Need help with port length / size based on my enclosure

Why do you want to hit 18Hz? You know that you rather feel those low frequencies than you actually can hear them? What you're hearing with most subwoofers at these frequencies are the way too much raised distortions.

Another point to consider is the listening room. With single subwoofer configurations this is often the limiting factor. In most cases you will not get a reasonably reproduction of frequencies under your first room mode. I say this from my personal experience. I myself had different subwoofers able to play down to 20 Hz and lower. But in nearly every room I tested them I wasn't able to hear these frequencies only in rooms with one dimension taller than 8 meters it was possible. A way around this would be a dipol subwoofer, double bass array or a single bass array with huge back wall absorption. Just mentioning...

Referring to your question: With your given enclosure dimensions you can reach good 19Hz (at the -3db point). If you're planning to run the Dayton with its rated 500 watts the port should be big enough in order to avoid audible port air velocity. I would choose at least the following internal port dimensions so you can keep the air velocity under 20m/s at 24Hz and up (with a peak of 27,5m/s at 17,5Hz): 35,36cm (wide) x 5cm (tall) x 50cm (deep). And no need to worry I already considered the port volume (liters) in my calculations and subtracted it from the box volume of 222,5 liters. Here is the max possible spl within the xmax of 14mm and power rating of 500 watts (sorry for the spaces in the link that's because my account is still limited):

A sealed 18" instead of the vented 15" doesn't make sense with regard to the aim of as much (quality) spl on the low end as possible. A fitting sealed enclosure for the Dayton RSS460HO for example would give you 8db less output at 20Hz than my suggested vented enclosure for the 15" Dayton. Here is a comparison of the two (red the RSS390HF BR and blue the RSS460HO CB):



In my opinion the vented Dayton RSS390HF is a fairly good starting point and if you actually need more output you could just build another identical one which would also give you a much more regular bass spreading in your room. And if you really want to impress with your setup you could upgrade to a single or even better a double bass array later on too...:thumb:

Referring to the inwall question: I don't see any huge drawbacks here. If you prefer the look a lot more I would definitely build it like this. One not to deny benefit of inwall speaker mounting is in fact that you will get less troubles with interferences caused by the speaker to wall gap. Of course you also have more pronounced room modes but it's a lot easier to correct those sonic rises than obliterations (sorry I don't know the correct expressions here). The by chrapladm suggested dual opposed construction (you mean impulse compensation right?) is ideal with freestanding speakers because this cancels out most of the enclosure vibrations. With a reasonable implemented inwall construction I think this could be obsolete if you make sure there is a really solid connection to the surrounding walls.

Just my thoughts on this...

TX1978 said:

As I am no longer constrained by the wall cavity, can I mod the enclosure dimensions as long as I stay at net 7.9 cu ft?

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The shape doesn't make that much of a difference as long as the volume stays the same. Only immoderate length in one dimension you should avoid because you then could get troubles with internal enclosure resonances within the subwoofer's frequency range.

TX1978 said:

Regarding ports, does round vs slotted make a difference as long as they are equal volume? Also, does it make much difference where they are on the enclosure? Same face as driver, 90 degrees to driver, etc.?

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Theoretically the port shape also doesn't matter if there is equal port volume. But I would build a slotted one anyway because in most cases it's more solid and you can already use three of the enclosure walls as spatial limitations so you have to add only one additional panel which at the same time is a enclosure stiffening too.

On the contrary the port positioning does make a minor difference. It's not like one position would mess up the entire performance but what it does is in fact influencing the port efficiency above your tuning frequency. As you probably know the sound radiation is 180 degrees in phase on the other side and thus would normally cause an almost complete sound cancellation in front of the chassis. Therefore one enclosure's purpose is either turning phase optimally to 360 or 0 degrees in order to use the back sound or destroying it (sealed box concept). Having said this one can conclude that with rising distance between chassis and port also the sound addition rises above the tuning frequency as their radiated sound waves are more in phase whereas the output around the tuning frequency stays the same because here the port resonance (delayed) already achieve this phase turn. According to measurements this turns out true but in nearly all cases compared with the tuning frequency's wavelength the enclosure's dimensions are so small that the impact here is relatively neglectable. With large floorstanding speakers the max possible port distance will give you a few extra db but not much.

Long story short - Choose it to your (visual) liking...

TX1978 said:

If the enclosure will reside on the floor, does it need feet?

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Which kind of feet are best sound quality wise depends on the underground. If your floor is made of solid material like concrete spikes could be the way to go because they achieve coupling of the enclosure to the ground which suppresses then most of the vibrations. But if your floor isn't solid enough and resonates by itself then it could be more intelligent to use decoupling from the ground while simultaneously destroying the vibrations within the feet. Viscoelastic materials like a rubber cork sandwich for example would reach this. Alternatively as already mentioned before you could just impulse compensate your subwoofer which would make all these actions obsolete. However you then will need a second identical chassis in your enclosure (of course this results in more output level too).

TX1978 said:

Would I gain anything by having the bigger enclosure (7.9 cu ft) plus port volume? If not, i will keep it at 7.9, including the port and add to it for driver and amp displacement.

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Now that there are no more dimension restrictions due to the inwall plan I made some new enclosure calculations for you:

• white: 214 liters (excluding port volume); port 38,9 x 5 x 56,4 cm (width x height x length)
• green: 222,5 liters (excluding port volume); port 38,9 x 5 x 53,9 cm (width x height x length)
• blue: 250 liters (excluding port volume); port 38,9 x 5 x 49,7 cm (width x height x length)

I thought increasing the port and thus enclosure width would make its look a bit more balanced cause the previous 35,36 cm (1,16 ft) is surely rather small for the Dayton 15" chassis (which is 38,84 cm). Besides that also the port's surface area increased hence further lowering its audible air velocity.

You can choose between these suggestions or ask me for another calculation if you would prefer still other enclosure dimensions (more specifically width). Sure you also could simply adjust the port dimensions by yourself considering the port's surface area and volume.

TX1978 said:

What do you mean about port radius? I will of course round off the edges of the exit and entrance, but the long edges will be flush with the enclosure sides so I cannot round them. Do you mean to take into account the material removed for the radius in the port volume calculation?

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Since a picture says more than thousand words please look up the following thread where chrapladm posted a very good picture lately explaining right this topic: Click Me

TX1978 said:

As I build the box, which will be on the floor via cork feet, would I see any benefit of having the driver firing at the floor instead of into the room? Also, I can have the port firing into the room, or towards a corner. Is one better than the other?

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This depends on your preference. Mounting your chassis facing the floor will give you more room shaking but in most cases poorer sound quality (in addition having a floor other than quite a solid one would suck off some of the hearable bass).

Referring the port positioning question: Personally I would place the port as near as possible to the backwall in order to have less interferences (and more room gain of course plus stronger room modes wherefore some would lapidate me now - To those: A few post above there is a short clarification) and additionally like explained before as far away as possible from the chassis. Not facing directly the port could also be an advantage if there should be some air velocity noises.

I will use the blue option; 8.8 cu ft

I am debating between two options. These are enclosure internal dimensions.

1)
55.87cm x 41.9 x 106.67

2)
55.87cm x 55.87 x 78.739

Both options will have a port width of 55.87cm

To these dimensions I will add volume for port, driver, amp, and baffles.