Cab ohms vs. amp watts

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mr p body

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This question has probably been posted a million times, but I couldn't find anything so here we go again...
I have a new Mesa Walkabout 300W (when driving a 4 ohm total speaker load) bass amp, and a Mesa Powerhouse 4 X 10 speaker cab (rated at 8 ohms).

According to the owner's manual for the amp, my power is reduced to 150W when driving an 8 ohm cabinet.

How does one calculate total speaker impedence loads when driving more than one cabinet? I have an old (70's vintage) SVT 8 X 10 cabinet (ohm rating unknown to me), my new Mesa cabinet, as well as a Peavey 4 X 10 cabinet also rated at 8 ohms.

If I drive the two 4 X 10 cabs, what is the total load, or how do I calculate total load for multiple cabs? I want to utilize all 300W of my amp without rewiring speaker cabs, etc.
 
If you plug both 4x10 cabinets (8 ohms each) into the Mesa, it'll create a 4 ohm load. Putting equal loads in parallel cuts the load impedance in half.
 
mr p body said:
This question has probably been posted a million times, but I couldn't find anything so here we go again...
I have a new Mesa Walkabout 300W (when driving a 4 ohm total speaker load) bass amp, and a Mesa Powerhouse 4 X 10 speaker cab (rated at 8 ohms).

According to the owner's manual for the amp, my power is reduced to 150W when driving an 8 ohm cabinet.

How does one calculate total speaker impedence loads when driving more than one cabinet? I have an old (70's vintage) SVT 8 X 10 cabinet (ohm rating unknown to me), my new Mesa cabinet, as well as a Peavey 4 X 10 cabinet also rated at 8 ohms.

If I drive the two 4 X 10 cabs, what is the total load, or how do I calculate total load for multiple cabs? I want to utilize all 300W of my amp without rewiring speaker cabs, etc.
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Ok, quick electronics lesson. Power (watts) is the voltage (volts) times the current (amps) accross a given load (ohms). If you reduce your load by half, you will double your power (4 ohms @300W = 8 ohms @150W). That is why your power rating will drop, but in reality, you are still delivering the same voltage and amps to the speakers. Another words, you will not change the amount of sound that comes out by reconfiguring your load, so 150W into 4 ohms will produce as much sound as 300W into 8 ohms. That is basically the simplified version and does not take intto account speaker efficiency and 100 other variables that will actually change the power into a sound level.

On the back of the Walkabout, I have one also, there are 2 8 ohms jacks. Just take your 2 8 ohm cabs and plugs one into each. You just need to make sure and keep the load the same on each jack (both 4 or both 8), don't mix them. If you don't, the 4 ohms cab will be twice as load as the 8 and you will probably fry the amp or more likely, the speakers.
 
juststartingout said:
Ok, quick electronics lesson. Power (watts) is the voltage (volts) times the current (amps) accross a given load (ohms). If you reduce your load by half, you will double your power (4 ohms @300W = 8 ohms @150W). That is why your power rating will drop, but in reality, you are still delivering the same voltage and amps to the speakers. Another words, you will not change the amount of sound that comes out by reconfiguring your load, so 150W into 4 ohms will produce as much sound as 300W into 8 ohms.
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If the loads doubles, then the amperage halves, it does not remain constant. Amps = volts/ohms. What is constant is voltage. Also, given the same sensitivity, 300W is louder than 150W.
 
Not this crap again! 300 watts is louder than 150 watts by 3db. Pretty insignificant. You will get more of a gain in volume because you just doubled the amount of speakers pushing air.
The bottom line is that you need not worry about the wattage as long as you get the impedance right. You really aren't screwing yourself out of 150 watts by using an 8 ohm cabinet.
The 810 cabinet should be 4 ohms.
 
mshilarious said:
If the loads doubles, then the amperage halves, it does not remain constant. Amps = volts/ohms. What is constant is voltage. Also, given the same sensitivity, 300W is louder than 150W.
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No, sorry, but 300W at 4 ohms is not louder than 150W at 8 ohms. They are the same. I have no idea what you mean by sensitivity, but if you mean that both loads are 8 ohms, then yes, 300W is louder than 150W.

Fairview is correct when he says that if you want more sound, then get more speakers. That will significantly increase how loud you are.
 
juststartingout said:
. I have no idea what you mean by sensitivity.
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Sensitivity is the 'so many db at 1 watt at 1 meter' spec. What mshilarious was trying to say was 'all other things being equal'
 
juststartingout said:
No, sorry, but 300W at 4 ohms is not louder than 150W at 8 ohms. They are the same. I have no idea what you mean by sensitivity, but if you mean that both loads are 8 ohms, then yes, 300W is louder than 150W.
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That wasn't what you wrote:

150W into 4 ohms will produce as much sound as 300W into 8 ohms.
Click to expand...
 
All other things being equal, 300W is louder than 150W, no matter what the impedance is. Cause it's more. :)

The thing to do is listen to the SVT and the 2 4X10 cabs and pick the one you like the best, and figure out which one you want to lug around.

Assuming the same sensitivity, you will get 6 db more volume with either of these set-ups than with a single 4X10.

Twice the power=+3db
Twice the cones=+3db
 
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boingoman said:
All other things being equal, 300W is louder than 150W, no matter what the impedance is. Cause it's more. :)
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The problem is that things are not equal. An 8 ohm speaker will take twice the power to move it the same distance, as a 4 ohm speaker does. This is what 'load' means.
 
juststartingout said:
The problem is that things are not equal. An 8 ohm speaker will take twice the power to move it the same distance, as a 4 ohm speaker does. This is what 'load' means.
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What? No! That is not what load means. It's not even true in any case. But I should have said "sensitivity being equal, 300W is louder than 150W, regardless of impedance."

I think you may be confusing the amount of power an amp can put into a particular load with the power needed to drive that load to a certain volume. They are different things.

An 8 ohm speaker does not inherently take more juice to move it than a 4 ohm does. It will limit the power a particular amp can put into it vs. a 4 ohm speaker, though, as it offers more impedance.

Sensitivity is the issue. An 8 ohm speaker with 98 db sensitivity will put out 98 db with a 1 watt input. A four ohm speaker with 98 db sensitivity will put out 98 db with a one watt input.
Two speakers with the same sensitivity will put out the same SPL at the same power, no matter what their impedance is.


Load does not inherently have any thing to do with movement, or the power needed to cause it. A resistor is a load. How many resistors flap around in your amp to do their job? :)

A load restricts current. A wire (voice coil) will restrict the same current whether it is moving or not.

Resistance is a non-reactive load, associated with DC. It is the same at any frequency.
Impedance is a reactive load, associated with AC. It changes with frequency, and is made of reactive and non-reactive elements.

The majority of a speaker's impedance comes from it's DC resistance, which is from the size of the voice coil(non-reactive). The rest comes from how the speaker impedes at a given frequency.(reactive)

The movement of the speaker cone affects impedance, but because the speaker has different impedance at different frequencies. An 8 ohm guitar speaker could range from 4-16 ohms as you play, depending on what notes go through it. We just call it an 8 ohm speaker. It's like an average.

Impedance doesn't set a certain level needed to do a particular task, like move a voice coil and cone.
 
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Dude, when they rate amps, the sensitivity or efficiency or number of speakers have nothing to do with it. The only thing they measure is the load on the amp.

Basic electronics formula E=I*R

so
16 volts = 2 amps * 8 ohms ------ 16 * 2 = 32 watts
or
8 volts = 2 amps * 4 ohms -------- 8 * 2 = 16 watts

So when you compare amps, make sure you compare ratings at the same load level.

Now, to me, speaker choice is far more important than the amp rating, when looking at the actuall dBs you will produce or the oveall sound quality. If you want to produce more dBs, run more speakers at the same power. 1 speaker or 4 speakers or 8 speakers, will all draw 100watts at 8 ohms, but there is a major dB difference.

Keep in mind, that the crowd doesn't carry dB meters, they don't care how many watts you can push, all they care about, is what the group sound like when they see them.
 
juststartingout said:
Dude, when they rate amps, the sensitivity or efficiency or number of speakers have nothing to do with it. The only thing they measure is the load on the amp.

Basic electronics formula E=I*R

so
16 volts = 2 amps * 8 ohms ------ 16 * 2 = 32 watts
or
8 volts = 2 amps * 4 ohms -------- 8 * 2 = 16 watts

So when you compare amps, make sure you compare ratings at the same load level.

Now, to me, speaker choice is far more important than the amp rating, when looking at the actuall dBs you will produce or the oveall sound quality. If you want to produce more dBs, run more speakers at the same power. 1 speaker or 4 speakers or 8 speakers, will all draw 100watts at 8 ohms, but there is a major dB difference.

Keep in mind, that the crowd doesn't carry dB meters, they don't care how many watts you can push, all they care about, is what the group sound like when they see them.
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Dude,
You are drawing all the wrong conclusions from all the right data.

You said it takes twice as much power to get the same amount of movement from an 8ohm speaker as it does a 4 ohm speaker. That is absolutely wrong, if they have the same sensitivity.

As far as volume goes, all that matters is the power to the speaker, and it's sensitivity.

A watt is a watt.

1 watt into a 4 ohm speaker is the same exact power as 1 watt into an 8 ohm speaker.
300W into a 4ohm is the same power as 300W into an 8ohm.

1 W@4ohms= .5A, 2V
1W@8ohms= .35A, 2.8V

The four ohm speaker gets more current, less voltage, but the power is the same. Given the same voltage, the 4 ohm speaker gets more power. That is the important part.

You gotta look at the input voltage, and it's resulting output voltage. Those are impedance-independent. An amp produces a given output voltage for a given input voltage, no matter the impedance. The power produced depends the current flow, which is set by the impedance of the load.


If you feed the same input voltage into an amp from a guitar, yes, the lower impedance speaker will be louder. Because it is getting more power at the output.


E=I squaredR is difficult to use for this purpose.
Power= voltage squared/resistance is much more helpful.

A lower impedance speaker lets the amp produce more power because more current can flow through it, given the same output voltage.

A four ohm speaker will sound louder when you plug it into your amp because it is getting more power from the same input signal it was getting at 8 ohms.
Not because the 8ohm speaker needs more power to move.(it doesn't)
 
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There seems to be some bad info on this subject. Let’s see if we can make this clearer.

Amplifiers deliver power to a load. Power is usually measured in Watts. As a general rule, if the impedance of the load goes up, the amplifier delivers less power.

The equation for power is P = IV, where I is the amps and V is the voltage. It can also be expressed as P = I2R, where R is the resistance (impedance) of the load.

The equation for computing impedance for two speakers is R = (R1*R2)/(R1 + R2), which is for when the speakers are in parallel. If they are in series, then R = R1 + R2.

An amplifier that delivers 300 W to a 4 ohm load will usually deliver something like 150 W to an 8 ohm load (which are the specs provided by the original poster)

Loudness is another factor, that while related to power, also depends on speaker sensitivity, cabinet design, speaker size, and some other factors.

Two same size same, same impedance, speakers from different vendors can produce very different volume given the same amplifier and power applied. This is where the speaker sensitivity comes in to play. Other factors like cabinet design also further complicate the expected results.

Suppose you have 2 identical 8 ohm speakers. If you wire them in parallel, the result is 4 ohms. Connect to the 300 W amplifier in question and 300 W of power will be delivered. Each speaker will receive 150 W and produce the same volume level from each speaker. The overall volume will be the sum of the volume from each of the two speakers that receive 150 W each.

Now connect only 1 of the two speakers to the amplifier. This results in an 8 ohm load. The amplifier will then only be able to deliver 150 W, which the single speaker will receive. If will produce the same volume as it did before, but only one speaker is working now, which is less overall volume.

Now connect the two speakers in series to the amplifier, which results in a 16 ohm load. The amplifier will now likely only be able to produce something like 75W, of which each speaker will receive about 37.5 W.

So in this simple test, 300 W into the 4 ohm load will be louder than when connecting the single speaker or the pair of speakers in series.

The original poster has three cabinets. Two were 8 ohm cabinets, and one unknown, but assumed to also be 8 ohms. Connecting one of the three cabinets to the amplifier will produce at most 150 W and some volume level.

Connecting two of the cabinets to the amp (the two known 8 ohm cabinets for example) will result in a total load of 4 ohms and 300 W. Since each cabinet receives 150 W, the result will be louder. The first cabinet will get the same 150 W it got before and produce the same volume. However in the two cabinet test, the second is also producing sound, and the overall result will be louder.

Ed
 
when using two enclosures with identical transducers, and each receiving the same wattage at the same ohm load as one speaker would have been receiving,
what % increase in db would you gain due to acoustic coupling?
 
With two enclosures, you would be moving twice as much air. You would be running at twice the power. As a general rule, going to twice the air or twice the power would result in about 3 db of volume.

Ed
 
Ed Dixon said:
With two enclosures, you would be moving twice as much air. You would be running at twice the power. As a general rule, going to twice the air or twice the power would result in about 3 db of volume.

Ed
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edit: content removed. :)
 
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Here you only have one combo. The twice the power usually refers to increasing the power in a given enclosure. That results in about 3db.

Here you are comparing two enclosures to one, each combo running the same number of watts. The difference is that you added twice as much air, so one would expect a total change of about 3db.

When viewed from the amplifier end, you get the same result. On the single enclosure case, you push say 150 watts. In the two enclosure case, you push 300 watts. So the amp end would expect a 3db change as the overall power doubled.

Ed
 
I think I'm following you here. let's say one power amp is driving one enclosure @150 watts. Then we stack another enclosure powered by the same amp, and maintain the wattage ouptput of the amp @ 150 watts. We gain only 3db acoustically, so to gather 6db we`d have to double the amps output to 300 watts?
 
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