828 Cabinet port dimension

[RogerR]

I am using 828 cabinets with 8 ohm 515G woofers. The horn flares are enclosed and *****d and the cabinet braced. The upper chamber is lined with Deflex pads and the floor is covered with 1.75" Black Hole pad. I currently have the vent set at the standard 25" X 8". Is this vent opening optimal? Thanks

[RClark]

roger-------the 825/828 boom like mad dogs due to the tuning effect of that giant port ------it is really high but i can't remember just what it is------by tuning it to the 30Hz range the sound is greatly improved--------i also can remember what size port will accomplish that but it is pretty small-------i do it with a duct to keep the opening large...........RC

[Fitts]

I built my pair of 828 cabinets a few years ago and experimented with port openings for a long time before I got them the way I wanted. I started with a 7 and 3/4" by 23" opening as recommended in the Altec plans that I had.

I didn't feel the recommended port was right and began reading up on the subject. According to the references I have, the optimum for a properly sized bass reflex cabinet is to have a port equal in area to the radiating area of the speaker. For a 15" speaker that area is about 130 square inches. A 5 and 3/4 " tall port yeilds a port of about 130 square inches so, I tried that size and noted a significant improvement. More control of the bass and better lower response.

The speaker opening in an 828 cabinet is cut off on the sides by the horn and the radiating area of the speaker is actually less than 130 square inches. I have measured mine to the best of my ability and believe the area is about 75 square inches. With this in mind, I made the port about 3 and 1/2" tall which gave me a port opening of about 75 square inches. I feel this size port made for an even better sounding system. Very tight bass, nice low end without being boomy and no peaks or resonances.

I did try reducing the port further but didn't find any improvement and have opted to stick with 75 square inches.

I think the 828 was intended for fairly large rooms and my small 12 by 16 room may have a lot to do with why the small port size sounds so good. I also don't have 515's or even 416's as I can't afford either. I am using Electro Voice speakers with T/S parameters that are seemingly a good match for the 828 cabinet.

I would encourage you to try a small port and see what happens.

Best wishes,

[Russellc]

RogerR:

I also have the 828 boxes that shut off the horns of the enclosure and also use 515-8Gs. Please post your findings as this is an area of interest to me as well. I thought I had read somewhere that these later boxes with the horns closed off were just about perfect for these drivers, but it didn't go into the port. You might see a post of mine a little while back when I was in a conversation with Eduard, who spoke concerning a question I had about his boxes, which obviously had reduced size ports. He gave another advantage, that it also reduces the size of the first impedance "hump" making it easier on ultra low powered tube amps. He included a page of an article from a french publication, which, while in French, you can seen the curve on the graph.

Hello Richard: could you please elaborate on this "duct"? what is it , how implimented, and what size, and finally what woofer are you using?

Thanks to all,
Russellc

[[email protected]]

FWIW: This subject has occupied me a great deal lately, because I'm building a pair of 828's. I have calculated the port size numerous times based on the volume of the later cabinet's smaller volume with the horn flare closed off and it always comes out the same - about 3-1/2 inches high. I've used WinISD and Unibox; both give similar results. After reading Jim D.'s comments in the "Small VOT" paper, I went ahead with the 7-1/2" opening per plan but now I'm certain that I was right all along. I'm going to cut a *****r to reduce the opening.

[Fitts]

Hi WunHaungLo,

I agree with you, all the formulas and information I have read about ported cabinets tell me that for the A7 the port should be 3 and 1/2 inches high and run the width of the cabinet for a total area of about 75 square inches.

I would be very interested in knowing if you think the smaller port sounds better.

P.S. What kind of woofer are you using?

[[email protected]]

The woofers are 515-G's I bought from GPA through Steve Burger.

[Russellc]

David:

Is this 31/2 inches ideal for all the altec drivers that were used in A-7s, or is this specific to the 515 woofer? I am talking here about the 828 box. My A7s have a board that shuts off part of the port. It is drilled so it can make the port either 10 x 25 or 7.5 x 25. It came with the larger 10 inch setting. I moved it to the smaller position and thing definately improved. Do you think then I will have further impovement by going to the 31/2 inch port? I have 5158g woofers, is this measurement "woofer dependent?

Thanking you in advance,
Russellc

[Fitts]

I'm Altec Green with envy. I'll be most interested to hear how you feel about the smaller port size after you make the changes.

[Fitts]

From what I have read, it seems that in the best possible vented enclosure the vent size should equal the radiating area of the speaker. Apparently, if the vent is larger than the speaker area, control is lost and if the vent is smaller, efficiency is lost. When you think about it this seems to makes sense. So, the speaker opening on any vented enclosure actually determines the optimum size of the vent. What we really need to know is how big or small to make an enclosure so that the vent can be the same area as the speaker opening.

The T/S parameters for a speaker dictate the ideal enclosure size needed so that the vent size can equal to the speaker radiating area. The formula I have seen most frequently for vented enclosures is this; the ideal volume equals fifteen times the Vas parameter in cubic feet times the Qts parameter raised to the 2.87 power or V=15*Vas*(Qts^2.87). For the Altec 515 T/S parameters this formula yields an enclosure volume that is very close to the internal volume of the 828 with the speaker mounting panel that runs the entire width of the cabinet. For the Altec 416 the ideal volume was slightly larger and more like the volume of the early A7 with the small square speaker mounting panel.

So, the volume of the A7 or 828 cabinet seems optimal for speakers like the 515 and 416 and this being the case, the area of the vent should be equal to the radiating area of the speaker. A 15 inch speaker has an area of about 130 square inches but the speaker cutout on the 828 or A7 is about 75 square inches. The 3 and ? inch opening gives you 75 square inches matching the speaker opening.

Sorry if this is a bit long winded. I do think you will find that a 3 and ? inch vent will give you additional improvement.

Regards