Originally Posted by Funky Waves
So you consider the SVS PB13 Ultra to be a bad design as well?
i could care less about SVS. i won't even bother looking it up. serious manufacturers are the ones who spend millions of dollars on R&D. you know - Harman, Genelec.
Originally Posted by Funky Waves
Vas, I am interested in your theory of why you think the design is bad, FYI as you can see it is something that has been built and tested. It works very well. As a general principle I do try and stay away from port bends in my designs as they do not perform quite as well. When done right they can work. Sometimes there is no other option.
ask and you shall receive !
basically since i am the only person with functioning intuition on this board i will have to translate my thoughts from my brilliant-think to something you people can understand - math. you should feel special - i developed a mathematical theory just now simply to make you feel bad about your box.
let's say at a port entrance 50% of the airflow comes in straight and 50% comes from the sides at 90 degrees. lets say at port exist 100% of air continues in straight path. in here entry and exit reverse depending on direction of airflow at any cycle. this is going to be our approximation. also let's assume that for any given angle of deflection the energy loss to turbulence for a given flow speed is equal to the square of that angle:http://en.wikipedia.org/wiki/Drag_(p..._high_velocity
if drag grows as the square of speed then you can consider angle as speed change so drag should also be a square of angle. ( not very exact calculation but for our order of magnitude estimates good enough )
so we have for a regular straight port 50% of air turns 90 degrees on one side of the port. let's introduce a number i will call UNITY DRAG equal to the drag in this situation.
since in a 90 degree bend 100% of the air turns 90 degrees we shall say the drag on this bend is 2X unity.
your port has two bends so your total drag is 5X that of a regular port with same airspeed. but your port is 2.2 times as long or so so air speed is 2.2 times as slow so 2.2^2 = 4.8 and your total turbulence is 5/4.8 = 104% drag of a straight port. you haven't lost much in the way of turbulence - you only lost a bunch of wood and a bunch of box volume. no biggie so far.
but now let's consider if you made only one turn. your port would be 2.0 long so you would have 2^2 = factor of 4 benefit due to speed reduction. the port configuration would only have 3X unity drag so you would in fact have 3/4 = 75% of the drag of a regular straight port.
so by adding that 2nd turn you went from 75% drag to 104% drag. you've INCREASED your drag by 38% while adding weight, complexity and cabinet volume.
your mistake was to add a 2nd turn which didn't add significant port length but nearly doubled the potential for turbulence.
now these calculations seem to favor port bends after all. but we forgot one little factor ! we forgot that port ends are flared which means that the actual airflow speed at port ends is LOWER than that in the bends ! ! !
so if a 4" port has a 6" flare that means the surface area is 2 times larger and the port and speed is 2 times lower and turbulence is 4 times lower ! ( and even if there is no flare the air still slows down as it exits the port, but it doesn't slow down in a bend )
that means that a 90 degree bend in fact has not 2X but 8X the turbulence of the port itself ! ! !
therefore the best way to do a port is a hyperbolic port - because it reduces airflow speed WHERE IT COUNTS that is at the entry and exit, without creating high speed sharp angle airflow obstructions in the process as bends do.
so we have:
hyperbolic port > flared port > straight port > port with elbows
now the calculations presented here should be properly understood not as calculations, but as a way to understand what is really going on. i didn't intend them to be something you can use to exactly calculate the amount of drag from any given port geometry but rather to explain why JBL uses a hyperbolic port and doesn't use the type of port that you built.
now a bend in the port CAN be used to increase performance if the bend is smoother than the flaring at the port ends. which is in fact what Genelec does. All the ports the Genelec bends they bend CONTINUOUSLY in which case the ANGLE of bending is virtually zero and performance IS increased.
the best port EVAR is this one:http://www.genelec.com/documents/ima...ory_deepLF.jpg
it is both continuously bent and it has hyperbolic expansion at the end.