IDSkoT
11-16-2008, 03:22 AM
My first EDU, so bare with me.
First, let me explain, very briefly, how speakers work. There's a magnet that's wrapped around a motor, inside the motor is a "voice coil" which is basically copper (or another magnetic conductor) wrapped around a cone. When a signal comes from an amplifier, or the source, it's in an AC current... as the current goes through the magnetic, it powers it intermittently. Causing the magnets to react by pushing up and down on the coil. This disrupts air waves, and creates sound. :clap:
The lower the frequency (Hertz), the slower the cone moves. For instance, a 40hz flat signal will cause the cone of any driver go 40 cycles per second. This gets crazy when you begin to think of wave-lengths up to 12K. 12,000 cycles per second. But, as one may assume, the slower the cone moves, the harder it is to produce sound since the cone isn't "cutting" the air. So, to do this, most enclosures require an "enclosure" to pressurize the cone (either the front, or the back. This will be explained a little later).
An enclosure is based off the power of the motor, the specifications of the coil, and the size of the cone. But, the manufacturer does all of these calculations for you. They will have a recommended sizes for enclosures (both sealed and vented). Of course, you can go bigger or smaller. They both have different effects. A smaller box will lower output, and puts a strain on the motor. Larger boxes normally have higher output, but can easily not put enough pressure on the cone, and blow your woofer.
And when I say "smaller" or "larger", I'm speaking in terms of full cubic feet. Fractions of a cubic foot will not make too much of a difference.
But, some manufacturers will recommend an enclosure that's simply too small (for practicality purposes)... and some people have just found some drivers that work better in smaller boxes or bigger boxes.
Types of enclosures and their effects:
Sealed:
This is a basic enclosure. This keeps the air sealed inside of the driver. The effect of this is a very "Flat" response.
Vented:
This enclosure has a tuned "port" which is based off a formula, and if you get into it, you can find the minimum size for your driver.
There are two different types of ports.
* Round ports (A variant is the aero-port): As the name suggests, it's just a round port. A common item to use is PVC piping. You can also buy them.
- Aero-ports are round ports with flared ends. This lowers the chance of something called "port noise" which is air being disrupted by the abrupt end of the port and can be described as a "whistling" sound. (I've personally have never heard it, so I can't comment.)
* Square or Vented ports: This is basically a vent in the shape of a rectangular prism.
The effect of this is a "peaky" response around the tuned frequency. I.E. If you tune it to 40hz, the output will peak at frequencies near 40 hz, and a flat response at other frequencies after a reasonable range. (It's not a quick drop off of output, just an FYI to try to clear it up.)
(I can't find any graphs. But, I hope I explained it properly)
Now, for the less used enclosures (due to one reason or another.
Bandpass: There are differ "order" band passes, which puts the port in different chambers. But the whole idea is that there's 2 chambers, and either one or both chambers are ported. This is usually done with round ports, but can be done with vented.
The result ends up being "peaky" around the tuning frequency, and a severe exponential drop-off at other frequencies.
Passive Radiator: This ends up being a test of physics, with a really poor outcome. It's cool, but usually isn't used due to it's ineffectiveness. Basically, it's a sealed box, with a "passive radiator" mounted on the box. A Passive Radiator is basically a speaker with out the magnet part. The box ends up being tuned based on how much the passive Radiator is pushed out, dampening the pressure in the box. This ends up with a really poor output as shown in the chart below:
(I used WinISD Alpha for this. I used Audiopipe TXX-15 drivers for the bassis of all of these graphs, but it shouldn't really matter. This is all just to show you each box's relation to each other. They're based on color.)
Sealed: Grey
Vented: Yellow
4th Order Bandpass: White
6th Order Bandpass: Pink
Passive Radiator: Orange
http://img.photobucket.com/albums/v714/TwistedShot/chart.jpg
EDIT:: A member on another forum pointed out to me something that's true, and I forgot wasn't common knowledge. These exact graphs will not be replicated with each driver. Some subs do better in sealed, or others. Some are pretty even. For instance... this is an IDQ10" with the default specs and the same passive radiator as the first "test".
http://img.photobucket.com/albums/v714/TwistedShot/chart2.png
I also made the lines thicker so they're easier to read. :D
But as you can see, this driver does well in nearly all enclosures, while the Audiopipe TXX-15 drivers do notably better in a vented enclosure.
BTW, the "Vented" enclosure isn't peaky because it was tuned to it's resonant frequency. So the result is a flat response. If you tune it different, the chart begins to have a mind of it's own and starts to look crazy.
Some other enclosures, which aren't too often used in vehicles due to the size restraints are:
Tranmission Line (T-Line) enclosures. This can most easily be described as a long port, where (as someone explained to me) the port area of any cross-sectional part is always equal to or less than the driver's cone area. (I.E. a true 12" cone has 113.04 cuInches of surface area, that means at any given point of the T-Line, it has to equal up to 113.04 CuInches. I'm not sure how real this is. I ordered a book online that should get here some time next week, I'll read up on it and edit this EDU)
Effects: A nice flat response with great gains in Dbs (Pressure levels of subs are measured in Decibels.)
Front and Rear Loaded Horns: I'm not sure what the difference is, but it acts like a giant port which exponentially gets larger. It works off the principle of compacted air gains velocity (and thus volume) as the port opens more and the pressure builds up behind it. [Or at least to my knowledge]. I'm trying to learn the concept and how to build these, but it's really confusing with a lot of terms I need to look up. :dogclap:
Now... to the math!
[My favorite part...]
Okay, I will give you the formulas. But they might not be totally right. Normally, I use a program called WinISD (Alpha). It's a free program that's pretty easy to use that will tell you the port length required based on variables you put in...
But let's take the formula from JL Audio's website:
http://mobile.jlaudio.com/graphics/Support/Tutorials/Ports/Fb.jpg
Let me give you the variables...
Av is the cross-sectional area of the port [In square inches], Lv is the length of the port [in inches] and Vb is the enclosures NET volume [in cubic inches].
You can use your Algebra skills and move the equation around. However, there are many other web sites that will give you the proper length of the port...
A popular one is:
RE Audio L-Port Calculator (http://www.reaudio.com/speaker_box/LPort_Box_Calc.html) (NOTE:: Ports don't have to be in an L shaped. This is just for pure ease purposes. They can be in a folded formation where the port is folded up against each other... much like this:
http://img.photobucket.com/albums/v714/TwistedShot/foldedport.jpg
The rule applies that the port has to be the same cross-sectional area at every given point of the port.)
BCAE1's Calculators (http://www.bcae1.com/spboxnew2.htm)
Here's a link to the free program which allows you to figure all of that stuff out:
WinISD's Website (http://www.linearteam.dk/) (NOTE:: I use WinISD Alpha. It allows input in the Imperial measurement system. However, WinISD Beta does metric. I'm not sure if it does the imperial system. Also, the driver in which you use will not effect the size of the port. You just edit the frequency and the cubic feet necessary for the box, and if it's a vented port, you put in the length of each side, and it will tell you how long it has to be.)
Now some rules I just realized I should include:
* When using more than one driver, in any enclosure, to figure out how much space you need for them to share an enclosure, you double the necessary space requirement for each sub. (I.E. If a sub needs 1 Ft^3 of space, for 2 drivers, you need 2 Ft^3, for 3 you need 3 Ft^3, etc.)
* When doing a slot port, the rule of thumb is to stay with in a 9:1 ratio (In inches). I.E. 18" x 2" would be pushing it, but possible. The reason for this is too much are rushing through a thin area makes the air turbulant, and causes port noise.
* Also, based on your sub is how wide a round port should be. Regularly, I use at least 4" tubes. I've seen some people use 2" and 3" diameter tubes successfully. However, I like to use at least 4" for drivers anywhere between 8-12" cones. Then I upgrade to 6" from 12-15. I've never worked with an 18", but I'd probably try to go bigger. However, the ports are all about cross-sectional area. So, you can try until you get the right sound for you.
*The port length is the MIDDLE line of the port. As in the median between the outside line and the inside line. This will be shown in a picture below.
Okay, so once you have your necessary space figured out, what kind of port you're gunna use, and your port length...
It's time to design.
For this, I'll use a simple 2 CuFt box (after port displacement) tuned to 35 hz with a vented port.
The port will have to be 29.68 inches long (It can be rounded up to 30. It lowers the tuned frequency to 34.85. But .15 herts isn't going to impact your system audibly) @ 12" high and 2.5" wide.
Now, since I couldn't get 30 inches of port length, I brought the port inside of the box. This is perfectly fine to get the proper tuning. However, now the port is taking up some of that 2 cuft of box. But that won't effect performance too much. (Not to mention that most of the time when a manufacturer gives you a box, it's including port displacement. Gross size of this box is roughly 2.5 cubic feet.
[http://img.photobucket.com/albums/v714/TwistedShot/2cuftbox35hz.jpg
Now, I could use this box for my car. However, I lack the wood, or money for wood, to go out and do it. So, I will be forced to just walk you through it.
There are many ways to design a box, for the sake of easy, I like to keep one size constant. For boxes, I keep all the "Height" lengths the same.
The cuts will go as followed:
QT: Lengths:
1 12"x12"
1 12-3/4"x12"
1 27-1/4"x12"
1 32"x12"
1 3-3/4"x12"
2 9-1/2"x12"
2 32"x13-1/2"
Now, when using a table saw, remember to watch your fingers. Also, remember to measure twice, or more, and cut once. I always use a tape measure from the most outside point of the saw to the fence. Both infront of the blade, and behind.
Once cut, you can use either screws, or Bar Clamps and glue to adhere each of the sides together.
Also, for an added measure of air-tightness, put a nice bead of rubber silicone on the inside of the box where the wood meets. Even if your wood cuts are perfect. It's better safe than sorry. $8 dollar bottle of silicone is worth having a subwoofer enclosure that sounds good.
Sorry if I left anything out, it's now 3:30, and I started this while installing Dead Space and still haven't played it yet. Updates will be made when I build my next box. Shouldn't be long. Thanks for reading along.
First, let me explain, very briefly, how speakers work. There's a magnet that's wrapped around a motor, inside the motor is a "voice coil" which is basically copper (or another magnetic conductor) wrapped around a cone. When a signal comes from an amplifier, or the source, it's in an AC current... as the current goes through the magnetic, it powers it intermittently. Causing the magnets to react by pushing up and down on the coil. This disrupts air waves, and creates sound. :clap:
The lower the frequency (Hertz), the slower the cone moves. For instance, a 40hz flat signal will cause the cone of any driver go 40 cycles per second. This gets crazy when you begin to think of wave-lengths up to 12K. 12,000 cycles per second. But, as one may assume, the slower the cone moves, the harder it is to produce sound since the cone isn't "cutting" the air. So, to do this, most enclosures require an "enclosure" to pressurize the cone (either the front, or the back. This will be explained a little later).
An enclosure is based off the power of the motor, the specifications of the coil, and the size of the cone. But, the manufacturer does all of these calculations for you. They will have a recommended sizes for enclosures (both sealed and vented). Of course, you can go bigger or smaller. They both have different effects. A smaller box will lower output, and puts a strain on the motor. Larger boxes normally have higher output, but can easily not put enough pressure on the cone, and blow your woofer.
And when I say "smaller" or "larger", I'm speaking in terms of full cubic feet. Fractions of a cubic foot will not make too much of a difference.
But, some manufacturers will recommend an enclosure that's simply too small (for practicality purposes)... and some people have just found some drivers that work better in smaller boxes or bigger boxes.
Types of enclosures and their effects:
Sealed:
This is a basic enclosure. This keeps the air sealed inside of the driver. The effect of this is a very "Flat" response.
Vented:
This enclosure has a tuned "port" which is based off a formula, and if you get into it, you can find the minimum size for your driver.
There are two different types of ports.
* Round ports (A variant is the aero-port): As the name suggests, it's just a round port. A common item to use is PVC piping. You can also buy them.
- Aero-ports are round ports with flared ends. This lowers the chance of something called "port noise" which is air being disrupted by the abrupt end of the port and can be described as a "whistling" sound. (I've personally have never heard it, so I can't comment.)
* Square or Vented ports: This is basically a vent in the shape of a rectangular prism.
The effect of this is a "peaky" response around the tuned frequency. I.E. If you tune it to 40hz, the output will peak at frequencies near 40 hz, and a flat response at other frequencies after a reasonable range. (It's not a quick drop off of output, just an FYI to try to clear it up.)
(I can't find any graphs. But, I hope I explained it properly)
Now, for the less used enclosures (due to one reason or another.
Bandpass: There are differ "order" band passes, which puts the port in different chambers. But the whole idea is that there's 2 chambers, and either one or both chambers are ported. This is usually done with round ports, but can be done with vented.
The result ends up being "peaky" around the tuning frequency, and a severe exponential drop-off at other frequencies.
Passive Radiator: This ends up being a test of physics, with a really poor outcome. It's cool, but usually isn't used due to it's ineffectiveness. Basically, it's a sealed box, with a "passive radiator" mounted on the box. A Passive Radiator is basically a speaker with out the magnet part. The box ends up being tuned based on how much the passive Radiator is pushed out, dampening the pressure in the box. This ends up with a really poor output as shown in the chart below:
(I used WinISD Alpha for this. I used Audiopipe TXX-15 drivers for the bassis of all of these graphs, but it shouldn't really matter. This is all just to show you each box's relation to each other. They're based on color.)
Sealed: Grey
Vented: Yellow
4th Order Bandpass: White
6th Order Bandpass: Pink
Passive Radiator: Orange
http://img.photobucket.com/albums/v714/TwistedShot/chart.jpg
EDIT:: A member on another forum pointed out to me something that's true, and I forgot wasn't common knowledge. These exact graphs will not be replicated with each driver. Some subs do better in sealed, or others. Some are pretty even. For instance... this is an IDQ10" with the default specs and the same passive radiator as the first "test".
http://img.photobucket.com/albums/v714/TwistedShot/chart2.png
I also made the lines thicker so they're easier to read. :D
But as you can see, this driver does well in nearly all enclosures, while the Audiopipe TXX-15 drivers do notably better in a vented enclosure.
BTW, the "Vented" enclosure isn't peaky because it was tuned to it's resonant frequency. So the result is a flat response. If you tune it different, the chart begins to have a mind of it's own and starts to look crazy.
Some other enclosures, which aren't too often used in vehicles due to the size restraints are:
Tranmission Line (T-Line) enclosures. This can most easily be described as a long port, where (as someone explained to me) the port area of any cross-sectional part is always equal to or less than the driver's cone area. (I.E. a true 12" cone has 113.04 cuInches of surface area, that means at any given point of the T-Line, it has to equal up to 113.04 CuInches. I'm not sure how real this is. I ordered a book online that should get here some time next week, I'll read up on it and edit this EDU)
Effects: A nice flat response with great gains in Dbs (Pressure levels of subs are measured in Decibels.)
Front and Rear Loaded Horns: I'm not sure what the difference is, but it acts like a giant port which exponentially gets larger. It works off the principle of compacted air gains velocity (and thus volume) as the port opens more and the pressure builds up behind it. [Or at least to my knowledge]. I'm trying to learn the concept and how to build these, but it's really confusing with a lot of terms I need to look up. :dogclap:
Now... to the math!
[My favorite part...]
Okay, I will give you the formulas. But they might not be totally right. Normally, I use a program called WinISD (Alpha). It's a free program that's pretty easy to use that will tell you the port length required based on variables you put in...
But let's take the formula from JL Audio's website:
http://mobile.jlaudio.com/graphics/Support/Tutorials/Ports/Fb.jpg
Let me give you the variables...
Av is the cross-sectional area of the port [In square inches], Lv is the length of the port [in inches] and Vb is the enclosures NET volume [in cubic inches].
You can use your Algebra skills and move the equation around. However, there are many other web sites that will give you the proper length of the port...
A popular one is:
RE Audio L-Port Calculator (http://www.reaudio.com/speaker_box/LPort_Box_Calc.html) (NOTE:: Ports don't have to be in an L shaped. This is just for pure ease purposes. They can be in a folded formation where the port is folded up against each other... much like this:
http://img.photobucket.com/albums/v714/TwistedShot/foldedport.jpg
The rule applies that the port has to be the same cross-sectional area at every given point of the port.)
BCAE1's Calculators (http://www.bcae1.com/spboxnew2.htm)
Here's a link to the free program which allows you to figure all of that stuff out:
WinISD's Website (http://www.linearteam.dk/) (NOTE:: I use WinISD Alpha. It allows input in the Imperial measurement system. However, WinISD Beta does metric. I'm not sure if it does the imperial system. Also, the driver in which you use will not effect the size of the port. You just edit the frequency and the cubic feet necessary for the box, and if it's a vented port, you put in the length of each side, and it will tell you how long it has to be.)
Now some rules I just realized I should include:
* When using more than one driver, in any enclosure, to figure out how much space you need for them to share an enclosure, you double the necessary space requirement for each sub. (I.E. If a sub needs 1 Ft^3 of space, for 2 drivers, you need 2 Ft^3, for 3 you need 3 Ft^3, etc.)
* When doing a slot port, the rule of thumb is to stay with in a 9:1 ratio (In inches). I.E. 18" x 2" would be pushing it, but possible. The reason for this is too much are rushing through a thin area makes the air turbulant, and causes port noise.
* Also, based on your sub is how wide a round port should be. Regularly, I use at least 4" tubes. I've seen some people use 2" and 3" diameter tubes successfully. However, I like to use at least 4" for drivers anywhere between 8-12" cones. Then I upgrade to 6" from 12-15. I've never worked with an 18", but I'd probably try to go bigger. However, the ports are all about cross-sectional area. So, you can try until you get the right sound for you.
*The port length is the MIDDLE line of the port. As in the median between the outside line and the inside line. This will be shown in a picture below.
Okay, so once you have your necessary space figured out, what kind of port you're gunna use, and your port length...
It's time to design.
For this, I'll use a simple 2 CuFt box (after port displacement) tuned to 35 hz with a vented port.
The port will have to be 29.68 inches long (It can be rounded up to 30. It lowers the tuned frequency to 34.85. But .15 herts isn't going to impact your system audibly) @ 12" high and 2.5" wide.
Now, since I couldn't get 30 inches of port length, I brought the port inside of the box. This is perfectly fine to get the proper tuning. However, now the port is taking up some of that 2 cuft of box. But that won't effect performance too much. (Not to mention that most of the time when a manufacturer gives you a box, it's including port displacement. Gross size of this box is roughly 2.5 cubic feet.
[http://img.photobucket.com/albums/v714/TwistedShot/2cuftbox35hz.jpg
Now, I could use this box for my car. However, I lack the wood, or money for wood, to go out and do it. So, I will be forced to just walk you through it.
There are many ways to design a box, for the sake of easy, I like to keep one size constant. For boxes, I keep all the "Height" lengths the same.
The cuts will go as followed:
QT: Lengths:
1 12"x12"
1 12-3/4"x12"
1 27-1/4"x12"
1 32"x12"
1 3-3/4"x12"
2 9-1/2"x12"
2 32"x13-1/2"
Now, when using a table saw, remember to watch your fingers. Also, remember to measure twice, or more, and cut once. I always use a tape measure from the most outside point of the saw to the fence. Both infront of the blade, and behind.
Once cut, you can use either screws, or Bar Clamps and glue to adhere each of the sides together.
Also, for an added measure of air-tightness, put a nice bead of rubber silicone on the inside of the box where the wood meets. Even if your wood cuts are perfect. It's better safe than sorry. $8 dollar bottle of silicone is worth having a subwoofer enclosure that sounds good.
Sorry if I left anything out, it's now 3:30, and I started this while installing Dead Space and still haven't played it yet. Updates will be made when I build my next box. Shouldn't be long. Thanks for reading along.