[tortugaranger]

I have a speaker project going where the "best" location from an overall aesthetic and practical construction standpoint is to place the port in the top of speaker cabinet pointing downwards. I've owned and built many speakers where the port was either in the front baffle or the rear. Is there any sonic quality reason why the port won't work equally well located in the top?

[JLM]

"...place the port in the top of speaker cabinet pointing downwards."  Huh?

Anyway, simple port formulas do not account for cabinet shape or driver/port locations.  Long thin cabinets behave like tubes.  In terms of musical instruments think clarinet or flute.  In terms of speaker types think pipes or transmission lines - check Martin King's MathCad application.

[mcgsxr]

No reason a top mounted port could not work.  In some cases it may even ease placement concerns as it will not load a surface nearby (such as a rear mounted port facing a wall).

Lots of designs out there with downward facing ports in the bottom of an enclosure.

[tortugaranger]

No reason a top mounted port could not work.  In some cases it may even ease placement concerns as it will not load a surface nearby (such as a rear mounted port facing a wall).
Lots of designs out there with downward facing ports in the bottom of an enclosure.

This is consistent  with my own understanding but always good to hear from others. Regarding downward facing ports how much room have you seen between the bottom of the cabinet and the floor to allow for the port to breath properly? Would a good 1/2" or so suffice for small set of bookshelf speakers or would 1-2" be needed? 

[mcgsxr]

That is a good question.

I feel we are rapidly swimming out of my depth though!

As I understand it, when you have a small distance from the port exit to the floor, you change how the port functions - and I am not clear on how to calculate that.

I would think that at least the same width of the port should be the bare minimum - and I think you'd want to leave lots of room around the port to allow for air flow.

You may find some info in this thread at DIY - http://www.diyaudio.com/forums/subwoofers/104190-down-firing-vs-side-firing.html

Here is a review of a Canton speaker that uses a down facing port - http://hometheaterhifi.com/reviews/speaker/floor-standing/canton-reference-52-dc/

It would be easier to use a port on top of the cabinet, as that will likely be quite far from the ceiling, unless your speakers are very tall.

[Quiet Earth]

Regarding downward facing ports how much room have you seen between the bottom of the cabinet and the floor to allow for the port to breath properly? Would a good 1/2" or so suffice for small set of bookshelf speakers or would 1-2" be needed?

Hi Tortuga,

I think your post might be confusing to some readers. Why would a small pair of bookshelf speakers be 2 inches off of the floor? How can the port be located on the top of the cabinet and fire downward? Maybe a picture would help to explain what your project looks like.

[mresseguie]

My thoughts on this are based on observations rather than any experience. I've seen several designs - from single driver speaker cabinets to three- and four-way designs that incorporate a downward firing port. Take a look at the Omega 3i here on AC for an idea. Its cabinet size isn't significantly different from the boat hull cabinet you are creating. Perhaps Louis could offer you some tips.

The other speaker cabinets I've encountered are Proac speaker's floor standing models and a large two-way monitor I saw this past weekend in Taichung City, Taiwan. This speaker is ported to the bottom with very small distance from bottom of speaker cabinet to its base plate. Let's see if this link takes you to the correct page: "Problem with link. I'm searching for correct link". A lot more air moves through the ProAc speaker's port because there are two or three midwoofers pushing/pulling in unison, so there is more clearance between the bottom of the cabinet and the base plate. In both the ProAc and Royce examples, the cabinet is supported by four short legs in each corner.
The Omega 3i lack the base plate of the others, but it does have legs that lift the cabinet from the desk or shelf the speaker sits on.

Hope this is helpful.

Michael

[Duke]

This is consistent  with my own understanding but always good to hear from others. Regarding downward facing ports how much room have you seen between the bottom of the cabinet and the floor to allow for the port to breath properly? Would a good 1/2" or so suffice for small set of bookshelf speakers or would 1-2" be needed?

Two issues arise when a downfiring port is near the floor.  The first is, an effective increase in port length.  This can result in your tuning frequency ending up lower than your modeling predicted, or conversely you can take advantage of it to get a bit lower tuning frequency than you otherwise would have. 

The second issue is, chuffing.  If the airflow is constricted the air velocity goes up, and when the air velocity goes up it tends to become turbulent, and that's what makes the chuffing, especially if the airflow also has to negotiate a sharp bend.   Make sure the cross-sectional area that the air of the air path right at the edges of the mouth of the port, between port and floor, is adequate.  A flared port can help prevent a bottleneck here and also smooth the airflow relative to having an abrupt edge.

If you want to do some math, first calculate the cross-sectional area of the port tube (Pi x Radius^2), and next the perimeter of the port opening (Pi x Diameter).  Then choose a Height such that Pi x Diameter X Height is at least 20% greater than the cross-sectional area of the port, and you should be fine.   For a flared port, you can use the Diameter of the flare itself, which allows you to get away with shorter feet.

[Letitroll98]

???  Am I missing something or didn't the op ask about putting the port in the top off the cabinet?  Separate from the excellent discussion about downward facing ports of course.  Perhaps the op could clarify or some of you guys could read over the op and point out what I'm missing?

[Duke]

???  Am I missing something or didn't the op ask about putting the port in the top off the cabinet?  Separate from the excellent discussion about downward facing ports of course.  Perhaps the op could clarify or some of you guys could read over the op and point out what I'm missing?

I was responding to a specific follow-up question by the OP, quoted in my post. 

[FullRangeMan]

Hi Duke,
I like port location on base of the side walls,
to the ports help form the central image. In
other words:
The left speaker w/port at right side wall,
Right speaker w/port at the left side wall;
there is any reason this never was made?

[Letitroll98]

I was responding to a specific follow-up question by the OP, quoted in my post.

Oh yes, of course, and your reply was great, I was just looking for an answer for the first question about the "roof" mounting of the port.  I can't think of a single drawback to it and was wondering if anyone else does.

[tortugaranger]

I want to thank everyone for their advice on this question. Good input.

To put this into context, this is for a custom cabinet we're building for a customer. It's a small 0.32 cubic foot bookshelf speaker with a "boathull" cabinet make of layered stacks of baltic birch that are CNC milled. Because of the boathull shape it's not practical to place the port in the rear. Placing the port in the front isn't workable either because there's very little room for one and it would probably look awful. Hence the question of top...or bottom....location.

Going to go with top mounted. Bottom mounting would require feet to give the port breathing clearance and want to avoid feet on such a small speaker.

If you're interested, here's our AC thread with some more pics of the roughed out enclosure before sanding or mounting the baffle. http://www.audiocircle.com/index.php?topic=137108.0

Cheers,
Morten

Also, here's a 3D CAD rendering of the driver/port openings with a top mounted port.

[mresseguie]

Dust trap?

[tortugaranger]

Dust trap?

That's a downside for sure. Owner says he can live with that.

[FullRangeMan]

That's a downside for sure. Owner says he can live with that.

He can turn the speaker upside down.
May be a change in the soundstage tough

[JohnR]

What if someone drops something into the port? (Makes me think of those little games where you have to get the ball bearing through the maze.)

[WGH]

I'm thinking cup holder.

[tortugaranger]

I'm thinking cup holder.

Gonna need a bigger port. 

[mcgsxr]

Fishing rod holder!  Sorry, the boathull theme struck me...

I am sure a decent woodworker (I am not) could work up some kind of small grill and use grill cloth to help prevent anything unwanted (car keys, change, pregnancy tests, cigars etc) from falling "into the hole"!