Port Placement in DIY Bass Reflex Cabinet

[Clipped and Shorn]

In a DIY bass reflex cabinet design, what are the factors determining the placement of the port? Once the net volume of the cabinet and the size and length of the port have been correctly determined (from crunching numbers with software and relevant parameters) what other guidelines should be considered regarding the placement of the port. Can the port just as easily be located at the top of the cabinet as the bottom? Are there critical considerations regarding this positioning?

-c7s

[Clipped and Shorn]

Clipped,

I found this bit while searching the web. thot it might be relevant:

Answers to C&S's Questions

"Where should I locate the port(s) with respect to the woofer?

Bass Reflex enclosures are usually designed to tune from about 100 Hertz and down. The length of sound waves at these low frequencies is over 11 feet, so port placement is not critical. Ports may be located anywhere on the baffle with no change in the bass performance; some designs even locate ports on the back of the enclosure which works well as long as the enclosure Is not close to a wall. On the whole, it is safest to locate the port somewhere on the baffle with the woofer far enough from the side walls to avoid interaction between the port and enclosure wall or the fibreglass insulation on the wall."

[Clipped and Shorn]

Thanks for the info.

In that case, I may locate the port up at the top of the cabinet to conserve space down below so that the horn and tweeter, already above two woofers (sub and low), do not end up higher than comfortable listening ear level. As it stands these cabinets will be 7 feet tall. With this arrangement, the 511B horn will be 50 inches up from floor level (to center of horn). The tweeter will be directly above the horn. The port now will be above that.

-c7s

[Stig]

by placing the port on the lower BACK of the cabinet you can take advantage of boundary (wall, floor, corner) effects,

it works...countless speaker designs use it

[fini]

C&S and I were talking turkey, gnawing the bone on this one after dinner last night. Any reason not to have an up-facing port? With a 7-foot high speaker, no one's going to see it, unless you have Manut Bol over for a listening session.

fini

[Clipped and Shorn]

Stig,

These cabinets will be flush up against the wall since every inch is critical in my limited space, so I think the port will better be in the front baffle, at the top. I do wonder if there may be some subtle or perhaps interesting effect from being on the top front of the baffle.

Fini's idea is also interesting with the port facing toward the ceiling which is less than 2 feet away. There is "acoustic" tile (or whatever it was called) on the ceiling, but that is not to say something more "reflective" couldn't be fastened to the ceiling in those corners where the speakers will be. Now fini has me wondering if there may be some definite bass boost by having the ports facing directly up toward the ceiling . Any thoughts?

-C&S

[Stig]

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On 11/29/2002 3:51:23 AM Clipped and Shorn wrote:

In a DIY bass reflex cabinet design, what are the factors determining the placement of the port? Once the net volume of the cabinet and the size and length of the port have been correctly determined (from crunching numbers with software and relevant parameters) what other guidelines should be considered regarding the placement of the port. Can the port just as easily be located at the top of the cabinet as the bottom? Are there critical considerations regarding this positioning?

-c7s

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On 11/29/2002 3:51:23 AM Clipped and Shorn wrote:

In a DIY bass reflex cabinet design, what are the factors determining the placement of the port? Once the net volume of the cabinet and the size and length of the port have been correctly determined (from crunching numbers with software and relevant parameters) what other guidelines should be considered regarding the placement of the port. Can the port just as easily be located at the top of the cabinet as the bottom? Are there critical considerations regarding this positioning?

-c7s

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Klipped,

with the cabinet being 7ft. tall you might get some reinforcement with the ports on top.

but...assuming you are using a 15in. sub driver at the bottom front of the baffle, your port will be almost 6 feet from the driver,

to me thats uncharted territory, might work fine, might not.

don't know how big your ports are, but in this case I would put the ports above or below the sub driver if there is room.

just my opinion...

curious, big cabinet, what material (MDF, plywood)and how much internal bracing are you using?

[Clipped and Shorn]

The port tuned to 24 hz will be a short piece of PVC pipe 6" diameter and 3" long. The cabinet will be made from 3/4" plywood, maple on one side, birch, the other. There will be a lot of cross bracing on the opposite sides and front panel in addition to the corner framing as well as insulation. The model and general method for this diy project is one that Mark Deneen made for his Altec 416B + 511B. He helped me with the calculations using a software program he has. My friend fini is an experienced cabinet maker and finish carpenter. I have also found good DIY information on the internet (the link I posted above). But as you say the port being on the top or aiming up to the ceiling is uncharted territory. It would be nice to have some speculative physics theorizing about this to motivate the decision. There is that statement quoted above which indicates that the port can be anywhere.

The two questions would be:

1. why not have the port farther from the woofer?

2. Is there some advantage or disadvantage to having a port directed to the ceiling. If having a port on the back bottom aiming at the wall is advantagious, why not aimed at the ceiling?

c7s

[fini]

Another thought: Why couldn't you put the 18" sub at the top? Not up-firing, but at the top of the stack? I'm assuming since I've never seen this arrangement that it has more to do with sonics than aesthetics, as there are quite a few avant garde speaker designs out there.

fini

[Stig]

Klipped,

As this is such a large and heavy cabinet, my last uggestion of mounting the port on the front was for practical purposes.

DIY cabinet building is sometimes by nature a trial and error affair. Sticking with known and accepted guidelines will be a good starting point.

I've made several 'why havent they done it this way' projects, most of them always ended up in a scrap pile out back. It's fun to experiment but again with something as big as your building I don't think you want to cart them out to the garage for port modifications everyday.

also one more thing... don't know what the cu.ft. volume needed for the 18in. sub is but it might be a an idea to seal it off from the rest of the system if possible.

have fun

[Clipped and Shorn]

Schtig,

Good advice. Thanks for the "been there" wisdom. I think I may have room afterall for the port next to the woofer in the front and still have the 511B horn at a reasonable height of about 50+ inches, and the JBL tweeter just above that.

Actually both woofers are 15" Altecs. The 416B is for the "sub" eg. below 100 -150 hz. The net volume 19.67 cubic feet has been calculated for this driver. The "low to mid" Altec 515C will be housed inside the cabinet in its own isolated (sealed) sub-cabinet. The net volume calculation has taken in consideration the volume of this extra box and all the framing and bracing as well as the horn. That was some fun algebra and calculating the approximate displaced volume of the uniquely shaped 511B was fun also.

The (PA style) speaker grills on the front are about 18" diameter in order to cover these Altecs, that was the confusion. You are correct about the weight of these monsters. I may have wheels on the bottom. I am contemplating an interesting paint job as well.

-c7s

[jnorv]

C&S, What is the actual internal volume that you are going to be using for your 515Cs. I have a pair of Bs with the same T/S parameters that are looking for a home. I have one now in a 10 ft^3 back chamber Karlson that sounds pretty good. Unfortunately I have roots in my sewer line so I have been off the speaker project for a while. I did discover 902s for 511Bs. much better high end than the 802s. If you are an old fart, with limited high end hearing, you can get by without a tweeter.

Jim N

[fini]

I know a lot of old farts who haven't used their tweeters in years, however none wants to get rid of them...just in case.

fini

[jnorv]

Ya never know. You might just get lucky.

Jim

[Clipped and Shorn]

Jim,

This somewhat extravagant tri amped speaker system is utilizing the Altec 515C in the 200hz - 1000+hz range and as such will be contained in a minimal compartment, a somewhat small sealed box within the larger speaker cabinet, probably no more than 2 cubic feet in volume. The bottom end below 200hz is to be handled by an Altec 416B and the critical bass reflex volume was calculated using its T/S parameters (worked out to 19.67 cubic feet tuned to 24 hz}. The Altec 515 was not the ideal choice for the bottom end in a bass reflex cabinet design, but was chosen for its beauty in the above 200hz range. In that range the enclosure volume is not critical, as I understand this.

Even though it is possible to make calculations for an Altec 515 in a bass reflex design one runs into some critical limitations. Its use in this design is not recommended. It is better used horn loaded. Tom Brennan, I believe, does this, but also has sub woofers in another bin to compliment it. Check out his website. For horn loading, you might want to either build or find some used Altec 825 cabinets {or whatever that number is that best utilizes the 515}.

Brennans Horn Loaded 515

Another Horn Loaded 515

The 515,515B,515C are similar enough that I think my comments apply to any of them. I do know that those original Altec 515 are particularly prized, if the prices on eBay recently are any indication. I'm sure I will be happy with the 515C (as opposed to a 515 or 515B}, although used in this configuration (rather than horn loaded) remains to be heard. I will hope for the best.

Since I will be using the JBL tweeter I will stick with the driver options I have for the 511B (802 or 806). Even with these, one could have a system without a tweeter and probably not miss it.

As I mentioned, the system I am building is extravagant, so I will add the JBL2404 tweeter. The whole system will use an active crossover in a three way mode. There will be a passive crossover between the 511B/802G and the tweeter... so when I get lucky, I will be able to get my tweeter up in the dark room with the warm glowing tubes. I plan on using an SS amp for the bottom, tubes for everything else. The whole shebang will be just dandy or just ducky or both.

-C&S

[WMcD]

I have limited knowledge about port placement. But perhaps I can add something of value.

This is not contrary to the JBL link. However, they've not explained a lot of what people want to know, in my view. A basic issue is how the ported, bass reflex design gives us bass boost through the port. Specifically, why does it work, and when it does work, what goes right, and wrong.

The port and the tube behind it is the mass part of a resonant circuit. The spring part is the air in the cabinet. The plug of air in tube only starts moving at the resonant frequency of the mass and spring. Therefore, the output from the port is limited to a narrow range of frequencies. This is typically where the driver bass output would be rolling off. If we can use the sound off the back of the diaphragm, maybe we can hold off the roll off for a bit of the frequency range.

You see the effect of the acoustic resonance in the electrical impedance of a bass reflex speaker, like the Cornwall. In a sealed box (like the Heresy), there is a peak of impedance at driver resonance (one peak). But in the bass reflex, there is a dip where the peak would be, and you get two peaks almost symetrically offset from the driver resonance. The loading via the resonant circuit put a dip in the middle of the peak.

Despite all the concern about different designs, Butterworth, quasi Butterworth, etc., the resonant frequency of the box and tube is always very close to that of the driver. Naturally this about 50 Hz or 40 Hz in most designs.

The odd thing about the design (and indeed its purpose) is that it acts as a phase reverser. The output off the back of the driver drives the resonant system, but the output from the port is 180 degrees out of phase from the back of the driver. As I've pointed out, an analogy is the toy of a paddle and ball on a rubber band. It is difficult to see exactly. But if you let the mass of the ball hang and move the paddle just a bit (back of the driver), the ball (air in the port) moves out of phase to the motion of the paddle. This means it is in phase with the front of the driver.

The bottom line is that at the resonant frequency, the port output is in phase with the front of the bass driver. To this, I say "wow". What a neat solution to use the back of the driver to reinforce the front.

Taking it all together, this means the port and the driver outputs reinforce at only low frequencies around 50 Hz or so. This corresponds to a very long wavelength, which I calculate to be about 22 feet.

Now. in a reasonably sized box, the port and the driver can't be much more than 2.2 feet apart measureing around the edges or any path. That is about 1/10th of a wavelenght. Therefore, in any event the port and speaker are "close". By placement (or moving of the the placement) of the port in the box, it is difficult to get them distant from each other in terms of the wavelength.

(I have a suspicion here that drone, passive radiators might work better because there is some mutual coupling via the large area of the drone, but that is another story.)

The bottom line here is that I think the placement of the port will not significantly effect the bass output because of a change of interaction.

On the other hand, designers do have some concerns that the physics of the port, or the mass of air in the tube, can be upset, or altered, if the port is up against a wall. Probably it is a matter that the plug of air is no longer what is in the tube, but rather is added to by the air trapped near the boundry with the wall. That could alter the resonant frequency of the system. It might be good if it could be designed for exactly. None the less, the design is for average, or a foreseeable condition. So you see comments that the wall should be several multiples of the size of the port away to maintain design conditions.

Therefore, if the port is in the back, don't get the box too close to the wall.

Another issue is chuffing. That is a non-bass noise caused by the movement of the plug of air at the port. At high levels, there is a lot of air being pumped through the port. Mr. Tomlinson Hulsman (Mr. THX) wrote that this leads to "port noise complaints." A very bad pun.

I suspect that these items give the speaker designer a couple of choices. If the port is at the back, the chuffing noise is less apparent. But then if the box is up against a wall, the chosen resonant frequency might be altered. If the port is at the front, the issue with the wall is avoided, but chuffing is more apparent to the listener.

If the port is large enough, there is less chuffing. I expect that the large port on the CW avoids cuffing, and therefore front placement was used.

Regards,

Gil

[jnorv]

Gil, very well put. But it does bring up the point that if I am cleaver than I can tune the box/port to accommodate a close wall placement. However if I move the wall, then I have to retune.

Jim N

[prodj101]

is it me or was C and S just talking to himself?

[jnorv]

C&S, if you slot load it next to the floor, you can tune it by adjusting the heitght off of the floor.

jim N

[John Warren]

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