What size Amplifier do you really need
It will depend on your speaker’s specifications. You will first need to look at the speaker’s data sheet and find the specification called “Continuous Power Rating” (also referred to as Power Capacity, Power Handling).
Amplifiers: The amplifier ratings that are important for audio technicians to know would be “RMS” Mean Power (root mean square / continuous) and “Peak” power ratings. These are ratings that are needed in order to find and match the correct speakers to your amplifier. Caution: You may have an amplifier that is rated at 50W continuous (or RMS), but when fully driven, it can put out 150-watt peaks. Make sure that you connect a speaker (or speakers) that can handle the power levels of this particular amplifier rating.
Speakers: Similarly, RMS Mean power, or root mean square (RMS) power handling, refers to how much continuous power the speaker can handle. The peak power handling value refers to the maximum power level the speaker can utilize in “short bursts.” For example, a speaker with a 30W RMS rating and a peak rating of 60W means that speaker can comfortably run with 30 watts of continuous power, with occasional bursts of up to 60W. Many speaker manufacturers give you both values for a given speaker. Both values are essential. See spec notes below using an Electro-Voice speaker.
Amplifier / Speaker Impedance Matching – Notes
1. Using higher impedances than those of the “rated” load of the amplifier will reduce the amplifier’s power being delivered to the speakers. Ex: Amp’s rated load is 200-Watts at 8-ohms, and you use a speaker that is 16-ohms – The amplifier will now only output around 100-Watts with this 16-ohm load. This will not affect the other characteristics of the amplifier (such as distortion).
2. Using Lower impedances than those of the “rated” load of the amplifier (depending on how low the impedance is) should be avoided, as this could cause the amplifier to “overwork.” This can cause some amplifiers to overheat. In some cases, too low of an impedance (under 1 – 1.5 ohms, for example) could be seen as a “short circuit” to the amplifier.
Tech Note – Wattages add whether they are in Series or Parallel. For example, if you have two speakers rated at 300-watts each connected in Series – the total wattage would be 600-watts. And if you connect these same two 300-watt speakers in Parallel, the total wattage would also be 600 watts.
Understanding how the Decibel (dB) relates to Amplifier Size
We learned that a +3dB change in Sound Pressure level is the minimal practical change most listeners can easily hear. With power amplifiers, in order to produce this +3dB change, you would need to “double” the power (wattage) of your amplifier. Yes, you would need to “double the wattage” of the amplifier you currently use to get this extra +3dB change!
Example: If you have a 200W amplifier, you would need a 400W amplifier to get the extra 3dB of sound pressure level. That’s all that doubling the power amplifier wattage will give you. With this being said – A 300W amplifier and a 350W amplifier would be relatively the same. Upgrading from a 300W to a 350W amplifier would be somewhat pointless regarding system “level” changes!
A +6dB change in Sound Pressure level is easy for listeners to hear. This change is considered the “Goal” for changes in system levels. To produce a +6dB change, you must increase the power by “4 times” your amplifier’s current power (wattage). Ex: If you have a 200W amplifier, you would need an 800W amplifier to get the extra 6dB of sound pressure level you need for this system-level change.
Do you need your system to be “Twice as Loud?” This will require a +10dB change in Sound Pressure level. To produce a +10dB change, you need to increase the power by “10 times” your amplifier’s current power (wattage). Ex: If you have a 200W amplifier, you would need a 2000W amplifier to get the extra 10dB of sound pressure level you need to hear the system as “Twice as Loud.” If you want to “Half” the loudness, you would require a -10dB change in Sound Pressure level (replace the 2000W amplifier with a 200W amplifier).
You should now have a better understanding of how the decibel relates to amplifier size. The best way to avoid these issues is to ensure you have the correct size amplifiers and speakers from the start of the project/installation. This is what is called the “Planning and Design Stage.” Find out how many speakers are needed, and what the power handling specs are, etc. Once you have this information, you can match the speakers with the correct size amplifier. This will ensure a smooth installation process.
Real-Life System Installation Failure – An audio contractor was hired to design and install speakers in a large building (a “Ride” inside the building) with 9-10 different zones. When it was time to “Commission” (test, evaluate) the system, our General Contractor walked the building with the audio contractor (they did not invite me), and all the levels were acceptable – so the General Contractor signed off on the installation. When I showed up, the audio contractor was gone, and I was told the system was mine to maintain. Since I wasn’t allowed to attend the first system walk, I asked for a second one. I was told it would be a waste of time, but I demanded we do the walk. We walked around the building and went through all the zones, and the levels appeared adequate. The General Contractor commented, “Well, that was a waste of time.” I asked if they could run the ride and turn on the AC system. With the Ride and AC system on – you could barely hear any audio coming from the speakers. I was asked to adjust the levels but found that the amplifiers were turned all the way up and were on the verge of clipping – there was no way to turn anything up! It was evident that the power amplifiers were incorrectly matched to the speakers. They had installed amplifiers rated at 250W per Channel, but the speaker specifications / raging was 375W continuous per Channel. This could have been avoided, but apparently, someone skipped the “planning and design” stage of the installation. A lot of extra labor and expense went into correcting this “preventable” mistake.
