Jim's questions in Live Sound,"How many WATTs is this sound system, mate?"

Jim Parsons, is the award winning dBs Music Programme Leader for the Live Sound degree course. He is one of the most experienced live sound engineers and academic audio professionals in the country. With decades of knowledge and experience, we've started a little feature where Jim discusses some of the frequently asked question in Live Sound, especially those interesting questions that live sound engineers get asked while doing their job. Over to Jim...

Jim+parsons+dBs+Music+Live+Sound+EngineerHow many WATTs is this sound system, mate?

"A deceptively simple question can sometimes be the most difficult to answer without resorting to pages of theory or complex mathematics. This is the case with the topic of loudness as framed in the comment above and the subtext of related questions such as  'why do you need all this equipment for our wedding party / political rally / album launch?;, 'if I stand next to this speaker will I go deaf?' or even 'why am I paying for all this kit, previously we had a lot smaller system?'

"A quick internet search will find erroneous rules of thumb like 'you need between 10 and 50 watts per person depending on music genre', however most audio professionals would say, 'it depends'. Unsurprisingly people expect a longer and clearer answer so here goes…


Some science...

"Sound is a change in pressure transmitted as a wave which is converted within the ear to an electrical signal which is sent to the brain. So, there is a sound source (the PA speaker) which transmits and your ear which receives. However, the human ear does not receive all sounds with equal sensitivity, so some sounds or frequencies appear to be louder than others. Furthermore, the relationship between sound pressure change and perceived loudness is logarithmic not linear. This is expressed as a decibel (dB) but more of this later. The second point is that sound obeys something called the Inverse Square Law so the further you are from a sound source the quieter it is, a lot quieter.

"What this all means, is that the person supplying a PA system has a lot of factors to take into account depending on the event type (speeches / band / DJ), the size of the venue, whether it is indoors or outside and how many people are likely to attend.

"Now we are armed with some basic acoustics knowledge let’s go back to decibels and look at some numbers....

Quiet bedroom at night = 25 - 35dBA
Café / office during day = 60 – 75dBA
Unamplified drumkit at 20m distance = 80 – 90dBA
UK festival (typical) at mix position = 98dBA
Screaming fans at an arena gig = 110 – 112dBA
Onset of bleeding / ear damage = 130 – 135dBA

"You will note that I have written dBA which is short for A weighted decibels which adjusts for the response of the human ear to sound (as discussed previously).

A practical example...

"Anyway, so now we are a bit more confident we can look at a (theoretical) speaker specification sheet and we see for example that it will produce 97dB with 1 watt of amplifier power measured at 1 metre distance and is capable of 128dB peak output when powered by 1,200 watts of amplification. So, if we want to make it louder we can add another loudspeaker and as long as we power it with a second 1,200 watts (or 1.2 Kw as normally written) amplifier our new 2.4 Kw system should produce 2 x 128 = 256dB peak power.
WRONG, remember we said sound was logarithmic, if we place the speakers to the left and right of the stage as is typical we will only gain 3dB giving 131dB max.

"However, another factor is that the audience do not all stand 1m from the PA speakers and actually may be anything from 20 – 50m distant. You will recall I mentioned that there was something called the Inverse Square Law, this says that every time you double the distance (x 2) from a speaker the loudness decreases by the square (x 4). So, if our speaker is producing 128dB at 1m, then at 20m we are down to 102dB and at 50m the level is 94dB. The conclusion is obvious, big spaces need a lot more boxes than small ones, to get to the same sound levels at distance we require an awful lot more speakers!

"So, if we want more noise then we could get more speakers, or instead we could get more efficient i.e. louder ones for the same amplifier power. For example, we could replace our original speaker with a different unit which produces 131dB (1watt/1m) which is the same as two of the original speakers. 

"To sum up, the number of watts is not important, the speaker efficiency is, and the number and type of speakers should be decided by the size of the space and the type of event. If you know what the efficiency of your chosen speaker is then you can plan how many speakers you require to deliver a consistent sound field across the audience. As I said, it all depends....


"Finally, above is an image that is widely shared in all live sound areas of the internet, I found it at the ‘PA of the Day’ website."


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