2/05, updated 11/05 - Jens Moller
General Thoughts about
Power ratings for Pro-Audio
Everyone builds thier own personal systems as it fits best for them, this is why there is no 'general' system that
will work for everyone. The various product vendors cover as many needs as possible and as a result, things can get
expensive fast. This write up is very general, and my not apply to your needs. I highly suggest that you ask many
other people's opionions and also try to see what is actually in use when you are at a live performance.
The concept of 'Headroom' is a topic that you need to be aware of. If you
drive a power amp beyond its capabilities, the
power supply will not be able to provide the level of current needed to drive the speaker system. When
that occurs, very bad things can happen to your speaker systems - if this power problem resorts to square
wave distortion, it will often damage voice coil based speakers (if you are burning out tweeters, and its not because
of feedback problems, this may be a contributer to the cause). There is a characteristic 'Splat' sound that
is definitely not part of your sound, but a good indicator of a power amp not being able to supply enough
current to your speakers - typically, deep bass is a cause of this. What your power amp is telling you is that you
are driving it beyond its capabilities, or your AC power source is having brown-outs (ie. the voltage to the
power amp is insufficient - thin power cables or low current Power circuits are problems in this area).
If you have a power amp that is capable of at least 2
times the power needed for your performance, you have gained a large percentage of protection from this
condition. This extra power is called headroom (A lot of people build systems in different ways to provide for it).
Figuring out the right power rating needed really takes more than just buying larger power amps,
in many cases its a good idea to visit venues that are similar to the type of size and music that you
perform at, and have a look at what is being used. Your choice of speaker systems and style of music will
determine the limitations that your system will have. Most all Musical Instrument speakers can handle 2 times
their rated RMS capabilities, however, not all speaker systems will handle power above their RMS ratings as
well as others; always refer to any documentation that you have for your gear and utilize your components within
their published specifications. As a general rule, PA Clipping = Damage to your speakers.
Headroom is very different than the desired coloration/distortion that you might want from a Guitar Amp.
You are trying to figure out what speakers you need for your amplifier, how do you match things up?
Lets pick a hypothetical case.
If your amplifier outputs 100 watts RMS, then a 100 watt (or higher wattage) speaker should be used.
Musical Instrument Speakers are rated for watts RMS - that means that they can handle peaks
of 2 times their RMS rating - this matches the characteristics of the amplifier.
Note: Car Audio Speakers have power ratings that often make no sense,
so looking at the information published about them often causes confusion. For Pro-Audio use, never use
anything other than High SPL Musical Instrument Speakers.
Alternately, you can use 2 50 watt speakers (or 4 25 watt speakers) to service a 100 watt amplifier.
Match the highest power impedance that the Power Amplifier supports (This will be lowest impedance), to get
the most power (highest volume levels).
You can always use higher powered speakers, but you should never use a combination (1 or more
speakers) that cannot handle the full RMS rating of your amplifier.
If you expect a lot of Bass, you should probably over rate your Woofers by at least 50% - in the case of the
100 watt power amp, that would suggest a 150 watt rating for the speakers. This is because Bass is where the power
is mostly used. The voice coils will get much hotter with a lot of Bass, and you want the speakers to be able to
handle the power.
I play guitar, what should I buy?
For Guitar, good sound depends a lot on the amplifier and the speaker choice. Some of
the 'Marshall' guitar sound is a result of using Celestion speakers (very high SPL -
100 db to 104 dB SPL). The front end preamp in a Guitar amplifier needs to match the
guitar pickups closely - the pre-amp stage on a good quality guitar amplifier is designed
for this purpose.
The 'tube' amplifier sound is partially caused by the pre-amp stage and largely affected by the
output transformer. The signal is compressed in the output transformer - it colors the sound
as a result of magnetic saturation of the iron core. Many companies attempt to simulate that
compression in solid state Guitar amplifiers, with varying degrees of success.
Usually, any good quality Musical Instrument Speaker will work with any good quality Guitar
amplifier. A good quality Guitar amplifier will color the sound (add distortion). Your Guitars
pickups greatly affect the tonality as well - Single coil versus Humbucking pickups; each
has their place (and people who prefer one over the other).
I am building a PA system, what should I buy?
For PA systems, the mixer stage is very different than that of a guitar amplifier (Guitars
sound very 'thin' when plugged into a PA Mixer directly). A good quality PA amplifier should
not color the sound very much.
The 70/30 rule
Your speaker system needs to cover more range than a Guitar or Bass Guitar, so it needs to be at
least a 2 way system (A Woofer and a Horn, or a Woofer and a Tweeter). A good rule of thumb is to
consider is that (on average) signals below 500 Hz will require 70% of the power and all signals
above 500 Hz will require 30% of the power. Sub-Woofers are stand alone and not considered in
the 70/30 rule.
If your PA system power amp is 100 watts RMS, then its peak output is actually 200 watts. While
the 70/30 rule implies that you could get away with 70 watt woofers, you should match the Woofer to
the PAs output if there is a potential for a lot of Bass in your signal (if you have a Keyboard
player plugged into the PA system, for example), to be on the safe side. If your PA is for Vocals only
(such as a lecture hall, auction house or outdoor location), then your signal does not fit the 70/30
rule; in these cases the ratio is closer to 50/50.
Why do your need these ratios? Because when you buy the horn or tweeter you will find that they cannot
handle as much power as the Woofer can - while it may seem that you should try to match these at
the same power level, the reality is that you won't. Following the 70/30 rule, in the case of a 100 watt
power amp, the signals above 500 Hz will only see 30% of the power - or 30 watts. The higher that you go
on the frequency scale, the less power the speakers will see. At 5000 Hz, the tweeters will only see
approximately 10% of the total power. This is why you see Midrange horns and tweeters at lower
power ratings than Woofers.
If you use Horns and tweeters, they need to have their frequencies limited by a cross-over network.
For passive cross-over network information see:
Be careful about feedback - these high frequency squeals can force 100% of the power that your power amp
has into a tweeter that can only handle 10% of the power - that tends to burn out the voice coils very
quickly. Look into getting a Feedback Eliminator to protect your tweeters (and your ears).
The frequency response of Musical Instrument Speakers is not flat - this is the price you pay
for high SPL. Get over it and realize that the room acoustics are usually horrible where you will
be running the PA system and you will need to EQ for it no matter what the capabilities of the
speaker system is - if anything, you'll find the coloration of the PA speakers a benefit for a live
band (and tolerable for playing back pre-recorded music).
These are a blend of Guitar amp and PA system speaker choices. Keyboards have a much higher range than
Guitars do, so they need to be at least a 2 way system. These may benefit from Sub-Woofers.
Always use Musical Instrument Speakers. Keep in mind that these are for the bands reference, and not for the
audience to hear. Sometimes these are turned up very loud - match the power as you would any other load -
to what the power amp is capable of. If you have 4 Floor Monitors and the power amp for these is 200 watts, then the
woofers should be capable of 50 watts each. The 70/30 rule applies for the tweeter.
If you need a Sub-Woofer (and not everyone does), match the Power amp wattage and add least 50% for the
Woofers wattage rating (higher is better). If you have a 500 watt RMS power amp for the 1 Sub, you should
consider a 750 watt RMS speaker. This gets expensive fast. If you have 2 Subs on that 500 watt power amp
(250 watts for each Sub), they should be able to handle 375 Watts each (250 + 125 watts). Be aware that lots of bass in large rooms = lots of power. Lots of spare headroom in your power amp will make a big difference.
Pro-Audio versus other Audio systems
Pro-Audio systems are creating music, not reproducing recorded music (very different design
goals). In all cases, for Pro-Audio you need to use high SPL speakers - see:
Speaker Design and how it affects your choices.
You never have the option of optimal placement of speakers when using a portable PA system, Guitar amp or
Keyboard amp, so you will be compensating for room issues thru equalization. When you choose speakers, you are
choosing how they color the sound - tone often depends on a good choice in this area.
Some good sources of Pro-Audio (Musical Instrument) Speakers are:
See what your local music shop has available, they often can assist in determining a match for your needs.
Support your local music shop!
NOTE: We do not work on Home or Car Audio. We work only with Pro-Audio applications. We cannot
help you with Home or Car Audio questions.
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