But if you want to know the real reason that virtuall all modern speakers are ported
This article covers some of the issues. But Mr. Feinstein started by asserting that the AR-1, now nearly fifty years old, was inherently superior to any modern speaker in terms of quantity and quality of deep bass. (I used to also believe this, passionately.) He then proceeds to explain why ported designs are so much better than they used to be, but hardly manages to identify any advantage of ported designs. You have to look hard for it. In the second page, he wrote, “My own feeling is that the lower 3dB down point of a small ported system and its higher efficiency are thought by many companies to be worthy advantages in today’s less critical, less hobbyist-driven audio environment.” He proceeds from there to talk about group delay, and even though he prefaced that by saying that he was about to contradict himself, he did not contradict himself at all. The group delay issue points very clearly to the superiority of acoustic-suspension speakers.
Unless you believe that the world’s finest, modern speaker companies are concerned primarily with speaker efficiency and the -3dB point, to the detriment of bass quality, the article does not really explain the reason that most of the world’s finest, modern speaker companies prefer ported designs.
It all comes down to one fact: cone excursion is vastly lower with ported designs than with sealed, acoustic suspension enclosures. This translates in a rather immediate way to reduced non-linear distortion in the deep bass. With the acoustic suspension approach, the strategy is to make the suspension very floppy, using the pressure differential across the cone to provide the force that opposes the motor force. The assumption is that the restoring force will be a linear function of cone displacement. But the obvious questions are with the linearity of the motor, and also the spider, which is part of the mechanical suspension. The voice coils are very long indeed, which leads to greater self-inductance, and also greater DC resistance and thus reduced sensitivity.
The alternative approach is to avoid the large cone excursion. This fundamental advantage gets lost in all the talk about -3B points and group delay. Group delay is accompanied by a time shift for output in the deep bass, relative to the rest of the spectrum, and becomes audible when, but only when, attenuation is severe. As long as the enclosure and port are tuned to adequately low frequency, such that you do not get the steep drop-off below the -3 dB point, it is not a problem at all. But there are other drawbacks. If the diameter of the port is not great enough, compression will occur, leading to reduced low-frequency output at higher volume levels. I.e., turn up the volume loud enough, and the bass disappears, and as it does so, the cone excursion increases. And this occurs regardless at frequencies below the -3 dB point, i.e., you still have high cone excursion and high distortion. There are other potential disadvantages that may or may not be significant, depending on who you ask: organ pipe effect and midrange leakage.
For medium-sized satellite speakers that rely on a subwoofer for deep bass, there is probably no significant difference between the two types, but it depends on the size of the satellite speakers, and also on whether equalization is used. With “home theater” speaker systems that use little tiny satellite speakers, the ported approach can be advantageous if used in conjunction with equalization, the reason being that you can use equalization to correct anomalies in the frequency response and extend the bass, while the port has the net, appreciable effect of reducing cone excursion, thereby reducing distortion. This same advantage, by the same reasoning, can apply to any speaker where porting is combined with equalization, whether the equalization is built in to the speaker as in the case of a powered monitor, or is accomplished by means of (ostensible) room-correction speaker setup in a modern home theater receiver.
Ported designs also make sense with very large speakers with very large woofers, since with these speakers, the benefit of reduced cone excursion still applies, as it invariably does. Most powered monitors are ported, because the driver itself does not displace sufficient volume to be capable of strong output in the deep bass. The required amount of excursion would be so great that high distortion would occur. With this type of speaker, you tune the enclosure and the port to a frequency well below the frequency that the alignment rules says that you should use, and then use equalization to correct the anomalies in the frequency response. This type of speaker is very smart from the additional perspective of the crossover, since by using separate amplifiers for each driver, the crossover is rendered immune from interaction with the complex impedance of the driver.
