CASE 1
Head: solid state (not tube), rated 800 Watts average (RMS) at 4 Ohms
Cabinet 1: Single 15", 8 ohm impedance, 300 Watt average (RMS) power rating, sensitivity 99 dB/1 Watt at 1 meter
Cabinet 2: Same
Wiring: Cabinets in parallel
Impedance of cabinets shown to amplifier = 1 / (1/R1 + 1/R2) = 1 / (1/8 + 1/8) = 4 ohms
Wiring the cabinet in parallel produces a 4 ohm load which is OK for this amplifier rating, so far so good. The 800 Watts the amplifier is capable of will be distributed equally to both cabinets, each will get 400 Watts. I’ll leave the math off of this since it’s sufficient to say that the cabinets must have the same voltage across them when wired in parallel and we know the power delivered is V-squared / R. Since they have the same R, they must have the same power delivered to each. The amp will not deliver more than it has, so that means each gets 400 Watts RMS, adding up to the total capability of 800 Watts.
The sound pressure level produced 10 meters away by each cabinet will be dB-SPL = Sensitivity + 10log(Power) - 20log(Distance in Meters) = 99 + 10log(400) - 20log(10) = 105 dB-SPL. Normally in PA systems when you stack two speakers you get a 3dB increase for them adding together (Trust me, I’m leaving off the math on that one). However, the bass frequencies are very large wavelengths compared to the size of the cabinet, so they will add together more efficiently, up to as much as 6dB rather than 3dB. Soooo, the total SPL from both cabinets working together will be 105 + 3 or 6, likely 110dB, maximum RMS level, peaks could be higher.
If the player is standing 1 meter away from their cabinet they will be exposed to 99 + 10log(400) - 20log(1) = 125 dB from one driver. The level could be as high as 131 dB for this person.
What about the idea that the cabinets are rated at 300 Watts and we have the capability to deliver 400 Watts to them? It’s probably inconsequential. How many more dB that is is given by 10log(P1/P2) = 10log(400/300) = 1.25 dB additional power in the amp. The cabinet also has a peak power rating of at least 2x the average power rating (for bass, 4x for normal PA) and we won’t be over that number.
Result: No smoke or destruction, possible hearing damage.
CASE 2
Wiring: Cabinets in series
Impedance of cabinets shown to amplifier = R1 + R2 = 16 ohms
Wiring the cabinet in series produces a 16 ohm load which is OK with the penalty of not being able to draw the full power from the amp. Our amplifier can only make its maximum voltage on the output and power is given by V-squared / R. If we change R from 4 to 16, we have reduced the power that can come out of the amp by a factor of 4. Therefore, we now have a Watts * R-Rated / R-Actual = 800 * 4 / 16 = 200 Watts amp. The 200 Watts the amplifier is capable of will be distributed equally to both cabinets, each will get 100 Watts. In this case both cabinets will have the same current and power is given by I-squared*R, so each will receive the same power. Another way to look at it is that the voltage across the series circuit will divide proportional to their load. Since they have the same R, they must have the same power delivered to each. The amp will not deliver more than it has, so that means each gets 100 Watts RMS, adding up to the total capability of 200 Watts for a total 16 Ohm load.
The sound pressure level produced 10 meters away by each cabinet will be dB-SPL = Sensitivity + 10log(Power) - 20log(Distance in Meters) = 99 + 10log(100) - 20log(10) = 99 dB-SPL. Normally in PA systems when you stack two speakers you get a 3dB increase for them adding together (Trust me, I’m leaving off the math on that one). However, the bass frequencies are very large wavelengths compared to the size of the cabinet, so they will add together more efficiently, up to as much as 6dB rather than 3dB. Soooo, the total SPL from both cabinets working together will be 99 + 3 or 6, likely 104 dB, maximum RMS level, peaks could be higher.
If the player is standing 1 meter away from their cabinet they will be exposed to 99 + 10log(100) - 20log(1) = 119 dB from one driver. The level could be as high as 125 dB for this person.
Result: No smoke or destruction, possible hearing damage.
Is that what you wanted to see presented @Gorch?