I cannot tell you the number of times I have heard a conversation about amplifier power go like this: “I hear that B&W speakers need at least 400W,” or, “I heard that Paradigm speakers are very power-hungry.” 

The fact is, there IS a way to figure out how much power your speakers will need and how to pair an amplifier with the speakers you already own or are looking to purchase. 

To determine the power your audio power amplifier will need, you will need the following items: 

1. Speaker sensitivity specification (typically dB SPL /1W /1m)
2. Number of speakers you are using
3. Distance from the speaker(s)
4. How loud do you want your music to sound, for general reference (2):

• 60dB – moderately loud (ordinary conversation, light traffic)
• 80dB – very loud  (garbage disposal)
• 100dB – uncomfortably loud (newspaper press)
• 120dB – painful (thunderclap, the threshold of pain)

Rules to Remember

1. Every time you double power or double the number of speakers, you increase the sound pressure level (SPL) by 3dB.
2. Every time you double the distance from the speaker, you decrease SPL by 6dB.
3. A speaker specification of 95 dB SPL/1W/1m means that at 1W of power, the speaker will deliver 95 dB SPL when you are located 1 meter away from the speaker.

Power Calculation Example 

Let’s use the B&W 703 S2 speaker, which has a specification of 89dB SPL /1W/1m. Most people will be using two speakers. This means double the power so it effectively makes the speaker specification 92dB SPL /1W/1m, or 3dB higher, as per rule #1.  Now let’s look at how rule #2 works. Every time you double the distance from the speaker, SPL decreases by 6dB. The table to the right shows how our 92dB from earlier decreases with distance.

Let’s say we will be sitting 12 feet away from the speakers. We can see how power will affect SPL, using the table on the left. 

My personal opinion is that 100dB SPL should be more than plenty. Consider for example that movie theaters aim for an average of 85dB SPL (1). With this, you can see that an amplifier needs to be able to provide somewhere around 120W per channel. 

Lastly, keep in mind that listening rooms will add gain as well, as there will be reflections from walls, floor, and ceiling. Room reflections will typically add about 3 to 6dB depending on speaker placement (2).

Summary

Don’t just listen to somebody about how much power you need; figure it out properly. This calculator is a very good starting point; https://myhometheater.homestead.com/splcalculator.html

References:

(1) https://www.ecoustics.com/articles/ideal-spl-home-theater/
(2) https://myhometheater.homestead.com/splcalculator.html