Old 03-27-08, 03:28 PM Thread Starter
Shackster
Bryan

Join Date: Mar 2008
Location: Victor, NY
Posts: 47

I have read and re-read posts here about tuning a port to a specific frequency. I searched the internet for the same info. I can find bits and pieces, and some highly analytical explanations of standing waves and pressures and springs. But let me try to condense this into my own language. Tell me if this is right:

Step 1: A bass-reflex cabinet has an open hole in it of a certain diameter with a certain length of tube that goes into the cabinet. The diameter, length of tube and cabinet volume determine the frequency the cabinet is tuned to. The 'tuned' frequency is the frequency where the cabinet internal air volume will vibrate in exact tandem with the woofer driver. Is this correct?

Step 2: Below the tuned frequency, the woofer will become unhindered by the cabinet air, and will be in danger of 'bottoming out.' So the idea is to tune the cabinet to a frequency right at or near the woofer's free-air resonance (Fs). Right?

Step 3: To change the tuned frequency of a cabinet, you shorten or lengthen the port tube (or change the diameter).

Step 4: To physically measure what the actual tuned frequency of a cabinet currently is, you...um... do what?

That's my understanding. Please correct me if I'm wrong. I'd really like to understand this in layman's terms so I can play with the port tubes on my DIY speakers.

Thanks!
Bryan
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Old 03-27-08, 04:09 PM
New Member
Platform

Join Date: Aug 2007
Posts: 2

It is correct that length and diameter as well as volume of the cabinet determine the tuning freq.

Below tuned freq. there is a possabilty of bottoming..could lead to damage...some use a subsonic filter to safeguard themselves

Changing the tuning you would change the volume or length of a port...the diamter needs to stay the same for the same driver...bigger driver = more air moved = bigger diameter port. If not you would experience port noise

Physically measure...use test tones...when the driver cone is not moving...meaning that the port doing all the work..that is the tuning freq.

Hope this helps....
Platform is offline
Old 03-27-08, 08:00 PM Thread Starter
Shackster
Bryan

Join Date: Mar 2008
Location: Victor, NY
Posts: 47

Quote:
Platform wrote: View Post
It is correct that length and diameter as well as volume of the cabinet determine the tuning freq.

Below tuned freq. there is a possabilty of bottoming..could lead to damage...some use a subsonic filter to safeguard themselves

Changing the tuning you would change the volume or length of a port...the diamter needs to stay the same for the same driver...bigger driver = more air moved = bigger diameter port. If not you would experience port noise

Physically measure...use test tones...when the driver cone is not moving...meaning that the port doing all the work..that is the tuning freq.

Hope this helps....
That's fantastic, thank you! Just to follow up on a couple things then...

- So a tuned port makes like a 'cushion' of air that is located at the tuned frequency. A speaker then should not be driven below that cushion because it's no longer supported with backpressure by the air.

- By contrast, a sealed enclosure has a finite 'cushion' of air (that is still at some frequency I assume) where a driver cannot get below that frequency. Right?

- And so again, the tuned frequency can be down to, but not below, the free-air resonance of the driver? What determines where this tuned frequency should be?

Thanks again for humoring me.

Bryan
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Old 03-27-08, 10:54 PM
Senior Shackster
Ben

Join Date: May 2006
Posts: 343

a ported enclosure can definitely and most certainly be tuned below the Fs of the driver, my AS-15 Klone
has a dual 4 ohm Sound Splinter 15" RL-p driver with an Fs of approx 27 Hz (IIRC), the enclosure uses a port and volume which provides tuning of approx 18 Hz.
Most of the "LLT" enclosures found on this forum are similar - but still tuned lower.
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Old 03-28-08, 03:10 AM
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Maverick

Join Date: Mar 2007
Location: Sandnes, Norway
Posts: 1,377

I use the Soundsplinter RL-p15D2 that has a Fs of 27Hz. My tubes are tuned to 13.3Hz, and they are the most awesome subwoofers I've heard so far.
atledreier is offline
Old 03-28-08, 07:10 AM
Shackster
Joe

Join Date: Jan 2007
Location: Greensboro, NC
Posts: 33

As already described, the port tuning frequency is often well below the driver Fs.

You might be able to think of it this way... The air in the enclosure needs to be compressed as the cone of the driver moves inward (and the opposite as it moves outward from its resting mid-point)

Below the tuning frequency of the port, the air can move fast enough through the port so that no "cushion" or "additional resistance to movement" is provided to the cone of the driver through its excursion max limits. Above the tuning frequency of the port, the volume of air in the port cannot move fast enough, and some additional resistance to movement is provided to the driver cone. this resistance to movement of the cone is what prevents it from over-extending (bottoming out)

The enclosure volume comes into play here too. A small volume enclosure is harder for a given driver to pressurize, since it has to pressurize the smaller volume of air to a greater degree for the same movement(excursion) of the cone. This will result in lower efficiency and the need for a larger amplifier to get the same excursion out of a given driver. A larger enclosure will need to be pressurized less for a given excursion of a driver, thus it will be more efficient as less work is needed for the driver cone to move to its excursion limits.

As set of really good animated graphics to show this are part way down the page at this link http://www.hometheaterhifi.com/volum...ayporting.html

In the animated graphics, the upper driver is the powered one, and the lower "driver" represents either the passive radiator, or the column of air in the port.

Joe L.
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Old 03-28-08, 08:16 AM Thread Starter
Shackster
Bryan

Join Date: Mar 2008
Location: Victor, NY
Posts: 47

OK, thanks again. So when you are determining what frequency to tune the port to, how do you make that decision? What frequency should a port be tuned to? Is there a rule of thumb? Or an absolute value based on the driver and enclosure? Or is it just up to the cabinet designer to pick any frequency he wants?

For example, atledreier, why did you choose 13.3Hz?

Bent, why did you choose 18Hz?

Why did Ed Frias choose 53Hz for his AR.com DIY bookshelf speaker when the Fs of the mid driver is 44Hz?
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Old 03-28-08, 08:36 AM
Shackster
Robert

Join Date: Aug 2006
Location: Southaven, MS
Posts: 42

Tuning frequency is deterimined by the designer and the goals of his/her project. Also, the larger the enclosure, the lower it can be tuned without port noise or port resonances. But by tuning low, you are sacrificing overall SPL. EVERYTHING in speaker design is a tradeoff. You have to find a balance of all factors that suit your needs.

-Robert
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Old 03-28-08, 08:38 AM
Elite Shackster
Maverick

Join Date: Mar 2007
Location: Sandnes, Norway
Posts: 1,377

Such a low tuning that I used is a somewhat special case. I tuned it that low because the driver/port/enclosure system would roll off about 6dB towards the tuning frequency. At such low frequencies this rolloff is countered by room gain (the room acting as an enclosure in itself), thus yielding a flat in-room response. Most systems aim to have flat response for the system itself. Read up on LLT (Large low tuned) and EBS (Extended bass shelf) for more information on this way of doing things. I ended up with 13.3 for my enclosures. I was aiming for around 14. The caps and port took less volume than I anticipated, and my internal damping was probably more efficient than anticipated. That said, they play very, very well.
atledreier is offline
Old 03-28-08, 08:38 AM Thread Starter
Shackster
Bryan

Join Date: Mar 2008
Location: Victor, NY
Posts: 47

Quote:
Robert_J wrote: View Post
Tuning frequency is deterimined by the designer and the goals of his/her project. Also, the larger the enclosure, the lower it can be tuned without port noise or port resonances. But by tuning low, you are sacrificing overall SPL. EVERYTHING in speaker design is a tradeoff. You have to find a balance of all factors that suit your needs.

-Robert
What advantage does tuning lower give you?

And I'd still like to know how Bent and atledreier determined their tuning frequency.
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