Port Measurement

[Mustang Marvin]

I am building a pair of bass reflex enclosures. I'm using 416's and need to determine the port size. I read in an Altec write up entitled "Loudspeaker Enclosures and Their Design and Use" that I got from this forum that the port size can be determined using an audio generator, resistor, and volt meter.

Can this be explaned and where can I get one of these at a reasonable price.
Thanks,
Marvin

[Altec Best]

What are the dims of the box and internal volume in cu.ft ? Check out this paper Altec Application notes AN-5 look at the different size enclosures WRT the 416 and the volume of each cabinet and the port size that determines the Box tuning.Cabinets explained are the 612,614,620,and the 930.All the duct lengths are 3/4" you can lower the tuning by lengthening the port length to 1",2",3",etc....... As GM has taught me on this I think, and what I have done on my project. :2thumbsup: I added 4" length of 6" PVC pipe on each port, 2 ports were required, one on each side of cabinet to get to a 25 Hz box tuning !


http://alteclansingunofficial.nlenet...Enclosures.pdf

[Mustang Marvin]

I can understand your and GM's method. But, the explanation in the article I mentioned states the method of using the audio generator as input to your amp along with a 500/1000 ohm 2/5 watt resistor and a volt meter in paralell with the bass driver will allow measurment of the impedence curve as plotted on a graph. (whew, long sentence) This method is stated as being able to tune ANY loudspeaker.

I presume this does not determine port size only tuning. Perhaps the method you recommend will get you in the ballpark and the final fine tuning can be made with the method discribed in the Altec publication.

I was just hoping someone could explain this to an old guy like me with very limited knowledge.
Box size is 36" H, 25" W, 19" D, or 9.89 cu. ft. before bracing. I'm using 416A 16 ohm drivers.

[Cal Weldon]

Hi MM,

Those are ID dimensions?

[Altec Best]

I'll assume they are Outside Dims.Well those dims (9.89) are close to a M19 (9.35) especially after bracing. I think which would give it a port size of 2.5"x 11" with a length of 3/4" the same on the M19 with a 30 Hz tuning.Also for a 416 driver. I hope that helps.For your method maybe GM or someone else can help in that regard.

BTW: Your not as limited knowledge wise as you think !

[Mustang Marvin]

Actually they are inside dimensions. Outside is 37 1/2 H, 26 1/2 wide, and 22 1/2 deep with an inch over hang at the front for a recessed front cloth covered panel.

True, it is close to the 19 dims, but there is no enclosed horn. The horn will be mounted on top. An 805 with a 399 compression driver. Crossed at 800 hz.

Bass is very important to me, so I am very concerned about the port. I want to go as low as possible.

Thanks

[GM]

I am building a pair of bass reflex enclosures. I'm using 416's and need to determine the port size. I read in an Altec write up entitled "Loudspeaker Enclosures and Their Design and Use" that I got from this forum that the port size can be determined using an audio generator, resistor, and volt meter.

Can this be explaned and where can I get one of these at a reasonable price.
Thanks,
Marvin

Well, it can be determined empirically by experimenting with different size vent areas, but it can't tell you how big/long it must be to have a low vent mach (acoustic loss) at the desired peak power.

Not sure how much explanation you're wanting, so will just list some links and you can 'fine tune' your Qs if you still have any:

Read Research - Articles (DL the various vented box papers)
The Subwoofer DIY Page - Port Calculations
Porting at Subwoofer-Builder.com

Ditto what you're specifically looking for at a reasonable price, but if you mean test gear, nowadays folks usually just use freeware speaker measurement computer programs, though of course portable or table model frequency generators are still available. Whichever you choose, Google is your 'friend'.......

GM