Speaker Box and Port Question

[Adam White]

Hello! I have some old parts from a set of Beta Sound speakers and I was thinking about building an Ojas style setup (which is really more Western Electric/Altec in origin, but you know what I mean) out of them. The bass driver is a CTS 12W38A and the only real info I could find about it was from this URL where there's a table of values (page 24): https://www.worldradiohistory.com/Archive-All-Audio/Speaker-Buider/Speaker-Builder-1980-01.pdf

I asked Chat GPT to summarize the doc and suggest a speaker box for the driver, and this is what it came up with. Can anyone check the robot's math/logic and let me know if this would put me on the right track?

According to the article in Speaker Builder (January 1980), for the 12W38A driver in a vented (bass-reflex) enclosure, the recommended vent specifications are:

Recommended Vented Box Design for 12W38A​

• Box Volume (VB): ~138 liters (4.9 ft³)
• Tuning Frequency (fB): 35 Hz
• Vent (Port) Size: 12.6 in²
  • Height: 3 inches
  • Length: 4.2 inches

This vent size is tuned to align the box resonance with the driver’s natural resonance (~35 Hz), providing optimal bass extension and flat low-frequency response.

Tip: Use a flared port if possible to minimize port noise (chuffing), especially at higher volumes.

PanelSize (inches):
Front/Back
30 × 18
Sides
30 × 14½
Top/Bottom
16 × 14½

Thanks for any help you can offer

BTW, the mids will be a setup exactly like these Atlas drivers and horns: https://community.klipsch.com/topic/218806-atlas-pd-5vh-compression-driver-with-k700-clone-horn/

 

[grindstone]

The SB/GPT box is provisionally OK, but we would need further details to refine. Will you reuse the Beta XO and tweeter as well or did you mean a 2-way (in which case the pd5vh+beta midhorn wont play high enough standalone)? Otherwise, if you wish the 2-way, you will need to choose other drivers + horns for the "top".

Tell us what you really want to build and what parts you have. What you have might not make the best/easiest/cheapest route. (Don't ask us how we know these things).

 

[painttoad]

i am curious how port size is calculated. 12.6 square inches? ok, sure 3x4.2 is 12.6 so that could be the opening of a square port but ports are expressed by volume so it needs to be cubic inches.(or liters or whatever measurement system one uses)

i did not see enough t/s parameters in that article to model a box in bassbox6 that i would trust.

i wouldn't trust chatgpt. if i was dead set on building a box for that driver, i would buy this:

Dayton Audio DATS V3 Computer Based Speaker & Audio Component Test System

Dayton Audio DATS V3 Computer Based Speaker & Audio Component Test System Dayton Audio DATS V3 Computer Based Speaker & Audio Component Test System The Perfect Companion for Audio Professionals When designing the DATS V3 measurement device, Dayton Audio took everything customers love about the...

www.parts-express.com

good luck.

 

[Gigantic]

this tread has files for the CTS speaker line from the time that speaker was available: https://audiokarma.org/forums/index.php?threads/looking-for-specs-for-12-cts-woofer-repost.637647/

ChatGPT is wrong 30-40% of the time and wastes obscene amounts of energy and resources for even minor queries and prompts. I would also advise against using it.

If you want to do an OJAS type speaker, why not just copy the original? it essentially uses a Heresy cabinet, those dimensions are widely available. From there, I'd recommend one of the woofers used in Super Heresy mods: Celestion TF1225, Eminence Delta Pro 12A or Eminence Kappalite 3012LF. Then, I'd select a mid horn and driver- the Altec 511B seems apropos and a B&C DE120 would be a good, affordable unit and cross it at a frequency that best matches the horn and driver, somewhere between 1000-1500Hz. based on the SH port tuning, you'd want a 4" x4-1/2" port; it's a simple matter of maths to convert that to a rectangular, front mounted opening. this will give you extension to about 42Hz. A larger box and bigger port will get you lower. Horn RESP app will help you determine cabinet VAS and port dimensions. As mentioned previously Dayton Audio's DATS is very useful, as is REW and a calibrated measuring mic.