Gallien Krueger RB800 - Add 8 ohm cab?

[Wasnex]

You are using the term sensitivity incorrectly.

How can that be when I specifically said sensitivity is not a factor? I am not using the term "sensitivity" at all. We are only dealing with the combined SPL of various correlated sound sources. It does not matter whether we have one driver or a thousand, the SPL produced by each sound sources remains exactly the same.

The sources SPL must be defined the same way for the calculator to return the correct numbers.

My understanding is the formula works with dissimilar SPLs. But I will assume I am wrong and play along. Let's consider correlated sound sources at 114dB.

Per the formula:

Four correlated 114dB sound sources results in 126.041dB
Six correlated 114dB sound sources results in 129.563dB​

The theoretical difference in SPL is 3.522dB

 

[agedhorse]

Note that the power is adding in addition to the mutual coupling gains. This is why the sensitivity normalizes this.

I went back and re-read your post #2 and think I see where this went sideways. You stated that the mutual coupling resulted in a 6dB change, but in reality the mutual coupling is responsible for a 3dB change. I can’t tell if you meant to say mutual coupling plus doubling of power applied.

 

[Wasnex]

Note that the power is adding in addition to the mutual coupling gains. This is why the sensitivity normalizes this.

I went back and re-read your post #2 and think I see where this went sideways. You stated that the mutual coupling resulted in a 6dB change, but in reality the mutual coupling is responsible for a 3dB change. I can’t tell if you meant to say mutual coupling plus doubling of power applied.

The doubling of power is assumed for net +6dB, but from an acoustic summing standpoint, you get +6dB over whatever SPL each driver is producing (assuming both are making the same SPL).

Example where applied power is held constant. Let's assume the cabs are separated to the point where there is no interaction between them. The 1st cab at our location is making 100dB. When you connect the second cab, the power is now being shared equally by both cabs. So the first cab is now making 97dB and the second cab also makes 97dB. There is no acoustic coupling at this point and the level at our location has dropped -3dB. Now we bring the cabs as close to each other as we can, the level goes up +6dB theoretically to 103dB. The net is only +3dB over the output of one cab, but we get a 6dB change from simply bringing the two speakers together.

For +6dB net, it true the applied power must double. Let's assume our cabs are separated and the cab at our location is making 100dB. When the second cab is connected the power doubles and is then shared equally by both cabs. So the first cab continues making 100dB and the second cab is also making 100dB. There is no acoustic coupling at this point and the level at our location has not changed, even though the applied power has doubled. When we bring the cabs as close to each other as we can, the level goes up +6dB to 106dB.

Perhaps it's more accurate to say +3dB from doubling the acoustic power and +3dB from mutual coupling. Either way you get a 6dB increase when the the two sound sources are brought together.

Let's keep in mind that Sensitivity is defined different ways: 1W/1m and 2.83V/1m. I don't want to get into it, but the conversation changes a bit depending upon which definition we assume.

I was under the assumption we were discussing the dB change when the number of drivers is changed from four to six. If the power is held constant the SPL goes up 1.76dB. However, AFAIK: if the voltage is held constant the SPL goes up 3.52dB-->more power and increased acoustic coupling. This is the part you said the formula gets wrong.

 

[Rich67]

I love the GK 800RB heads.

 

[agedhorse]

The doubling of power is assumed for net +6dB, but from an acoustic summing standpoint, you get +6dB over whatever SPL each driver is producing (assuming both are making the same SPL).

Example where applied power is held constant. Let's assume the cabs are separated to the point where there is no interaction between them. The 1st cab at our location is making 100dB. When you connect the second cab, the power is now being shared equally by both cabs. So the first cab is now making 97dB and the second cab also makes 97dB. There is no acoustic coupling at this point and the level at our location has dropped -3dB. Now we bring the cabs as close to each other as we can, the level goes up +6dB theoretically to 103dB. The net is only +3dB over the output of one cab, but we get a 6dB change from simply bringing the two speakers together.

For +6dB net, it true the applied power must double. Let's assume our cabs are separated and the cab at our location is making 100dB. When the second cab is connected the power doubles and is then shared equally by both cabs. So the first cab continues making 100dB and the second cab is also making 100dB. There is no acoustic coupling at this point and the level at our location has not changed, even though the applied power has doubled. When we bring the cabs as close to each other as we can, the level goes up +6dB to 106dB.

Perhaps it's more accurate to say +3dB from doubling the acoustic power and +3dB from mutual coupling. Either way you get a 6dB increase when the the two sound sources are brought together.

Let's keep in mind that Sensitivity is defined different ways: 1W/1m and 2.83V/1m. I don't want to get into it, but the conversation changes a bit depending upon which definition we assume.

I was under the assumption we were discussing the dB change when the number of drivers is changed from four to six. If the power is held constant the SPL goes up 1.76dB. However, AFAIK: if the voltage is held constant the SPL goes up 3.52dB-->more power and increased acoustic coupling. This is the part you said the formula gets wrong.

But the voltage at rated power isn’t the same for the two sources because the rated power is different (300W and 100W).

 

[Wasnex]

But the voltage at rated power isn’t the same for the two sources because the rated power is different (300W and 100W).

Yes, and I did my best build these nuances into the example I created. It's why the drivers in the 410 received 75W each and made 115.8dB while the drivers in the 210 received 50W each and made 114dB.

I did a workup treating the 6 drivers as individual sound sources:

So we have 115.8 + 115.8 + 115.8 + 115.8 + 114 + 114 = ~130.8dB*

* These are the figures reported by the calculator.

I also did a workup treating the 410 and 210 as the individual sound sources, and got the same predicted SPL:

I don't think we can use the ~1.76dB increase in the sensitivity rating because the speakers are not receiving equal voltage. Remember 300W total to the 410 and 100W total to the 210.

410 [Log(300) x 10] + 103 = ~127.8dB
210 [Log(100 x 10] + 100 = 120dB

Per the calculator and formula: Mutual summing of 127.8 and 120 = ~130.8dB...same as above.