[truwarrior22]

I bought a Qlogic 1.5 cubic foot box and tossed a rockford fosgate 10" HE sub in it then stuffed it with pollyfill. Sound good but the box is really made for ported enclosures. So...
1. How does port lengh affect sound?
2. What about port diamiter?
3. What about port location?

Don't diss my pick of equipment. I got it all for under 80 bux and the RF sub is actually fairly nice. Box ain't to bad either... Just take out the plastic ring cuz it sux.

 

[1loudtalon]

Port length and diameter detemine the frequency that the enclosure is tuned at. Keep in mind that a Q-logic box is not the exact enclosure size for your sub, but a well rounded universal choice. As far as location of the port, i woulkd recomend putting it on the same side as the sub is facing, with plenty of clearance from the sub. I prsonally like the sound of a sealed box better, so borrow some of your friends ported boxes and listen to them and see what you like. For better info, visit www.rockfordfosgate.com. They have exact box measurments for your application.

 

[Redsand187]

Like mentioned Rockford has the specs. One thing that I should add, is a ported box will need to be bigger. A sub needs more air space in a ported enclosure, and you also have the port in there to take up more volume.

 

[truwarrior22]

Well the box is 1.5 cubic feet... My HE recommends 1.0 for ported so I'm kinda screwed either way I guess. I knew I should have got the smaller sealed box. Just got to good of a price.

 

[1loudtalon]

After you add the sub and depending on the port size, it may be a lot closer to 1 cubic foot then you thought. Look up the specs and use the volume calculator and see i you can get a proper port size from them.

 

[truwarrior22]

It's a 3 inch wide port and about 13" long. I don't understand how you figure the port takes up volume... It's a hallow tube with both ends open. It would hardly take up .01 cubic feet since the carboard is thin and in cylinder shape.

 

[stevep]

It's a lot easier to reduce the internal volume after the fact than it is to increase it. Just fill up the extra space with a sandbag or something that won't slide around.

Steve

 

[1loudtalon]

the air inside the port does not count.

 

[alanseijas]

Vented boxes class 101!

A vented enclosure is not much more complex than a sealed box. It consists, basically, of a box with a hole in it. However, despite it's simple design, vented boxes are considerably harder to get good performance from than sealed boxes-- although many times the extra effort can be worth it.
The vent in the enclosure interacts with the volume of air in the cabinet and the driver to help increase output and reduce cone excursion at and around the tuning frequency. In fact, at box tuning, almost all the bass is produced by the vent-- not the woofer.
The trick in building a vented box is to get the right size enclosure and the right size vent. You can't be too far off on either of these factors or your speaker's performance will suffer. In particular, using a too-small box or a too-high vent tuning frequency can eliminate bass instead of increasing it. Porting a sealed box that is too small usually does nothing to improve frequency response. The vent's placement within the enclosure is also important. You must have at least the equivalent of the vent's diameter between the vent and any inside wall. For example, you would not place a vent with a 3" diameter within 3" of any wall. The same is true for clearance between the vent opening and the bottom of the enclosure.
The advantages of vented enclosures are:
1. Reduced cone excursion and reduced distortion around vent tuning.
2. Increased output capabilities around vent tuning.
3. Vented boxes allow for specialized sound qualities such as the "bump" often preferred for rap and rock & roll.
The disadvantages of vented enclosures are:
1. Total loss of cone movement control below vent tuning, which can result in high distortion and driver mechanical failure.
2. Midrange sound coming from inside the box through the vent can produce unpleasant sound coloration.
3. Vented enclosures are more sensitive to changes such as temperature, humidity, and driver fatigue.
4. Enclosure design is more complex and the enclosure itself must be more solidly constructed because of internal pressure at frequencies around vent tuning can be nearly twice as high as a sealed enclosure.
5. Vented enclosures usually don't sound as "fast" as sealed boxes because of the vent tuning which is always slightly out of phase with the driver's output.

 

[truwarrior22]

Is it ok to put in polly fill in the ported box? I bought some and don't really want to waste it..

 

[alanseijas]

It's really a lot more complicated than just thowing polyfill in because you don't want to waste it. I assume you've accepted the premise that a raw speaker (no box) radiates sound from the back as well as the front. So if a speaker is placed in a sealed box, a lot of useful radiation from the speaker is lost inside the cabinet. By placing a vent or port in the cabinet, the acoustic energy from the rear of the speaker is allowed to escape and reinforce the front energy.
If the sound emerges out of phase with the forward radiation, there will be cancellation rather than reinforcement. But the box interior has "capacitance," or the capacity to store the flow of acoustic energy, much the same as an electrical capacitor in an electrical circuit. The box must "fill up" with sound from the back of the speaker before it is released into the port. By the time the rear acoustic energy exits the port, it is in phase with the front-radiated energy, but only between box tuning and about one octave above.
Acoustic energy (sound waves) have difficulty negotiating a small opening, especially through a tube so the capacity of the box to store and delay sound waves is aided by the "inertance" of a tube or duct at the port. By adding the tube with it's inertia effect to the port opening, the box size may be diminished while maintaining the same "filling up" capacity. For various reasons, the port only reinforces low frequencies. In effect, the capacitance and inertance of the box and port tube tend to "filter" out the hight frequencies from exiting the port.
The acoustic capacitance of the box and acoustic inertance of the port tube are variable and therefore become a tunable circuit. Because a speaker moves back and forth, the volume of air inside a box and how easily it exits the port tube has a direct effect on the acoustic resistance seen by the speaker. By controlling the size of the box (acoustic capacitance) and the length and diameter of the port tube (acoustic inertance), we can choose the frequency at which the speaker moves most freely, or it's resonance frequency. By adjusting the variable "capacitance" and "inertance", we control the compliance of air in the box and the mass of air in the port. So the enclosure becomes part of a system, tuned to resonate with the loudspeaker with which it is to work. In fact, one can think of the mass of air as a "piston" or "diaphram of air" which resonates along with the speaker, thereby enhancing output.
If the resonance frequency of the cabinet is tuned closely to that of the loudspeaker, resonant peaks and "boominess" are reduced and smooth tight sounding bass is the result. Futhermore, when the box is tuned to the speaker's free-air resonance (fs), the damping action of the port permits the use of a more highly efficient woofer and also improves the transient response of the bass because of the reduced cone excursion. Increasing the duct length increases the mass of vibrating air which permits lower frequency tuning. Remember that the larger the port area, the longer the duct must be for a given frequency.
Now you've got me rambling on.......... You can half-fill your box with polyfill, and see if you like the extra "boominess" caused by the woofer believing you have increased your box size.