That old saw: A7 port size

[Audio_by_Goodwill]

Originally posted by aeyb701:
In reading many old threads, as well as 40-plus year-old Altec literature on the topic there seems concensus that 75 square inches is an optimal port size for the 825 or 828 cabinet
for tuning at about 45 Hz.

<font size="2" face="Verdana, Helvetica, sans-serif">Hmm, I wasn't aware that Altec literature specified a 75 square inch port. Can you post a link to what your reading?

Why then did Altec make their ports on the order of 200 sq. inches?

<font size="2" face="Verdana, Helvetica, sans-serif">Until I measured my port, I didn't realise how much bigger than 75, the factory ports are. I just calculated over 211.

[aeyb701]

Not that Altec directly specified 75 sq in for the A7-but I infer that from the graph in:

http://www.lansingheritage.org/image...ure/page04.jpg

using 10 cu ft to be a ROUGH volume for the A7.

I have a similar graph from ca. 1964 Altec speaker brochure copy I got on eBay-but no link.

Meantime I covered half the port area from behind the cloth and am listening-initial impression is there's lower bass but maybe more peaky at other frequencies...I'll listen with it like that for a few days.

Jon

[GM]

Greets!

75"^2 vents was chosen by various forum members based on their in-room listening experiences, though FWIW me and a few others prefer much lower tunings for HIFI apps.

Altec OTOH tuned them for PA and/or cinema apps, which have a considerably different performance criteria that dictates a higher tuning, though it's preferable for a HIFI app if a sub system is used.

GM

[Audio_by_Goodwill]

Well, that graph is for a purely bass reflex cabinet. Maybe it doesn't work for a combination cabinet.

[Audio_by_Goodwill]

problem posting, ended up triple posting.....

really wish I could delete my extra posts!

<font color="#FFFFFF" size="1">[ May 04, 2008 03:10 PM: Message edited by: Audio_by_Goodwill ]</font>

[Audio_by_Goodwill]

problem posting, ended up triple posting

<font color="#FFFFFF" size="1">[ May 04, 2008 03:09 PM: Message edited by: Audio_by_Goodwill ]</font>

[GM]

It's a simple Helmholtz resonator, so it doesn't matter whether it's for a BR, ducted port or combination cab, though the aspect ratio of the cab and the distance between the driver and port does affect tuning to some extent, so for the math to be the most accurate the vent should be acoustically right next to the driver.

GM

[Richard C.]

Someday I am going to have to listen to some REAL VOT (A7 at least!) in a small room; say 15" x 20".

I built a copy 30 years ago, and I didn't like it at all! It was fun to power it with a tiny transistor radio (9V), so loud that we couldn't hardly talk over it. The problem is I had to be 20' away to really appreciate it. I read that many people have them in "normal" sized rooms, but I just don't see how.

I love both my 14's & my 19's, and I don't see how I could improve on them. BUT..., like I said, I would someday like to hear what you guys hear.

I didn't play with the port size of my A7 copy at all, because the Altec plans didn't mention it.

Richard C.

<font color="#FFFFFF"><font size="1">[ May 04, 2008 11:25 PM: Message edited by: Richard C. ]</font></font>

<font color="#FFFFFF" size="1">[ May 04, 2008 11:28 PM: Message edited by: Richard C. ]</font>

[aeyb701]

Hello once again:
In reading many old threads, as well as 40-plus year-old Altec literature on the topic there seems concensus that 75 square inches is an optimal port size for the 825 or 828 cabinet
for tuning at about 45 Hz.
Why then did Altec make their ports on the order of 200 sq. inches?
Can someone opine?
Thanks,
Jon Archibald,
Indian River, Ontario

[Altec19]

I have a pair of A7-500's in a 15x25 room. They produce a very large sound stage but the bottom end is rather modest. Keep in mind that these cabinets are designed to project into a movie theater space. The bass presentation just seems to need some distance in front of the cabinet to get it together. These are not near-space cabinets. I also have a pair of UREI 811A's (604-8K drivers) sitting on top of the A7's. With a decent subwoofer in place (the .1 as in 7.1) the UREI's put the A7's to shame in this space. But I have to give the A7's credit. They make almost perfect stands (sturdy, correct height) for the UREI's. And what better speaker to have on display in a home theater than the "Voice of the Theater".