Hey guys.
I just want to know, does the calculated optimum internal volume of a ported box INCLUDE or EXCLUDE the volume enclosed by the port tube?
ie. If I have a box design for 34.2L say, and the port tube is 8.2cm diameter by 29.9 length (giving an encolsed volume of 1.579L), do I then have to compensate and make my box 35.779L, or does the volume enclosed by the port get included in the 34.2 that I have calculated to be the optimum box volume for my driver?
Your replies are most appreciated!!
I just want to know, does the calculated optimum internal volume of a ported box INCLUDE or EXCLUDE the volume enclosed by the port tube?
ie. If I have a box design for 34.2L say, and the port tube is 8.2cm diameter by 29.9 length (giving an encolsed volume of 1.579L), do I then have to compensate and make my box 35.779L, or does the volume enclosed by the port get included in the 34.2 that I have calculated to be the optimum box volume for my driver?
Your replies are most appreciated!!
Just backing up Pete. Exclude. As one poster here once described it, pretend the port is a piece of solid wood.
Ideally, this means you should increase cabinet size to account for the space you lose to the port. As a practical matter, if your cabinet is already built, you can consider the space taken up by the port as being negligible if it is 10% or less of the volume of the cabinet.
Finally, a flared port acts essentially the same as an unflared port that is twice it's cross-sectioanl area. Not twice it's diameter-twice it's cross-sectioanal area. So a 3 inch flared port is approximately equivalent to a 4 inch unflared port, yet takes up much less room in the cabinet.
Just thought I would throw that in in case you are worried about the port volume affecting your tuning and F3.
Ideally, this means you should increase cabinet size to account for the space you lose to the port. As a practical matter, if your cabinet is already built, you can consider the space taken up by the port as being negligible if it is 10% or less of the volume of the cabinet.
Finally, a flared port acts essentially the same as an unflared port that is twice it's cross-sectioanl area. Not twice it's diameter-twice it's cross-sectioanal area. So a 3 inch flared port is approximately equivalent to a 4 inch unflared port, yet takes up much less room in the cabinet.
Just thought I would throw that in in case you are worried about the port volume affecting your tuning and F3.
Hi Les
Drivers are usually mounted flush to improve high frequency diffraction, the smoother surface giving less corners/edges to reflect from. These reflections can cause small anomalies in the response pattern of a speaker.
Sometimes this technique is also used to time align the drivers, by rebating one or more drivers to get the accoustic centres of the drivers as close as possible. This will improve the phase response of the system.
Drivers are usually mounted flush to improve high frequency diffraction, the smoother surface giving less corners/edges to reflect from. These reflections can cause small anomalies in the response pattern of a speaker.
Sometimes this technique is also used to time align the drivers, by rebating one or more drivers to get the accoustic centres of the drivers as close as possible. This will improve the phase response of the system.
Thanx guys. You just saved me a world of woe. Looks like I'm gonna have to tinker with some interior dimensions again. By the way, for a ported design just how big is the margin for error when setting up the interior volume? I've tried to account for everything that's going to be inside the box, but it's pretty hard to get an exact figure for the volume of the back of a driver, or the volume of the x-over etc. I mean, do I need it to be correct to 0.1L, 0.01L, 1/2L or what?
Craig:
Relax. Even if you are off by a huge 25% of box volume, your F3 will only be a quarter of an octave higher. For instance, if you planned on a 2 cubic foot box that will have an F3 of 30 Hz, and it turns out the internal volume is only 1.5 cubic ft., then your F3 will be 35.5 Hz. Hardly a disaster. And it is pretty hard to be off by 25%.
Relax. Even if you are off by a huge 25% of box volume, your F3 will only be a quarter of an octave higher. For instance, if you planned on a 2 cubic foot box that will have an F3 of 30 Hz, and it turns out the internal volume is only 1.5 cubic ft., then your F3 will be 35.5 Hz. Hardly a disaster. And it is pretty hard to be off by 25%.
I also had an issue with port volume/box volume and a question hit me. Why not make the port outside the box? For example part of the speaker stand. My current speaker project calls for a 3.3L box with a port that just will not fit in the box without bending it.
What kind of issues can I expect with locating the port totaly outside the box?
What kind of issues can I expect with locating the port totaly outside the box?
I dont know if I quite understand this. do you mean that you can tune the length of a 3" flared port as if it was a 4" unflared?
Or is it just that you can have a lesser diameter port when flared because the turbulence becomes less audible. but that you will have to tune the port length to match the "new" lesser diameter port ?
Keld
Tune the 3" flared port as if it was a 3" unflared port.
I have not used a flared vent yet, but this is what I have read from those who have.
The tuning will be just a tiny bit off because the flare counts not quite as straight piece of pipe, but counts for something. F4ieer has some formula worked out, but if the port is 6 inches or more, the tuning will be only lightly affected whether you count the flare as a full length of pipe or not.
For instance. A 3.2 cu ft box is tuned to 25.5 Hz with a 6 3/4" length of 3 inch diameter pipe. If we knock off 2 inches, to make the pipe 4 3/4" long, we only go up to 29 Hz tuning. Such a difference will rarely throw the speaker off very much.
Flares are normally about an inch or so long.
So you see, once the port gets to be 6" long or so, it almost doesn't matter if we count the flared portion as being part of the pipe, not part of the pipe, or something in between.
Perhaps the best compromise is to count the flared portion of the vent as half as much as a piece of straight pipe of equal length. So a 1 inch portion of flared vent equals one half inch of straight pipe. Regardless, if the vent is 6" or more, the tuning won't get too far thrown off whatever we do.
Here is the paper on flared ports by F4ier:
http://www.geocities.com/f4ier/vents.htm
I have not used a flared vent yet, but this is what I have read from those who have.
The tuning will be just a tiny bit off because the flare counts not quite as straight piece of pipe, but counts for something. F4ieer has some formula worked out, but if the port is 6 inches or more, the tuning will be only lightly affected whether you count the flare as a full length of pipe or not.
For instance. A 3.2 cu ft box is tuned to 25.5 Hz with a 6 3/4" length of 3 inch diameter pipe. If we knock off 2 inches, to make the pipe 4 3/4" long, we only go up to 29 Hz tuning. Such a difference will rarely throw the speaker off very much.
Flares are normally about an inch or so long.
So you see, once the port gets to be 6" long or so, it almost doesn't matter if we count the flared portion as being part of the pipe, not part of the pipe, or something in between.
Perhaps the best compromise is to count the flared portion of the vent as half as much as a piece of straight pipe of equal length. So a 1 inch portion of flared vent equals one half inch of straight pipe. Regardless, if the vent is 6" or more, the tuning won't get too far thrown off whatever we do.
Here is the paper on flared ports by F4ier:
http://www.geocities.com/f4ier/vents.htm
I should probably mention that I haven't had the chance to correct the flared vent section of that page (mentioned by kelticwizard), but I will, I promise 
School just ended last week, so I'm looking forward to updating my pages when or before the next versions of subsim and xoversim are finished.
In short, the flared vents will be updated with information from Maximizing Performance from Loudspeaker Ports. It's an AES preprint [thanks Janne for the heads up], but it was recently republished in a 2002 JAES article, I forget which one
Isaac
School just ended last week, so I'm looking forward to updating my pages when or before the next versions of subsim and xoversim are finished.In short, the flared vents will be updated with information from Maximizing Performance from Loudspeaker Ports. It's an AES preprint [thanks Janne for the heads up], but it was recently republished in a 2002 JAES article, I forget which one
Isaac
LOL, I hope I don't cause more cranial rotation, but if I were doing it I guess I would count the flared portion as one half the length of the straight pipe.
Of course, if you do an impedance test-which can be performed by an online freeware tone generator and a simple resistor-you can eliminate all doubt. Your box is tuned to the frequency of lowest impedance. Just start off with a vent that your calculations show is too long and slice off pieces until you hit the right frequency, (apparently the flares are designed to slip over standard size pipe).
I know you know how to perform this test, Keld, but for anyone reading this, to measure the impedance valley in a vented box, look at the setup in the following link:
http://sound.westhost.com/tsp.htm
All you have to do is start at a frequency somewhat above your projected box tuning frequency and work down. Where the voltage drop across the RESISTOR-not the loudspeaker-is greatest, that is your tuning frequency.
For a freeware online tone generator, go here:
http://duncan.rutgers.edu/physicsfreewares.htm
Just "blue over" the frequency written in the box and write in your own frequency. Then click "Start".
Of course, if you do an impedance test-which can be performed by an online freeware tone generator and a simple resistor-you can eliminate all doubt. Your box is tuned to the frequency of lowest impedance. Just start off with a vent that your calculations show is too long and slice off pieces until you hit the right frequency, (apparently the flares are designed to slip over standard size pipe).
I know you know how to perform this test, Keld, but for anyone reading this, to measure the impedance valley in a vented box, look at the setup in the following link:
http://sound.westhost.com/tsp.htm
All you have to do is start at a frequency somewhat above your projected box tuning frequency and work down. Where the voltage drop across the RESISTOR-not the loudspeaker-is greatest, that is your tuning frequency.
For a freeware online tone generator, go here:
http://duncan.rutgers.edu/physicsfreewares.htm
Just "blue over" the frequency written in the box and write in your own frequency. Then click "Start".
Oh, one thing. When using that frequency generator-don't forget your sound card has a frequency response, too. When I checked the response of my sound card-which I had trouble installing-I found that I had some midrange sound and nothing else.
I reinstaled my sound card and got decent response, but still. I am 3 dB down at 33 Hz and 6 dB down at 20 Hz-something to consider when running tests on a speaker, especially one with good bass response.
When doing the impedance check at the link I gave above, be sure to test both the voltage acrosss the resistor ad across the loudspeaker.
Oh, one more things-to make the math easy, use a 1 ohm resistor for the test.
Here is the response of my sound card, even after I re-installed the driver.
The following response is from the Right Mark Audio Analyzer, a freeware downloadable program to test your sound card available at:
http://audio.rightmark.org/
All it requires is a 1/8" stereo plug to 1/8" stereo plug connector to be put between the Line In and Line Out of your sound card. In the USA, Radio Shack sells them for three bucks, # 42-2497.
I reinstaled my sound card and got decent response, but still. I am 3 dB down at 33 Hz and 6 dB down at 20 Hz-something to consider when running tests on a speaker, especially one with good bass response.
When doing the impedance check at the link I gave above, be sure to test both the voltage acrosss the resistor ad across the loudspeaker.
Oh, one more things-to make the math easy, use a 1 ohm resistor for the test.
Here is the response of my sound card, even after I re-installed the driver.
The following response is from the Right Mark Audio Analyzer, a freeware downloadable program to test your sound card available at:
http://audio.rightmark.org/
All it requires is a 1/8" stereo plug to 1/8" stereo plug connector to be put between the Line In and Line Out of your sound card. In the USA, Radio Shack sells them for three bucks, # 42-2497.
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