Speaker Buyer's Guide

·         Impedance: Technically, impedance is the opposition to current flow in an alternating current circuit.  Its significance in audio is a way to ensure that components work together correctly; the audio input needs to use up the current from an output circuit.

Typically, input impedances are higher than the output impedance of the circuit they are connected to.  So, a powered speaker system is taking in a line level input and should have high impedance.

·         S/N Ratio: The difference between the nominal program level, or speaker volume, and the noise floor, or underlying hiss and static in an electronic circuit.  The larger the S/N ratio, the better.

Higher S/N will be very noticeable.  The sound will be cleaner with less noise during playback, and when the speakers are turned on but not in use, quieter with less residual hiss.  For critical listeners, this specification should carry some weight.  If your music, or games, have a broad dynamic range (the difference between the loudest and quietest portions of the program), it is very important to insure that the S/N ratio is larger than this dynamic range.  If is not, then either the quietest portion will be lost beneath the hiss of the noise floor, or the loudest portions will overload the speakers and cause clipping and distortion.  Either way, the listening environment will not be satisfactory and deduct from your overall computing experience.

·         Output Power: How much power the amplifier provides to the speaker system.  The amplifier is often integrated into one or all of the speakers in a configuration known as “powered speakers,” as opposed to having an independent amplifier like in a home stereo system.  A larger output power will directly lead to a louder speaker system.  Also, in a multichannel system, the principal stereo speakers should be more powerful than the surround speakers, which are just used for spatial effects.  Finally, in a sub woofer, higher powers will lead to a punchier system with more presence.  When examining speaker’s handling of power, sometimes the speaker’s efficiency is presented.  As a speaker is a transducer, its job is to convert forms of energy, from electrical to physical movement of the air.  The efficiency measure how much energy is actually transformed instead of lost to heat.  Given two equal speakers, one with a higher efficiency will attain higher volume levels at lower power and will perform better than the less efficient speaker.

The trick with output power is how it is measured.  When not indicating the standard used in measuring, any specification talking about output power can be deceitful.  The most common, and fairly honest, standard is to use the RMS (root mean square) testing method.  This technique takes the average of an alternating current signal, such as the power sent to speakers, and gives a specification that would be equivalent to the power dissipated in a comparable direct current circuit.  If a company chooses to instead list peak power, this doesn’t always reveal the full truth of the speaker.  Sound is produced by alternating current with ever-changing levels, and the RMS value gives a much better idea of what the system can really do, instead of the instantaneous peaks in power.  Some other methods, such as describing “music power” or “program power” will vary based on the signal used to create the specification and can prove to be more deceptive, yielding higher ratings than the actual performance in normal listening conditions.

Another specification bundled into output power is the Total Harmonic Distortion, or THD.  This is a measurement of the purity of the audio signal.  Like digital artifacts in imaging, electronic equipment can introduce distortion in the form of harmonics, or frequencies not present in the source, but reproduced as integer multiples of the source frequency (i.e. a 100Hz source produces an output with 100, 200, 200 Hz and so on).  Besides the loss of quality, the human ear is bothered by these imperfections.  The distortion can be presented in either of two ways—by presenting the difference in signal between source and harmonics in dB, or by giving a percentage to indicate the ratio between the harmonics and the original source.  Therefore, the lower of either two numbers, the higher quality of the audio.  Given a typical reading of so and so watts at 0.1% THD, this means that the artifacts are going to be 1/1000^th of the program level, or 30 dB lower.  Translated into speaker pressure, that’s 1/64^th of the program source in perceived loudness.  What does this all mean?  If the program’s dynamic range is greater than 30 dB, then the harmonics are going to be evident in the program as they fall within the dynamic range.  However, the dynamics would be present within the range only under the loudest portions, which would drown them out.  This makes 0.1% THD reasonable, but a lower value would mean a better speaker.

When these specifications are tested, there is a quick way and there is a thorough way; the two don’t go together.  When checking distortion, it is possible to examine just one frequency and how the system reacts to it, or to look at the full spectrum and present the system’s overall distortion.  THD across the entire spectrum will indicate a more accurate picture of the speaker’s quality.  And seeing as sound is much more than just a test tone, this gives a more accurate picture of how well the speakers will actually sound.  Also, computer manufacturers aren’t subjected to the requirement to present all of their specifications from stereo mode, like on home receivers.  So their measuring environment may not resemble an actual listening environment; instead it may be optimized for good specs.