How do you calculate the tuning frequency of a port? (1 Viewer)

[Mike Dr]

My current sub has 2 3"(diameter) x 9" (length) ports and i just wanted to know what that tunes to (non-flared).

 

[Marv]

Mike post the enclosure inside dimensions and I can calculate it for you.
-Marv

 

[Mike Dr]

18" x 20" x 20" is the outside measurement.. keep in mind tehre's a plate amp in the back and a 15 driver in front (this is a Tannoy PS115).

Thanks.

 

[Ron Shaw]

I find that the calculated tuning frequency is still off by some amount. The port area/length is directly linked to enclosure volume. To get it right on, you also need to calculate displacement lost to the driver, port, bracing, glue blocks, etc. If you want to know exactly what it is tuned to, you need to measure it. This can be done with a volt meter and a signal generator. If you have these, or can borrow them, hook the output of the signal generator to the + and - terminals of your loudspeaker, set it for a range that covers, say, 10 to 100Hz. Connect your voltmeter to the speaker terminals, just like the signal generator (in parallel), set it on AC, and set it for a low voltage range. Sweep the signal generator from the highest frequency down, and you will observe the voltage rise to some level, then go down again, and then peak again. The bottom of the dip, between the two peaks, is the tuning frequency, which you can now read off the signal generator. Many (if not most) of you probably dont have a signal generator or volt meter, but if you are serious about speaker building, these are good items to purchase. You will use them for years. This is the same way I tune passive radiators, also. I glue weight to the backside, and measure the result, until I get to my desired tuning frequency.

 

[Jack Gilvey]

Excellent info, Ron. I'm familiar with that procedure from various texts, but don't own either of those pieces of equipment. Would those be items (volt meter,signal generator) that might be found at Radio Shack, or do you have a particular model/source that you recommend?

 

[Brian Bunge]

Jack,
I believe you should be able to use just about any DMM that Rat Shack or anyone else offers. As far as a tone generator, the NCH tone generator software should work. I tried it before, but like an idiot was trying to measure impedance (which you can't do with a DMM). Needless to say, I'll try it again and this time set the DMM to AC voltage instead of resistance!
http://www.nch.com.au/action/index.html#106
Brian:b

 

[Jack Gilvey]

Thanks, Brian, that tone generator is pretty cool. One day, I've got to put a computer upstairs where the sound stuff is, all kinds of neat stuff to play with. (At first, I downloaded the "Tone Detector" by mistake. I'm sitting here saying "Ok, now WTF does he want to do with this"? )
Is there a specific range I should look for with the DMM, or are they basically all the same?

 

[Ron Shaw]

As far as meters, I prefer ones with analog meters to the digital types, because its easier to see trends (you see the needle go up, then back down, then back up), than to interpret a bunch of numbers (but Im old. I prefer analog watches, too!). A digital one will work, of course, if you already have one, or prefer that type. It should have a high input impedence (I imagine they all do, these days). As far as signal generators, I dont have any real preference. Just make sure it covers the audio band, and not an RF generator for adjusting receivers. You really just need a sine wave for this, but even the basic ones will probably generate sine, square, and triangle waves. If you have a Frys Electronics around, they should have a few for $100-200 or so. I imagine you can get something from RS as well. If you dont have anyone around that carries them, a search on the net should locate a bunch of vendors.

 

[Brian Bunge]

Jack,
I imagine just about anything would do. I have an old $100 RS true RMS meter that I bought when I was in college. Earlier this year I bought one from PE that measures inductance and capacitance as well. Should come in handy if I ever try some crossover design work! I think it was around $80.

(At first, I downloaded the "Tone Detector" by mistake. I'm sitting here saying "Ok, now WTF does he want to do with this"?

As my mother said many times, "Those who can read have a definite advantage in life over those who cannot!" Dammit, I always hated it when she was right!

 

[Jack Gilvey]

Thanks, guys.

 

[Mike Dr]

wait wait.. shouldn't the port length and diameter always be equal to some resonant frequency regardless of what enclosure it's in? i just want the port tuning.. not the whole thing... i dont see how the resonance of a port itself will be different with a differen enclosure.. thanks

 

[Patrick Sun]

If you had a CD with 1/3 octave test tones from 10Hz to 20000Hz, you could just "feel" the cone as you plays tones of frequencies 10, 12, 15, 17, 20, 22, 25, 28Hz at low volume levels. When you get to the frequency where the cone doesn't move much at all, that'll be the resonant frequency, give or take 2Hz.

 

[Mike Dr]

but woudlnt that be the CONE's resonant frequency? .. i'm looking for the port's frequency ... i see it as a pipe organ .. and no matter wehre the pipes are, tehy'll produce the same note.. hence PORT TUNING .. am i missing something?

 

[Marv]

18"x20"x20" outside dimensions and assuming 3/4" material and the volume taken up by an average 15"er and two 3"dia x 9"long ports, BassBox is showing a tuning freq of about 32.5hz.

Hope this helps.

-Marv

 

[Brian Bunge]

"but woudlnt that be the CONE's resonant frequency? .. i'm looking for the port's frequency ... i see it as a pipe

organ .. and no matter wehre the pipes are, tehy'll produce the same note.. hence PORT TUNING .. am i

missing something?"

No, that's not the cone's resonant frequency. At the tuning frequency of a ported enclosure you get very little actual output from the driver and almost all the output from the port. So at the tuning frequency the driver will just barely move.

And yes, you are missing something. If the port diameter and length are fixed and you change the volume of the enclosure, the tuning frequency changes as well. The tuning frequency goes up if you mae the box smaller and down if you make the box larger.

Also,if you make the box smaller, the port has to be longer to get the same tuning frequency. Make the box bigger and the port will be shorter for the same tuning frequency. That's why it's so hard to build a small, ported enclosure. The port length required for a given tuning becomes too long to fit in the enclosure.

Does this clarify things any?

Brian

 

[Jack Gilvey]

i see it as a pipe organ .. and no matter wehre the pipes are, tehy'll produce the same note.. hence PORT TUNING

Mike, this is a very good question, and the answer is not necessarily intuitive (at least not to me).

There are two different "resonances" we're talking about here. There's the "Helmholtz" resonance (the one we want to use), which is determined by the relationship between the compliance of air in the box and the mass of the plug of air in the port. At the Helmholtz frequency (Fb), the air in the port vibrates in-phase with the driver, acting as another "driver", and augmenting output. Since the air in the port is compressing the air in the box at the same time the driver is compressing it, the motion of the driver is almost completely damped (stopped) at the tuned frequency, which is why Pat's method can work for determining that frequency.

The undesireable "organ pipe" resonances are something else, and, as I understand it,are purely a function of the length of the pipe. These resonances get lower in frequency as the port gets longer, and, if the port is long enough, can actually intrude upon the sub's bandwidth, or at least be close enough to be excited by the sub. Avoiding these noises is the main reason to keep ports short.

 

[Mike Dr]

MARV - now we're getting somewhere.. this is interesting..

When i was tuning my sub using SpectraPlus, there's a HUGE dip at around 32hz .. the level drops off by at least 12db.. more i think (dont recall right now) .. now if this is the tuned frequency, why would the level drop off so much .. are my ports not doing what they should? i mean it kinda makes sense since you say teh driver almost wont do any work at that frequency, but why would the level drop off so dramatically?

 

[Brian Bunge]

Mike,

That makes sense! With a tuning frequency of 32Hz you get a 24dB/octave rolloff below that. This is a function of ported enclosures. It can also be a drawback if tuned too high. That's why you won't get much output below 32Hz.

Brian

 

[Mike Dr]

well actually i DO get output below 32.. but AT 32 the drop off is huge! .. i'll try to post the graph later . basically it's steady from 100 down to about 50 then drops off severely to 32 .. then raises up a good amount at about 25 and drops off a little to 20... the sub is a good performer but that huge dip at 32 and all the port noise

 

[Jack Gilvey]

I wouldn't bother with it anymore. A proper Tempest sub will be so much better in every area.