If there’s one PA-related question we get asked more than any other it’s “How many Watts will I need?”. I will start by suggesting that comparing sound-systems based entirely on the manufacturers advertised power rating isn’t a reliable measure. Let’s start by looking at the various ways in which wattage is rated:
Here we use speaker ratings as an example but amplifiers are rated in the same way.
RMS : This is the ‘true’ power rating of a speaker. Sometimes referred to as a ‘continuous’ rating, the RMS is effectively the level at which a speaker can operate continuously for an indefinite period of time. If we are to take the popular Peavey Pro 15 (pictured left) as an example, these speakers are rated at 300w RMS. This means you could push 300w of power through the speaker all night long and it would play comfortably with no distortion and no risk of damaging the speaker.
Peak : This is the other most common rating you will see advertised. Peak figures are often used in marketing as it will give you the highest rating but, beware, peak figures are often exaggerated and are not a good measure of a speakers’ true performance. In essence a peak figure is the amount of power you can put through a speaker over a short period of time without melting the voice coil or, in simple terms, making it go ‘pop’. If we refer back to our Peavey Pro 15, this has a peak rating of 600w. In essence, this means that you could put 600w of power through it and, in the very short term, not cause any damage. The speaker will never produce 600w of output and anything over the RMS rating will generally cause distortion and will eventually cause damage to the driver.
Program : Program power is an old term that isn’t really relevant today (although some manufactures still insist on using it). Most manufacturers simply quote twice the RMS but, personally, I would ignore this rating altogether. The only practical use for this measure can be in amplifier selection. For example, a speaker with 300W RMS rating and 600W programme power (2x300W) might use an amplifier with 600W output.
Others : Certain manufacturers use their own ratings for power output. These ratings can generally be disregarded as they cannot be used to effectively compare two systems. These ratings (depending on manufacturer) range from the realistic to ‘pie in the sky’ headline figures which, despite being a dream for marketing people, can be generally disregarded. One manufacturer notoriously rated their amplifier range in PMPO (which stands for ‘Peak Momentary Power Output’ or something similar) which meant that a 250w amp could suddenly be rated at 3kW on the basis that the amp could handle 3000w of power for a fraction of a second without bursting into flames.
So…that’s power ratings explained but how much do you need? Well, unfortunately, this isn’t simple either (typical). It all depends on what you’re are using your PA system for. Take a crowd of 150 people for example (in my years of DJ’ing this is a good yard-stick). If you are playing background music during the meal at a wedding then a pair of active 12” speakers such as the best-selling Mackie SRM450’s would more than adequate. Come the evening function, the DJ is playing a variety of music genres the SRM450’s (producing 900w between them) may be sufficient but, to be sure, you may want to add an active sub (eg. The best selling QT18SA) which adds 500w of bass (bass is easily lost in large crowds) to ensure decent performance when the sound is cranked up.
1400w may sound a lot BUT in a different scenario such as a nightclub or outside event where much more bass is required (due to the nature of the music or the sound that is ‘lost’) you could easily add another 1000-2000w of bass reinforcement to get the desired effect with the same amount of people.
As a rule there are two things which will affect the amount of power you require, people and space. People soak up a huge amount of sound so 200 people will always require a bigger system than 100 people. If you’ve ever had a house party you will have noticed that your home HIFI (which seems really loud when you’re listening to your tunes on your own) can suddenly seem underpowered when confronted with a room full of guests. Similarly, sound (particularly bass) is easily ‘lost in space’ (nothing to do with the camp 70’s TV show). Large venues WILL require bigger systems and outdoor functions require huge amounts of bass reinforcement to get the same effect. Always consider what type of gigs you will be playing and be realistic about the amount of guests you will likely to have at your functions when choosing a sound system.
Naturally this is only a brief guide and they are many other factors to consider when building your PA system. If you need any advice on the best system for you then please don’t hesitate to contact our knowledgable sales team on 01206 855010 who will be only too happy to assist with your enquiry. Alternatively check out our PA packages section on the website where you will find a number of pre-made kits put together by our resident audio experts.