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.

While they might sound cool in the movies, you can't evaluate loudspeaker accuracy with warp engines...yet.

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.

With fundamental notes reaching down to 16Hz, pipe organ music can be a grueling test of bass extension.

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.

Evaluate Loudspeakers for Sound and Accuracy - Continued

If you’re going to understand anything about audio—how speakers work, the effect of room acoustics, how to set your subwoofer’s level, the concept of bass management/low-pass filters, anything about audio—you have to understand frequency response. This is the cornerstone of everything in audio. It all starts and stops with frequency response. Amplifiers, receivers, speakers, CD and DVD players, they all have to have as close to perfect frequency response as possible, or else they just won’t sound right. Nothing else matters if the frequency response is no good. Can’t make it any clearer than that.

Why Frequency Response is important: An audio device with good frequency response is able to play all the low, middle, and treble tones correctly—and in the proper proportion to each other—and that’s what tells our ears whether or not this is a high-fidelity unit with rich, vibrant, lifelike sound.

To understand frequency response, remember this: the loudness of sound is expressed in a unit of measure called the decibel, or dB. To define frequency response, we specify a range of frequencies, and then we state by how many decibels (dB) the equipment varies from perfect. For example, a speaker may be said to have a frequency response of 40 Hz-20 kHz (that’s the range), ± 3dB (that’s the variation).

Now it’s time for another hard part: Frequency responses are almost always shown as a graph. This graph is known as the “Frequency Response Graph.” (Clever, no?) You have to know how to read a graph, no excuses. If you paid attention in Mr. Kelleher’s 6th grade class, great. If not, you’ll be sorry now.

Now look at figure 2. The black line is a speaker with excellent frequency response. The frequency response curve (so-called because a speaker’s frequency response curves, or drops off, in the low bass and high treble) is pretty flat (“flat” is good, because it means the device is accurate with very little up or down variation), with no serious peaks or dips. For speakers, ± 2 or 3 dB is considered very good. Electronic equipment (receivers, CD players, etc.) should be within ± .5 dB.

The red line in Figure 2 shows a speaker’s frequency response with a big 7 dB peak (so-called because the graph looks like a mountain’s peak) in the upper midrange around 6 kHz, which will make it sound harsh and irritating.

Figure 2—Speaker frequency response curves—Good and Peaked

Figure 3 shows what speaker frequency response curves look like that correspond to various subjective descriptions.

Learn these terms and learn how they look as frequency response graphs, and you’ll be on your way to a meaningful understanding of loudspeaker sound.

Figure 3—Subjective descriptions shown as frequency response curves

Try to listen in an environment similar to your home listening space because room acoustics have a tremendous influence—good and bad—on how a speaker sounds. Some rooms are too “live”, with lots of hard, reflective surfaces, which can make a speaker sound too bright and shrill. Conversely, rooms can be “dead,” or overly absorptive, like a room with thick carpeting, heavy drapes and overstuffed furniture. Speakers in a room like this can sound dull and lifeless. So bear in mind that a crowded, noisy open showroom floor or a small sound room filled to the gills with speakers and other equipment will sound nothing like your 12 x 15-foot den or your 17 x 27-foot living room with vaulted ceilings.

Room Acoustics—“Live” (left) and “Absorptive” (right)

One advantage of buying from an Internet Direct company with a home trial arrangement is being able to listen to speakers in the actual location where they’ll be used. Many ID companies have a trial period where you can live with the speakers for a week or more and return them if you don’t like how they sound in your home. This is actually a major improvement over the way speakers were typically purchased in years gone by, when customers would listen to several models in a retailer’s sound room. The problem with that, of course, is that a dealer’s sound room bore almost no acoustic resemblance to a consumer’s actual listening room, so people often got an unpleasant surprise when they got their new speakers home and discovered that they sounded quite different than they did at the store.

Subwoofers

Subwoofers are certainly important, especially in home theater systems. These days, few people have a two-channel “music only” system with full range speakers and no subwoofer, so listening to a sub is part of the overall equation. Use the guidelines for evaluating bass quality, extension and clarity that we previously outlined, but add a few more for a subwoofer. Perhaps the most important thing about a subwoofer’s sound will be its ability to play loudly enough for your needs without noise or audible distress. Once you have determined that the sub in question has the quality of sound you’re looking for (by listening to it with music), you need to determine if it has the quantity of sound as well.

Play a movie soundtrack with a lot of deep bass special effects—explosions, crashes, dinosaur roars, etc. Play the movie slightly louder than you think you’d ever play it. Listen to the bass—does the sub fill the room with clean output or is it weak and struggling? If it sounds good, then replay the same sequence again—only this time, turn off all the other speakers and listen just to the sub by itself. Does it sound pretty clean, without any really noticeable distortion and is it free from severe distracting mechanical noises? It’s ok if there’s a little audible distortion and a small bit of mechanical noise, because remember, this is at a level higher than you’ll ever really play the system and when the other speakers are playing, they’ll mask the vast majority of the sub’s bad sounds. But….if the sub sounds pretty good all by itself at really loud levels without the other speakers to “cover for it,” then you can be pretty confident that the sub will do a great job in your room, playing your most demanding material at the loudest levels you’re likely to play.

Velodyne subwoofer

Center Channel Speakers

Center channel speakers are critically important in home theater systems because typically 50-75% of a movie’s sound track is handled by the center channel speaker.

To evaluate the center channel speaker, it’s best to listen two ways:

1. Play music through it and evaluate its sound like any other speaker, using same standards we outlined previously. Chances are, you’ll only have one speaker to use, so set the receiver to “mono” so the single center speaker receives all audio channels.

2. Use the center as the left or right speaker of a stereo pair, with one “main” speaker as the other speaker in the pair. Use the receiver’s balance control to go L to R to see how tonally similar they are. They will not be identical twins, but they should be at least close relatives. Then evaluate them as a stereo pair. Is the image stable? That will tell you if their radiation patterns are similar enough. Do they sound “right” as a pair? That will tell you if their tonal signatures are similar enough. A similar tonal signature is absolutely essential for the front stage image to be believable as sounds pan across the front three left/center/right (LCR) speakers.

Atlantic Technology 1400C center channel speaker

Source Material

Use good old-fashioned stereo music CDs. Don’t use compressed iPod MP3 files, because the audio quality is simply not good enough to use as a test source. Develop a collection of music that you know really well, and that covers different genres and musical formats. You’ll want music that tests for female vocals, male vocals, dense, detailed percussion, deep bass, acoustic piano, horns, guitars, strings, etc. Cover every musical scenario and performance venue. Studio recordings should sound tight and immediate; live recordings should have a realistic, believable sense of “air” and three-dimensionality, etc.

This is important—don’t pick test material based on your favorite music, because if you really love the music you’re playing, it can make the speakers sound better than they really do.

This is also very important—Use the same CDs over and over. The scientific method dictates that you keep the variables to an absolute minimum and try to isolate and identify the data points you’re after. Become very familiar with the sound of a few CDs and use them on a wide range of speakers. You can’t compare the bass performance of different speakers using Steely Dan’s Aja on one speaker and Saint-Saëns Organ Symphony on the next. I’ve come up with my collection over about a 20-year span. They work really well for me, and not necessarily because they’re “perfect” recordings, but because I know what is on them and I can consistently evaluate the sound. If the vocalist on a particular CD sounds laid-back and very smooth on 9 out of 10 speakers, you can be pretty sure that’s what the recording is like. If speaker number 11 makes the vocalist sound too forward and edgy, you can be pretty confident it’s the speaker’s fault.  That’s the advantage of knowing your CDs—they should become like reliable test equipment. The CDs listed in Audioholics’ equipment reports are a very good starting point for a good test collection.

Conclusion

If you follow these guidelines and learn this basic information, you’ll be well equipped to evaluate speakers and be confident about your impressions of their sound:

•  Use music, not special effects, as your “test” material for listening
•  Listen in two-channel stereo. Multi-channel music can fool the ear with distracting sonic “trickery.”
• Try to listen in an environment similar to your home listening space
• Use CDs, not compressed MP3 files
•  Choose a wide range of musical genres and formats, to give the speakers a complete workout
•  Use the same CDs over and over, to ensure consistent, easily-comparable listening results