In general, if the amp is 4 ohms minimum, many people would choose an 8 ohm cab. This gives you the potential to build an expandable modular rig. Then, if you have a relatively quite gig, take one cab. Need to play louder, take both cabs.
If your amp makes the same power at 8 to 4 ohms, you get +3dB from adding the second (matching) cab.
If the amp doubles the power when you go from 8 to 4 ohms, you get +6dB from adding the matching cab.
Some amps make more power when the load is changed from 8 to 4 ohms, but they don't double the power. Under this circumstance you somewhere between +3dB and +6dB when you add a matching cab.
Now let's consider what happens when you trade an 8 ohm cab for a 4 ohm cab. The first thing to consider is can the cabs handle the full power of the amp. Some amps generate twice as much power at 4 ohms.
The second consideration is the sensitivity of the cabs. When companies make both 8 and 4 ohm variants of the same driver, it's not unusual for the 4 ohm version to be 1-2dB less efficient. Best case scenario is the amp makes twice as much power with the 4 ohm cab. Doubling the power is a +3dB change. However, if the 4 ohm cab is -2dB less efficient, you only net +1dB. 1dB is not generally consider statistically relevant, because it's hard reliably hear a 1dB change.
If the amp does not double the power, it's possible the 4 ohm cab will not play as loud as the 8 ohm cab.
My point...don't focus solely on power. The volume comes from useable power and the sensitivity of the speaker system you are using, so you need to consider both factors.
I don't recommend using an amp that is rated for twice as much power as the speaker. Speakers have different types of power ratings. The RMS power rating is essentially the thermal rating of the voice coil. In other words how much power the voice coil can handle long term without overheating. Program power is generally 2x the RMS rating. This is probably where the advice comes from. The advice is relatively safe for mid range frequencies but not frequencies below 100hz. Also, if the long term average power exceeds the RMS rating for two long, the voice coil will get burned.
In all honesty, the Program power rating is the max power the speaker is expected to be able to handle and survive. Some of the tests only require the driver to survive for about 2 hours, and the driver is not required to meet specs after the test. So for long term reliability, you probably don't want to slam a driver will it's full program rating.
Before I go further, when you are dealing with bass amp, you also need to be concerned with mechanical power handling, which is most relevant for frequencies below 100hz. Mechanical power handling is determined by how far the woofer can safely vibrate without being damaged. The amount the driver moves is determined by how the cab is tuned. My point is if you put the same driver in different cab designs, the mechanical power handling will vary from one cab design to the next.
It's not unusual for the mechanical power handling to be significantly lower than the driver's RMS rating. In other words you can damage the drive with less than the RMS rating. Using an amp that can produce 2X the speaker's RMS power rating makes the risk of damage much more likely.
AFAIK, the reason for RMS, Program, and Peak power ratings is because speakers are rated with a 6db crest factor. Crest factor is the ratio between the peak value of a waveform and the RMS value. So many companies will post a RMS power rating (long term average in RMS Watts), Program Power (short term average in RMS Watts and 2X the RMS rating), and Peak Power (instantaneous power in Peak Watts).
You might see something like Power Rating: 200W RMS, 400W Program, 800W Peak.
The tricky part is the relationship between the Peak rating and the RMS and Program ratings. Peak power is measured in peak volts and peak Watts. RMS and Program power are measure in RMS volts and RMS Watts.
Notice the peak power rating appears to be 4x the RMS rating and 2x the Program rating. But if you do the math, the Peak power is exactly the same amount of power as Program power.
Here is our Power Rating: 200W RMS, 400W Program (RMS), 800W Peak.
Let's convert the Peak value to the equivalent RMS value.
We need to know the impedance to determine power. So assume these rating are at 4 ohms. Let's find the peak Voltage rating for 800Wp at 4 ohms.
Voltage = sq rt(power x impedance)
Voltage = sq rt (800 x 4) = ~56.57Vp.
Now we will convert this to the RMS voltage value.
~56.57Vp/(sq rt of 2) = 40V RMS
The salient point I want you to see is
40V RMS = ~56.57Vp...it's the same value using different measurement units.
Now we determine how much power is dissipated at 4 ohms with 40V RMS applied.
Power = (Volts^2)/Impedance
Power = (40^2)/4 = 400W RMS
This shows that 400W RMS is the same as 800Wp
400W RMS = 800Wp
Remember I mentioned a 6dB crest factor is used to test speakers.
- 200W RMS to 400W Program RMS is 3dB
- 400W Program RMS to 800Wp is 3dB.
- So the RMS to Peak crest factor 3 + 3 = 6dB.
But the 3dB difference from Program to Peak power is really a conversion of measurement units rather than an actual change.
Hopefully this will make it a bit more clear.
A pure sine wave has a 3dB crest factor.
View attachment 5131733
The RMS voltage is ~0.707V and the peak voltage is 1V
RMS Watts at ~0.707V RMS and 4 ohms.
(~0.707^2) x 4 = 2W
Peak Watts at 1Vp and 4 ohms
(1^2) x 4 = 4Wp
So if you measure the RMS value, you get the blue line and if you measure the peak value you the red line.
View attachment 5131741
The waveform does not change only the units of measure.
- So 2W RMS to 4Wp is a 3 dB crest factor
- However, 2W RMS = 4Wp
Yes it blows my mind too.
My advice. Disregard the Program rating and Peak rating. An amp rated for about the same as the speaker's RMS rating is relatively safe...but you still may have enough power to destroy the drivers if you push the rig hard enough.
In other words if you get an amp rated for 400W RMS, you need a speaker rated for at least 400W RMS.
You can certainly use an 800W RMS amp with a 400W RMS speaker...but you have to use more discretion and restraint. Often speakers sound great right up to the point where the go silent. Meaning you can't always tell you are pushing a speaker too hard.
