Understanding power ratings and impedance (Ohms) in pa / sound systems.

This article aims to provide the complete beginner with a basic understanding of loudspeakers & amplification equipment. In addition to power ratings and how ohms (impedance) affect this, the reader will attain a basic grasp of amp bridging and how and why to allow “head room”. The main objective is to provide the layman with knowledge of how to match up speakers with amps in any pa / sound equipment set up.

A loudspeaker is a fairly simple piece of equipment. It’s a wooden box with a piece of hardware known as a “driver” screwed into it. The driver is made up of two ring magnets one within another, attached to a cone (the familiar round black thing with a bump in the middle). When power is supplied to the outer ring the inner magnet moves back and forth which in turn moves the cone creating sound waves that humans can hear. The faster the cone moves back and forth the higher pitched the sound that is heard.

All loudspeaker drivers have power ratings which are given in watts. This can be confusing because the wattage can be quoted in three different ways:

RMS continuous power, program power and peak power.

Peak power is the maximum a loudspeaker can handle for a very short time (feedback spikes etc.)

Program power is a relatively safe range but you wouldn’t want to run into this for too long.

RMS continuous or AES is the actual power the speaker driver can supposedly handle continuously without overheating.

Program Power is double the RMS and Peak is double the program power.

Whenever you are sizing up a pa speaker always look for the RMS or AES continuous power rating.

A high power pa speaker might be rated like this:

450 Watts RMS, 900 Watts Program, 1800 Watts Peak.

Anyone who knows their stuff and isn’t trying to con you will always quote the RMS or AES rating.

Most pa speakers are rated at either 4 or 8 ohms.

Amplifiers will usually handle loads of 4 ohms or more. Some good quality amps can handle loads of 2 ohms.

Amp manufacturers provide power ratings for their products at 4, 8 and sometimes 2 ohms. The amount of power an amp will supply depends on the speakers “ohms” that you have hooked up to it.

A 4 ohm speaker would get (very approximately) twice the power that an 8 ohm speaker would get from the same amp.

Loudspeakers have an input on the back and an output so that you can link speakers together (sometimes called daisy chaining). If you “daisy chain” two speakers together and connect them up to an amp you effectively halve the load (ohms). From the amps point of view, two 8 ohm speakers linked together is the same load as one 4 ohm speaker.

Don’t worry if you’re already confused.

Let’s look at an example:

The Peavey PV 2600 amp is fairly common. It has two channels A & B. Each channel is rated at 900 watts @ 4 ohms and 550 watts @ 8 ohms.

In this example we are just using channel A.

If we hook up a 4 ohm speaker to channel A on the PV 2600 it would get 900 watts. If we plug in an 8 ohm speaker it would get 550 watts.

Now, if we “daisy chain” two 8ohm speakers together then the amps would share the 900 watts between the two speakers so they would kick out 450 watts each.

Although the power output is the same – this arrangement would actually sound “louder” than one 900 watt 4ohm speaker.

Only a top quality professional 18” Sub driver rated at 4 ohms could comfortably handle 900 watts RMS.

If we were to daisy chain four of the 8 ohm speakers onto channel A of the Peavey PV 2600 then this would be equivalent to a load of 2 ohms. The amp cannot handle this and would very quickly overheat and break down. 

Some stage monitors like the “galaxy hot spot” are rated at 16 ohms. This means that you could “daisy chain” four of them together and connect them to channel A on the PV 2600 because this would be equivalent to a 4 ohm load on the amp. 900 watts divided amongst four speakers equals 225 watts per speaker – this is just about right for galaxy hot spots which are rated at 200 watts RMS.

Touring companies or large scale pa hire companies will choose very high power, good quality amps that can handle loads down to 2 ohms.

As another example the Peavey CS4000 amp can handle a load of 2 ohms. This particular model supplies 2000 watts @ 2 ohms 1350 watts @ 4 ohms and 800 watts @ 8 ohms. This means that if you “daisy chain” four 8 ohm speakers onto a CS4000 you get 2000 watts shared amongst four speakers, 500 watts each.

It’s not usually a good idea to run amps at 2ohms as they get very hot. It’s a bit like buying a new car and thrashing it around town. It won’t last very long. It’s the same with amps – running them cool at 8 ohms is the best way to protect them and make sure they last.

Sub woofers always need the most power and you can buy drivers like the Eminence Kilomax series or the PD 2150 that will happily handle 1000 watts RMS all day long.

To drive these you either need a bad ass amplifier which will cost you big bucks or you can do what is known as bridging. This is where you take an amp like the Peavey PV1500 which is rated at 500 watts @ 4 ohms and 300 watts @ 8 ohms and combine channels A + B. This is done by pushing a switch on the back of the unit. You then, effectively, have four times as much power output. (It’s actually double but you have also halved the impedance (ohms).)

So you only have one channel (A), but it is chucking out 1000 watts into an 8 ohm speaker (amp thinks it’s 4 ohms) and 1500 watts into 4 ohm speaker (amp think it’s 2 ohms). You could plug in one 1000 watt 8 ohm speaker and drive it perfectly or you could “daisy chain” two 750 watt 8 ohm speakers and drive both of them safely. 

Don’t ever hook up a 4 ohm speaker onto a bridged amp. I have never seen a 4 ohm speaker that handles 1500 watts! Don’t ever “daisy chain” two 4 ohm speakers onto a bridged amp as this would be equivalent a load of 1 ohm and the amp would probably melt through the floor!

The 15″ / 12″ or 10″ main driver in your speaker can vibrate back and forth producing most sounds from very low frequencies 35 Hz up to about 2000 Hz or 2 kHz. Hz is simply a measure of how fast the cone is moving back and forth. Humans can hear from about 40 Hz up to about 18 kHz (18,000 Hz).

Full range speakers always have a compression driver or “tweeter” to fill in the gap from 2 kHz – 18 kHz. These are like mini versions of the main driver usually 1″ in diameter. These can produce all the higher pitched sounds you need so that when combined with the bass driver the speaker cabinet is truly “full range”.

Full range speakers provide all the detail in sound reproduction so it’s best that they are kept “comfortable”. They need to be running cool but with a bit more power than they need. This is called “head room”. It’s like cruising in a Ferrari at 70 mph, the engine is “purring” with plenty of power left.  You would use an amp that is approximately 20 % more powerful than the RMS rating on the speaker but it’s very important NOT to crank the mixer to the max. 

Don’t go “daisy chaining” all your full range cabs so that the amp runs at 2 ohms. For the best sound quality you need to run the sound system cool. So you want the amps at 8 ohms (usually one speaker on each channel), with 20 % more amp power than the RMS rating on the loudspeakers.

The subs provide all the “thud” in a big sound system and need lots of power to move the big 18″ or 15″ drivers. It’s ok to run these at 2 ohms in fact you may have to get enough power out of your amps.

Hopefully you now have a basic appreciation of how sound amplification works and you understand which speakers go with which amps. You also know how to safely bridge your amplifier and you understand about allowing head room in a full range system.

Ben Igoe (16/05/07)

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