Port area vs efficiency.

dustman96

Member
2011-07-17 11:42 pm
In my numerous hours of reading of late on porting I can't seem to find much about port area's relationship to output efficiency. My brain is telling me that a larger port area is equivalent to a larger radiating surface which equates to more efficiency. Would someone with experience mind elaborating on this subject? :scratch:
 

OscarS

Member
2011-01-02 10:44 pm
yup you missed something. You just need to put 2 and 2 together....

My brain is telling me that a larger port area is equivalent to a larger radiating surface which equates to more efficiency.
the problem with what your brain is telling you is that your brain is not telling you that even though the port functions as an acoustic radiator, it is not an electrodynamic loudspeaker. Therefore its efficiency is not governed by the same laws that govern electrodynamic loudspeakers because it is not converting an electrical signal to acoustic energy into the air. Minimizing port compression is efficiency.
 
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dustman96

Member
2011-07-17 11:42 pm
yup you missed something. You just need to put 2 and 2 together....

the problem with what your brain is telling you is that your brain is not telling you that even though the port functions as an acoustic radiator, it is not an electrodynamic loudspeaker. Therefore its efficiency is not governed by the same laws that govern electrodynamic loudspeakers because it is not converting an electrical signal to acoustic energy into the air. Minimizing port compression is efficiency.

Thereby reducing resistance to "energy flow" out of the port! :yikes:
 

18Hurts

Member
2010-11-21 11:20 pm
It talks about compression but doesn't seem to address port efficiency as related to area. Good article though. Did I miss something?

Port compression IS losing efficiency! The graph indicates just at the start of compression there is a slight gain but it drops off quickly.

A flared port won't be louder than a straight port, they will remain the same until the straight port starts compressing. Flared ports don't give you a gain, they just ensure you don't lose anything at higher flow rates.
 

Zero D

Member
2009-08-06 11:11 am
@ dustman96

Hi, i think i know what you're getting at ;) If you're thinking that a larger port produces more bass, = more effecient, then you're sorta right. But it all depends ! You could have a Very large port that is or isn't deep. The area of the port/s together with the volume of the box = fb. This f could be anything, low or high. If the airflow through the port/s is impeeded too much by incorrectly sizing the port/s, it will lead to port compression.

Originally Posted by OscarS

Minimizing port compression is efficiency

Spot on :)

So at what point would there be no more to gain by increasing the size of the port?

Have a look at my screenie, it shows the maximum Port Velocity of a particular design of mine. The lower it is the better. Higher would mean more port noise = less effecient.
 

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@ dustman96 ...If you're thinking that a larger port produces more bass, = more effecient, then you're sorta right...

Well, NO, not really. OK, we're splitting semantics here, I don't think what you meant to say is wrong but it could be misread.

We gotta define what the heck is "efficiency":
1) SPL per watt coming out of the speaker
2) In the case of a port, I suppose you could consider a better flowing port "more efficient" BUT please pretty please let us NOT use such phrasing. "Efficiency" of speakers is used to refer to #1. Let's keep it that way to not confuse newbies.

For a given port tuning, the size and shape of the port won't affect the basic efficiency SPL per watt meaningfully.

If the port is not up to snuff, it may choke at high volumes, and reduce the maximum SPL, but that is not really "efficiency" as the term is used.
 

OscarS

Member
2011-01-02 10:44 pm
We gotta define what the heck is "efficiency":
1) SPL per watt coming out of the speaker
2) In the case of a port, I suppose you could consider a better flowing port "more efficient" BUT please pretty please let us NOT use such phrasing. "Efficiency" of speakers is used to refer to #1. Let's keep it that way to not confuse newbies.

Sorry, but that is incorrect. Efficiency of any system is a unitless number less than 1 and greater than or equal to 0. Please refer to any college level Physics Textbook on Thermodynamics.

#1 as you state is sensitivity. Sensitivity is dB(SPL) per watt (or specified voltage) @ a specified distance---you cannot forget that last part because it's part of the unit for sensitivity.

back to the original question----to speak of a port's efficiency you're gonna get in to fluid mechanics. I've read up on it while getting my Physics undergrad. degree, but unless you're an expert on it, lets just say that anything and everything you need to know is still in that PDF that you looked at. You don't need to guess at anything---I clearly laid it out for you after I quoted a portion of your previous post. You even said it yourself...
It would seem there is more to it than compression...
...except there is no "seem"---that's it, minimize compression and you just maximized efficiency.

This next question that you posed is not easily answered without either genius-IQ together with gedankenexperiment OR lots and lots and lots of testing...

So at what point would there be no more to gain by increasing the size of the port?
the only and complicated answer is----the point at which you stop compression. So now we're back to square one---how do we define compression? Go back and read the PDF.
 
let me jump in to clear some confusion about sensitivity & efficiency.

the main point you need to realize is the fact when a speaker does not need to move much at all to blast out loud decibel levels.

it's important because efficiency is related to the strength of the magnet and thus the amount the magnet will push the voice coil to move the speaker cone.

there is a break in the logic clearly stated with the explanation of sensitivity & efficiency.
to be more sensitive, at first thought is to move more air to get more decibels.. but as i said, the cone doesn't need to move to be loud.
it's kinda the difference between vibration and movement of travel.. because if your eyeballs can't see it moving, that doesn't imply the cone isn't vibrating.

the efficiency might make the cone travel in & out much to the eyeball, but it doesn't imply there is enough sensitivity to move the cone in & out with the intended soundwave shape.

yet another point to prove a gap is the amount of slew coming from the speaker.
where sensitivity has been implied that the amount of sensitivity helps control the shape of the soundwave, it says nothing about how many soundwaves can be output at the same exact time.

that is clear how a lower sensitivity speaker can sound improved because there is literally more soundwaves coming out of the speaker all at once.
not all sounds are complete soundwaves, but the fact that the number of soundwaves coming out at the same time is higher.. it simply provides opportunity for the extra details of resolution to come out of the speaker.

i don't think the specification is released to the public because people would look for that value specifically & jump for those speakers over the competition most of the time.
because the amount of slew has been something steadily improving each decade.
 
when you look at the problem & become more confused, that doesn't imply everybody has the same problem.

splitting the difference between the two words while elaborating their details shouldn't be confusing.

adding the point of slew and where it fits in is simply rhetorting a bad choice of words (when thinking of multiplicity) as to what they are aimed at defining.

a person might confuse sensitivity with slew
a person might confuse efficient with slew

a person might of reached out for one word over the other looking for slew and become disappointed or even feelings of being lied to without my clarification.
 
You're on the right track!

dustman:
My brain agrees with your brain (at least on this subject)! Although the referenced JAES article makes some very good points w.r.t. port entry losses/turbulence/etc., I also admit to a gut feeling that something is still missing from current vented enclosure design theory. As far as "laying it out for you", I hope to write a thesis on the subject when I get some spare time. (i.e. - don't hold your breath!)
In the meantime, I do encourage you to keep thinking & experimenting on this problem. Since I haven't yet worked up a rigorous analysis on the subject, I'm a bit hesitant to speak with authority, so please treat my following thoughts as the musings they are:
We've always been told that horns have to have a large mouth opening to efficiently produce low frequency output, yet vented box theory says the port (mouth) opening size is irrelevant (as long as port velocity/compression is kept below some limit). Sorry, but I side with horn theory on this one! My hunch is that vented box theory assumes the air velocity exiting the vent is perfectly converted to a pressure wave in order to produce the calculated SPL at the listening position. If so, I believe this assumption would be inaccurate. Hence, it makes sense to me also that a bigger port area = lower port velocity = more efficient conversion to pressure wave = more efficient sound radiation. Better yet - I think - would be to make the port with an expanding profile like a horn. This would take some work to integrate with vented box theory, as it would obviously affect the box tuning equations.
Hope this helps.
Wilf
 
But, anwaypasible, this thread is about the port...
...A flared port won't be louder than a straight port, they will remain the same until the straight port starts compressing. Flared ports don't give you a gain, they just ensure you don't lose anything at higher flow rates.
If I may try to add to this explanation: Flares reduce turbulence at the in and/or out of the port.
But compression is still the same in the port itself. The only difference is the turbulence, which also reduce port output a little, because any energy used to put out something different then the intended sound, is a waste.
 

OscarS

Member
2011-01-02 10:44 pm
Better yet - I think - would be to make the port with an expanding profile like a horn. This would take some work to integrate with vented box theory, as it would obviously affect the box tuning equations.
Hope this helps.
Wilf

This is covered in that paper--sorta. It turns out that the elliptical port profile that Bose started using in the 80s is the most efficient of all. Not a coincidence that the NFR=0.5 very closely matches what would "appear" to be an elliptical profile. This type of profile almost approaches a loss-less situation as long as the minimum cross-sectional area is of adequate size.

To expand on that with an actual horn-profile can be done in Akabak---in which case you end up with what's called a scoop, aka, rear loaded horn type of situation where there is no longer a rear-chamber like a true "vented box" alignment. It's kinda like, do you want a ported box? or do you want a rear loaded horn? One fundamental difference is that one is a helmholtz resonator that can be mathematically modeled as a damped double-spring double-mass system that I hope you're familiar with from differential equations and or college level physics.

Now, attempting to do a true vented box, an actual helmholtz resonator with an exit like a horn, but an entry like a standard port would likely lead to some harmonic distortion, as I'm sure you saw in that paper that from an entry conditions standpoint, there are less losses when the NFR approaches 1. This weird "different entrance different exit" would likely have you running in circles against yourself. It would be best to stick with the info presented in page 22, where it is clear that a slow port taper leads to more laminar airflow at the exit. This is evidenced not only by the port profile Bose used in the Acoustimass 8th order bandpass of long ago, but also by JBL who still currently uses a port profile that very very closely resembles that of an elliptical port profile with a very slow taper on both ends.
 
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But, anwaypasible, this thread is about the port...

well in that case, here..


as orders of harmonics add up, it is essentially pressure (or PSI) building up.
the properly tuned order of harmonics can be presented perfectly to the port, and then the size of the flare (as well as the shape) can be perfectly matched to literally unfold (or combine) the harmonics to produce tiny sonic booms that increase the decibel level.
when the harmonics are lined up with the port, the harmonics are modeled inside the port.. that is how they know which flare size & shape to use to seperate or combine the harmonics.
it is easy if you think about looking at rolling smoke in a clear tube.
because there are rolls, there are spots with smoke and spots with gaps.
the flare simply does one or the other:
1. combines all the spots of smoke together to form a bigger 'puff'
2. seperates the gaps from the spots of smoke to produce an output (that output might be aimed at a clean direct soundwave, or two harmonic soundwaves ment to combine to achieve the perceived soundwave .. the same thing the harmonic bass maximizers do to get more bass from smaller speakers)

i believe horn loaded speakers have all sound coming from one port without the speaker cone making contact with the air.
and that means when doing number 2, the one harmonic might be the actual raw soundwave.. while the other harmonic is the same impulse of the room (or the opposite of the impulse.. or the same impulse, but the opposite phase).

the difference between horn loaded ports and regular ported speaker cabinets is the horn port works with pressure, while the regular speaker port works with neutral pressure.

the reason for working with pressure?
because you know those plastic whistle keychains they sell for safety, people are supposed to blow on them when they are in trouble.. and the chamber is designed to work with the pressure to make the output excessively loud.

if your horn port doesn't have pressure, you don't have a horn.. you've got a fancy port shaping the reflections of soundwaves until they sound appropriate, and people do the same shaping inside the speaker cabinet with blocks and smoothed edges to give the speaker more resolution.