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Old 04-29-2008, 11:03 PM   #3
Big Daddy Big Daddy is offline
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Jan 2008
Southern California
Slot versus Flared Round Ports
Many people question what the difference is between flared ports and slot ports. There are fairly basic differences between the two, and depending on your situation, one might suit you better than the other. The common purpose of the two ports is that they must be the correct length for the particular subwoofer box to achieve the desired sub box tuning. There are a few differences that may help in your decision between flared and slot ports for your custom subwoofer box.
Flared ports are very good at virtually eliminating port noise. Port noise is the noise created by the friction of air moving through the port. If you have sharp edges on a port (as with standard round ports, which we do not use), you are going to have significantly more port noise than with a flared port. A flared port requires the smallest acceptable port area for a subwoofer box. Since the ends are flared, and the inside of the port is round (no sharp edges), it is unlikely you will be able to notice any port noise from the subwoofer enclosure. However, if you are running a large, high-powered subwoofer, you will want to use two flared ports. A single flared port is still capable of creating port noise when very large amounts of air pass through it.

With a slot port, you can use as much port area as needed for your application, which is especially helpful in SPL applications. A slot port requires more port area than a flared port to minimize port noise. We generally use a rule of 12 square inches of port area per cubic foot of internal volume of the custom sub box. You may go with a larger port area, but this will increase the total volume and length of the port. In turn, a large port area will mean an increase in the total size of your subwoofer box. Slot ports are the standard for our ported custom subwoofer boxes.

If you are not limited on space, we would recommend going with a slot port for your subwoofer box. If space is tight, and you have a few extra bucks, you'll probably want to use a flared port (or two, depending on your subwoofer size and power). It is possible to achieve the same result from both subwoofer boxes, so the decision between flared ports or slot ports depends on your specific needs for your custom subwoofer box.
Port Tuning
Tuning of a ported subwoofer box is determined by a combination of port area, port length, and net volume of the subwoofer box. Tuning changes the frequency at which the frequency response peaks (is loudest) and can change the way a subwoofer box sounds in your setup. Your tuning choice will be determined by the goal of your system install.

If you're looking to achieve an SPL setup your car (getting as loud as possible), you'll want to tune fairly high. High tuning for SPL is usually somewhere around 45Hz or possibly higher. At this high of tuning, the sound quality will not be very good, but it will be louder than if you were to tune low.

A SQ (sound quality) based setup will call for a tuning fairly low - even down around 25Hz. This will also allow you to hit the ultra-low notes with your system. Lower-tuned boxes produce better sound quality, more like a sealed box. The downside to lower tuning is that it flattens the frequency response, which will cause the box to lack the boost around the tuning frequency.

To achieve a good mix between SPL and SQ with a ported box, we have found that it's good to be in the lower 30's for tuning. If you tune up around 35Hz, you'll get a fair amount of SPL out of the box and still have fairly good sound quality. If you tune closer to 30, the box will yield better sound quality but have a little less output than the 35Hz. A range between 30 and 35 Hz is generally good for most general setups.

If you want SPL, tune high. If you want SQ, tune low. If you want somewhere in-between the two, we have found a good tuning is right at 33Hz. This is why we default the port tuning to 33Hz on our customization pages for our ported enclosures. When choosing a ported subwoofer box, the freedom to tune the enclosure is a great way to get your system sounding the way you want it to sound.
Sealed vs. Ported
When choosing a subowofer box, the decision between sealed and ported will determine how the final install will sound. The general rule for this choice is fairly simple.

Sealed subwoofer boxes generally are much smaller than ported enclosures. If you're limited on space, it would probably be a good idea to go with a sealed enclosure. Not only is there a size difference, but there is also a difference in sound between the two enclosures. Sealed enclosures tend to produce tight, accurate bass and have a flat frequency response curve. They are also generally the enclosure of choice when looking for a SQ (Sound Quality) oriented setup.

Ported boxes can get very large and have many varying factors that will determine their size and sound output. Ported enclosures produce louder bass than sealed enclosures, and allow you to tune the box to a specific frequency to determine how the bass will sound. Higher tuning on a ported box will get louder, but at the expense of sound quality. Lower tuning will still get louder than a sealed box, and at the same time will yield fairly good sound quality.

Port area and port type also plays a role in ported enclosures. Too little port area for the subwoofer box can result in port noise, which can severely detract from the sound quality of your setup. To much port area can also have a negative impact on the sound quality. However, if your goal is to get loud and not care about sound quality, then a ported box, tuned high, with a large amount of port area is the way to go.

Personal preference and size limitations will both play a role in the decision between sealed or ported enclosures. If you have the room, and you want to get loud, go with a ported box. If you're limited on space or are looking for the best sound quality possible, it would generally be best to go with a sealed box. Either way, as long as the specifications of the subwoofer box are within the subwoofer manufacturer's recommended box specifications, you should be safe with either choice.
In conclusion, I need to make a few general statements:
  • We need to distinguish between a subwoofer's output (SPL) and its sound quality. People with a background in car audio tend to be interested mainly in SPL, while people interested in home audio and home theater applications tend to care more about sound quality.
  • Our ears are more sensitive to upper bass frequencies than the lower bass frequencies. We generally feel the ultra low bass frequencies. We do not hear them.
  • In most averaged-sized rooms, upper bass (mid-bass) is best reproduced when the subwoofer is near the listener.
  • The ultra low bass is most efficiently reproduced when the subwoofer is in the corner of the room and not near the listener. See the following diagram.

  • When a subwoofer is placed near the listener, the high direct to reflected sound ratio of nearfield placement helps to reduce room effects, will have excellent headroom (high output with relatively low power), and low distortion.
  • A subwoofer driver that is optimized for low bass reproduction is not necessarily the best for mid-bass to upper bass reproduction.
  • A driver with a heavy cone material is best for low bass, but this type of driver will have reduced mid to upper bass efficiency.
  • A subwoofer driver with a very light cone material, a low inductance voice coil, and a strong magnet will have superior transient response with high efficiency (relatively low power will be needed).
  • Also, let us not to ignore the effect of the cabinet on a subwoofer's performance, but that is another story.
Having said all of that, manufacturers specializing in HT equipment do not make large (15" or 18") subwoofer drivers to optimize their mid-bass to upper bass performance. They tend to do that with smaller drivers. We can therefore conclude that as a general rule, large drivers are better in generating ultra low bass frequencies, but they may not be as good in generating the upper bass frequencies. In that respect, 12" drivers are considered to be better than 15" drivers.

Last edited by Big Daddy; 01-11-2013 at 07:32 AM.
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