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SEALED BOX

Sealed box subwoofer enclosures have advantages that appeal to beginners because of the simplicity of box design and construction, and to experts because of the compact enclosure dimensions, very good power handling, and outstanding transient response. They are noted for their tight "non-boomy" sound, and relatively small size compared to other designs. The driver will tend to have a fairly low free air resonance (Fs), long excursion capability (xmax), and loose suspension (the air loading of the small box provides restoring force). A bigger version of the contemporary compact sealed box is the classic "infinite baffle" which, because of the increased compliance of the air in the larger enclosure, requires a stiffer mechanical suspension to provide loading for the woofer cone. The larger cabinet volumes for infinite baffles are not always practical for all applications. All sealed box designs, large or small, tend to have lower sensitivity and higher bass roll off characteristics than other designs.

VENTED BOX

Vented, ported, ducted, bass reflex; these terms all describe the same type of enclosure. Vented designs have been around for quite a while, but really came on with a bang after acoustic researchers devised a way to effectively identify speaker parameters (the mechanical and acoustical characteristics of the driver in question) and use those parameters as elements in mathematical formulas that consistently model or predict the response of a given driver in a given enclosure. Simply put, the driver is matched to what is essentially a tuned resonant air chamber. As the driver goes lower in frequency, the driver excursion is reduced and the air in the vent proportionately increases its pressure on the air outside of the enclosure. This "high pressure air" in the port is just as effective a diaphragm at those frequencies as an actual hard diaphragm made of solid material. Talk about getting something for nothing! The vented box has a lot to offer, with advantages like flatter response down to the cutoff frequency or lower limit (although this is generally higher in frequency than a sealed box), reduced cone excursion near the box resonance frequency, and overall higher efficiency than most sealed box designs. There are also some significant limitations to this type of design. The transient response of the subwoofer is generally not as good as a sealed box design, but this can be minimized by careful driver selection and critical box tuning. Another problem is that there is no acoustic loading on the driver below the specific tuning frequency of the enclosure, meaning there is no control over cone motion beyond the subwoofer's own mechanical suspension at those frequencies below the box resonance. This results in "bottoming out" and the ultimate destruction of the driver. A simple solution is to use a low cut or rumble filter before the amplifier to prevent the subsonic frequencies from affecting the speaker below cutoff. The overall design of a vented box can be a bit trickier for the casual speaker builder, but modern software programs like BassBox Pro or the technical support staff at Parts Express can help you to develop a high performance project.

PASSIVE RADIATORS

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SUBWOOFER FAQs

"What's the difference between a woofer and a subwoofer?" See the answer to this and many other questions on subwoofers.

Passive radiators are an alternative to vented designs and are in fact part of the same family of enclosure design. It is the simple substitution of an actual diaphragm (usually similar to a speaker without a magnet and voice coil) for the virtual diaphragm of air in the vent that we just described in the last section. Advantages would include the absence of wind noises from a vent, and the ability to block higher frequency sound reflected from the back of the driver and out through the vent. They are also useful when a particular tuning requires a vent length longer than what the enclosure can accommodate. Some disadvantages are inferior transient response due to the actual mass of the passive radiator diaphragm, and greater difficulties in tuning compared to vented designs.

BANDPASS BOXES

Bandpass boxes are designs that use multiple chambers to create an acoustical filter that will focus and increase sound output within a specific frequency range. The most common of the many variations is the 4th order bandpass, which consists of a woofer loaded in a conventional sealed box enclosure, but the output of the woofer is directed through a second vented chamber in front of the driver. The port increases gain in a fairly narrow range, but also acts as a low pass filter to help reduce high frequency output above the bass frequencies. The actual vent diameter should be as large as possible or practical, as these designs are particularly susceptible to vent noise- the whole output of the woofer is passing through that opening! Bandpass designs offer transient response approaching that of a conventional sealed box, combined with output that can actually be greater than a vented box. The downside of this family of boxes is a tendency toward an unnatural or "one note" sound, due to the peaky nature of their operating range.

TRANSMISSION LINES

The transmission line, or TL, is also known as an acoustic labyrinth because of the long and somewhat complicated path that the rear radiation of the speaker must pass through before exiting the enclosure. The idea is to use the long "tunnel" to selectively absorb higher bass frequencies through the use of damping material, while tuning the labyrinth length to equal a quarter wavelength of the resonant frequency of the driver. The rear pathway creates a 90 degree shift in the speaker's rear radiation to constructively combine with the front radiation, while damping and controlling cone motion at the driver's resonant peak. They are many variations on the basic TL concept, but not as much data to mathematically model or predict the performance of a particular design compared to closed box or vented systems. The concept demands especially careful analysis of loudspeaker parameters, a fair bit of testing and tuning, and the space to accommodate a design that may turn out to be somewhat large and heavy. As a result, the transmission line enjoys a bit of a cult following, but when properly executed the results can be very impressive.

HORN LOADED

If we looked back through the mists of time, we would see small power amplifiers and low wattage speakers requiring a little bit of a lift from the Laws of Physics to get the kind of sound pressure level demanded by many playback environments. The use of horns to boost audio output certainly became very popular during that era, and horns still have a place in some specific modern audio applications. If you have ever cupped your hands in front of your mouth to amplify your own voice ("Hey, you kids, cut that out!") you will have employed basic horn loading techniques. Horns increase the gain or output of a source, and also improve its directivity or directionality. The horn serves as a transformer that matches the high acoustical impedance or pressure presented by the driver at the small throat of the horn to the lower acoustical impedance seen by the air at the larger mouth of the horn. Linearity is maintained by a carefully calculated "flare rate" for the horn path that ensures that the sound will travel with consistent expansion. The overall directivity and low frequency response of the horn are both determined by the length and mouth area of the horn. A large low frequency horn can be made smaller by "folding" it, which can get some of the larger designs down to a more manageable shape and size. A particular "K" brand has certainly become identified with the successful employment of horn loaded designs in high quality home stereo systems. Although horns have many attributes including high efficiency, excellent transient response, and great dispersion control, their size and complexity tends to make them practical for only a very small minority of home stereo and home theatre users.

BASS ACTUATORS

The small but mighty bass actuator does not require an enclosure of any kind, but still has a place in our general discussion concerning sub bass performance. A bass actuator is a small servo device or motor that converts an audio input signal into a mechanical force that is placed in direct contact with a solid surface. Conventional subwoofers transmit information through air, while a bass actuator delivers low frequency program directly through solids. The sensation is very much the same as it would be if the sub bass had come from a speaker first and then vibrated the solid surface, so actuators can be useful in situations where space is too limited for large speaker enclosures. The actuators connect to an amp just like a speaker, but you should be prepared to use them in multiples because the coverage of the effect is limited by the nature of the solid object to which the bass actuator is attached.

 
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