What @Gorch says.
Also:
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LOL. I love it
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HOW MUCH POWER DO YOU NEED? Not as much as you think. For the musician, TONE is king. we’ll get back to this statement in a bit. Just some real world application first, then we’ll talk about the bass player’s place in it.
If you end up playing in any size venue at all then you will have a PA system complete with a sound guy to run it. You will have somebody like @DaveT there who knows the science behind the function. You must trust your sound guy in these situations because what you hear on stage isn’t what the audience is hearing. Your sound guy will make you stand out. He will make you sound good because making you sound good makes him sound good. With that in mind, you really don’t have to worry about power or dB or levels or anything else. Let the sound guy do the heavy lifting. That’s what they do.
After saying all that, all you need to be concerned with is your sound, your TONE. If you don’t sound good to you then your not going to be able to just roll with it. So make sure you sound like you want to sound then send a signal to your sound guy and he will take from there. So now the long awaited answer. How much power do you need? Enough to reach across the stage and still have room to turn it up some. Anywhere from 250 to 500 watts will do everything you need to do. Most of the time you will be nowhere near taxing your amp. In fact, if your too loud, the sound guy will have you turn it down some. That size of amp will fill a good size room by itself. Think small church size or school gym size. You can dial your controls back and use it in your music cave or crank them up and play a fund raiser at your kid’s school. Two recommendations here, make sure your amp has an XLR out so you can plug in to the house system when available and never run your amp maxxed out. Your sweet spot for tone will be somewhere in the middle unless you play metal in which case you will have your gain cranked as well as several other gadgets in place to distort your sound. If you absolutely have to have your amp dimed out just to keep up, THEN look at getting a bigger amp. Again, this is based on experience, not science. Participation and opinions welcome.
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Awesome! Probably want to go one more octave there too 
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I could do the math and show you the numbers why your answer works for the situations you describe, but you are correct, it wouldn’t really matter because what you have found through experience is what works for you. That’s really the advantage of having this forum, lots of experience to draw from and use as a starting point until someone gets their own experience. I find it particularly useful that collections of people are using the same gear and talking about it, specifically the Rumble line and the Zoom processor. It really gives a lot of perspective on how that gear behaves for a variety of people attempting to do a variety of different things.
The answer to any audio engineering question anyone asks me is usually, “It depends.” What kind of music? What kind of stage? Who else is on the stage? Who needs to hear you? How are they going to hear you? Who doesn’t want to hear you? How big is the audience? What shape is the audience? How far is the stage from the audience? How far are you from other people on the stage? What is the shape of how everyone is placed on the stage? What microphones are being used where? How is stage monitoring accomplished? Are the musicians wearing in-ear monitors?
Anyone who has played in a similar situation to the one someone is getting ready for is going to be able to give way more useful and readily understandable information than I can with math. My advice would be to copy a rig from the experienced and then make your own modifications from there.
As a public service announcement I’d strongly encourage anyone experiencing high SPL levels on stage to consider the options for in-ear monitoring.
The power topic is one of the most misunderstood among anyone, including engineers. I could talk about it for hours. Often times manufacturers don’t follow a standard when measuring their gear and quoting power numbers. It used to be in pro audio that amplifiers were pushed to some reasonably acceptable THD (total harmonic distortion) for a given test signal and that was considered their average power rating and their peak power available for short transient sounds would be 6dB higher. That means that a 100W amp average power rating could put out as much as 400W, but only for a very short time. With digital amplifiers they often quote a higher power number than with analogue amplifiers because they are capable of sustaining their peak power for longer. Sooooo, many digital amplifiers will say their power rating is 400W, but that’s it, it isn’t going as much above 400W for a peak, that’s all you get ever, but unlike an analogue amp you can get it for long enough that it can be rated higher. Many manufacturers of music products get their amplifier boards from a company that came out of Bang + Olufsen, the IcePower product. Open any mini-amp and there’s probably a chance this is inside. Each manufacturer makes their product special by adding the cab sim, integrating an effect or making a tone control people like. Companies don’t always invent their own audio amplifier boards. IcePower History
Something else to look for when considering amplifier power is what impedance load you are driving. Some amplifiers will put out full power with either a 4 Ohm or 8 Ohm load. Many will advertise the 4 Ohm power rating and put out half as much with an 8 Ohm load, a 3dB performance drop.
I know this is all head spinning, which is why I say use a rig that someone likes.
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Just to prove to myself I didn’t make this up, I found the bit that takes off the cover to a Darkglass Microtubes 500. The board in the center is the IcePower ASX125. The board on the back of the front panel Is by Darkglass and does tone control and overdrive. The board on the back is by Darkglass and handles line drivers for the effects loop and DI outs. The speaker output has a piggyback board with some giant resistors to pad down the main power output for headphones.
This is a fabulous quality board if you find it in your amp!
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Thank you @DaveT. As you can see, there is a lot to the science end of things. That’s why I say to let the sound guy do the heavy lifting.
Think about it. If you the bass player, had to worry about all of that stuff every time you did a gig then you probably wouldn’t gig very much because it isn’t any fun.(unless your a sound engineer) They get down to the nitty gritty of it so you don’t have to.
On another note, he mentioned monitor options. I deliberately avoided that subject but without going into it too deep, a monitor is simply a way for you to hear whats going on.
You can use your amp as a monitor if the whole stage can hear it, you may have wedges, (speakers on the floor pointed at you) with a sound sample of the product, or you may have portables, (a personal monitor just for you) or you may have an in ear system,(ear pieces plugged in to a device that lets you adjust the mix you want to hear) but suffice to say, your sound guy will handle that too. There will be monitoring of some sort. Don’t sweat it.
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Next up, SPEAKERS. 10s,12s,15s, 410 cabs 212 cabs… What’s best???
NOTE: I’m not going to talk about impedance here (4 ohm vs. 8 ohm) There is already a good thread on that subject on the forum. I will simply say buy matching gear or a combo and thus you don’t have to worry about it.
To recap a little, TONE is KING. This is your passion, sound the way that best pleases you. If you do that, you will enjoy playing and others will enjoy playing with you and listening to you. We will get a little deeper into how to get your tone later on but the speaker configuration you start with has a lot to do with it. Going back and forth over is this set up better than that is an argument with no solution like Ford vs. Chevy or Ruger vs. Glock. It’s totally a preference thing and will probably change over time.
First, let’s talk about sound in general. A frequency is a sonic wave at a given speed, the faster the speed, the higher the pitch. The slower the speed, the lower the pitch. A freq is what you hear when you get a hearing test. It’s constant with no unique character what so ever. A SOUND is a collection of different freqs with variable amplitudes( just a fancy word for how strong a particular freq is compared to the others) With that in mind, a speaker generates SOUND in a given FREQUENCY RANGE( from the lowest pitch it will discern to the highest pitch it will discern) based on it’s sensitivity. Different speakers respond more readily to some freqs more than others in their design range. This is why you can generate the same sound out of three different speakers and get three different sounds. Some are more low end sensitive, some are more high end sensitive. All of these differences are due to things like how a speaker is made and what it is made of. Already there is tons of variety and we haven’t even discussed size or configuration yet.
Now lets look at speaker diameter, 8 vs.10 vs. 12. vs. 15. For this you will have to imagine a graph of some sort showing the response over the given frequency range. Since all of the speakers we are talking about are designed for use in bass guitar amps, we are going to assume that the freq range of all of the speakers is the same (for the most part, this is true)
If we hit our A string and generate a sound out of a 15" speaker, we will see on the graph a gradual uphill slope to the point of highest sensitivity then a gradual downhill slope to the other end. Wide speaker, wide graph. Now if we do the same thing with a 12" speaker we would see the uphill and downhill slopes are a little steeper and the point is a little higher.
This is because you are putting the same frequency range into a smaller area. The smaller you go, the steeper the hill and the sharper the point. Again, this is non scientific general information, not exact design specs. Anyhow, what this means is on a 10" speaker, for example, you may have to boost you lows and highs a bit more and reduce the mids on your amp in order to make the hill on your graph more flat. When you adjust the EQ on your amp, this is essentially what you are doing. You are compensating for the frequency response of your speakers in some fashion to bend the sound to your liking. In the case of the 15" speaker, you may not have to make as much adjustment to your EQ but you will have to push the speaker a little harder to bring the level up to where you want it to be. This is the trade off. Nothing is free. You can put a 110 cab in your music cave, set your EQ to taste and it will sound good at low volume and your neighbors will like you. If you take it outside, you will have to really crank it up and dial back your EQ a bit to get the same “perceived” result.
If you put a 115 in your music cave, you will have to crank the volume just to get enough response to set your EQ.
Having said all of that, know that companies that design and market bass amps and cabs know what you need and they are going to make products that will work for you. They are ALL designed with a “sweet spot” somewhere in the middle of the amps capability. That’s the nuts and bolt of speaker SIZE. I’m going to take a break for a minute but part two will be speaker configuration.
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This kind of makes sense for class D power amps. The tonal qualities come from the preamp and any effects or stuff you put on it, but for the power amp stage all ypu really care about is it is a transparent power boost and doesn’t distort audibly, due to how class D amps work.
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Bravo mate,
Well said.
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That makes so much sense.
I’m impressed
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Seriously dude, talking the talk like that, why are you doing a course?
It sounds like you know what’s what.
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WTF!
I have no idea what you just said.
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Dude, you explain things so well even a dumb ass like me can understand what you are saying…sort of
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It is deceptively simple.
The sensitivity indicates how sensitive a speaker is to energy being put into it. The more noise a speaker produces per watt of input, the more sensitive it is.
The unit of energy being fed to the speakers is expressed in watts, and the amount of noise coming out of a speaker is expressed in decibels (dB). In order to compare the sensitivity of speakers, we measure the number of dBs being produced at a 1 meter distance for a 1 Watt input.
Unfortunately, the scale for sound pressure (noise) is not as you would expect. 93 dB is in fact twice as loud as 90dB, but it is only perceived as “a noticeable difference, but not more”, even though you do need twice the air movement to get at it. For this reason, the scale for sound pressure is what it is.
So… if you would want to hear a noticeable increase in noise, you need to double the energy you put into your speaker: double your amplifier’s power. Or… you could, you know, work with a speaker that has a sensitivity that is 3 dB higher than the one you have.
And that is why a 3 dB step up in sensitivity equals the noise level of an amplifier with twice the power.
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Thanks buddy.
I sort of get it now but I’ll have to reread it all a couple of times for it to really sink in.
I need to learn about it though because it’s important.
Thanks again.
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Ok, here we go. A non scientific approach to speaker configuration (multiple speakers in the same box)
A speaker is essentially an air hammer. It projects sound by pushing on a column of air. the rate of “push” you get is measured as decibels (dB). I’m not going to get into dB here because it isn’t necessary in order to understand the principle. Nor am I going to get into the math involved in determining how big of a column of air you are pushing on. It isn’t necessary to understand the relationship between column size, power applied, and push achieved.
When you apply a signal to a speaker at a given power level, the speaker will flex in and out or “oscillate”. The more power you apply, the more it will flex, the more it flexes, the harder it pushes on the air in front of it, the harder it pushes on the air, the more intense the effect on your ears. which produces what we perceive as VOLUME.
All speakers work on the same principle so they all have the same limitations. Mainly, they can only flex so much therefore, for a given size, they can only handle so much power before something has to give.
For this reason you will note that a 110 amp will be marketed in a given power range and a 115 amp will be rated in a higher power range. Does this mea that a 15 is louder than a 10? NO. It means that you can apply more power to a 15. I should say it this way, It takes more power to drive a 15 with the same intensity as a 10. Does that mean that the 10 is louder? NO
It means nothing is free. It takes more power to push on more air. More air moved= more sound produced. Not louder, just more. You say that doesn’t make sense. To that I can only say that ohms law is in full effect. Volts X Amps = watts. That’s how it is. You simply get the same “per watt” overall effect, it’s just spread over a different size area depending on the speaker size in question. Bigger speaker, softer push. Smaller speaker, harder push. Overall amount of force imposed on air is the same, more or less.
So that means if you want more sound, you need to move more air, in order to move more air, you need to apply more power. Since speakers are limited as to how hard they can push before they break, you need MORE SPEAKERS so you can push on MULTIPLE COLUMNS OF AIR.
PLEASE HEAR ME: I’m not saying more speakers is better. I’m not advocating everyone getting an 815 cabinet and buying a generator to power the amp. Remember we are playing for the human ear and all that is overkill. The ear has a given frequency range and a thresh hold for intensity.
The most common speaker configurations are 115, 112, 212, 210, 410, 810, and 208. I’ll be back later to put the speaker size and speaker configuration together by talking about the general particulars of each. Hopefully, this will give you an idea how to most efficiently produce YOUR UNIQUE TONE in multiple acoustic environments.
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Interesting discussion, @Gorch . . . 
A single 15" speaker has about the same amount of area as 2 x 10" speakers. So they should be fairly even in the amount of air they push . . . right?
I always thought that a smaller 10" or 12" speaker would react faster than a larger 15" or 18" speaker would. Put another way, they would be more responsive to an input signal, thus making them slightly clearer and “punchier”.
Did I get that right?
Cheers, Joe
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Yes. Exactly. Also, earlier I talked some about frequency response. Certain size speakers have certain inherent characteristics. Starting with a configuration that naturally delivers a sound closer to what you are going for is much more efficient than starting with something contrary to what you want simply because it’s cool, then having to wrestle with your tone all the time. For the sake of conversation, a smaller speaker has a greater sensitivity to a certain frequency band within it’s design range so naturally it will enhance those frequencies. Punchy, as it’s called.
The genre of music you play figures in as well. That’s why there is such variety in the market place. That’s also why I preach tone over power. Tone matters but after a certain point, power really doesn’t.
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