PORT Thoughts

[SurferJoe46]

I was talking to my son who designs cabs and speaker enclosures for home and auto sound systems and we got onto the idea that a slot port is less limited and works better for a wider frequency range.

In other words - by it's design it is more broad-ranged and less limited in it's ability to vent (if that's the right word) the cab and keep barometric pressures in better control than a tuned round port.

OK - all well and good - but then afterwards I got to thinking of a slot port with an internal angled entry like below:::



If the angled cut-off is figured to match the range of frequencies you want to have less attenuated, would this be a good idea?

[billfitzmaurice]

Originally Posted by SurferJoe46

I was talking to my son who designs cabs and speaker enclosures for home and auto sound systems and we got onto the idea that a slot port is less limited and works better for a wider frequency range.

That would be a possibility if ports had a wide frequency range, but they don't. Unless improperly located or the cab isn't properly damped port radiation takes place over a very narrow bandwidth. Your son needs to go back to the books on this one.

[Mr. Foxen]

I don't think you are picturing how ports work correctly. The main relevance of port shape is to do with turbulence on the airflow.

[SurferJoe46]

Originally Posted by billfitzmaurice

That would be a possibility if ports had a wide frequency range, but they don't. Unless improperly located or the cab isn't properly damped port radiation takes place over a very narrow bandwidth. Your son needs to go back to the books on this one.

His statement - although prolly missed by me - was that a slot port is less frequency-narrow than a round or tubular port.

We were actually discussing bell-mouthed round ports verses straight tubular ones, and that the bell-mouthed version would have to be (as far as I understand it anyway) not a linear flare style but rather be an exponential flare in design.

Not that they were good or gooder --- just different.

[silky smoove]

A properly designed port handles a pretty narrow range of low frequencies. If the port's frequency handling is too wide, it means that it's improperly designed.

[SurferJoe46]

This is confusing - what is the parameter for this 'range' of frequencies that we want to work with for a bass guitar with four strings?

I want to make sure that I get equal and balanced sounds from all the freqs a bass can create - but especially in the 40HZ area where I feel the very lowest notes require the most help in getting them heard.

So - that in mind a port that works best at 40HZ f'rinstance will start to fall off on both ends - but where or at what freq range does this happen?

[seamonkey]

Grab a set of headphones, hook up to your PC, grab audacity, or any DAW with a graphic EQ. and play some bass lines. Headphones are perfectly capable of doing 40 hz no problem. Tweak the EQ to do a hard shelf at like 50hz, and you'll still hear wonderful bass.

Look up missing fundamental on Wikipedia. You don't need to output the fundamental if all the harmonics are reproducing good. No need to do 40hz, or 31 hz for low B.

MaxxBass built into to Peavey DSP amps does just this. It adjust the harmonics to make the bass sound lower. It doesn't boost 40 hz to make 40 hz louder, it tweaks the upper harmonics.

[SurferJoe46]

So then the true note is missing and what you hear is sortta a residual of the secondary or tertiary frequencies?

Is the correct word: 'ORDER of MAGNITUDE' for talking about the tones you hear that are not the fundamental and you are fooled into thinking it's the signatory or struck note?

[Passinwind]

Originally Posted by SurferJoe46

So then the true note is missing and what you hear is sortta a residual of the secondary or tertiary frequencies?

Is the correct word: 'ORDER of MAGNITUDE' for talking about the tones you hear that are not the fundamental and you are fooled into thinking it's the signatory or struck note?

Take a look in the Amps Forum FAQ at this thread.

[SurferJoe46]

10-4.


Looked through that and the largest thing that strikes me is that although the science involved in acoustic engineering is an absolute - the actual field results are pretty much all over the board.

Just f'rinstance - putting staples in the internal sound mat verses gluing the material on the internal walls makes an arguable - however moot - difference that I don't think can be seen on a slide rule nor extrapolated from computerized acoustic sound designs, nor the best proficiency in mathematics.

I was just gathering wool and wondering that IF a port is so narrow in application, why not make it more broad ranged?

[fdeck]

Originally Posted by SurferJoe46

Looked through that and the largest thing that strikes me is that although the science involved in acoustic engineering is an absolute - the actual field results are pretty much all over the board.

Just f'rinstance - putting staples in the internal sound mat verses gluing the material on the internal walls makes an arguable - however moot - difference that I don't think can be seen on a slide rule nor extrapolated from computerized acoustic sound designs, nor the best proficiency in mathematics.

If it's real, then it can be measured, and if it can be measured, then it can probably be modeled. The only question is whether modeling is feasible or worthwhile. In the case of the low frequency, small-signal behavior of basic speaker designs, empirical results are in excellent agreement with the electromechanical model that is in widespread use. AFAIK most designers -- especially DIY'ers like myself, are limited to using empirical data to model the higher frequencies.

I was just gathering wool and wondering that IF a port is so narrow in application, why not make it more broad ranged?

I suspect that if it were practical to control the bandwidth of a port, it would find its way into the modeling software. In fact, this should be possible. In the simplest speaker model, the port is assumed to have an infinite "Q" factor, and the bandwidth of the port is controlled by the bandwidth of the driver resonance. You could model an additional Q factor into the port, and in fact I think there's a modeling program out there called Unibox that does this. You could also fiddle around with the design equations and software at my website.

A drawback is that a Q factor implies a dissipative element (for instance the resistive heating of the voice coil), so you will lose efficiency and hence sensitivity. The same adjustment to sensitivity could be done with EQ.

I think that you could build a broadband port by turning a second driver into a passive radiator and controlling its Q with a resistor across its terminals.

[SurferJoe46]

Originally Posted by fdeck

If it's real, then it can be measured, and if it can be measured, then it can probably be modeled. The only question is whether modeling is feasible or worthwhile. In the case of the low frequency, small-signal behavior of basic speaker designs, empirical results are in excellent agreement with the electromechanical model that is in widespread use. AFAIK most designers -- especially DIY'ers like myself, are limited to using empirical data to model the higher frequencies.



I suspect that if it were practical to control the bandwidth of a port, it would find its way into the modeling software. In fact, this should be possible. In the simplest speaker model, the port is assumed to have an infinite "Q" factor, and the bandwidth of the port is controlled by the bandwidth of the driver resonance. You could model an additional Q factor into the port, and in fact I think there's a modeling program out there called Unibox that does this. You could also fiddle around with the design equations and software at my website.

A drawback is that a Q factor implies a dissipative element (for instance the resistive heating of the voice coil), so you will lose efficiency and hence sensitivity. The same adjustment to sensitivity could be done with EQ.

I think that you could build a broadband port by turning a second driver into a passive radiator and controlling its Q with a resistor across its terminals.

Isn't the Q-factor the swept volume during pink noise of the cone itself or am I getting that crossed up in my mind?

If I am assuming correctly, then that resistor would be a very healthy one (read: HUGE) - correct?

[billfitzmaurice]

Originally Posted by SurferJoe46

So - that in mind a port that works best at 40HZ f'rinstance will start to fall off on both ends - but where or at what freq range does this happen?

That can be clearly seen on the frequency response chart of port output (as opposed to total output) in most modeling programs, which today are used by virtually all cabinet designers, amateur and pro alike. How does your son design his cabs?

I was just gathering wool and wondering that IF a port is so narrow in application, why not make it more broad ranged?

Because that's not how ports work. A ported cabinet is a resonant device, and it can only resonate at one frequency.

[SurferJoe46]

Originally Posted by billfitzmaurice

That can be clearly seen on the frequency response chart of port output (as opposed to total output) in most modeling programs, which today are used by virtually all cabinet designers, amateur and pro alike. How does your son design his cabs?
Because that's not how ports work. A ported cabinet is a resonant device, and it can only resonate at one frequency.

He uses some sorta program - maybe the ones you spec'd. I dunnow - he's 600 miles from here and a phone call isn't like being there in person.

I know he's doing all sorts of boom boxes and lo-pass stuff that's technical lightyears away from where I am and maybe even want to be.

I'm just looking for a somewhat Reader's Digest version of this tube-slot idea and I still wonder why if they only work for one frequency they are even IN a cabinet anyway.

I know there's the displaced volume and the loss of effectiveness of a cone if it is building pressure in the cab and that stifles the cone motion - so with all the notes a bass can hit - why have a port that only works for one note?

That's just confusing - and special thanks for taking me through acoustics kindergarten, Bill.

[fdeck]

A concise description of how ports work is in my speaker theory article. I can't figure out how to make it less math-intensive, but I attempted to make the text fairly descriptive.

[fdeck]

Originally Posted by SurferJoe46

Isn't the Q-factor the swept volume during pink noise of the cone itself or am I getting that crossed up in my mind?

If I am assuming correctly, then that resistor would be a very healthy one (read: HUGE) - correct?

It's confusing, so don't feel bad. A driver will have Thiele-Small parameters, including the Q factors, that are for just the driver. When installed in a box, the resulting system will have its own Q factor. Going from one to the other involves a bit of math, that the design software takes care of.

Real speaker designers know how to look at those Q factors and make educated guesses about how the system will behave. I don't. Instead, I just use those numbers as practically arbitrary inputs to the analysis process, and then I make all of my design decisions based on the resulting graphs.

The resistor might have to be substantial. Its purpose is to convert input power into heat. That's why I think it's more practical to let the port be a lossless port, and adjust the response curve via EQ. It's my understanding (and this could have been due to comments from Bill F) that the big dogs design speakers under the assumption that the fine tuning will be done electronically.

[Mr. Foxen]

Originally Posted by SurferJoe46

I'm just looking for a somewhat Reader's Digest version of this tube-slot idea and I still wonder why if they only work for one frequency they are even IN a cabinet anyway.

The peak of the response bump from a port is one frequency, but the bump is an octave wide, so you put the bump where the dip from the box/driver's response is, extending it a bunch.

The pendulum diagrams here are good if you picture things like that: Essay

[SurferJoe46]

Originally Posted by Mr. Foxen

The peak of the response bump from a port is one frequency, but the bump is an octave wide, so you put the bump where the dip from the box/driver's response is, extending it a bunch.

The pendulum diagrams here are good if you picture things like that: Essay

Aha!

I was thinking that it was just one note or frequency that was going to get a clean shot at being reproduced and all the others could just lump it for themselves.

It's an octave! Now it makes more sense.

The thought of just one freq was a big mental obstacle to me.

Well --- I have other mental obstacles - but that's a different subject.

I'm going to your speaker theory treatise now. Be back - maybe.