port question

[revboden]

I can explain it but I need a better understanding of what you are doing.

mostly how big is the box volume, what tuning have you chosen?

is 3.6cm the radius of a pipe or a side of a square port?

edit: what driver are you using?

 

[sreten]

wait am i getting confused here..
if one port has 10x10 by 55 how can four ports be a lower mach and be 3.6x20 each??? is something wrong with the program? cos you could fit 4 40mm pipes into a 8cm square with space to spare, so if there is a obvious reduction in the overall size of the vent hole, and the length is shorter to match... how can the program say the mach of the 3.6mm by 20cm pipes is less??

does length effect mach 😱 this makes no logical sence, going to have to think about this

Hi,

Good question. And a problem with simulator assumptions. I don't
know the answer but its probably to to with end corrections for the
real effective size of the port and those corrections not considering
multiple port interactions for closely spaced multiple ports.

rgds, sreten.

 

[sreten]

wait am i getting confused here..
if one port has 10x10 by 55 how can four ports be a lower mach and be 3.6x20 each??? is something wrong with the program? cos you could fit 4 40mm pipes into a 8cm square with space to spare, so if there is a obvious reduction in the overall size of the vent hole, and the length is shorter to match... how can the program say the mach of the 3.6mm by 20cm pipes is less??

does length effect mach 😱 this makes no logical sence, going to have to think about this

Hi,

Good question. And a problem with simulator assumptions. I don't
know the answer but its probably to to with end corrections for the
real effective size of the port and those corrections not considering
multiple port interactions for closely spaced multiple ports.

But even so its not enough to explain the discrepancy I think.

rgds, sreten.

Collo's DIY Subwoofer Enclosures

Is good reading, but note your not building a subwoofer, in reality
a low tuned port needs to be able to handle 10% to 20% of the
total power available to a bass / low mid unit.

 

[revboden]

wait am i getting confused here..
if one port has 10x10 by 55 how can four ports be a lower mach and be 3.6x20 each??? is something wrong with the program? cos you could fit 4 40mm pipes into a 8cm square with space to spare, so if there is a obvious reduction in the overall size of the vent hole, and the length is shorter to match... how can the program say the mach of the 3.6mm by 20cm pipes is less??

does length effect mach 😱 this makes no logical sence, going to have to think about this

I just sim'd a generic driver in a generic box using 4 round 3.6cm x 20cm ports vs. one 10cm x10cm by 55cm port. The velocity of the 4 smaller ports is almost three times that of the larger single. Something is wrong with your sim.

Length effects tuning frequency in conjunction with cross area. Cross area alone determines mach.

edit: 100cm^2 is 2.59 X 38.48cm^2. So the velocity will be greater than 2.59 x that of the larger port.

 

[WaVeInFoRm]

sorry for the inactivaty, im off on holiday tomorrow, had to force myself to take a break

igot my driver units and am overjoyed, i had to plug em in and test them free field at a low volume, they all work fine

the build quality of these drivers is superb, esp the midrange, tweeter dosent look lovely, because its faceplate is plasticy

now i have observed something that could throw a spanner in the works,

for the mid its fine because it will be sealed

but for the bass,

in all drivers i have seen the spider coveres the voice coil

for some reason with these you can see the copper coil, and the magnet gap, im just wondering, in a ventet cabinet could this not get bunged up overtime with dust as other bits,

see pic

An externally hosted image should be here but it was not working when we last tested it.

image hosting

 

[WaVeInFoRm]

my final box, before i put pencil to wood

tuning volume 90liters
dimentions 75x55x35external 70 50 30 internal
box volume 105liters
port tuning 27.5 (low note on piano)
square port...
dimentions 36x10x10 internal 33.5x15x15external
port volume 7.54liters
left for bracing and driver 7.46liters
strait firing port 180rotation from woofer, inother words lower back of the box
material all 25mm mdf

all things concidered over time, i decided to listen to those who said dont curve the port. i also decided to go between peoples views on not tuning too high or low and decided on 27.5hz

all bracing as it ony need apply stability to its dimention will be 2.5x2.5xtbc sections of spare mdf in a grid cross pattern with a t section for the woofer

as a guess i am leaving 1liter for the woofer itself

everything in order?🙂

 

[WaVeInFoRm]

my only minor concern is the 16.5cm distance between the port and the inside of the box front. is this a issue? i read alot and think it should be fine

 

[Tomom]

There are two advantages of having round port; by having front and back of port flared 1. turbulances will be reduced 2. you can make port significantly shorter. This is in regards to stright ports. If you manages to make contoured port, that gives 3. increased maximum output from the port by up to 8dB or so..

 

[WaVeInFoRm]

thanks for the reply
i can round it off a 0.2-1cm with a router. but i like the idea of a square port for some unknown reason i just do

also none of the above where advangages of a round port, more a flared one

 

[Tomom]

That is ok, just food for thought...

 

[Tomom]

What I wanted to say was it is much easier to flare round port than square one

 

[WaVeInFoRm]

ah ok.
also the volocity is acceptable for my driver, according to previous posts. and since i am tuning a tad higher i dont see how it can make much differance

win isd says port velocity is 12

 

[Tomom]

you were worried about distance from port to the front of box; since flareing the port efectivly extends its' length, you can make actual port shorter...but you may know all this

 

[Tomom]

And actual speed in rectangular port is smaller than in round port; and the smaller the ports are, the greater that difference is

 

[WaVeInFoRm]

not worryed just want the answer 😀😕😀

An externally hosted image should be here but it was not working when we last tested it.

very quick representation. has the port got enougth space to breath?

last little thing before i can have fun making it...

 

[fastbike1]

Not necessarily. Depends on the aspect ratio of the port.

And actual speed in rectangular port is smaller than in round port; and the smaller the ports are, the greater that difference is

 

[Inductor]

my final box, before i put pencil to wood

tuning volume 90liters
dimentions 75x55x35external 70 50 30 internal
box volume 105liters
port tuning 27.5 (low note on piano)
...

Sorry to (have a) say in your thread but I think you have a too big of a speaker.
I would point more for 71 Liters internal volume (SEAS CA26RFX) with a natural more rounded output curve at lower frequencies. Tunning frequency is fine for this internal volume.
In fact after simulating your 90 Liters (int. vol) box it doesn't look too bad. For sure it doesn't look like a sealed as you mentioned in your firsts posts. Probably it will not sound that good (take it with a grain of salt). So if I'm right, I hope not, you will end up with a sealed cabinet with your port(s) canceled from necessity. Shall see (with respect for others designs) maybe only what a final audition can tell.

 

[WaVeInFoRm]

making the box slightly larger raises the efficency slightly. obviously the cone will be moving a tiny bit more to compensate.
can you provide a reason why you think the box is too big. what makes you think that it would sound bad? im not understanding,
im happy with the final box as long as the port has enougth space, which i am still being an impatant beaver and awaiting conformation on...

 

[WaVeInFoRm]

🙁

 

[DBMandrake]

making the box slightly larger raises the efficency slightly.

Making the box bigger does nothing to change the efficiency, it just changes the cutoff frequency and roll off shape. The efficiency is always determined by the driver and it's associated T/S parameters. (Efficiency can be calculated from T/S parameters alone with no reference to box size)