HT Measurements Explained: We've Got Some Explaining To Do - Speaker Measurements

Wondering what those confusing charts in our gear reviews are really telling you about a product? Just ask senior technical editor Mike Wood. This month, he explains speaker measurements.

Unless you're looking at a powered speaker (with built-in amplification), the power-handling rating (which is often incorrectly referred to as the number of watts a speaker has) will tell you little about how the speaker integrates into your system, let alone how it sounds. This isn't to say that the spec is useless. After all, some people like to play music really loud—I'm talking head-near-the-speaker-stack-at-a-rock-concert loud. In those rare cases, this specification may be useful. However, for the rest of us, this is probably the least necessary information, even though it's usually the most common question we get about speakers.

Although at some point, in extreme circumstances, you may need to know the power-handling capability of a speaker, its frequency response, sensitivity, impedance, and phase angle will likely tell you more about the product's performance and how it will work with your system. So, in this article (the first in a series designed to interpret our technical measurements), I'm going to explain our speaker measurements.

To begin with, frequency and amplitude are the major components in a speaker's frequency response, which is one of the more-important tests of a speaker's performance. The frequency-response measurement can give you an idea of the speaker's tonal quality—be it bass-heavy, overly bright, or anywhere in between. Driving the speaker with test tones at many audible frequencies from the low bass to the high treble and measuring the relative volume (aka amplitude, sound pressure level, or SPL) of the output gives us an objective idea as to how the speaker performs. Since each tone is fed to the speaker at consistent amplitude, we know that any variance at the output is an aberration and will affect the sound. An ideal measurement, therefore, would look like a flat line and is usually referred to as a flat response.

Unfortunately, it's nearly impossible to build a speaker with an ultimately flat frequency response, but this may not be entirely necessary. Most speaker measurements will look more like a squiggly line that stays relatively even across all frequencies. Studies by Harman International's Dr. Floyd Toole and others show that the audibility of each squiggle, or deviation in response, depends greatly on its frequency, bandwidth, and amplitude. For example, low-frequency anomalies, such as those below 500 hertz, are easier to detect by your ear than higher-frequency distortions. However, these lower frequencies are also considerably more affected by room placement and more difficult to measure with most conventional test equipment. Move a speaker a small distance in any one direction, and the low-frequency response will change dramatically. Second, a response error with wide bandwidth (one that affects many frequencies) and little amplitude may be just as noticeable, or potentially more so, than a deviation in response that affects only a narrow group of frequencies, even though the latter error may have a considerable spike in amplitude. (See Figure A for details.)

Figure A.

An example of less-than-perfect frequency responses. The peaks at "A" (particularly at 2 kilohertz) are high enough to make certain instruments, voices, and notes stand out, while the dips at "B" may cause other sounds to seem distant or too quiet. The intersection at "C" between the sub and the main speaker may cause the bass to sound disjointed and unattached from the rest of the music or soundtrack.

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