A reader wrote in to our Ask-a-Pro forum for help with understanding speaker specifications. The reader asked: “What are the best spec’s to look at when buying surround sound speakers. If a speaker’s frequency response is 47hz - 30khz and another is 58hz - 30khz which is better and why?”

Our resident audio authority Bob Archer, from CE Pro Magazine, offered this advice: 

Unfortunately, determining a speaker’s applicability to a given installation is not as easy as looking at the specifications.

Frequency response can be a useful number in giving you a guideline in how dynamically the speaker will perform (it’s ability to create low frequency, midrange and high frequency content) , but there are other specs such as efficiency and impedance that are important too.

With the numbers you’ve provided (speaker A 47Hz to 30kHz vs. speaker B 58Hz to 30kHz) the frequency response numbers tell us that in theory speaker A plays a little bit deeper than speaker B. That is of course as I mentioned a guideline that’s subject to factors such as the room environment the speakers are placed in.

Speaker response, along with impedance, speaker sensitivity rating and to a lesser extend power handling are to many the specifications worth looking at.

Quickly going over these numbers: Impedance is the “load” the speaker presents to the amplifier. Look for numbers such as 8 ohms as this indicates the speaker is easier to drive than a 4 ohm speaker.

Sensitivity indicates how easily a speaker converts power into sound. In theory a speaker rated with a 91dB level should play twice as loud as a speaker rated at 88dB. So, for example it would take a 300 watt amp to drive an 88dB speaker to play as loud as a 91dB sensitive speaker driven by a 150 watt amplifier.

Power handling is a number that everyone defines differently, so this can be a tough number to go by, but in theory you wouldn’t want to use a 400 watt amp to drive a speaker rated to a maximum power handling of 80 watts.

Read Acoustics Matter: Tips for Home Theater Speakers

The most important factors to go by when evaluating a speaker for rear surround is the application in which you intend to use the product. There are three basic types of rear surround speakers: direct radiating, bipolar and dipole. Direct radiating speakers are traditional speakers the disperse sound directly from their front baffles (where the speaker drivers are located). Bipolar speakers are designed to disperse sound in a spherical pattern to fill a larger space and dipole speakers look nearly identical to bipolar speakers and they accomplish a similar goal of filling wide spaces, but they work through a dispersion pattern that looks like a figure-eight pattern. Bipolar and dipole speakers features drivers on the front and back of their cabinets or if they are specifically designed for rear surround they may have a flat backside that designed to hang on a wall, a front with a midrange/woofer driver and angled sides that employ tweeters that spray higher frequencies to the sides.

Read more about bipole and dipole speakers here.

The major difference with a dipole and bipolar speaker is that a dipole’s side or rear drivers are wired out of phase from the front drivers. A bipolar speaker’s drivers are all wired in phase with one another.

Your goal will be through the use of specifications and application scenarios will be to determine if you want a direct radiating speaker, which tend to sound more focused or a bipolar or dipole speaker that tend to sound more diffuse, but are capable of filling a larger space with sound.

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