You need to know the nominal Impedance of the speaker you are going to use and the power the speaker is capable of handling. This information is available from the manufacturer or on the specification plate on the cabinet.
The impedance will be
measured in ohms or Ω and the power in watts
or W. You require the continuous power rating of the loudspeaker, normally this
is indicated with the letters RMS (Root Mean Squared)
Depending on what you intend to use your amplifier/speaker system for. The amplifier should be rated between one and a half and two and a half times the rated power of the speaker. The impedance of both the speaker and the amplifier should be the same.
Amplifier rated at 150W into 8 ohms. Speaker should be between 60 and 100 Watts
If your system is mainly used for background music and announcements the amplifier power should be about one and a half times that of the speaker. If the sound system is going to be used for live music or in a club or disco the amplifier should be about two and a half times the power rating of the speakers.
You have a speaker cabinet
rated at 100 Watts @ 8Ω installed in a restaurant
your amplifier should be capable of producing about 160 Watts into 8Ω. If the
same speaker were to be used for live music in a pub the amplifier should be
able to deliver about 250 Watts.
For all the methods shown we will assume the speaker cabinet has an impedance of 8Ω and the amplifier can deliver in to a 2Ω load. The calculated speaker cabinet wattages are shown as a guide depending upon your system application. The most common method of connecting speaker cabinets to a stereo power amplifier is one speaker for left channel and another for the right hand side. Use the heaviest gauge cable that you can afford and keep the cable run as short as possible.
Amplifier power = 600
Watts into a 8Ω load, therefore each speaker cabinet
must be able to handle between 240 and 400 Watts.
To increase the coverage of your amplifier/speaker system you can add a second speaker to each channel. If both speaker cabinets are 8Ω the amplifier will see a load of 4Ω so it must be able to deliver at that impedance.
Amplifier Power = 1050 Watts into a 4Ω load, therefore each speaker cabinet must be able to handle between 210 and 350 Watts.
You can increase the load on the amplifier further by adding two more speaker cabinets. If the cabinets are 8Ω each and you wire them in parallel the amplifier must be able to deliver into a 2Ω load.
Amplifier Power = 1400
Watts into a 2Ω load, therefore each speaker cabinet
must be able to handle between 140 and 230 Watts.