It has come to my attention that displacement based divisions is a bit forward thinking in the world of car audio competition. In fact, no other competition body that I am aware of does this, and for that reason there is a lot of debate about whether this is a good idea or not. So I want to explain my reasoning and offer a few examples of systems to illustrate why I think this is a better idea than simple surface area, or square inch, models.
SPL is a measure of pressure (thus, Sound Pressure Level) and is represented in decibels (db). In order to make this pressure, a system must alternately compress and thin the air in the listening environment. In our case, this is the cabin of a car. The devices we use to create this pressure are called speakers, woofers, drivers or transducers (depending on how technical you want to get). All of these devices move in two directions - forward and backward - and this creates compression and rarefaction of the air in the cabin. This effect causes the build up of pressure waves in the cabin of the car, and these waves are analyzed by test equipment and represented as decibels of SPL (db/SPL).
Now, it stands to reason that the more ability a system has of compressing and rarefying the air in a cabin, the higher the SPL reading will be. This is what we call "louder" - more decibels means more pressure, which our own test equipment - our ears - interpret as louder. So the question becomes how does a system create pressure, and moreover how does it do this in an effective enough manner to be "louder" than another system?
Simply put, the more air a system can act upon, the louder it is going to be. Systems act on the air in two ways. First, let's talk a little about the driver. It is essentially just a piston, with the cone of the driver being the bore that contacts the air and the motor of the driver being the linkage that pushes the bore against the air. Compressing the air against the bore, then, becomes a function of these two forces - the size of the bore by the distance it travels.
There are two effective ways to increase the effect of compression. The first, and more traditional, method is increased bore size. Larger diameter speakers simply play louder than smaller diameter speakers because they can act on a larger amount of air. Some manufacturers have even played around with different shapes - like squares - to increase bore size (as a side note, bore size in a woofer is more often called surface area, and is represented in the speaker's Thiele/Small Parameters as SD). The effect, however, is always the same - the more surface area a speaker has, the more air it can contact and the louder it will play.
This is the extent to which most every car audio competition goes in dividing woofer classes against each other. Here is a link to DBDrag Street class guidelines in which this is a little more explicit in it's description. You'll notice that they simply pair systems together that have similar SD sizes. This is a valid way to do things and I don't mean to knock the system that really established the sport - DBDRA has stuff figured out and clearly it works. However, there are some clear shortcomings with this two dimensional perspective, most obviously in the example that follows.
The limitation with a surface area model is simply that not all woofers of equal size are, in fact, equal. A Funkasonic 15" from Wal-Mart that cost $39.99 probably isn't as capable a bass machine as an MTX Thunder T9500 15" which costs $829.99. Under the standard SD based rules, however, these woofers would be pitted head to head in the lanes, and the guy with the deeper pockets (who bought the more expensive, and vastly more capable, MTX woofer) would take the trophy home. That hardly seems like an accurate way to play the game. The size of your wallet should never decide the outcome of a competition. The skill you bring to the table should. That said, there is another dimension as regards pressurization that is entirely left out of this equation.
Here's an example: a JL 10W7 is fairly high end 10" subwoofer. By contrast, a Rockford P1S415 is a pretty run-of-the-mill 15" subwoofer. If we look solely at SD, would anyone question that the 15" speaker will play louder? The JL has a clear disadvantage due to it's comparatively small SD (59.8 in^2 versus 135.7 in^2). Which woofer will make more SPL at rated power - the JL or the Fosgate?
Here's the results:
First off, you'll notice I am using WinISD, which is a great program for evaluating subwoofer systems and enclosures without having to go and build a bunch of them first. If you haven't used this program, I recommend you start. It's a real advantage and a tool that every serious subwoofer user should be familiar with. Here's a link to the download page.
The important info is this: the white line represents the JL 10W7 and the red line represents the Rockford P1S415. Both are in their rated sealed enclosure and both are powered by manufacturer's rated peak power (more on this later). Notice, then, that the JL is marginally louder than the Rockford (119.7 db versus 119.3). Does this mean the JL will outperform the Rockford under every conceivable condition? Absolutely not. The difference will come down to a handful of factors - power, enclosure design, charging capacity, and reinforcement of the cabin. This, of course, is exactly what we want to happen. Those previous factors require skill to implement properly. This means that the winner should, and likely will, be the more talented installer.
But why, you ask, is a 10" performing on par with a 15," like in the previous example? Well, simply put, it's the 3rd dimension that we eluded to earlier: excursion.
The second method for improving compression is to make the woofer cone move further. This can have the same effect as increasing surface area by allowing the woofer to push the air further instead of simply pushing a larger diameter of air. The volume of air acted upon by the bore can be the same for a small woofer with more movement as for a large woofer with less movement. The movement of the cone in forward and backward, or positive and negative, motion is represented in a woofer's T/S parameters as Xmax, also called one way linear excursion. As another interesting side note, while compression of the air occurs when the woofer moves forward and pushes against the air, rarefaction occurs when the woofer moves away from the compressed air, causing the air to thin out, or rarefy. This produces the negative pressure which, when combined with the positive pressure achieved from the compression cycle, results in the conditions required to create a wave.
What we can then evaluate from these numbers is how much air a woofer is actually capable of acting upon. By evaluating the actual volume displacement in cubic inches, instead of just square inches, we discover just how much compression a woofer system should be capable of, and in fact we have a much better idea of which woofers can play louder than others. The end result is that we can now pit woofer systems of comparable capability together based on their calculated displacement and not just on their generic diameter/perimeter.
As per our previous example, the 10W7 would be pitted against the P1S415, simply because they exhibit similar displacement characteristics (the JL makes 41.1 in^3 versus the Rockford's 50.2 in^3).
So by adding Xmax to the equation, we make sure that the Funkasonic, JL W7, or Rockford P1 woofer play ball with other woofers of their capability, not just with other woofers of their size. The same is true for the big stuff. Diameter is simply an insufficient real world indicator of a subwoofer's actual output potential. Let me throw some numbers out that make this point a little more clearly.
We'll use a few common woofers of a 12" variety but of differing price points. For the time being, we'll also be using the same amount of power for each woofer, without respect to manufacturer's ratings (again, more on this later).
Alpine SWR1243D - Type R 12" Dual 4 ohm woofer
Xmax=.79 in.
SD=74.4in^2
Displacement=58.78 in^3
JL Audio 12W3v3-2 - W3 12" Version 3 2 ohm woofer
Xmax=.51 in.
SD=80.6 in^2
Displacement=41.1 in^3
Rockford Fosgate P3D412 - P3 series 12" Dual 4 ohm woofer
Xmax=.63 in.
SD=88.4 in^2
Displacement=55.62 in^3
These woofers all fall into the $200-$300 price range and are considered excellent speakers by any measure. But notice that the Alpine, which has the smallest piston area at 74.4 in^2, displaces the most air at 58.78 in^3. This is simply because it has the highest excursion capability at .79 inches. So based on this model, we would expect the Type R woofer to play the loudest (albeit probably neck-and-neck with the Rockford) because it can act on the most amount of air, or in other words cause the greatest amount of compression/rarefaction in the vehicle cabin. But is this true?
Not exactly. In this case we see a discrepancy in the projected outputs, but not one that we couldn't have predicted. The JL, which should have been the quietest of the group, is actually the loudest. The Alpine and Rockford are neck-and-neck, as we expected, but the JL is clearly out front. The answer lies in the woofer's sensitivity, or efficiency. In this example, all 3 woofers are being fed 500 WRMS. On that amount of power, the JL will make the most output due to it's higher efficiency (87 db @ 1w/1m versus the other woofer's 85). So does this mean the displacement model is shot? Does woofer performance really come down to efficiency and that's it?
Not hardly. If that was true, the trick to making a loud woofer would simply be to make it crazy efficient and then pump as much power as humanly possible into it. But we know that this isn't so simple. This is the reason for power divisions. In the lower power applications, the lower excursion, higher efficiency woofers tend to do better. Low excursion drivers tend to be more efficient because it is easier to compress a lot of air a very small distance than it is to compress a little bit of air a long distance (more interesting side notes: The Romans had this theory nailed with their application of hydraulics in the Colesseum). But this doesn't mean that high efficiency drivers will always play louder. It just means they'll play louder until there's enough energy to move the woofer through it's entire range of motion (called Xpeak - the total movement of the woofer in both positive and negative phase). A low excursion driver will hit it's mechanical limits much earlier than a high excursion driver, not to mention it's thermal limits, and thus there is only so much output a low excursion driver can provide. By contrast, a high excursion driver, which will compress more air as we have previously proved, takes more power to reach this point, but will also handle this greater amount of power with a greater degree of control than will a low excursion driver. It is for this reason that the higher power applications will also commonly employ the higher excursion drivers. Thus, we divide the the low power from the high power and then we see that this effect is reduced.
Now don't get me wrong here. Every woofer can be driven beyond it's mechanical limits and forced to absorb more power with the intent of producing more SPL. However, that doesn't mean that any given woofer can be forced to play louder and louder simply by adding power to it. That JL W3 will hit a point where more power only hastens woofer failure but does nothing to increase volume. Every woofer has that same point, however some woofers are designed for SPL applications and have a very high mechanical power handling. They also tend to exhibit very impressive Xmax ratings. That high mechanical power handling and extreme Xmax's go hand in hand on SPL drivers is no coincidence.
Also consider this: at 500 WRMS, the JL 12W3 is very near it's thermal limits. In fact, JL voids the warranty on that speaker at 600 watts. That's the same as Alpine's RMS power rating. The SWR1243D doesn't reach it's limit (according to the manufacturer) until 1800 watts. So let's look at that graph again and put each speaker at it's rated limit:
Notice now that the Alpine is slightly ahead of the Rockford, and comfortably ahead of the JL. Once we accomodate for efficiency, the displacement effect becomes very evident. If we tried to run that JL at the power levels the Alpine is designed to handle, the W3 would tear itself to pieces.
The reasonable conclusion to this example is this: if you intend to build a daily driver system, or intend to compete in low power events, a high efficiency driver is a good choice. Of these three woofers, the W3 is the one I would choose for my own car because I know that on the 300 WRMS I would have available to power the thing, it would give me the best performance in daily, all-purpose applications. However, if I intended to compete in an SPL event and I had much more power at my disposal, the Alpine is certainly the way to go.
Now let's include some more aggressive speakers and see how they compare.
JL 12W7-3 - W7 12" 3 ohm woofer
Xmax=1.15 in.
SD=84 in^2
Displacement=96.6 in^3
Rockford Fosgate T2D412 - T2 series 12" Dual 4 ohm woofer
Xmax=.87 in.
SD=66.7 in^2
Displacement=58.03 in^3
MTX T9515-44 Thunder 9500 series Dual 4 ohm woofer
Xmax= 1.01 in.
SD= 83.05 in^2
Displacement=83.88 in^3
While the Rockford is not the powerhouse it's price might indicate, the other woofers exhibit some pretty impressive displacement numbers. In fact, that 12W7 exhibits over twice the displacement of it's little brother the 12W3. That said, one might expect a single 12W7 to perform on par with a pair of 12W3's. We'll run some SPL numbers and see if this is true.
This is almost exactly what we would expect to see. The W3's have a slight advantage still due to their efficiency, but the W7 exhibits almost identical output. Clearly there is something to be said for this displacement model.
But let's go back to our 3 woofer comparison:
Again, the JL is the white line, the Rockford is the red line, and the MTX is the blue line. At this point, is anyone really surprised by the results? While the MTX should probably be a little higher in the graph, this about the exact response we could have predicted based on the displacement numbers. Let's compare the 12W7 against the T212D4 for a moment. The JL's extra ~40 in^3 of displacement, under these conditions, equates to an additional ~5 db of output. That is huge! Especially when you consider the power differential. All 3 woofers in the simulation are seeing manufacturer's rated peak power (again, controlling for efficiency). This means the JL is running on 1500 watts versus the Rockford's 2400. Even with almost an additional 1000 watts of power, the JL's vastly superior displacement puts it head and shoulders ahead of the curve.
Now let's not forget the importance of enclosure design and application. After all, that's the goal of this event - to make sure the best applied system wins. The Rockford and MTX could both outperform the JL under the right conditions. Nothing here is meant to imply that the JL W7 is the greatest SPL woofer out there. I use that simply because it has a great deal of displacement potential and, when compared with other woofers of it's size, efficiency and capability, it should be the clear performer. That said, there can be no doubt that when paired against something with some serious power handling, the JL will probably meet it's match.
So how about we put that theory to the test? How about some big custom stuff? Some SPL woofers?
Fi N312 - BTL series 12" Dual 1 ohm woofer
Xmax=1.1 in.
SD=74.4 in^2
Displacement=81.84 in^3
RE XXX12 - XXX Series 12" Dual 4 ohm woofer
Xmax=2.12 in.
SD=74.4 in^2
Displacement=157.73 in^3
We'll throw in that 12W7 for comparison, too.
If these above examples and the explanations I have offered don't convince you that I am right, come out to Lynn's Summer SoundOff on Saturday, the 2nd of July 2011 and see for yourself what will actually happen. I'm either right or I am wrong, and after the 2nd we'll know for sure.
Risk
SPL is a measure of pressure (thus, Sound Pressure Level) and is represented in decibels (db). In order to make this pressure, a system must alternately compress and thin the air in the listening environment. In our case, this is the cabin of a car. The devices we use to create this pressure are called speakers, woofers, drivers or transducers (depending on how technical you want to get). All of these devices move in two directions - forward and backward - and this creates compression and rarefaction of the air in the cabin. This effect causes the build up of pressure waves in the cabin of the car, and these waves are analyzed by test equipment and represented as decibels of SPL (db/SPL).
Now, it stands to reason that the more ability a system has of compressing and rarefying the air in a cabin, the higher the SPL reading will be. This is what we call "louder" - more decibels means more pressure, which our own test equipment - our ears - interpret as louder. So the question becomes how does a system create pressure, and moreover how does it do this in an effective enough manner to be "louder" than another system?
Simply put, the more air a system can act upon, the louder it is going to be. Systems act on the air in two ways. First, let's talk a little about the driver. It is essentially just a piston, with the cone of the driver being the bore that contacts the air and the motor of the driver being the linkage that pushes the bore against the air. Compressing the air against the bore, then, becomes a function of these two forces - the size of the bore by the distance it travels.
There are two effective ways to increase the effect of compression. The first, and more traditional, method is increased bore size. Larger diameter speakers simply play louder than smaller diameter speakers because they can act on a larger amount of air. Some manufacturers have even played around with different shapes - like squares - to increase bore size (as a side note, bore size in a woofer is more often called surface area, and is represented in the speaker's Thiele/Small Parameters as SD). The effect, however, is always the same - the more surface area a speaker has, the more air it can contact and the louder it will play.
This is the extent to which most every car audio competition goes in dividing woofer classes against each other. Here is a link to DBDrag Street class guidelines in which this is a little more explicit in it's description. You'll notice that they simply pair systems together that have similar SD sizes. This is a valid way to do things and I don't mean to knock the system that really established the sport - DBDRA has stuff figured out and clearly it works. However, there are some clear shortcomings with this two dimensional perspective, most obviously in the example that follows.
The limitation with a surface area model is simply that not all woofers of equal size are, in fact, equal. A Funkasonic 15" from Wal-Mart that cost $39.99 probably isn't as capable a bass machine as an MTX Thunder T9500 15" which costs $829.99. Under the standard SD based rules, however, these woofers would be pitted head to head in the lanes, and the guy with the deeper pockets (who bought the more expensive, and vastly more capable, MTX woofer) would take the trophy home. That hardly seems like an accurate way to play the game. The size of your wallet should never decide the outcome of a competition. The skill you bring to the table should. That said, there is another dimension as regards pressurization that is entirely left out of this equation.
Here's an example: a JL 10W7 is fairly high end 10" subwoofer. By contrast, a Rockford P1S415 is a pretty run-of-the-mill 15" subwoofer. If we look solely at SD, would anyone question that the 15" speaker will play louder? The JL has a clear disadvantage due to it's comparatively small SD (59.8 in^2 versus 135.7 in^2). Which woofer will make more SPL at rated power - the JL or the Fosgate?
Here's the results:
The important info is this: the white line represents the JL 10W7 and the red line represents the Rockford P1S415. Both are in their rated sealed enclosure and both are powered by manufacturer's rated peak power (more on this later). Notice, then, that the JL is marginally louder than the Rockford (119.7 db versus 119.3). Does this mean the JL will outperform the Rockford under every conceivable condition? Absolutely not. The difference will come down to a handful of factors - power, enclosure design, charging capacity, and reinforcement of the cabin. This, of course, is exactly what we want to happen. Those previous factors require skill to implement properly. This means that the winner should, and likely will, be the more talented installer.
But why, you ask, is a 10" performing on par with a 15," like in the previous example? Well, simply put, it's the 3rd dimension that we eluded to earlier: excursion.
The second method for improving compression is to make the woofer cone move further. This can have the same effect as increasing surface area by allowing the woofer to push the air further instead of simply pushing a larger diameter of air. The volume of air acted upon by the bore can be the same for a small woofer with more movement as for a large woofer with less movement. The movement of the cone in forward and backward, or positive and negative, motion is represented in a woofer's T/S parameters as Xmax, also called one way linear excursion. As another interesting side note, while compression of the air occurs when the woofer moves forward and pushes against the air, rarefaction occurs when the woofer moves away from the compressed air, causing the air to thin out, or rarefy. This produces the negative pressure which, when combined with the positive pressure achieved from the compression cycle, results in the conditions required to create a wave.
What we can then evaluate from these numbers is how much air a woofer is actually capable of acting upon. By evaluating the actual volume displacement in cubic inches, instead of just square inches, we discover just how much compression a woofer system should be capable of, and in fact we have a much better idea of which woofers can play louder than others. The end result is that we can now pit woofer systems of comparable capability together based on their calculated displacement and not just on their generic diameter/perimeter.
As per our previous example, the 10W7 would be pitted against the P1S415, simply because they exhibit similar displacement characteristics (the JL makes 41.1 in^3 versus the Rockford's 50.2 in^3).
So by adding Xmax to the equation, we make sure that the Funkasonic, JL W7, or Rockford P1 woofer play ball with other woofers of their capability, not just with other woofers of their size. The same is true for the big stuff. Diameter is simply an insufficient real world indicator of a subwoofer's actual output potential. Let me throw some numbers out that make this point a little more clearly.
We'll use a few common woofers of a 12" variety but of differing price points. For the time being, we'll also be using the same amount of power for each woofer, without respect to manufacturer's ratings (again, more on this later).
Alpine SWR1243D - Type R 12" Dual 4 ohm woofer
Xmax=.79 in.
SD=74.4in^2
Displacement=58.78 in^3
JL Audio 12W3v3-2 - W3 12" Version 3 2 ohm woofer
Xmax=.51 in.
SD=80.6 in^2
Displacement=41.1 in^3
Rockford Fosgate P3D412 - P3 series 12" Dual 4 ohm woofer
Xmax=.63 in.
SD=88.4 in^2
Displacement=55.62 in^3
These woofers all fall into the $200-$300 price range and are considered excellent speakers by any measure. But notice that the Alpine, which has the smallest piston area at 74.4 in^2, displaces the most air at 58.78 in^3. This is simply because it has the highest excursion capability at .79 inches. So based on this model, we would expect the Type R woofer to play the loudest (albeit probably neck-and-neck with the Rockford) because it can act on the most amount of air, or in other words cause the greatest amount of compression/rarefaction in the vehicle cabin. But is this true?
Not exactly. In this case we see a discrepancy in the projected outputs, but not one that we couldn't have predicted. The JL, which should have been the quietest of the group, is actually the loudest. The Alpine and Rockford are neck-and-neck, as we expected, but the JL is clearly out front. The answer lies in the woofer's sensitivity, or efficiency. In this example, all 3 woofers are being fed 500 WRMS. On that amount of power, the JL will make the most output due to it's higher efficiency (87 db @ 1w/1m versus the other woofer's 85). So does this mean the displacement model is shot? Does woofer performance really come down to efficiency and that's it?
Not hardly. If that was true, the trick to making a loud woofer would simply be to make it crazy efficient and then pump as much power as humanly possible into it. But we know that this isn't so simple. This is the reason for power divisions. In the lower power applications, the lower excursion, higher efficiency woofers tend to do better. Low excursion drivers tend to be more efficient because it is easier to compress a lot of air a very small distance than it is to compress a little bit of air a long distance (more interesting side notes: The Romans had this theory nailed with their application of hydraulics in the Colesseum). But this doesn't mean that high efficiency drivers will always play louder. It just means they'll play louder until there's enough energy to move the woofer through it's entire range of motion (called Xpeak - the total movement of the woofer in both positive and negative phase). A low excursion driver will hit it's mechanical limits much earlier than a high excursion driver, not to mention it's thermal limits, and thus there is only so much output a low excursion driver can provide. By contrast, a high excursion driver, which will compress more air as we have previously proved, takes more power to reach this point, but will also handle this greater amount of power with a greater degree of control than will a low excursion driver. It is for this reason that the higher power applications will also commonly employ the higher excursion drivers. Thus, we divide the the low power from the high power and then we see that this effect is reduced.
Now don't get me wrong here. Every woofer can be driven beyond it's mechanical limits and forced to absorb more power with the intent of producing more SPL. However, that doesn't mean that any given woofer can be forced to play louder and louder simply by adding power to it. That JL W3 will hit a point where more power only hastens woofer failure but does nothing to increase volume. Every woofer has that same point, however some woofers are designed for SPL applications and have a very high mechanical power handling. They also tend to exhibit very impressive Xmax ratings. That high mechanical power handling and extreme Xmax's go hand in hand on SPL drivers is no coincidence.
Also consider this: at 500 WRMS, the JL 12W3 is very near it's thermal limits. In fact, JL voids the warranty on that speaker at 600 watts. That's the same as Alpine's RMS power rating. The SWR1243D doesn't reach it's limit (according to the manufacturer) until 1800 watts. So let's look at that graph again and put each speaker at it's rated limit:
Notice now that the Alpine is slightly ahead of the Rockford, and comfortably ahead of the JL. Once we accomodate for efficiency, the displacement effect becomes very evident. If we tried to run that JL at the power levels the Alpine is designed to handle, the W3 would tear itself to pieces.
The reasonable conclusion to this example is this: if you intend to build a daily driver system, or intend to compete in low power events, a high efficiency driver is a good choice. Of these three woofers, the W3 is the one I would choose for my own car because I know that on the 300 WRMS I would have available to power the thing, it would give me the best performance in daily, all-purpose applications. However, if I intended to compete in an SPL event and I had much more power at my disposal, the Alpine is certainly the way to go.
Now let's include some more aggressive speakers and see how they compare.
JL 12W7-3 - W7 12" 3 ohm woofer
Xmax=1.15 in.
SD=84 in^2
Displacement=96.6 in^3
Rockford Fosgate T2D412 - T2 series 12" Dual 4 ohm woofer
Xmax=.87 in.
SD=66.7 in^2
Displacement=58.03 in^3
MTX T9515-44 Thunder 9500 series Dual 4 ohm woofer
Xmax= 1.01 in.
SD= 83.05 in^2
Displacement=83.88 in^3
While the Rockford is not the powerhouse it's price might indicate, the other woofers exhibit some pretty impressive displacement numbers. In fact, that 12W7 exhibits over twice the displacement of it's little brother the 12W3. That said, one might expect a single 12W7 to perform on par with a pair of 12W3's. We'll run some SPL numbers and see if this is true.
This is almost exactly what we would expect to see. The W3's have a slight advantage still due to their efficiency, but the W7 exhibits almost identical output. Clearly there is something to be said for this displacement model.
But let's go back to our 3 woofer comparison:
Again, the JL is the white line, the Rockford is the red line, and the MTX is the blue line. At this point, is anyone really surprised by the results? While the MTX should probably be a little higher in the graph, this about the exact response we could have predicted based on the displacement numbers. Let's compare the 12W7 against the T212D4 for a moment. The JL's extra ~40 in^3 of displacement, under these conditions, equates to an additional ~5 db of output. That is huge! Especially when you consider the power differential. All 3 woofers in the simulation are seeing manufacturer's rated peak power (again, controlling for efficiency). This means the JL is running on 1500 watts versus the Rockford's 2400. Even with almost an additional 1000 watts of power, the JL's vastly superior displacement puts it head and shoulders ahead of the curve.
Now let's not forget the importance of enclosure design and application. After all, that's the goal of this event - to make sure the best applied system wins. The Rockford and MTX could both outperform the JL under the right conditions. Nothing here is meant to imply that the JL W7 is the greatest SPL woofer out there. I use that simply because it has a great deal of displacement potential and, when compared with other woofers of it's size, efficiency and capability, it should be the clear performer. That said, there can be no doubt that when paired against something with some serious power handling, the JL will probably meet it's match.
So how about we put that theory to the test? How about some big custom stuff? Some SPL woofers?
Fi N312 - BTL series 12" Dual 1 ohm woofer
Xmax=1.1 in.
SD=74.4 in^2
Displacement=81.84 in^3
RE XXX12 - XXX Series 12" Dual 4 ohm woofer
Xmax=2.12 in.
SD=74.4 in^2
Displacement=157.73 in^3
We'll throw in that 12W7 for comparison, too.
The first thing to notice is the odd shape of the response curve. This looks very different from the other graphs. The reason for this is simple: the Fi BTL series woofers are not sealed box speakers. The comparison, then, using those against the others in a sealed application would tell us nothing. There are a multitude of factors that determine actual subwoofer performance, not simply SD and Xmax, and we must do our best to accomodate for all of these when comparing speakers. In this example we have put all three speakers in the recommended ported enclosure and power them at the manufacturer's mechanical ratings.
Now to the meat of the issue. Not surprisingly, the RE exhibits the most output under these conditions. It has by far the most real displacement. But the real story here is the W7 versus the FiBTL. While the W7 has more displacement than the Fi, the Fi is actually louder than the W7. This is again coming from the power handling difference. The JL can only absorb 1500 watts at the limit, where the Fi can take 3000 RMS and 6000 at it's limit (as in this example). But what does mechanical power handling have to do with increased Xmax? Simple: making big SPL is, and always has been, a process of accepting distortion. A subwoofer begins to distort when the motor no longer has control over the voice coil, and this happens more and more as the woofer cone moves further and further. Eventually, the coil will essentially leave the magnetic gap of the woofer motor and exhibit huge amounts of distortion. While we would never want to listen to this under normal conditions, making SPL is about getting big numbers, not sounding good. That said, if we can push a woofer well beyond it's magnetic limits and force to it compress more air, even at the expense of increased distortion, we can get higher SPL numbers. And that is exactly what we are after here. So why is the Fi louder? It's mechanical limits are greater than the W7's, and even though it takes 6000 watts to get it there the Fi can physically move further than the JL can.
Basically this means that the rated Xmax numbers, which are acquired under standard T/S protocols, don't tell us how far the woofer can actually move with these extreme amounts of power. This is why we divide power classes and accomodate for woofer efficiency. Does this mean calcualted displacement is shot? No, it just means that we can't foresee every possible contigency and every imaginable system configuration and make classes for them. Here's what I mean.
Calculated displacement, as we have seen from the previous examples, gives us a very good baseline by which to divide systems against each other and make these comparisons a little more fair. But we also know that if two similar woofers, by which I mean woofers of similar displacement and similar power handling, are pitted against each other, the one with more power is going to play louder. If I had an FiBTL with 6000 watts and you had one with only 2000 watts, and otherwise our systems were the same, my Fi is going to play louder because I have the power to push that woofer well beyond it's rated excursion. However, if you had the W7 and tried to power it with 6000 watts to force it to outperform my Fi (which, following the calculations, it should be able to do) the chances are good that your woofer would self-destruct before it got that far because it doesn't have the mechanical ability to do what you're asking it to do. This doesn't mean the W7 is a bad woofer. It just means it's not an SPL woofer (which, incidentally, JL never claimed it to be). This also doesn't mean the Fi is the best woofer on the planet. It just means it was built to make incredible SPL and handle lots of power that can move it beyond it's rated limits. Both woofers have their design and purpose and the trick is in applying them properly to excel at that purpose.
Here's the bottom line: from almost any conceivable perspective, Xmax is as important a consideration as regards potential SPL as is SD. Whether it's small, high SPL drivers or huge power monsters, the ability of the woofer system to move further gives it the ability to compress more air and thus play louder. It doesn't matter if this compression is happening with 40% distortion and the voice coil hanging all the way out of the gap or whether the woofer is in perfect control and barely moving a muscle. The fact is that net displacement is the only accurate indicator of the woofer's actual performance potential.If these above examples and the explanations I have offered don't convince you that I am right, come out to Lynn's Summer SoundOff on Saturday, the 2nd of July 2011 and see for yourself what will actually happen. I'm either right or I am wrong, and after the 2nd we'll know for sure.
Risk
No comments:
Post a Comment