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| Understanding Loudspeaker Power Ratings
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| | Most people when assessing what speakers to use for a venue will take a look at the Power Rating of the speakers - quoted in Watts (W). Certainly this is an important figure when matching the speaker against an amplifier. However, it is not the most important figure to take into account.
| | If you look at the manufacturer's data sheet for any speaker, it will quote a figure named Sensitivity. This is quoted in Decibels at 1 meter (dB/1m) - that is the sound level measured at a distance of 1 meter from the speaker. Different speakers with the same power rating will have differing sensitivities. This makes sensitivity a much more important parameter to consider.
| | But don't forget about the power rating, especially when it comes to matching the speaker against an amplifier. Both speakers and amplifiers have a maximum power rating and you should never drive a speaker with an amplifier which is capable of delivering more power than the speaker can handle - i.e. don't connect a 300W speaker to a 500W amplifier! But there is another parameter that should be looked at here - Impedance.
| Impedance is a measure of electrical resistance and the higher the impedance of a speaker, the less electrical energy can be driven into it. Impedances are quoted in ohms (W) and speakers usually come with an impedance of 8W. There are some available, however, with an impedance of 4W. The amplifier's Power Rating is expressed as a combination of power and impedance, e.g. 500 Watts into 4W. What does this mean? Well, it means that if you connect a speaker with an impedance of 4W to the amplifier, the amplifier will be able to deliver a maximum power of 500 watts to the speaker. But what if you connect an 8W speaker to the amp? Well, you are doubling the resistance of the speaker to electrical power so you should get half the power, i.e. 250 watts. This inverse relationship between power and impedance isn't always so neat and tidy. Most amplifier manufacturers will quote test figures showing the actual power delivered into 2W, 4W and 8W speaker loads. If you don't have access to these figures then you can estimate with the principle of double the impedance, half the power.
| So, be careful when selecting speakers and amplifiers and make sure that you take into account the impedance of the speakers.
| | One more thing. What happens if you connect 2 speakers to a single amplifier channel in a daisy chain formation - i.e. connecting the second speaker to the back of the first? Now you would think that this would increase the overall impedance, i.e. 8 + 8 = 16. This would certainly be the case if the speakers were wired in Serial. However, due to the way speakers are internally wired, connecting two together actually wires them side-by-side or in Parallel. In this case, assuming both speakers are the same impedance, then the overall resulting impedance will be half of one of them - e.g. if two 8W speakers are wired together, the overall impedance will be 4W. The maths gets more complex if the speakers are of different impedances but I'm not going to go into detail here. I would suggest consulting a good book on electrical theory and Ohms Law - great bedtime reading.
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