comlicated port/driver combo question. (long read)

[soongsc]

We had some other projects where an individual contractor did a heat dissipation and airflow analysis for us, but vibrating waves are quite different in nature.

[rcw]

The most optimum coupling between the driver and the Helmholtz resonator causes a peak, i.e. the best coupling is the boom box.
There are a class of reflex alignments that use this peak in conjunction with a high pass filter to produce an overall flat responce and these are known as the QB5 class II alignments.
These optimise excursion limited power handling, and are much used in sound reiforcement, the reason being that they allow both the full voice coil and the full excursion capabilty to be used simultaniously, giving a very large amount of sound, the drawback is that the speakers f3 needs to be at least twice the cut off frequency, for the best extension /size/power handling.
Next are the class I alignments, these are generally similar to a B6 alignment described by Don Keele in which you take a standard B4 box, reduce the port tuning by .7, and feed it from a Q=2 filter tuned to the same frequency.
This results in optimum bass extension in a given volume, but the power handling is less and is usually limited by cone excursion, subwoofers of this sort are widely used in such applications as large monitoring systems.
Richard Smalls testing showed that if you keep the air velocity in the vent at not more than 4.5% of the speed of sound no vent noises are heard, in Win Isd if the vent velocity is below 10m/s.,
Low tuning means big vents and if the box is small they are difficult to accomodate, Peerless for instance makes passive radiators for its xxls series because of this.
On the matter of pipes, a pipe is essentially a transmission line that is periodic, it will resonate at frequencies at which the amplitude of the ratio of the forward and reflected wave is unity, a Helmholtz resonator is the acoustic equivalent of a parallel resonant electrical circuit and will resonate at the equivalent parallel resonant frequency, if the inductance is in the form of a pipe this will have pipe resonances.

[exhausted mule]

yeah. you just found a complicated way of saying something i allready knew...


my original question and current dilemma was the lenth/size of port and if there were limits to their design application in real life.

as i stated.. i'll be using a relitively small box with a very low tuning... two tunings in fact. one at 16-18 hz- the other at 30-35 i have yet to complete my crossover as that will be a big deciding factor as to exactly what tune it will end up to be.

i didn't, however, know that there was a specific mach at was a port would start to "whistle" at. although... acquiring a flow modeler will help in the end design and will most certainly affect my current design.

i believe i'm in the final stages of my first draft design for my prototype. i will post pics of my acad and hopefully a flow model gif (but i doubt it) asap.


you'll have an idea of what's going on here.

[rcw]

I suggest you look at Bill Collisons site.

[exhausted mule]

thanks, i will.

i'm gonna look up the other two names mentioned in your thread as well.

books are still the way to go when studying anything, however, and hopefully there's something more than the usual out there in the way of theory and application pertaining to this issue.

[Artmaster]

Originally posted by exhausted mule
[B]my original question and current dilemma was the lenth/size of port and if there were limits to their design application in real life.

gday!
I know a few ways for port optimazed. for example you can set up port outside of box. even a few pcs. why not? the same theory and formula.

[Artmaster]

I'd tested the next:



then next:



even next:



but the best way for port noise minimize was:



it's translucent for some waves material, so it seems like indefinite diametr of ports.

[rcw]

The most comprehensive treatment if ports I know about is a paper that appeared in the AES Journal, Vol. 50 No.1/2, Jan/Feb, 2002.
The point that I was attempting to make before is that what you require is a musical subwoofer that goes down to the lowest frequencies. Most of the designs you will see such as shelving alignments, are for home theatre, and are intended to make large low frequency noises that sound somewhat like the sound effects people intended, and they have large amounts of non linear distortion and tend to use ports that are too small.
A subwoofer that will give a genuine 16Hz. note with very little distortion at a level that you can actually hear is another beast entirely, and unless it is very large electronic assistance is almost mandatory, and it depends very significantly upon the amount of room gain you have.
In short first ascertain your room gain by measurement and then design a subwoofer that will give the required output.
Also note that a shelving type reflex box is cheap and easy to do, and it might well satisfy your ears although its objective specs might leave something to be desired.

[Artmaster]

Originally posted by rcw
first ascertain your room gain by measurement and then design a subwoofer that will give the required output.

absolutely agree.
but first - you have to choose music style, then - room acoustic environment, third - loudspeaker concept.

[BassAwdyO]

A subwoofer that will give a genuine 16Hz. note with very little distortion at a level that you can actually hear is another beast entirely, and unless it is very large electronic assistance is almost mandatory

If you're looking for 16hz at a level that you can hear you cant design your subwoofer around your listening environment. The listening environment must be designed to accomedate such intense sound pressure. Otherwise you will just hear the house rattle and shake and vibrate and well you might THINK that its 16hz that you're hearing but you're wrong. The human ear loses its ability to recognize pitch below 20hz anyhow.... 18hz 16hz 10hz they all sound the same, its just the amount of SPL it takes for you to 'hear' it

Electronic assistance is Almost mandatory.... I have an Adire Tumult... If I had 3 good 18" PR's on it then it could pump out a very low(comparably) distortion 16hz tone in less than 5 cubic feet at over 110db Anechoic. No EQ needed