How to Match Speaker Wattage to an Amplifier

By David Lipscomb

Speaker wattage is a guideline, designed to give a rough frame of reference for amplifier matching. Most well-built speakers will be able to handle well in excess of their power rating, but staying close to the RMS (root mean squared)-or average-power rating is sound if there is any doubt. For the best performance, a good rule of thumb is to have twice as much rated RMS amplifier power as the speaker's rating. Traditionally, speakers blow under too little power, as clipped waveforms (or heavy distortion) heat up voice coils, causing premature failure.

Step 1

Examine the speaker's documentation, available on the manufacturer's website, and from local dealers of the speaker in question. For roughly the past 20 years, well-built speakers use rubber (or Santoprene) as the surround, and a treated cloth or rubber around the tweeter. These materials are superior at keeping the driver's cones centered as they move forward and back. This translates to better efficiency, and means that the speaker is less likely to encounter mechanical distress under power. Regardless, discovering the speaker's power rating is a useful piece of information, but does not necessarily dictate whether the speaker will "blow" without examining the build quality of the unit as well.

Step 2

Locate and study the amplifier's specifications, aiming for 200 percent of the speaker's RMS rating as a benchmark. Keeping the power ratings similar is fine, but that only will provide base-level performance with minimal dynamics and punch. Amplifier distortion ratings tell the true story, and it is desirable to keep these figures in the 0.0005 percent range or better. This quality of distortion means that the input signal and output to the speakers are virtually transparent to one another.

Step 3

Inspect the nominal impedance capability of the amplifier, vs the impedance spec for the speakers. Impedance is the "load", or amount of stress applied to an amplifier's output stage by whatever is connected to it. Most home amplifiers are rated at 8ohms, with better models having the ability to drop into the 4ohm range. Impedance fluctuates based on frequency, meaning that sharp treble and low bass cause the largest stresses to an amplifier. Most music is in the middle however, so these short-term stresses are typically not an issue. However, along with power ratings, it is critical to ensure that if a low-impedance speaker is chosen, that the amplifier is suitable to drive it.

Step 4

Research electrical theory surrounding amplifier power and speaker output. One common misconception is that a 100 watt amplifier is twice as loud as a 50 watt device. Similarly, since speakers are generally passive devices, they do not produce power, only transfer the power from the amplifier into something useful and audible. Therefore, selecting a 400 watt speaker will not be beneficial over a 100 watt speaker under normal usage. These examples are a few of many that is needed to distill what's important and what isn't.

Step 5

Look at the efficiency ratings of the speaker. Because efficiency is rated as a logarithmic function, a rating of 96db/W (96 decibels at one meter, driven with one watt) is nearly twice as efficient as one rated at 90db/W. What this means is that, although the speaker may not subjectively sound better, the amplifier driving it has a far easier time performing its task. Selecting an efficient speaker is beneficial if using a lower-power amplifier, or if loudness is a significant issue.

Tips & Warnings

  • When auditioning amp/speaker combinations, pay attention to what is driving what. This will impact the perception of loudness and overall performance.