Tuning a ported cab for JBL K130- what freq?

Lynn Kasdorf · From: Waterford Virginia, USA

I am building a slant-front cabinet for a nice old 15" JBL K-130 for use with my Stereo Steel amp. I am using a pretty much random design, but I figured I'd try to use a little science when it comes time to do the porting.

I have found a nice online calculator for determining port size and length based on cabinet volume and desired tuning frequency. port calulator

So- the big question- for steel guitar use, an 8 ohm K130, and a cabinet with a volume of 1.4 ft3, what would the ideal tuning frequency be?

Or is a tuned cab a bad idea, since these sound great in an open cabinet like a session 400?

I'll also be building a smaller cab for a nice JBL 12" speaker (with a MASSIVE magnet- don't recall the model).

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"You call that thing a guitar?"

Jack Stoner · From: Kansas City, MO

Go to the JBL site. They have several documents that can be downloaded on porting (and cabinet size).

The porting is vs cabinet size. They may have something you can use for the size cabinet you have. However, the size of the cabinet is as important as the amount of porting.

Blake Hawkins · From: Florida

Lynn, There is no exact frequency to which you tune the port.
The purpose of the port in a bass reflex enclosure is to smooth out the response of the speaker at low frequencies.
Each speaker has a resonant frequency at which it will be louder. This can cause a boominess in the sound.
The idea is to get rid of the boom.
The design criteria of these cabinets is based on the assumption that you want to reproduce music, such as a symphony orchestra.
A steel guitar is a special case. Depending on the guitar, amplifier, and speaker you are using, you may want a resonance somewhere to give you the "tone" you want.
A method of determining this is to cut the port larger than the design calls for, then use a sliding wooden shutter to change the size of the port while you evaluate the sound.
If you like, I can give you the engineering technique for tuning the port but it requires an audio oscillator and an AC Voltmeter which has response at audio frequencies above 60 Hz.
Blake

Dan Tyack · From: Olympia, WA USA

The calculations supplied by the speaker vendors for the port vs. cab dimension are to obtain as close to a flat frequency response as possible. IMHO this is the last thing you would want for a musical instrument cab. My favorite 15" cab is built by THD (see http://thdelectronics.com ), and it is ported, but the port is designed to emphasize the most 'musical' frequencies, and also to allow maximal on stage dispersion of sound from the cab. My next favorite cab is an open backed cab which is very good sized in relation to the speaker (not just big enough, like the Mesa Boogie or Tube Works cabs).

I have owned and played through many different types of ported cabs (theil designs, others built to JBL specs) and IMHO they just don't compare.

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www.tyacktunes.com

[This message was edited by Dan Tyack on 08 May 2001 at 07:41 AM.]

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