JBL GT120D
Owner's Manual

Choosing an Enclosure JBL GT Series
Thank you for purchasing a new JBL GT Series subwoofer. Subwoofer installation requires woodworking skills and some experience disassembling and reassembling automotive interiors. If you lack the tools or necessary skills, have your subwoofer installed by an authorized JBL dealer. Warning: Playing loud music in an automobile can permanently damage your hearing as well as hinder your ability to hear traf�c. We recommend listening at low levels while driving. JBL accepts no liability for hearing loss, bodily injury or property damage resulting from use or misuse of this product. GT Series subwoofers are optimized to perform best in small, sealed, vented and prefabricated bandpass enclosures. While in�nite-baf�e mounting of GT Series subs is possible, power handling will be greatly compromised because there�s no enclosed volume of air to prevent the speaker�s cone from moving past its limit. For this reason, we do not recommend in�nite-baf�e mounting for GT Series subwoofers. You should choose the enclosure you will use based on the type of music you listen to, how much ampli�er power you will use for the subwoofer and how much space inside the vehicle you can devote to a subwoofer enclosure. Because a sealed enclosure provides the most control over the woofer�s movement, a woofer mounted in a sealed enclosure will handle more power than a woofer mounted in another enclosure type. Sealed enclosures provide more accurate sonic reproduction than other enclosure types, so they are well suited to all types of music. Sealed-enclosure construction is straightforward and there are many prefabricated sealed enclosures available. An optimum sealed enclosure is always smaller than other types of enclosures optimized for a particular speaker, so they require the smallest amount of space inside the vehicle. Vented enclosures provide better ef�ciency in the 40Hz � 50Hz range but this ef�ciency comes at the expense of sound in the lowest octave (below 40Hz) and at the expense of some control and power handling. If you are using a small ampli�er, a vented box will provide more bass output from less power. Vented enclosures are also well suited to a variety of music types. Because vented enclosures require the volume of the enclosure and the size of the port to have a speci�c relationship with the characteristics of the woofer, the enclosure must be built exactly to the speci�cations provided. While there are some prefabricated vented boxes available, matching a prefabricated box to a particular woofer is dif�cult. If you wish to use a vented enclosure, we strongly recommend having your authorized JBL dealer build it or verify that your design is correct if you wish to build it yourself. An optimum vented enclosure is always larger than the optimum sealed box for the same woofer and will require more space inside the vehicle. Bandpass enclosures often provide the most output available from any ampli�er and subwoofer combination at the expense of sonic accuracy. If sheer SPL (sound-pressure level) is what you desire most, choose a bandpass enclosure. Bandpass-enclosure design is very tricky and the aid of a computer and enclosure design software is necessary. If you are an experienced installer or have some woodworking experience, you may wish to build the enclosure described in the enclosure design sheet included with this woofer. Fortunately, there are many prefabricated bandpass boxes available and they are all optimized to extract the most output possible from any woofer. Bandpass enclosures can be quite large and may require a lot of space inside your vehicle.

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