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[Jasen Chung .]

Happy Holidays Everyone, 

So I'm a hobbyist being asked to check a sound system for church and please correct me if I'm wrong, but as a rule of thumb shouldn't the amp be double the wattage as the speaker for efficiency? Then I saw the features of the amp and got a bit confused with the options

- I have 2 EAS S250Z that run at  1050W (4 ohm) and 525W (8 Ohms)
- The amp being used is a QSC PL325 that pushes 500W @8 Ohms/ 850W @4 Ohms/ 1250W @2 Ohms.

1) should we be getting a higher powered amp?
2) if budget doesn't permit, what is the best setting for max bass output?

Thanks in advance experts!

[Ivan Beaver]

Happy Holidays Everyone, 

So I'm a hobbyist being asked to check a sound system for church and please correct me if I'm wrong, but as a rule of thumb shouldn't the amp be double the wattage as the speaker for efficiency? Then I saw the features of the amp and got a bit confused with the options

- I have 2 EAS S250Z that run at  1050W (4 ohm) and 525W (8 Ohms)
- The amp being used is a QSC PL325 that pushes 500W @8 Ohms/ 850W @4 Ohms/ 1250W @2 Ohms.

1) should we be getting a higher powered amp?
2) if budget doesn't permit, what is the best setting for max bass output?

Thanks in advance experts!

Efficiency has nothing to do with it.

There is no "standard" rule for amp/speaker power-but the more "common" (whatever that is) standard is having an amp twice the size of the continuous rating of the loudspeaker.

HOWEVER this depends greatly on the actual usage-the dynamic range of the source material and so forth.

In some "heavy" cases (think long term dance club), an amp half the rating or equal to the continuous rating is a good choice-due to the long term heating.

In other cases-it is recommended to have an amp 4 times the continuous rating (so it can pass 6dB peaks).

Of course the REAL answer is to have an amp of the proper size to provide the needed SPL for a particular application.  In some cases this might be only 10% of the rated power.

Believe it or not-but a sound system DOES NOT ALWAYS need to run at max output. 

[duane massey]

You need an amp that provides enough power to deliver enough spl for your needs. That could be 10 watts, or it could be 2,000 watts. Since you are using one amp, I assume you are driving each speaker with one channel, so you are driving a 4ohm load. If you are not clipping the amp, and it is loud enough, you are good. If you are clipping the amp and it is not loud enough, you need more power. Ivan said it better in technical terms, but my response is "It depends"....

[Luke Geis]

There is a general rule of 2-4 times the continuous/rms rating of the speaker. If you think of it like this, you will see why it makes sense. A speaker who's rms ( or continuous )  power is 500 watts, can in essence be powered by an amp that will provide 500 watts. However you don't want to power it with an amp who's peak power is 500 watts. That means that the amp would be working pretty hard all the time...... Another general rule for amp power is to halve it's peak power to arrive at it's rms/continuous power rating. That means an amp that is rated for 1000 watts peak should be able to push 500 watts pretty easily all day every day. But wait there is more..........

 Lets suppose we have pretty decent idea of what the MAX SPL the speaker can produce? Lets say for giggles that it's 129db calculated ( just a rough round number for a 500 watt rms  speaker ). We take into account that the speaker sensitivity is 96db at 1W/1M. That means to acquire the theoretical 129db SPL we would need an amp capable of pushing 2,000 watts!!!!! That is right at the probable peak power of that speaker. We don't want to be working a speaker that hard. But we probably don't need 129db SPL to acquire our needed SPL level at listening position. Lets say that we only need 105 at 50' from the Speakers. My calculations show that an amp capable of providing 1,200 watts should do the job. But we need some lee way in the amp still. In theory we should be able to get away with an amp that is pushing out roughly 1,600 watts to get the desired SPL at FOH. The amp is still fairly high in power for the speaker and quite likely will only just do the job without destroying the speakers. Here is why.......

Diminishing returns...... Power compression and the cables natural tendency to absorb power from the amp will reduce the actual performance of the speaker. At some point you can throw all the power at the speaker you want and it won't increase in volume, but rather it will melt down and quickly quit. Most of the performance numbers of a speaker are theory ( although some are fairly truthful ). I always assume that the speaker will provide close to the stated performance, being realistic to the true needs. A speaker with a 136db MAX SPL will probably only actually produce around 133db if powered with an amp half it's peak rated power. I don't feel like pushing my amp into clip so another 3db loss in volume from there is expected. So a realistic SPL for a speaker rated at 136db peak is roughly 130db in actuality!!!!! That means I need to be able to acquire the volume I want with that speaker provided I don't go over 130db at 1m from the speaker. In the scenario from the above paragraph the speaker had a calculated max SPL of 129db. It will probably only actually produce 123db at 1m if you run it in the safe zone. In theory you should be able to get about 100db at 50' from the speakers. With an amp pushing 1,600 watts you should never be in worry of running out of headroom and should give you enough power to reach the 105db at FOH if desired. Keep in mind this is working the speaker pretty hard as you will be running the speaker well into it's likely " PROGRAM " power rating which would be right around 1,000 watts.

But you get a little more back when you have more than 1!!!!!!! so all these numbers were factoring only 1 speaker. Another cool thing about audio is that when you double the power you in theory gain 3db in perceived volume! When a speaker is in a free space you loose volume at 6db per doubling of distance ( the classic inverse square law ). Indoors that changes a little since the sound has less space to disperse to. That means you will likely have more level at FOH than the calculated numbers, how much more is the question? So now you have just gained back some of that 6db that you lost from the calculated max SPL. and running the amp in the safe zone should yield you the desired SPL level without fear of running out of headroom!

To put things in perspective for your specific situations and speaker choice, we don't really have enough info. Is the rating peak power, or rms? What do the specs say for Peak SPL? Are they separated or placed together? Do you mean that each speaker is rated at 8ohm's and that the 4 ohm rating is with the two paired together? My guess is that the speakers are really 4ohm boxes and that the number on the back is the peak power at that ohm rating? I would plan on providing that 850 watts as a safe bet. The real question is if you are getting the SPL needed without clipping the amp? If the answer is yes then this amp should be fine. If not then you have some research to do. Hopefully this information helps.

I factor speakers and amps like this. I plan on only running up 75% of the amps available power and the rest is safety cushion. If my speaker is rated for 1,600 watts continuous and 6,400 watts peak, I plan on powering it with 1.5 times the continuous as the usable range and the last .5 as being worst case scenario. So I want to have 2,400 watts going to that speaker at any given point in time with the last 800 watts as spare energy. An amp with a 3200 watt peak running at 75% should produce 2,400 watts. I'm lucky that I have several options around that power range!!!! This is of course the low down and dirty way to conclude an amp to speaker power ratio. It goes much deeper than that. And this is only the accepted low end of determining power needs. If you factor that an amp will only actually produce half it's peak power, you would want an amp that is 4 times the continuous rating of a speaker. This should yield a true ( or closer to it ) 3200 watts of available power.

This link described what I will generalize: http://www.prosoundweb.com/article/print/power_lines_getting_the_proper_flow

The 10db crest factor is a pretty good practice. It basically prescribes that most amps are rated for a sine wave as being the peak power creator. What this means is that an amp rated for 1000 watts will actually only produce about 100 watts of power factoring the 10db crest scenario. So in order to power a speaker with 1,600 watts of power using the 10db crest factor you would need and amp capable of producing power that exceeds the speakers peak power rating! Diminishing returns much....... It's not uncommon for Larger sound companies with deeper pockets to power their speakers with amps that are higher than the peak rating of one speaker. This power is of course often distributed into several speakers which can handle all of that power. There is a higher level of responsibility that comes with that too. You can't fool around with that kind of expense......

Another good link to assessing power needs: http://www.crownaudio.com/how_much_power.htm

[g'bye, Dick Rees]

 Lets suppose we have pretty decent idea of what the MAX SPL the speaker can produce? Lets say for giggles that it's 129db calculated ( just a rough round number for a 500 watt rms  speaker ). We take into account that the speaker sensitivity is 96db at 1W/1M. That means to acquire the theoretical 129db SPL we would need an amp capable of pushing 2,000 watts!!!!! That is right at the probable peak power of that speaker. We don't want to be working a speaker that hard. But we probably don't need 129db SPL to acquire our needed SPL level at listening position. Lets say that we only need 105 at 50' from the Speakers. My calculations show that an amp capable of providing 1,200 watts should do the job. But we need some lee way in the amp still. In theory we should be able to get away with an amp that is pushing out roughly 1,600 watts to get the desired SPL at FOH. The amp is still fairly high in power for the speaker and quite likely will only just do the job without destroying the speakers. Here is why.......

Oh boy are you in for it now....... 

[Ivan Beaver]

Let's look a little bit closer at where the loudspeaker "ratings" come from.

The wattage numbers are those that a manufacturer "assigns" to a loudspeaker.  It comes from a noise source that has a 6dB crest factor-and is basically the rating that the loudspeaker can "handle" without damage.

NOT how good it sounds-just that it is not damaged.  So the USABLE/LISTENABLE wattage may be a bit lower.  And may not be the same for all freq-as the noise has a shaped response in an attempt to simulate "music"-whatever that might be.  Techno is different than jazz which is different than rock etc.  So which one is "right"? Hence the reason for so many different "standards".

The nice thing about standards is that there are so many to choose from.

6dB (for those that still don't know is a power rating of 4 times the average or RMS level.  YES YES I understand that average and RMS are NOT the same thing when talking about the points on a since wave signal-but I am simply trying to keep it "simple" for the discussion.

NOW just to dig a little bit deeper.  The wattage numbers presented ARE NOT the actual wattage being dissapated by the loudspeaker.  They are a result of the VOLTAGE applied and a CALCULATED wattage.

Once the loudspeaker starts to go into power compression (typically 1/4 to 1/2 the rated "continuous or RMS (wrongly termed) wattage". 

Since the impedance increases during power compression-the actual "wattage" is less than what is expected.

The "wattage" ratings are NOT wattage ratings, but rather ideas of the size of amplifier that might can be used.

Yes there are guidlines-and there are exceptions.  It is important to understand all the "little things" that go into proper amp size choice.

As usual-there is not simple answer.

[Ivan Beaver]

Lets suppose we have pretty decent idea of what the MAX SPL the speaker can produce? Lets say for giggles that it's 129db calculated ( just a rough round number for a 500 watt rms  speaker ). We take into account that the speaker sensitivity is 96db at 1W/1M. That means to acquire the theoretical 129db SPL we would need an amp capable of pushing 2,000 watts!!!!! That is right at the probable peak power of that speaker. We don't want to be working a speaker that hard. But we probably don't need 129db SPL to acquire our needed SPL level at listening position. Lets say that we only need 105 at 50' from the Speakers. My calculations show that an amp capable of providing 1,200 watts should do the job. But we need some lee way in the amp still. In theory we should be able to get away with an amp that is pushing out roughly 1,600 watts to get the desired SPL at FOH. The amp is still fairly high in power for the speaker and quite likely will only just do the job without destroying the speakers. Here is why.......

Oh boy are you in for it now.......

So I will 'take the bait"--------------------------------
105dB.  WHAT EXACTLY DOES THAT MEAN??

A level as measured by an SPL meter?  What weighting?  What time response?  If that is A weighted-then the C weighted response could easily be 15-120Db louder than that.  So now we have to do our calculations based on 120dB or more.  That starts to change EVERYTHING.

If that is an "average or slow" response-then we need to add ANOTHER 6-10dB-so now we could be at 130dB needed from the subs.

So the calculations arrived at for 105dB would be VERY LACKING if the actual need SPL is closer to 130dB.

As usual-it depends-and before stating a "simple number" it is VERY IMPORTANT to understand that that simple number actually means.

[Nicolas Poisson]

The most commonly found wattage for speakers now is “AES”, which is determined using pink noise with a 6 dB crest factor. A good thing is that this standard, at least, tends to become standard. Max SPL is not.

Basically a 500W AES speaker powered with a 500W RMS amplifier will be able to be used at its maximum (if required) for any signal with 6dB crest factor. Both speaker and amp reach their limits at the same point.

What if the crest factor is lower? The amplifier will be able to put out more mean power than the speaker can stand. If the amp is ran right below its limit, there is a strong risk of coil burn.

What if the crest factor is higher? Then the amplifier will start clipping before the speaker has reached its maximum capacity. This is not a problem in itself, as long as you do not run into clip. This is not a problem at all if the level produced is sufficient for the application. Indeed, this is a common practice for home stereo. However if you need more level, the amp cannot deliver while the speaker could stand it.

If you want the speaker and the amp to match, i.e. to reach their limit at the same point, you need to know the crest factor.
For typical live sound the crest factor is around 12dB (it can be much greater if you do not compress anything, up to 20-24dB). Let’s take a 500W AES speaker powered with a 1000W amp:
- the AES power already includes a 6dB crest factor
- the amp can deliver 3dB more wattage, which means 6dB more in level
This means the system is perfectly selected to reproduce the 6+6 = 12dB crests of live music. If you want to match speakers and amp considering an 18dB crest factor, you need four times the speaker AES power.

As a simplified rule, one often consider:
- Dance music, DJ, etc: amp power = speaker AES (low crest factor)
- Live music: amp power = 2x speaker AES (12dB crest factor)

Please note that nothing in this rule tells you that you will have “enough rig for the gig”. It just tells both amp and speaker will reach their limit at the same time, not that this limit is adequate for the application.

[Ivan Beaver]

The most commonly found wattage for speakers now is “AES”, which is determined using pink noise with a 6 dB crest factor. A good thing is that this standard, at least, tends to become standard. Max SPL is not.

Basically a 500W AES speaker powered with a 500W RMS amplifier will be able to be used at its maximum (if required) for any signal with 6dB crest factor. Both speaker and amp reach their limits at the same point.

Just for some clarification.

The AES signal is NOT a normal pink noise signal.  It is shaped pink noise-with both the HF and LF tapered off.

The statements about a 500 watt speaker and a 500 watt amp is NOT correct.  Because of the 6dB crest factor- when the continuous (or misused RMS) is 500 watts, the amp MUST be able to pass 2000 watt peaks-or else the signal would be compressed/clipped so the full swing of the applied signal would not be able to be applied to the loudspeaker.

If you use a 500 watt amp and put 6dB pink noise (shaped or not), the the highest continuous (AES or other) signal would be be 125 watts.

[Ivan Beaver]

- the AES power already includes a 6dB crest factor
- the amp can deliver 3dB more wattage, which means 6dB more in level

Just to address these two points.

To be honest-I have not read up on all the details of the AES test.  HOWEVER-if it already included the 6dB crest factor in it-then how come so many loudspeaker manufacturers who state the AES "wattage" ALSO inlcude a 6dB peak wattage-ABOVE the AES rating?  That would indicate 12dB above the continuous rating.

I am pretty sure the AES rating is arrived at by using the "average" voltage applied-squared-then divided by the rated impedance of the loudspeaker.  This would put the peaks 6dB higher than the continuous rating.

How do you figure that an amp can deliver 3dB more in wattage than its rating?  Yes some amps can provide a little bit more on peaks-but 3dB?  That is a whole bunch of extra "wattage" that the marketing department does not know about.  I bet they would find a way to put that into the spec sheets if that was the case.

Do you have a specific example of an amp that can do this?  Actually deliver 3dB more on peaks than its RATED output.  I am not aware of any-but I have don't know every amp out there.

I am really confused by how a 3dB increase in wattage is a 6dB in level.  Would you care to explain that a little bit more?  I have never heard of this.  A dB difference is a dB difference-whether it is in wattage-or voltage or VU units.  How they are arrived at is different-but the result is the same.

Doubling wattage gives 3dB increase.  Doubling voltage gives a 6dB increase.

[ProSoundWeb Community]