Flared ports and turbulence

[Coolin]

I guess you havent read the info that has been provided. Its not THAT directional. Its still a dipole just more radiation on the front than the back.

The spacing beween front of port and driver is also quite large, i'm guesing 3-4 feet.

The cancelation is because the two are out of phase. If i'm not mistaken regular ports are also "mostly" out of phase compared to the driver output.

There are also others that do it this way with two drivers that are adjustable. Even a commercial product but i dont remember which one.

What i do wonder about is fow effective this will work when placed in a real room. The speaker will be near a wall and this will distort the cancelation abilities.

[maiky76]

Hi!

For the directivity, it is easy to understand. Let's refer to microphones: you can turn any kind of mic into any king of mic!
Take an omni capsule the pattern is O, do some pinch holes on the back of the housing and you have a cardio caps!
To have a figure 8 pattern, do a hole of exactly the same area than the actual caps. you can continuously shift from an O to a 8 including any kind cardio (super, hyper….).
The reciprocal is true, with a cardio you can do an omni, just by sealing the holes.
The pattern is due to the balance between the pressure that reach the front and the rear of the capsule’s diaphragm at the same time (you can add some time delay between the two sides too but it is not our aim here).
The same for the BR boxes, under the Fb the box has a funny directivity depending on the mutual coupling of the driver(s) and the port(s).
So BassAwdyo, yes any BR box as a dipole pattern directivity (or cardio or 8 depending upon the area of the port…) if the driver and the port are on opposite sides. I think that Jean Kergomard issued a paper in the JASA I think about that! The trick: for a midrange driver tuning (stupid habit but…) in a 3ways the point is to tune it high and put the port exactly on the back of the unit to improve room coupling ….
The nozzle use this in a special way, see AES pdf and my former mails.

@+
Maiky

[Collo]

Hi again!

I finally finished testing with three different diameter ports to see how the useable velocity might change.

Whilst I was at it, I re-tested the five 86mm ports using voltage measurement to determine input power as suggested earlier in this saga by RonE.

The tests with DVD sequences and SPL meter were abandoned.

This gives more accurate determination of input power and eliminates possible errors from:
-SPL meter adjustments
-Room nodes and meter placement issues
-Conversion of WinISD predicted SPL for "half-space" into a value for the test environment.

The re-testing has given a slightly different set of results to those that were published earlier which were based on SPL readings

The new comparison between ports of three different diameters has shown that larger ports are able to use higher velocity

Testing was done to detect just audible "chuffing" close to the port. At the seating position an extra 2dB is required for it to be just audible with a sine wave test. For normal content, this rises to 3dB, which is equivalent to an increase in port velocity of around 40%. I have factored this in to my recommendations so that ports are no larger than they need to be in a real world situation

An all-encompasing equation has been developed, and for those who don't like maths, I have supplied a downloadable calculator.

All the info has been written up on the flare testing page, along with a link to download the calculator.
http://www.users.bigpond.com/bcolliso/flare-testing.htm


Attached is a new table of maximum design velocities (in metres per second) for common port and flare combinations

as before,
all constructive criticism is welcome

Collo

[simon5]

Nice update, I hope to see it in the next AES paper hehe!

[paulspencer]

Very nice work! I'm impressed!

Can this be right? I entered a 100mm vent with 72mm flare radius and it says 42.9m/s is ok!!!

[Collo]

Thanks for those comments guys!

Paul, that velocity is correct.

There is of course the principle that the unflared section of the port also has a speed limit.


I suspect that larger ports have a higher allowance than smaller ones, but have not been able to find any definitive literature on it.

The closest I have come is a statement in the AES paper "Maximising Performance in Loudspeaker ports" where they
indicate that heavily flared ports suffer from more compression, but they don't expand this into a general principle.

I suspect that a minimally flared port runs into trouble before the unflared section has a chance to get play up. Adding a flare allows you to lift the speed to the point where turbulence and compression in the straight section become an issue.

It would be nice to know what velocity this occurs at for different diameter pipes. Perhaps yet another round of building test ports.

I have looked at most of the commercial flares and all seem to be 30mm radius or less.

Consider the monster 6 inch Aeroports:
http://2loud4you.ca/shop/index.php?m...461d9f56376034

The unflared section is 6 inch diameter and the outside of the flare is 9 inch. Subtracting 1/4 inch all round for the mounting flange and dividing by two gives a flare radius of 1-1/4 inch (ie 30mm)

The precision ports and lightning ports are similar.

I suspect that they are not available in larger radii because of the afforementioned problems.

In the meantime, I'll stick with the notion that anything over 35 m/sec is questionable.


I also would have liked to have tested a 6 inch port to check the accuracy of my predictions, but the test box just won't push enough air!

Maybe I should add a "Warnings and limitations" paragraph to my notes!

regards
Collo

[paulspencer]

Collo, so what you are saying is that according to the program, 43m/s is acceptable for that port? Of course, as you say 34m/s is going to probably be the absolute limit. I wonder if there is a way you could limit the answer to 34m/s ... nice work ... it's not something I know how to do!

[Collo]

A beta version of the calculator gave color coded warnings as the speed increased. I took this out because the 34 m/sec limit is based on "conventional wisdom" rather than hard data.

I can easily add it back in if people think it would be a good idea!

[paulspencer]

I'd say include some kind of warning, just don't make it out to be a hard limit, you might note that it might be possible to make it work with a higher velocity ...

[Collo]

done