Please help me understand tuning a port

[cassb]

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

[Platform]

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....

[cassb]

Platform wrote:

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

[Bent]

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.

[atledreier]

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.

[Joe L.]

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.

[cassb]

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?

[Robert_J]

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

[atledreier]

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.

[cassb]

Robert_J wrote:

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.

[atledreier]

By modelling in WinISD. I knew from previous projects what would work. WinISD suggested 260liters for EBS with my driver. I knew also from previous projects that that driver needs at least a 6" port. This can be determined from WinISD by looking at the port air velocity plot as you change tuning, enclosure size, port size and port length. You can also observe the spl plot, group delay, driver excursion as you change variables around. It's basically a matter of knowing what you want from a subwoofer. This comes with experience. I've had sealed enclosures, small ported, fairly large ported, some tuned for maximum SPL, some for good sound quality.

For my current project I wanted it all. By sacrificing room space I got it. The low tuning ensure that the port only contribute to output at very low frequenzies. The system is basically a sealed system down to below 30Hz, with the benefits that give (transient response, group delay), and get the help it needs very low. Also, the low tuning frequency means it won't unload (go below tuning) for the majority of material. Also, the natural roll-off you get with this low tuning (Gradual fall in spl from around 80Hz match the room gain pretty nicely.

See where I'm going with this? Go LLT!

[Joe L.]

The lower tuning frequency allows lower frequency sounds to be reproduced at a higher sound pressure level. This is an advantage for many of today's movie sound-tracks where low-frequency effects routinely occur between 10 and 20 Hz. Although most people cannot hear much below 20 Hz, it can be felt and adds a lot to the immersive movie experience. Generally, low tuning works with drivers that can move a lot of air. You need to model the driver, enclosure, and port in a loudspeaker-computer-aided-design program to see the overall frequency response.

As described in the paper I linked to earlier, output drops off rapidly (at 24 db/octave) below the tuning frequency in a ported enclosure. A sealed enclosure has a frequency response that drops off at a much slower rate. It still has significant output well below the -3db point of the system.

It is all a trade off. The higher tuning frequency of the DIY.com bookshelf speaker allows a much higher efficiency. Its smaller drivers would not be able to move enough air at lower frequencies to reproduce loud low bass (much under 55Hz) to keep up with the tweeter in any way. So... the designer could have elected to build padding into the crossover to make a speaker that could play lower frequencies, at lower SPL, but needing a higher power amplifier, or design a more efficient speaker that could not play as low.

Joe L.

[Joe L.]

atledreier wrote:

For my current project I wanted it all. By sacrificing room space I got it. The low tuning ensure that the port only contribute to output at very low frequenzies. The system is basically a sealed system down to below 30Hz, with the benefits that give (transient response, group delay), and get the help it needs very low. Also, the low tuning frequency means it won't unload (go below tuning) for the majority of material. Also, the natural roll-off you get with this low tuning (Gradual fall in spl from around 80Hz match the room gain pretty nicely.

See where I'm going with this? Go LLT!

I did much the same simulations using loudspeaker CAD programs to design my subwoofers for my home theater. I ended up with two sealed enclosures, each about 12.5 cubic feet in volume, each with an 18" Ascendent Audio Avalanche driver capable of displacing 6.5 liters of air. My trade-offs were lower efficiency (because they are sealed enclosures and cannot get an extra 3 dB or so of output from the port), and a pair of huge enclosures, those trade-offs resulted in a frequency response that rolls off somewhere in low teens and still has significant output between 5 and 10 Hz.

[cassb]

That's a great article -- I just finished reading it. I still don't quite understand some of it, but it's a lot clearer now. So you're right, the 55Hz tuning really is meant to dampen the lower frequencies of the 6.5" driver, kind of like a filter or gate in a way. I see now that you don't have to tune a cabinet to the Fs of the woofer if that's not the designed purpose of the overall speaker. So I can assume that Ed's design intended to reproduce frequencies from 55Hz and up very well, and to leave anything below that to the subwoofer.

Which, by the way, I am about to embark on. I have my sights set on the Dayton RSS315HF-4 driver and the Dayton SA240 plate amp, and a sealed enclosure, since this system is mostly for music and not LFE. Wish me luck!

Thanks for all your help!

Bryan

[Bent]

In my case, I didn't specifically chose 18 hz, This was my first project, I had lots of time and selected an existing design. ThomasW and John Marsh had chosen to make a larger version of a 12" Ariel Acoustics commercial subwoofer, and wound up with what they called the AS-15 (Ariel-Stryke 15" subwoofer). They used a HE 15 driver manufactured by TC sounds for what was then called Stryke Audio in what was then considered a large(ish) enclosure.
They determined via testing as well as Unibox predictions prior to building that they would acieve an 18.56 hz tuning frequency.

Mine was built with an ondoard Adire Audio ADA 1200 plate amp, and I extended my enclosure by approx 1.5 inches - and in confirming my tuning with a Fluke 187III and instrumentation grade adjustable frequency current/voltage source, I determined mine was also 18 Hz.

[cassb]

Bent wrote:

[...]and in confirming my tuning with a Fluke 187III and instrumentation grade adjustable frequency current/voltage source, I determined mine was also 18 Hz.

So can I determine my tuning frequency with a common multimeter, or do I need a specialized instrument like that?

Bryan