Boxes with holes
Woofers are in boxes so the front wave doesn't get canceled by the out of phase back wave. So, why do some speaker boxes have holes that let out that threatening back wave? Those holes are called ports and to understand what's going on let's start with the problem of putting a woofer in a box. Pressure. The pressure resisting the movement of the woofer is greater when it is mounted in a box. The smaller the box, the greater the resistance to movement. One way to relieve some of that pressure is by punching a hole in the box. That's the port. But, it cannot be any sized or shaped hole. As with most things engineering the hole has specific qualities to tune what comes out of the hole and what sound the speaker produces. If done properly, the pressure exhausting the port both adds to the bass as well as enables the woofer to breathe a bit easier. The net result is lower bass with fewer watts than from a sealed box. Many designers, including myself and my speaker mentor, Arnie Nudell, are not fans of ports. Asked why Arnie used to roll his eyes and encourage the questioner to place their ear near a port for the "farting" it produced. In all his many years of speaker designs, Arnie rarely tolerated ports. One series Infinity had them in, the SM line (officially Studio Monitors but around Infinity's hallowed halls more properly Sado Masochist), made plenty of bass. They were most often demonstrated at shows reproducing the sound of a 747 landing and rarely music. There are certainly well behaved ported loudspeakers on the market. Some, like Bud Fried's famous IMF line, used a type of front port called a transmission line. The idea of the transmission line was a long, folded acoustic path for bass frequencies that delayed the port's output by 180˚ so it was in phase. Though Arnie and Bud were lifelong friends, the two never agreed on the use of the port. Other ported schemes include the passive radiator you often see in older Polk Audios, Definitive and modern Golden Ear products. If you want to learn more about ports I have a video to watch here.
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