Speaker Power/ Impedance Question

[Ed Stanley]

Hypothetical : Two identical 8 ohm cabs are being run in stereo and getting 300 watts each. If this set up was changed to two of the exact same cabs on each side, now a 4 ohm load, and getting 500 watts each side would there be a significant increase in volume? A techie aquaintance of mine said that while you would be moving more air with the latter arrangement that the actual volume would only increase slightly due to the lower impedance and he had a compelling techie argument to support his contention but my old school sensibilities still tell me that more speakers + more power should equal more volume. I've heard this sort of theory bandied about before but am not really as up to speed on the subject as I should be. Any of you electronics savvy types care to confirm or destroy? Thanks.

[JeffLearman]

Yes.

 

Let's simplify the math a bit and assume 600W into 4 ohms. (The difference between 500 and 600 watts is less than 1dB.)

Then Let's calculate the SPL, as far as I can go.

 

First, double the power, that adds 3dB.

Second, double the number of cabinets. That adds 6dB (I believe, and the bigger the cabs are, the lower the amount you actually get, due to neighbor phase cancellation.)

 

So, you get 9dB, which is almost twice as loud.

 

To get twice as loud simply by adding power, you'd need to use 10 times more watts. So, doubling your speakers is almost as good as going with 3000W into your existing cabs (but only if they could handle it, which is unlikely).

 

[ITGITC]

LearJeff,

 

I was hoping that you would see this post and reply.

 

You are da MAN.

 

Tom

 

PS - Sorry I couldn't make the blues jam the other night... problems with significant other + empty nest/separation anxiety.

 

 

 

[MikeT156]

Ed:

 

There has been a number of discussions on Harmony-Central Live Sound Forum about this subject. There are several resident audio engineer/gurus on that forum that really know their stuff. Several folks have been servicing PA gear for decades.

 

Briefly, if you want more volume, you need to add more speakers AND more amplification. Its not an either/or situation. To me, a PA sounds better when you put more watts of power into the continuous sine wave rating of the speakers. Somewhere between continuous sine wave (sometimes referred to as RMS) and 'Program', which is typically twice the rating as continuous sine wave. Example, I use a Yamaha P7000S, 700 watts at 8 ohms into a newer set of Peavey SP2's that are rated 500 watts continuous and 1000 watts program. This amp has considerable more 'punch' than the amp I replaced it with. It is more than double the watts of power rating. Is it twice as loud? NO. But it is a mark improvement over what I was using before.

 

I don't think its a great idea to use the same power amp to push a 4 ohm load instead of a 8 ohm load. If its a large increase in volume you need, then duplicate what you already have (amp AND speakers) OR biamp what you have. That will allow you to take your largest power amp to push the lows, get an electronic cross-over and a small power amp to push the horn(s). That would give you an opportunity to push more power through your existing speakers and get more 'punch'. If you over-do the power or push the speakers too hard.....poof. So be careful and get a qualified person to help you select the right components to match up with your existing speakers and amp.

 

Mike T.

[Ed Stanley]

Thank you all for your replies. Mike : Why would using the same amp be unadvisable? Typically their respective 4 and 8 ohm ratings are consistent with what the cabs require. In the context I am considering it would be 8 ohm cabs rated at 175 watts each but which are presently getting 325 each (with the builders blessing). For two cabs the builder recommends 400 to 500 and the same amp delivers 550 at 4 ohms. Sounds reasonable, no?

[mate stubb]

I don't have any problem with doubling the power each channel of a power amp delivers by halving the impedance.

 

If you were going from a 4 ohm load per side to a 2 ohm load though, you would have to check that the amp could handle it. All modern amps handle 4 ohm loads per channel just fine.

 

Sven is right. Doubling the power to a given set of speakers gives 3 db more, doubling speakers adds 6 db.

 

However, now your amp is delivering close to 1000 watts, which may cause it to pull enough current to blow a breaker all by itself!

 

If you are using a QSC PLX 1804, be aware that the specs say it can pull 32 AMPS at full power with 4 ohm loads, so be careful turning it all the way up!

[J. Dan]

I always thought that the reason they said doubling the speakers adds 6dB was because you increase sensitivity by 3dB and half the impedance which doubles power, the other 3dB. If that's the case, then adding the other 3dB for the extra power would be adding 3dB too much.

 

Also, one thing I do know, to realize the advantage of adding speakers they have to be right next to each other to avoid comb filtering effects. This is easier to achieve with low frequency drivers than high because the wavelength is so much longer.

 

In terms of running the same amp, most amps these days are fairly efficient and can handle the lower impedance. But keep in mind that it's the current that kills an amp due to overheating. Low impedance, high current - make sure it's not running hot and you'll be fine.

[JeffLearman]

OOPS, I WAS WRONG.

 

Someone whose opinion I respect says you only get a 3dB increase.

 

Given that you're not changing the speaker efficiency by adding more of them, it's all about the power, which you're only doubling. And doubling power only adds 3dB.

 

Still, 3dB matters. It's noticeable. I have a selector on my powered mixer (nice Yamaha EMX5000 with decent power, not like the typical box powered mixers) that can cut the power in half. When I do that, I get enough volume in general, but I lose the headroom my piano sound needs, and at loud blues jam leves it sounds like crap. (110 dB SPL on stage.)

 

BTW, you do get a good bass boost by doubling the speakers, if you put 'em close enough together. Pretty much extends you down an octave, under ideal circumstances.

 

The volume boost for doubling the area is something speaker engineers take into account, along with a buncha other mathy stuff, and it's meaningful, but I think not in this context.

 

One of these days I'll try it with an SPL meter. I'd do it now, but my meter wandered off so I need to get a new one.

 

[JeffLearman]

Here's Mac's answer (after he gave me a clue and made me figure it out for myself, dammit):

 

 

Trying to get something for nothing?

 

Never works out, man.

 

The voice coil and the cone can only provide LOSSES due to heat, no matter how that heat is derived.

 

When dealing with power, "What goes in is what must come out" -- minus frictional losses, which are always present.

 

If you increase the power available by 3dB, then that's all you've got to work with, period. Speaker cone area is equivalent to the same physics you would encounter with a hydraulic piston in which you increased its surface area by a factor of two. Understand that the driven piston is not the source of power. That comes from elsewhere, hydraulic pump in most cases. Now substute electromotive force for the hydraulic juice and you may be able to draw a visualisation analog.

 

You ain't gettin' somethin' fer nuthin'.

 

You may not know Mac, but he's a bitchin' Hammond player, in addition to trumpet & guitar, and he used to make jingles for a living. In addition to a lot of other stuff, including gear repair. He knows his E=IR stuff pretty well. A ham radio man too. One of the lynchpins at "http://audiominds.com", which is a good site for friendly help about home recording and stuff like this.

[JeffLearman]

So, here's what it boils down to:

 

Do you want to get 3 more dB by

 

1) doubling your power (of course, you'd need new speakers if that's much over their rated capacity)

 

2) doubling your speakers (more to carry)

 

To get twice as loud (10 dB louder), you need TEN TIMES the watts. Not to mention speakers that can handle the load.

 

Them's the options. Keep in mind that it rarely hurts to have amp power that's a nice margin HIGHER than the speaker's rated capacity, as long as you don't crank that baby and blast full steam and solid. The speaker's capacity is limited by its ability to cool the voice coils (among other things, but that's usually the main one in the long haul).

 

So, if you're pushing your power only in the peaks (like piano does), between them you're giving the coils time to cool back off. This is why many speakers have two ratings; a peak and a steady.

 

Another thing is, if your amp is clipping, you're really killing your speakers. You see, those ratings I mentioned above assume a normal music program. If you're sending clipped stuff (or raw square or triangle waveforms from a synth!), you're sending stuff with a frequency mix that's far different from what they assumed when they made the specs, and you can sail into trouble.

 

So, it helps to have plenty of headroom -- you're LESS likely to damage your speakers. Just don't be push it by cranking the amp and playing sustained sounds at full volume.

 

[Mr. Nightime]

Sending a clipped signal or a raw square wave can be detrimental to your speakers because of heat. When you clip the signal, there is a moment of DC to the voicecoil, so the current is going through with no voice coil movement, allowing the voicecoil to heat up. After a while the coil can get hot enough to break down, and you've got a blown speaker on your hands.

[J. Dan]

Here's the deal with distortion:

 

Power is area under the voltage curve. So if you think of a sine wave, RMS power ends up being [(peak Voltage)/Sqrt(3)]*current. If you think of a square wave, the peak voltage and RMS voltage are the same. So you don't have that Sqrt(3) factor in there - just peak voltage times current. Much more actual power to the speaker. A severely clipped wave form approaches a square wave, so RMS power approaches peak and you run more continuous power to the speaker, which equals more heat. Heat is what kills a speaker, the voice coil melts. Not often any more that you see a speaker fail due to mechanical damage due to over excursion.

[Dave Horne]

... and that's why it's always better to have amplifiers with much more output than actually needed. An underpowered amp will clip and destroy voice coils. An overpowered amp will more than likely damage your ears before the it damages the voice coils.

 

 

 

 

[MikeT156]

Quote by Ed Stanley:

 

"Thank you all for your replies. Mike : Why would using the same amp be unadvisable? Typically their respective 4 and 8 ohm ratings are consistent with what the cabs require. In the context I am considering it would be 8 ohm cabs rated at 175 watts each but which are presently getting 325 each (with the builders blessing). For two cabs the builder recommends 400 to 500 and the same amp delivers 550 at 4 ohms. Sounds reasonable, no?"

 

Ed, typically, the main reason I don't like using an existing amp that is currently sounding great running at 8 ohms and changing it to run at 4 ohms is that the additional load on the amplifier will make it run hotter, have a greater chance of sending a clip to the speakers, and shorten the life of the amplifier. Other posters have mentioned the current draw running at 4 ohms on the QSC amp mentioned is a lot of juice and you might not get that out of a typical circuit, in a house or a club.

 

Many amps can run at 4 ohms safely, but probably more amp manufacturers publish unreliable ratings based on a dubious rating system at best than those that rate their amps honestly. If you are using a QSC amp, then I would say the ratings they provide will be conservative, and accurate.

 

For me, I like the sound I am getting out of my small PA system running at 8 ohms, and the amp is not even "breathing hard" at this point. The setup is simple, and it sounds great. Why mess with it? When you get into biamping a set of speakers, you have to know what you are doing and you are depending on the specs of the mfg to be accurate enough so you don't overload either your amp or speakers or both. Setting up an electronic crossover is not rocket science, but you have to be sure that the amps you use to drive the speakers and the cross over point(s) are correct.

 

If you choose to just add speakers (no bi-amp arrangement) and just drive your current amp harder, your amp has to have enough output transistors to drive the load safely without overheating or clipping. If you do that, I would like to know if you feel your speakers still have the "punch" at 4 ohms that they do at 8 ohms. I've gotten 30 years out of my old amp running it at 8 ohms, and the only reason I took it out of service is because newer speaker systems can handle more watts than it can safely deliver, and some newer amps have a built in compressor to help protect against overload, auto shut down, and damping filters to protect your speakers from transient surges that can "pop" your speakers when you power on the amp. That little pop is not good for speakers. My old amp didn't have any of those features. Whatsmore, my new amp delivers 700 watts at 8 ohms, my old amp was rated at 220 watts at 8 ohms, not a good match for my new speakers.

 

This is a great topic, and I hope that the information everyone provided has been helpful.

 

Cheers,

 

 

Mike T.

 

[mikeNZ1]

There is a lot of misinformation about speaker and amp power floating around. If you want some good expert advice, download Part Two for the EV bible from here:

 

http://www.electro-voice.com/tech-library.php?pt=general

 

Their conservative recommendation for a non-expert is the amp at 0.5-0.7 times the power rating of the speaker, and keeping a wary ear out for distortion and clipping. At this rating the spaker will never burnout from normal signals, and even if you do clip or distort it will take a while before damage will occur.

 

For a 'skilled expert' (read seriously good sound engineer) they recommoned 2-4 times speaker power. EV says "this arrangement is only for experts". How many of us are 'skilled experts' and ride the mixer controls all night?

 

If you have speaker protection and a properly setup speaker management system then you can run your system harder and longer, and use larger amps for your speakers. These days, a speaker mangement system such as the dBx driverack series is a very good investment, IMO far better than buying an oversized amp. Powered speakers have a lot of built in protection, and are usually harder to destroy through incorrect use.

 

Speakers blow up because they are setup or operated incorrectly, or due to avoidable accidents, such as dropping mics or equipment start-up transients. The way to avoid this is to learn the skills to properly setup and run the system.

 

Michael

[JeffLearman]

Good one, Mike! Anyone interested in live sound should read this stuff.

[mikeNZ1]

They have loads of articles in their 'PA Bible' series, with many 'must reads'.

 

Rane also has some excellent material. I highly recommend RaneNote 135 "Setting Sound System Level Controls". If you get this right, you are well on your way to getting great sound, and will be unlikely to cause clipping at any point in the signal chain.

 

http://www.rane.com/note135.html

 

Michael

[JeffLearman]

However, interesting that in the intro (written recently, unlike the rest) it says that with today's systems, overloading due to clipping is not so much a problem.

 

I'm happy to hear that, but I'll pretend I didn't!

 

[MikeT156]

If you KEEP THE VOLUME at reasonable levels, you probably won't damage your equipment OR your ears. The audience will appreciate that too.

 

Mike, thanks for the links, great stuff!

 

Mike T.

[JeffLearman]

No kidding, Mike. As I've told y'all, I play a lot of blues jams. Every now and then, there's some gear problem like with one of the guitar amps, and while they're fussing with it, I'll start a quiet instrumental tune for the rest of us to do while they get it sorted out. Since it's not "really playing", the guys generally honor the quiet mode and play along.

 

It's often the best sounding tune of the night, and the audience always loves it.

 

Now if only I could get 'em to play like that for "really playing"!

[Byrdman]

OOPS, I WAS WRONG.

 

Someone whose opinion I respect says you only get a 3dB increase.

 

 

 

Actually you were not entirely wrong.

 

At low frequencies if you put the two speakers close together they couple so each is working into a greater resistance in the air. Hence the efficiency of transferring energy into the air goes up. Its the I^2R rule but in air rather than circuits. You get the same effect by having the speakers close to the floor.

 

So by using two speakers close together you get 6db but only at low frequencies - up to a few hundred hertz for 8" speakers.