minimum diameter (inches) > 39.37(411.25(Vd)/(fb^.5))^.5
Vd = cone displacement in cubic meters
fb = tuning frequency (Hz)
What's the surface area of an 18" something like .125 m^2
multiply that by the xmax in (be sure to convert to meters)
estimating can't kill ya...
hope this helps
-andy
(taken from the loudspeaker design cookbook - Vance Dicakson)
Vd = cone displacement in cubic meters
fb = tuning frequency (Hz)
What's the surface area of an 18" something like .125 m^2
multiply that by the xmax in (be sure to convert to meters)
estimating can't kill ya...
hope this helps
-andy
(taken from the loudspeaker design cookbook - Vance Dicakson)
The rule of thumb I read was to keep the max wind speed below 5% of the speed of sound: 17 m/s. I wish I could remember where I read that. I do remember that it was one of the big names in acoustics back in the 60's. Some JBL guys did some distortion measurements at different volume levels for different degrees of rounding off of the port ends. After making some calculations to convert to m/s the JBL charts did seem to back up the 5% rule. As I recall distortion was a few % at that point for either a straight or rounded port. Where as for 5m/s distortion was pretty minimal for a well rounded--still high for the straight port. Don't quote me on that, though--better go to the source, which was published in the JAES in 2002.
John
John
Oh, a couple other things...wide-band noise is certainly one result of vent turbulence, but you also have to worry about non-linear distortion and vent compression (the acoustic resistance of a vent rises with velocity). Putting the vent in the back won't help you out there, unfortunately.
John
John
hancock said:
Yes. I'm aware with that. (Thanks!) I have an
oppertunity of getting 6 Double Eighteens
(Dirt Cheap) However, the vents are in the
front, and, I needed a ballpark measurement
when I scale them in my program, the precentage
of port noise I will get at 1200 watts a box.
(Conservative Power)
Thank You Gentlemen
Best Regards,
no one can know how much noise it will produce,,-can they ?
u can just minimise it.
winisd says to keep it below 3% speed of sound
www.jlaudio.com
bose-
u can just minimise it.
winisd says to keep it below 3% speed of sound
www.jlaudio.com
bose-
bostarob,
You absolutely correct.
The only downfall of having a very large port, it reduces
the internal chamber for the speaker.
Say you have a 14 cubic foot box,(Internal counting
the driver) and your using a 24 x 22 x 12 inches in
depth port. You will have no port noise, however,
your internal chamber is now 9.5 cubic feet.
So, there's always a trade off on large ports.
You absolutely correct.
The only downfall of having a very large port, it reduces
the internal chamber for the speaker.
Say you have a 14 cubic foot box,(Internal counting
the driver) and your using a 24 x 22 x 12 inches in
depth port. You will have no port noise, however,
your internal chamber is now 9.5 cubic feet.
So, there's always a trade off on large ports.
Are you sure? Then why is it that when you have a ported speaker with the vent hanging outside of the box, (for testing purposes) Fb is barely effected? I'll try and find something more substantial.
hmm.. Dickason talks about pipe resonances as the only disadvantage of having long port. He does say that "Larger cross secional areas will always produce better linearity in any given situation. For high powered applications, such as the speakers designed for stage performance, it is desirable to use vent areas as nearly equal to the driver area as possible." He also says that problems asociated with pipe resonances are not nearly as troublesome as those of undersized ports...But he does not mention that port volume subtracts from total box volume. If I am overlooking something please tell me.
-andy
hmm.. Dickason talks about pipe resonances as the only disadvantage of having long port. He does say that "Larger cross secional areas will always produce better linearity in any given situation. For high powered applications, such as the speakers designed for stage performance, it is desirable to use vent areas as nearly equal to the driver area as possible." He also says that problems asociated with pipe resonances are not nearly as troublesome as those of undersized ports...But he does not mention that port volume subtracts from total box volume. If I am overlooking something please tell me.
-andy
bostarob said:
If you have a speaker in a lets say 6 cubic foot box,
and, your port takes up 3 cubic feet out of the box,
in which, the speaker needs 6 cubic feet, to achieve
40 Hz. You will sacrafice some lower octaves to
achieve less port noise.
Scale a 16 cubic foot box.
Use any 18 inch driver you like.
Make the vent 24 H by 22 W by 12 Depth.
Now, try to get 40 Hz (- 3dB) out of the box.
You can't, for the port has taken up too much space.
However, make the port smaller, you can achieve
40 Hz. But, will have more air passing through the
port than the larger port.
bostarob said:
Think of it this way...
The box volume forms an acoustic capacitance. This means that a volume velocity ((vibrating driver) acting into the box will tend to compress and decompress the air in the box. It is the compliance of the air that forms the acoustic capacitance. The port, on the other hand, is an acoustic inductor. This means that a volume velocity acting into the port will not compress the air in the port and the air in the port moves like a solid column of air. It is the mass of this solid column of air that forms the inductance.
Now if you put a port inside a box, you are dividing the total volumn of the box into two regions. In one region, the acoustic capacitance, there is compression and decompression, but no partical (air) velocity. In the other region (the port), there is no compression and decompression, but there is partical velocity. Now if you make the port bigger and keeping the box the same size, you are decreasing the arrea of the box that acts as an acoustic capacitor. The only way around this is to use a passive radiator. A passive radiator works exactly like a wort. The only difference is that a port relies on the mass of the air inside the port to form the acoustic inductance, while the passive radiator relies on the mass of the radiator to form and acoustic inductance.
Hope that helps, John
I think the port sizes of most P.A. speakers are a compromise anyway.
While there is general demand for high accuracy sound even at high SPLs there are also econcomy requirements: The boxes have to be reasonably sized.
I think most of the vented P.A. subs generate quite a bit of port noise when driven to their peak SPL capabilities.
But because a P.A. should have some reasonable headroom they are only driven at these levels during short peaks.
OTOH there is music with almost no dynamics at all and P.A. systems might be fed with this stuff at almost full power endlessly.
But then it is not the vents, that generate most of the noise anymore
Regards
Charles
While there is general demand for high accuracy sound even at high SPLs there are also econcomy requirements: The boxes have to be reasonably sized.
I think most of the vented P.A. subs generate quite a bit of port noise when driven to their peak SPL capabilities.
But because a P.A. should have some reasonable headroom they are only driven at these levels during short peaks.
OTOH there is music with almost no dynamics at all and P.A. systems might be fed with this stuff at almost full power endlessly.
But then it is not the vents, that generate most of the noise anymore
Regards
Charles
I believe Hancock is right; 17m/s is probably very close to max, I know this form some experiments I’m doing right now. I’m building a pair of Maelstrom Subs and have been using WinISD Pro to model the enclosure in (355L Vented @ 20Hz) and I’ve made a jig that allows me to change out ports on the fly so to speak. My first model used a 6” diameter port and they all had port noise ranging from severe to medium, the longer the port the less severe the port noise. Then I tried an 8” diameter port and to my surprise, I still got some audible port noise although it’s probably not audible in a room while playing music. So I went back and checked my WinISD modeling and found that I’m right at 19m/s with the 8” port and the 6” ports had anywhere from 35m/s (shortest port 4”long) to 29m/s for the longest port (30”).
So based on this I believe that the 17m/s figure is a good max level, also I was playing a pure 20Hz test tone at very loud levels to get this port noise with the 8” port at 19m/s velocity, so this probably wouldn’t be a problem with any music material.
The lower the tuning freq the longer the port, the longer the port the slower the velocity, also the bigger the box the higher the air velocity of the port. So the box size and the port length and diameter all play a role here.
So based on this I believe that the 17m/s figure is a good max level, also I was playing a pure 20Hz test tone at very loud levels to get this port noise with the 8” port at 19m/s velocity, so this probably wouldn’t be a problem with any music material.
The lower the tuning freq the longer the port, the longer the port the slower the velocity, also the bigger the box the higher the air velocity of the port. So the box size and the port length and diameter all play a role here.
Attachments
bostarob said:
That's why I gave you an example, for you to try
in your program.
OMNIFEX said:
Box Dimensions: 48 H x 24 W x 24 D
This should convince you.
If your new, its going to take some time to edit your
post. It has happened to all of us
If your a regular poster, just click on the "Edit" next
to the blank box that says Quote.
OMNIFEX said:
If your new, its going to take some time to edit your
post. It has happened to all of us
If your a regular poster, just click on the "Edit" next
to the blank box that says Quote.
Ahh, so eventually I'll see an "edit option" at the end of my posts. I guess I'll have to proofread before submitting. Don't want to make any more posts about worts...
John
subtracting volume
I did some measurements in my car with a 4 inch port 15 inches long. If the port sticks out of the box the resonance drops. It pretty well matched what I expected in winISD, after I add cabin gain for the car. This tells me that the port volume is subtracted from the volume of the box.
If the volume of an external port were added to Vb I would have observed an 18hz resonance and -2db at 18hz. However, resonance was at 20hz just as expected with external, and ~21.5hz with it tucked in. Just as reported by winISD with subtraction of the port volume I get a flat response to 23hz, -3db at 20hz.
Just my tests, trying to get the car sounding good. Try working with a van someday... Lots of room but the shape/resonances suck. It is much easier to get flat 20hz in a sport coupe! At much louder volumes too!
I did some measurements in my car with a 4 inch port 15 inches long. If the port sticks out of the box the resonance drops. It pretty well matched what I expected in winISD, after I add cabin gain for the car. This tells me that the port volume is subtracted from the volume of the box.
If the volume of an external port were added to Vb I would have observed an 18hz resonance and -2db at 18hz. However, resonance was at 20hz just as expected with external, and ~21.5hz with it tucked in. Just as reported by winISD with subtraction of the port volume I get a flat response to 23hz, -3db at 20hz.
Just my tests, trying to get the car sounding good. Try working with a van someday... Lots of room but the shape/resonances suck. It is much easier to get flat 20hz in a sport coupe! At much louder volumes too!
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