less commonly seen. The efficiency of a speaker driver is given by: where the variables are Thiele/Small parameters. Deep low-frequency extension is a common goal for a subwoofer and small box volumes are also considered desirable, to save space and reduce the size for ease of transportation (in the case of sound reinforcement and DJ subwoofers). Hofmann's "Iron Law" therefore mandates low efficiency under those constraints, and indeed most subwoofers require considerable power, much more than other individual drivers. So, for the example of a sealed speaker box, the box volume to achieve a given is proportional to : where Therefore, a decrease in box volume (i.e., a smaller speaker cabinet ) and the same will decrease the efficiency of the subwoofer. Similarly the of a speaker is proportional to : As the efficiency is proportional to , small improvements in low-frequency extension with the same driver and box volume will result in very significant reductions in efficiency. For these reasons, subwoofers are typically very inefficient at converting electrical energy into sound energy. This combination of factors accounts for the higher amplifier power required to drive subwoofers, and the requirement for greater power handling for subwoofer drivers. Enclosure variations (e.g., bass reflex designs with a port in the cabinet) are often used for subwoofers to increase the efficiency of the driver/enclosure system, helping to reduce the amplifier power requirements. Subwoofers are typically constructed by mounting one or more woofers in a cabinet of medium-density fibreboard (MDF), oriented strand board (OSB), plywood, fiberglass, aluminum or other stiff materials. Because of the high air pressure they produce in the cabinet, subwoofer enclosures often require internal bracing to distribute the resulting forces. Subwoofers have been designed using a number of enclosure approaches: bass reflex (with a port or vent), using a subwoofer and one or more passive radiator speakers in the enclosure, acoustic suspension (sealed enclosure), infinite baffle, horn-loaded, tapped horn, transmission line and bandpass. Each enclosure type has advantages and disadvantages in terms of efficiency increase, bass extension, cabinet size, distortion, and cost. Multiple enclosure types may even be combined in a single design, such as in computer audio with the subwoofer design of the Labtec LCS-2424 (later acquired by Logitech and used for their Z340/Z540/Z640/Z3/Z4), which is a (primitive) passive radiator bandpass enclosure with a bass reflex dividing chamber. While not necessarily an enclosure type, isobaric (such as push-pull) coupled loading