80 Hz or so. Typically this allows the other speakers to provide better sound quality more effortlessly since less of their input power is needed to produce low-frequencies. Additionally, the 5.1 digital surround sound systems allocate a specific low frequency effects channel for subwoofers. Subwoofers produce the lowest frequencies, those below 80 Hz or so, with the greatest detail, accuracy, ease and power adding visceral impact and oomph to almost any audio system.