JBL C36 configuration 02 load...should cabinet be sealed?

[Steven]

My C36 project came with the 075 driver and no lf driver, but included an adapter ring for the 12" D123 driver...which I installed. This left a third, covered hole in the baffle for a third driver. I left it as is, thus no vent anywhere in the cabinet. Is this correct? I see Viscounts and such with rectangular vent hole or an open hole where a third driver could be installed. Please enlighten me! I hear cabinets need "tuning"

[Horn Fanatic]

Post a photo, if you can.

[Steven]

inside of cabinet will three openings... 075 and D123 and factory cover

[Horn Fanatic]

inside of cabinet will three openings... 075 and D123 and factory cover

The hole plug is for symmetrically mounting the tweeters. Otherwise referred to as left and right hand.

[Steven]

Yes it is...but the question remains....should the cover be removed thus venting or porting the cabinet, OR should it remain as it is in the photo, effectively sealing the cabinet air tight? Isn't the sound of the cabinet and the bass response affected greatly by whether or not the cabinet is ported? Thanks...if my configuration is fine with the experts I won't pull the cover off.

[Horn Fanatic]

Yes it is...but the question remains....should the cover be removed thus venting or porting the cabinet, OR should it remain as it is in the photo, effectively sealing the cabinet air tight? Isn't the sound of the cabinet and the bass response affected greatly by whether or not the cabinet is ported? Thanks...if my configuration is fine with the experts I won't pull the cover off.

You can remove the cover if you want, but an educated guess tells me doing so will not improve the sound. Doing so could do more harm to the sound than good at low frequencies. With respect to ported enclosures, as the frequency increases, the port basically is non-existent. There will be a point above a certain frequency when the enclosure behaves as if it is sealed.

The combination of the enclosure volume and port parameters result in a Helmholtz resonator, no different than a soda bottle. Even a soda can behaves as a resonator. Resonators create an anti-resonance. An anti-resonance of an equal but opposite frequency to the free air resonance (fs) of the speaker. A frequency where the speaker is operating at it's peak efficiency based on the mechanical mass and compliance of the cone assembly.

Also, too small of a port can result in a high mach number. An undesirable situation. In the old days, JBL used four inch diameter ports. The hole diameter for an 075 is around 3.125 inches. Too small for a speaker such as a D123, which has an fs of 45 Hz.

Some manufacturers tune their enclosures a bit above, or a bit below fs. I prefer tuning to fs. The goal of a bass reflex enclosure is to control the speaker at resonance, meaning there should be virtually no movement of the cone at resonance. A sealed enclosure cannot provide that type of control, as the system resonance will always be higher than fs.

The volume of the C36 & C38 enclosures is approximately 2.7 cubic feet. Without calculating the system resonance of a C36 sporting a 3.125 inch diameter port, I couldn't tell you what that tuning frequency would be.

HF

P.S. Just curious. Does your cabinet have the old style 'L' shaped legs that were on the C36 when it was introduced, or the screw on type which came later? The age of the box may explain why there is no port. An option for the very early C36 was for a single D216 with no tweeter. Another option was the D131.

P.S.S. I'm not suggesting, nor do I advocate butchering a vintage enclosure for the sake of installing a port.

[Steven]

Thank you Horn Fanatic for a clear explanation of cabinet resonance and speaker cone movement. I'm certain 1950's engineers carefully calculated exactly how much if any "porting" would be optimal. I specifically chose the 12" D123 full range driver to avoid booming bass. It sounds amazing as is...intending to leave it alone....thanks again for the info....hope it helps others with similar questions...assuming most all of the original c30+ series loudspeakers were cannibalized in the 60's and 70's for PA and musical instrument amplification. Glad I went with the 02 configuration. I found it without legs...it had 1/4-20 t-nuts on the bottom so I added reproduction aluminum legs and cleaned up the nicks and scratches in the mahogany veneers to make it look nice again.

[Steven]

Here is a photo of my 50's mono tube system. Bogen R660 tuner, Bogen DB130 integrated amp and C36 loudspeaker

[Odd]

Steven.

Where did you buy new legs?

[Steven]

Legs came from Alpha Furnishings thru Amazon. Similar to the originals...solid machined aluminum with a nice bright satin finish. Be aware the Alpha legs have a 5/16-18 stud in the center of each which doesn't match the 1/4-20 threaded inserts in the original cabinet bottom. Paid about $8 each

[Odd]

Thank you very much!:)

[Steven]

You are welcome! Give me a holler if you run into a problem putting them on...I thru bolted them with a washer and nut inside the cab....rock solid and the legs are solid aluminum too..really heavy. Cheers!