Re: JBL SRX / QSC PL236 underpowered?
It sounds then, that you have enough power to run the subs at the needed level. That being true, then there is not much to worry about. The math is only important if your really looking to run at the optimum performance a system can provide, or are in need of knowing what your system can do. The math is also important in figuring out how to budget your purchases for performance reasons. Like I said getting that last 3db may be cost prohibitive?
You don't have to run the subs under the tops, but this is a the most common way to do it at the weekend warrior level. The side effect of deploying subs in this manner is that you get lobbing and the famous power alley. Basically it makes your low frequency coverage look kinda like a hand and can leave people standing in a certain spot to have little sub impact, while another person nearby is getting plenty. Centralizing the subs simply makes coverage even in all directions and nets you a 3db gain at all reproduced frequencies ( no comb filtering ). The common practice is to place the subs on the floor in front of the band, or you could simply place booth subs on one side of the stage. I will agree it looks funny, but it sounds better in all seats in the house when done this way. It is only slightly louder on the side where the subs are as the downside.
The way each manufacturer produces spec's is a mystery, but don't weed out the possibility of bias towards their desired equipment line. JBL works closely with Crown and the suggested power amp size may coincide with the amount of power available in the Crown amp line? Qsc does the same thing. They also have probably factored that the average user will not be into spending more money on amplification, than the speaker they are powering? They spec for an amp that will easily fall into the budget of many users and still provide adequate performance. I find that if your looking to squeeze all the performance you can out of a speaker, your probably going to have to power it with an amp that sets right about 2/3rd's of the way to the speakers peak power rating. For example, a 100 watt peak speaker, powered with a 70-75 watt amp. Not always the case, but probably true.
As for believing that a x-over is doing the job it needs to in freeing up headroom, it's more complicated than that. All a X-over does is funnel frequencies put in, to either highs, or lows ( in the case of a two way X-over ). But vocals, guitars, keys and all the other instruments may still have energy that is produced below the X-over point ( usually around 80-100hz ). Keep in mind that the slope of the X-over is referenced in octaves. So a 12db per octave slope at 80hz still has energy coming into it from 160hz ( although at 12db lower ) and still very little energy coming in from 320hz ( at 24db lower ) etc. This energy is still being produced by the subs. Now with an aux fed sub, you can make it so only the desired instruments energy is being applied towards them. You will still need a X-over, but now your only sending kick, bass and perhaps some keys to the subs now. This will free up headroom and allow for the subs to reproduce those items with more potential volume. Removing the amount of energy going in, free's up headroom. This means that more energy can be used to reproduce the desired signals. This theory is easily noticed by setting up monitors. A frequency that is hot will feedback. You cut that frequency and you can get a little more volume in the speaker before feedback occurs. This is why tri amp systems get a little louder and sound a little better. Each range is controllable and can be fine tuned just a little more and since each element can be designed for it's specific task, it's more efficient at it.
There is an old adage that says you should have about a 2:1 ratio of subs to tops wattage. So if you have 100 watts to the tops, you should have 200 for the subs. This is a very rough rule and is flexible, but truthful to a large degree. I like to have twice the power allocated to subs as I do tops if I can, but it is not always needed. Indoors you can get away with less, but outdoors you will probably need at least a 2:1 ratio for full impact at distance. The other way to work around this is to have a more efficient sub. A sub that is capable of 3db more volume than the tops is basically the equivalent of twice the power. However there is not many subs at the low to mid level that outperform the tops that accompany them.
It sounds then, that you have enough power to run the subs at the needed level. That being true, then there is not much to worry about. The math is only important if your really looking to run at the optimum performance a system can provide, or are in need of knowing what your system can do. The math is also important in figuring out how to budget your purchases for performance reasons. Like I said getting that last 3db may be cost prohibitive?
You don't have to run the subs under the tops, but this is a the most common way to do it at the weekend warrior level. The side effect of deploying subs in this manner is that you get lobbing and the famous power alley. Basically it makes your low frequency coverage look kinda like a hand and can leave people standing in a certain spot to have little sub impact, while another person nearby is getting plenty. Centralizing the subs simply makes coverage even in all directions and nets you a 3db gain at all reproduced frequencies ( no comb filtering ). The common practice is to place the subs on the floor in front of the band, or you could simply place booth subs on one side of the stage. I will agree it looks funny, but it sounds better in all seats in the house when done this way. It is only slightly louder on the side where the subs are as the downside.
The way each manufacturer produces spec's is a mystery, but don't weed out the possibility of bias towards their desired equipment line. JBL works closely with Crown and the suggested power amp size may coincide with the amount of power available in the Crown amp line? Qsc does the same thing. They also have probably factored that the average user will not be into spending more money on amplification, than the speaker they are powering? They spec for an amp that will easily fall into the budget of many users and still provide adequate performance. I find that if your looking to squeeze all the performance you can out of a speaker, your probably going to have to power it with an amp that sets right about 2/3rd's of the way to the speakers peak power rating. For example, a 100 watt peak speaker, powered with a 70-75 watt amp. Not always the case, but probably true.
As for believing that a x-over is doing the job it needs to in freeing up headroom, it's more complicated than that. All a X-over does is funnel frequencies put in, to either highs, or lows ( in the case of a two way X-over ). But vocals, guitars, keys and all the other instruments may still have energy that is produced below the X-over point ( usually around 80-100hz ). Keep in mind that the slope of the X-over is referenced in octaves. So a 12db per octave slope at 80hz still has energy coming into it from 160hz ( although at 12db lower ) and still very little energy coming in from 320hz ( at 24db lower ) etc. This energy is still being produced by the subs. Now with an aux fed sub, you can make it so only the desired instruments energy is being applied towards them. You will still need a X-over, but now your only sending kick, bass and perhaps some keys to the subs now. This will free up headroom and allow for the subs to reproduce those items with more potential volume. Removing the amount of energy going in, free's up headroom. This means that more energy can be used to reproduce the desired signals. This theory is easily noticed by setting up monitors. A frequency that is hot will feedback. You cut that frequency and you can get a little more volume in the speaker before feedback occurs. This is why tri amp systems get a little louder and sound a little better. Each range is controllable and can be fine tuned just a little more and since each element can be designed for it's specific task, it's more efficient at it.
There is an old adage that says you should have about a 2:1 ratio of subs to tops wattage. So if you have 100 watts to the tops, you should have 200 for the subs. This is a very rough rule and is flexible, but truthful to a large degree. I like to have twice the power allocated to subs as I do tops if I can, but it is not always needed. Indoors you can get away with less, but outdoors you will probably need at least a 2:1 ratio for full impact at distance. The other way to work around this is to have a more efficient sub. A sub that is capable of 3db more volume than the tops is basically the equivalent of twice the power. However there is not many subs at the low to mid level that outperform the tops that accompany them.