The amount of watts your PA has probably doesn't matter nearly as much as you think it does.
We often get questions from our customers asking "Should I get the higher wattage amp to power my loudspeakers?" or "This studio monitor only has 40 watts, is that going to be enough?". Many people assume that the higher wattage system is going to sound better or give them significantly more volume. Given this common misconception, I thought I'd write a little about how the output from your power amplifier relates to the performance of your loudspeaker or studio monitor.
Doubling the wattage of your amp will not double the perceived volume of your sound system!
When I was a teenager browsing my local music shop for a PA, I remember looking at the 1000w amplifiers and thinking, "man, if I could just afford to get that instead of this 500w amp, I could really blow em away!" What I didn't know then was that for most common applications, that extra power was going to make a very minimal difference in how loud I could crank it. Essentially, doubling the power output from the amplifier increases the sound pressure level (SPL) output of the speaker by 3dB. For reference, a 3dB increase in output is like turning up your car stereo by a 2-4 clicks, or something said to be 'slightly louder'; not exactly a huge difference.
Without going too far into the details, power (wattage) doesn't translate well to our ears and perceived loudness, which is specifically why the decibel (dB) was created. The dB is a ratio that works on a logarithmic scale. It translates the world of electricity and acoustics into units that we use to better represent differences in volume. While the human ear can hear something as quiet as a 0.000001 pascal (sound pressure units, abbreviated as Pa) to perceiving pain at about 20 pascals, this huge set of numbers is not very practical for simple communication to each other. Rather than saying "Turn up the PA another 35,000,000 micro pascals" its a lot easier to say turn it up by 6dB. Not to mention if we wanted to turn it up by 6dB again, this time we'd have to say "Now turn it up 70,000,000 micro pascals." Put another way, as the power output increases exponentially, our ears hear a steady linear increase in volume!
So how does this relate to your the watts of your PA? Well, since watts increase exponentially for a linear increase in perceived volume, doubling the power output makes only a small notch up the scale our ear hears. So for most consumer level PA systems (i.e. any system that is not installed in a large venue or reinforcing for more than 400 people), the extra cost in purchasing amplifiers with 150%-200% more power is rarely worth it.
So when do the watts make a difference?
If you turn your amplifier up to just before it starts clipping, and you still don't have enough volume, then at that point yes, you might need an amplifier with higher wattage. But in this scenario, as we mention above, you're not going to get much from going from your 500 watt amp to a 1000 watt amp, you'll likely need to upgrade to at least 2000 watts before hearing much of a difference (providing your loudspeakers can handle this increase in wattage). In fact for a perceived doubling in volume, you'll need 10 times the power (10dB increase), or 5000w! At this point, you'd likely be better off just buying speakers with a better sensitivity rating. Loudspeaker sensitivity ratings tell you how many dB-SPL you can get out of a speaker using 1 watt measured from 1 meter away. These sensitivity ratings can often swing a good 3-6 dB between different models and manufacturers. A loudspeaker that has a sensitivity of 96dB 1watt/1meter, is going to give you 6 more dB of output than one with a sensitivity rating of 90dB 1watt/1meter, given both are receiving the same wattage. These speakers may be in a very similar price range, and could buy you that 6dB increase in volume for much cheaper than upgrading your amp from 500w to 2000w.
On a similar note, if you aren't getting enough volume out of your PA due to not enough wattage, and you try to force it out of your amp by increasing the amp's input sensitivity (volume knob), you may clip the input, causing a square wave to pass on the amp's output. These square waves can quickly destroy a loudspeaker's compression driver or "tweeter". Keep in mind that in this situation, it is not that the amp had too little power that blew your speaker, its that you turned the input sensitivity up until it clipped the signal it was receiving, thus sending out a square wave and frying the tweeter. The only reason this is less likely to happen with an amp with more power is that you are less likely to turn up the input sensitivity that high because its already putting out more volume. I may write a post soon clarifying this misconception of how under powering your speakers can fry them.
What about watts and how it relates to my studio Monitors?
When it comes to active (powered) studio monitors, checking out how many watts it's on board amplifier has is in a large part useless. Given the two topics we talked about above (3dB increase in volume when doubling wattage, and speaker sensitivity ratings) you really can't gather much about the monitor's performance from a watts specification. If fact, here at our office there is a pair of monitors rated at 140w that get significantly louder than a pair we have rated at 180w. I also know that I like the sound of that 180w monitor much more than one of its competitors that is rated at 225w. If you are concerned with how loud the monitor is capable of getting, take a look at its maximum spl rating, or SPL at 1 meter rating. These specifications will provide you much more insight into how loud the monitor will get than will the wattage spec.
Please feel free to leave a comment below if you have any other questions or thoughts related to the topic.