How to Evaluate Loudspeakers for Sound and Accuracy

What do we really mean when we talk about a loudspeaker`s [sound?" When we hear a speaker, how do we know by simply listening to it whether or not it`s a good speaker? What should we listen for? Obviously, a speaker`s job is exactly the same as any other audio component, like an amplifier or disc player-its job is to reproduce the input signal as accurately as possible, with minimal distortion or deviation from the original. Loudspeakers are the least perfect of audio devices. But because they are transducers-they convert one kind of input (in this case an electrical signal) into a different kind of output (acoustic sound waves)-they have a more difficult job than an amplifier or CD player.

So, how should you judge a speaker`s sound to determine if it`s doing its job correctly? What should you listen for?

To start with, play a wide variety of different kinds of music with which you`re familiar. Listen for a natural, balanced tonal presentation. Your general first impression should be that the sound is wide range, with strong bass, clear vocals and instruments, and a sparkling treble region. It should have a general feeling of sounding natural, as music does in real life, and no one tonal area should stand out over another. Everything should simply be [there," with an effortless, relaxed sense of detail-and visceral impact if the music calls for it.

Acoustic instruments and vocals are the best test material because people are most familiar with the sound of real instruments and voices. Heavily-processed electronic synthesizers have no real-life reference of naturalness, since those sounds don`t exist in nature. Same thing for movie sound effects-no one knows what an Exploding Death Star or the Enterprise`s warp engines are supposed to sound like, because they don`t exist in real life.

Very important: Listen in two-channel stereo, because the acoustic novelty of music coming at your ears from all around you from a surround sound system will often fool you into thinking the sound is better than it really is. Regular two-channel stereo is free from aural gimmicks and distracting special effects like a guitarist suddenly coming in over your left shoulder. That may grab your attention for the moment, but it diverts your concentration and good judgment away from listening for tonal accuracy. Instead, multi-channel sound can fool you with audible trickery. Some people may disagree with the [two-channel rule," and that`s fine. Disagreements are the lifeblood of any hobby. But at the very least, start off with critical, concentrated two-channel listening. If you feel the need, you can move on to multi-channel music or video soundtracks later. But remember-a speaker that sounds great with demanding, difficult musical material will do a fine job with video soundtracks. However, the reverse is definitely not automatically true.

Listen for the depth and fullness of the bass. A speaker with good bass response conveys much of the power, weight and impact of real, live music. Even though bass should be strong and powerful, it should always sound clean and articulate, never [thuddy," [boomy," or like it`s simply one indistinct bass note repeating itself (known as [one-note" syndrome). A good thing to listen for is whether you can follow the bass line in the music in spite of the busyness of all the instruments and vocals taking place on top of it. An upright acoustic bass in a jazz quartet is a good test (this is known as a walking bassline, because the repeating bass notes sound like someone is taking a stroll down the street). If you can follow the bass notes clearly, and they are strong and impactful, that`s a very good sign. Many speakers fall down on this. This test really separates the contenders from the pretenders.

Another great test is a well-recorded pipe organ. Pipe organs can get as low as 16 Hz and the low C in the opening seconds of the [2001" theme is 32 Hz. This is lower than the vast majority of so-called [full range" speakers can play on their own without the aid of a subwoofer. The lowest note on a 4 string electric bass, which is the lowest musical note in most popular music, is low E at 42Hz and Bo at 30Hz on a 5 string bass. Even 42 Hz is a tough test for an all-in-one full-range speaker. What we often think of as bass when we hear the repeating bass lines in rock music or hip-hop is really in the 50-60 Hz range. A good full-range speaker should have no problem with that. Again, listen for clarity and distinct notes, not just a dull thud.

What a speaker doesn`t do when playing bass is just as important as what it does do. If a note (like low C at 32 Hz) is well below the speaker`s usable frequency range, a well-designed, well-behaved speaker will not produce objectionable audible distortion or emit extraneous mechanical noises trying to reproduce that note. A vented speaker should not produce port chuffing or a distracting whooshing sound. The speaker`s woofer should not produce a clicking or thwacking noise as it runs out of excursion or its voice coil bottoms against the backplate. Good speakers simply reproduce the bass they`re capable of reproducing, and they should pretty much ignore the bass they can`t deliver without any distracting audible distress or other drama.

Now, shift your attention to the midrange area of the sound. This is the region of sound where most of the things we can actually identify are: vocals, guitars, saxophones, footsteps on the floor, slamming doors, hand clapping, violins, etc. A good speaker will make this sound very realistic. Your favorite singer should sound just like your favorite singer. A speaker that is not accurate in the midrange will make your favorite singer sound like she`s got a head cold or will make a familiar male vocalist sound like he`s singing from the bottom of a deep barrel. Their voices will take on an unnatural [coloration" that is very obvious and quite objectionable.

Similarly, instruments should sound like they do in real life: a tenor saxophone should have that nice reedy [bite," but it shouldn`t be shrill or annoying. An electric guitar should have a nice sharp twang, but it shouldn`t take your head off. Music played loud should sound exciting and detailed, and, well, lifelike. If your first inclination when playing a speaker loudly (assuming the amplifier isn`t running out of power and distorting) is to turn it down because the sound is grating on your nerves, that`s a sign that something about the speaker`s sound is amiss. Massed choral groups and multi-part vocal harmonies are great tests of accurate midrange reproduction. You should be able to pick out and follow all the individual voices.

Lastly, listen to the very highest tones, or what we call the treble. This is the frequency region reproduced by the tweeter. A speaker with good high-frequency response has a silky, sparkling sheen to its sound. Again, be on the lookout for sound that`s too [hissy" or edgy. Good speakers deliver sparkle and detail without sounding shrill.

Two things will tell you if the speaker has a good, accurate treble region:

1. The upper-range instruments (like triangle, tambourine, cymbals, etc.) have a sense of [air" around them, as if you can visualize them existing in their own three-dimensional physical space. Cymbals and triangles have a long decay-after they`re struck, their sound lingers on for a while, diminishing in loudness as the seconds pass. You should be able to follow the initial strike and then hear the decay as it fades away, even with all the other music going on around it.

2. A speaker with good high-frequency response has what speaker engineers call good dispersion, which means that you can hear the high frequency sounds even if you`re well off to the side of the speaker. Speakers with poor off-axis response tend to emit high frequencies like a flashlight beam-pretty much only straight ahead. Good speakers have wide high frequency dispersion, which imparts a sense of [space" and liveliness to the sound. However, be careful of speakers that are [omnipolar" (they send out equal low, mid, and high frequency energy in all directions) , because speakers like that (which have too wide dispersion) will not present a spatially-believable sonic image, meaning instruments and voices will sound vague and indistinctly-positioned.

Now is a good time to present a few basic definitions for the sake of this conversation. We`ve talked a lot about bass, midrange and treble, so let`s define more precisely what we mean. Sound waves that vibrate or oscillate through the air are measured in cycles per second, or Hertz (named after German scientist Heinrich Hertz), commonly abbreviated as [Hz." The frequency of sound waves audible to humans range from 20 Hz in the bass to 20,000 Hz (20 kHz) in the treble. (Well, women and young children can hear that high; a middle-aged man is lucky to make it much past 13-14 kHz. Your grandfather needs his hearing aid because his ears are toast by 3 kHz. That`s why he`s always saying, [Huh?") See figure 1 for an explanation of what we generally refer to as bass, midrange and treble, expressed as Hz (LFE stands for [low-frequency effects").

As you can see, the midrange-from roughly 200 Hz to 3,000 Hz-is where most of the [action" takes place. If an audio device can cover this range with reasonable accuracy, then people will regard it as doing a pretty good job of conveying intelligible information. Most TVs, phones, AM radios, PA announcement speakers in supermarkets, etc. cover this frequency range. It`s a wide enough frequency range that people can easily understand what is being said or recognize musical melodies, but it`s nowhere near wide-ranging enough to be considered high fidelity.

For that, the audio device-in this case, speakers-must be able to reproduce far deeper into the bass and reach much higher into the treble. However, the range of frequencies covered by an audio device is just part of the story. Just as important-even more so-is the concept of flat, accurate frequency response.