Porting information please!

[GM]

Hi,

>Greetings all!
There seems to be a mound of information on correct porting calculations. Four years ago, I built a pair of boxes modelled after the later Altec studio monitor cabinets. I was working from brochures and my own technical knowledge.
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Care to be more specific, as in which model monitor?
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>The components used were the A7-800 kits (1974) with the 416-8A bass driver. I have learned that its Fs=25Hz.
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That's the published spec, measured specs will vary somewhat for a variety of reasons.
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>According to an old Altec chart, my 2 ports should be 4 9/16" each in diameter. They also say that the box should be tuned to the free air resonance of the driver, which is 25 Hz.
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This is probably pre-T/S alignments, so don't take either as 'gospel'.
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>My existing ports are 3 1/2 inches in diameter and the bottom end seems weak.
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The smaller the cross sectional area of a vent for a given length = a lower Fb, so little wonder it sounds 'weak', especially if the horn's not attenuated enough.
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>I'm not using any tubes as in the old Valencia cabinet.
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Hmm, the Valencia cab, at least the one's I'm familiar with, don't have vent tubes, but a crude reflex vent that the horn mounts in: http://www.lansingheritage.org/image...ure/page10.jpg
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>Can ANYONE help me? I'm so confused.
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Sure, if I get enough info, like cab design details, the desired in-room Fb, the room dims/details, where in the room the speakers are located, are they SS or tube driven?
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>I've even had a speaker designer from a very successful speaker manufacturer tell me that the Fs doesn't really matter when calculating ports.
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Correct, cab tuning is strictly a function of Vb and vent specs, the driver's specs only dictates how the cab affects its performance.
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>According to him, my boxes are tuned to about 35 Hz. He also said that the 416-8A has good compliancy and my existing ports are good??

>My cabinets are 6.3 cubic feet with the 811B horn and driver mounted inside. the flange of the 811B is about 3" behind the LF mounting ring, similar to the mounting used on the Valentia.
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According to some measured specs I have, this is a little small for an SS driven T/S max flat (6.67ft^3 net/37.5Hz Fb), but not seriously so, and if yours are actually tuned to 35Hz, then it's 'close enough'.

I'm confused though, if you have the same horn mounting same as in the link above and dual vents, then Fb will be very high, which will also yield a very 'weak' LF response. Or did you build it like this: http://www.lansingheritage.org/image...ign/page31.jpg

Anyway, assuming a 6.3ft^3 net/35Hz Fb, then this alignment begins rolling off around 80Hz in half space with the measured specs I have/SS amp, which if up against a wall usually sounds pretty well balanced if there's some room gain once the horn is attenuated enough.

Several feet out from the wall and it's going to sound 'weak' without significant EQ response shaping. Really, for HI-FI apps (other than apt. or condo), a 9-10ft^3 cab tuned to 27-32Hz positioned ~up against a wall or in a corner is the Hot Ticket.

GM

[Fitts]

Hi,

GM's last sentence sums it all up. A 416 in a 10 cubic foot cabinet in the corner of the room is heavenly.

If you decide to make such a cabinet, try a simple 75 square inch rectangular vent instead of a tuned port hole.

Regards,

[acre]

Dear GM

Many thanks for such a detailed response! I think the box is correct and I was referring to the 1974 Valencia. (I didn't realize there was an earlier one.) How do I attach a picture of my monitors to this post? Thanks again.....Lee

[acre]

Greetings all!
There seems to be a mound of information on correct porting calculations. Four years ago, I built a pair of boxes modelled after the later Altec studio monitor cabinets. I was working from brochures and my own technical knowledge. The components used were the A7-800 kits (1974) with the 416-8A bass driver. I have learned that its Fs=25Hz. According to an old Altec chart, my 2 ports should be 4 9/16" each in diameter. They also say that the box should be tuned to the free air resonance of the driver, which is 25 Hz. My existing ports are 3 1/2 inches in diameter and the bottom end seems weak. I'm not using any tubes as in the old Valencia cabinet. Can ANYONE help me? I'm so confused. I've even had a speaker designer from a very successful speaker manufacturer tell me that the Fs doesn't really matter when calculating ports. According to him, my boxes are tuned to about 35 Hz. He also said that the 416-8A has good compliancy and my existing ports are good??
My cabinets are 6.3 cubic feet with the 811B horn and driver mounted inside. the flange of the 811B is about 3" behind the LF mounting ring, similar to the mounting used on the Valentia.
Thanks in advance to anyone that can help me!! Lee Batchelor