Idiot question #3. Ports - continued

[filmboydoug]

How much unencumbered space does one need around a port?

[Xover]

Hi Again Doug

Good question and not real easily answered but I can tell you how to find out.

Since I responded to IQ #2 I suspect that you have a deeper concern or project can you tell us more? Maybe that will help me answer this one.

[DrWho]

At least one port diameter from any surface is the general rule of thumb...

If you get too close you end up effectively making the port longer

(which lowers the tuning point). And if you get even closer you start

to plug up the port which really reduces the output of the port (thus

losing low frequency extension).

[filmboydoug]

Specifically, I am wondering if it would work to have the mouths of 2 bass bin ports oriented straight up into a backless cabinet that would hold a horn. See attached crappy art.

[Xover]

That should work. Be sure to leave as much room on top of the ports to vent as possible. Also a angled plate above the ports to help direct the sound into the room might help.

Some other thoughts on this. I have had good results venting at the bottom especially bottom back or bottom back corners to take advantage of boundary reinforcement of the port output. This assumes the cabinets will be resting on the floor near a back wall and are of a single chambered design the most common situation.

Now back to how to tell if the ports have enough unencumbered space. Referring to my responding post on IQ #2:

"This phenomenon can be observed by taking a bit of white label and sticking it on the woofer cone then cut a 1/2 in strip of index card and tape it so it extends about halfway into the port opening. Now sweep a low frequency and you see the woofer cone nearly stop when the card strip reaches maximum motion."

Set up this condition at the port tuned frequency and observe the white dot on the woofer cone as you move the obstruction closer. You will see the cone start to unload IE cone motion increase when the obstruction is getting too close.

BTW these are good questions and you haven't seen my art.

[DrWho]

Specifically, I am wondering if it would work to have the mouths of 2 bass bin ports oriented straight up into a backless cabinet that would hold a horn. See attached crappy art.

Where would the driver go in such an arrangement? You can't simultaneously port the rear wave of a driver and rear horn load it....but if you have a driver inside a cabinet firing down the horn, you can totally port the rear chamber. However, the rules for porting will change slightly due to the different impedance of the horn...so be sure to allow for some flexibility with the port dimensions. You mention a backless cabinet....not sure where that is.

Also, how high up in frequency will the sounds coming through the horn be? Having the ports hanging down will cause some reflections inside the horn mouth, which is one of the things good horn design is trying to avoid...If your wavelengths are significantly bigger than the outer diameter of the ports (say 8x bigger) then you should be fine and not notice anything at all.

edit-hmmmm, maybe I looked to quick at the picture...for some reason I thought you had chimneys hanging down into the horn mouth (as an attempt to maximize the air space behind the driver). Either way would work...I'm not sure how tight you are on space.

I wouldn't worry about "reflecting" the sound of the port because the port output is pretty much omni-directional and it looks like you'll be within 1/4 wavelength of any boundaries (which means you're already getting those gains).

[filmboydoug]

Where would the driver go in such an arrangement? You can't simultaneously port the rear wave of a driver and rear horn load it....

DrWHo, you have fallen victim to my crappy art. Let me try again.

[DrWho]

Oh...the woofer isn't hornloaded. I get it now.

Technically your idea would "work" but a few things to consider...

Think of a port as a speaker that plays over a very small range of frequencies. Would you ever place a speaker where you are hoping to put your ports? You can get away with what you are planning to do if the tuning frequency of the port has a wavelength greater than 4x the distance to the front of the speaker. A 45Hz tuning has a wavelength of 24 feet...divded by 4 that is 6 feet. So if your cabinet is under 2 feet deep and 2 feet wide, then your idea will work (2 feet from the port to the rear of the cabinet, 1 foot to the side, 2 feet to the front of the cabinet and then 1 foot back to the middle).

In your current design I would look into mounting your ports at an angle...perhaps coming through the front off to the sides of the tweeter or at the very least going out the back at an angle to get the mouth of the port as far back as possible.

Another thing with ports is that you also have rear wave reflections from the active driver making their way out...the enclosed cavity that you want to put the ports into is going to resonate at a frequency and it will sound bad if the rear wave refelctions excite it. (and with the nature of resonating system it won't take much to make it sound bad....).

well...gotta catch a bus...hope this helps.

[Xover]

Me again, that is exactly how I pictured your system from the first drawing.

Do you know what your tuned frequency is going to be? It really doesn't make any difference in port placement I am just curious.

Something else that I notice and you might already have this covered. The sides of the top portion are likely to vibrate and find some resonance unless you put some braces in. Since they are open this could done later.

[Marvel]

I have another port question, that perhaps can be answered here or I can start another thread.

Say I have a cabinet of a given size and know the port size for round ports. What if I want to change to a single shelf port, as in the Cornwall? There you have the bottom of the cabinet as part of the port. The port volume won't be the same or act the same.

Bruce

[DrWho]

Well first off...don't think of "port volume" - the analogy I like to

use is blowing through straws: pick a short fat straw and then pick a

long skinny straw such that they both have the same volume. Which is

easier to blow air through? The tuning of a port is dependant upon its

resistance to air flow. However, increasing the volume of the port

increases the mass of air trapped inside - which will lower the tuning

point: more mass on a spring resonates slower. So to keep the tuning

point the same, the port needs to be longer, which effectively

increases the springiness of the spring: a higher spring constant

causes a spring to resonate faster. (keep in mind that making the port

longer will also increase the mass so it's gotta be that much longer

again...not exactly a linear system).

If you could provide the exact effective internal volume (internal

volume minus the space taken up by the drivers and ports) and the exact

dimensions of the ports (diameter and length - flared or not flared),

then it would be but a trivial task to run some numbers and come up

with the dimensions for a slot port. I dunno why you'd wanna do that

though...what project you working on?

[filmboydoug]

Do you know what your tuned frequency is going to be? It really doesn't make any difference in port placement I am just curious.

29 Hz.

[Marvel]

If you could provide the exact effective internal volume (internal volume minus the space taken up by the drivers and ports) and the exact dimensions of the ports (diameter and length - flared or not flared), then it would be but a trivial task to run some numbers and come up with the dimensions for a slot port. I dunno why you'd wanna do that though...what project you working on?

I am actually thinking of the LS bass mod. I want to make a base but not use round ports. I think it would be more aesthetically pleasing. I can't remember what the internal volume is with the bottom cabinet added, but the ports Dennis suggests are two 4 inch diameter x 7 inch long ports. I would like to place a Cornwall style port across the bottom instead.

I found Dennis' post, where he says the internal volume is about 4.8 cu ft. The stock doghouse is about 2.25 cu ft.

Bruce

[Xover]

Hi Doug

29 hz Cool looks like you are shooting for a F3 in the mid 30s should sound great.

Do you have what you need to tune the port? (Signal source @ that frequency range)

[Xover]

Bruce

"If you could provide the exact effective internal volume (internal volume minus the space taken up by the drivers and ports) and the exact dimensions of the ports (diameter and length - flared or not flared), then it would be but a trivial task to run some numbers and come up with the dimensions for a slot port. I dunno why you'd wanna do that though...what project you working on?"

In order to figure the port dimensions one needs:

Volume of the box.

Desired resonant frequency of the box.

Desired Port width for example the tube diameter or the inside width of the cabinet for a slot port.

If you want to determine the resonant frequency of an existing system go see my post in response to Doug's Idiot question #2.

Usually the port needs some final tuning to get it spot on.

[DrWho]

If you could provide the

exact effective internal volume (internal volume minus the space taken

up by the drivers and ports) and the exact dimensions of the ports

(diameter and length - flared or not flared), then it would be but a

trivial task to run some numbers and come up with the dimensions for a

slot port. I dunno why you'd wanna do that though...what project you

working on?

I am actually thinking

of the LS bass mod. I want to make a base but not use round ports. I

think it would be more aesthetically pleasing. I can't remember what

the internal volume is with the bottom cabinet added, but the ports

Dennis suggests are two 4 inch diameter x 7 inch long ports. I would

like to place a Cornwall style port across the bottom instead.

I found Dennis' post, where he says the internal volume is about 4.8 cu ft. The stock doghouse is about 2.25 cu ft.

Bruce

Ya know....you could just target the same cross-sectional area which

allows you to go with the same portlength. Two 4" ports have a

cross-section of 25 square inches so any slot type port with a mouth of

that size will need to be 7" long.

[filmboydoug]

Hi Doug

29 hz Cool looks like you are shooting for a F3 in the mid 30s should sound great.

PE box and Win ISD tell me about 31 Hz.

Do you have what you need to tune the port? (Signal source @ that frequency range)

No, I was going to be relying on google when I get to that point. []

[Xover]

31 hz is better yet. Not knowing your other variables my guestimate was 34hz.

If when the time comes you need the signal source let me know and I'll try to get a useable reference from the function generator recorded to CD for you.

[filmboydoug]

Thanks Xover, I'll keep that in mind.

Another question re the one port width rule of thumb, I am assuming that is side to side. Further assuming a front firing port, is there a rule of thumb re how much space is needed between the butt end of the tube and the back cabinet wall?

[DrWho]

You want the port to be at least one port-diameter-width away from ANY

surface/boundary. So a 4" port needs to be at least 4" from the side

walls, at least 4" from the floor and at least 4" from the rear wall.

Any design of your own needs to be of variable length so that you can

fine tune the system once it's built...there are a lot of other

variables that can't be accurately anticipated (all of which can affect

the ideal tuning).