Measuring Loudspeakers, Part One Page 2

Subjective loudspeaker performance is thus a multidimensional phenomenon. However, to make objective measurements that are both meaningful and practicable involves a subjective choice about what parameter to plot against one, or at most two, other parameters. All other parameters have then to be held constant. If you plot, say, a loudspeaker's sound-pressure level against frequency for a given input voltage, the result is the typical amplitude or "frequency" response. But this measured response will only be valid on the chosen axis in an anechoic chamber at the chosen sound-pressure level at one instant of time. How typical will it be of what the loudspeaker does with music in a real room played at widely differing spls? It is important, therefore, to keep in the back of your mind that to make "objective" measurements involves subjective choices!

A list of measurements that I typically perform in connection with the loudspeaker reviews published in Stereophile includes:

• Voltage sensitivity on the chosen axis.

• Electrical impedance (magnitude and phase).

• Impulse and step responses.

• Amplitude and phase response on the chosen axis in the farfield.

• Nearfield amplitude response (at low frequencies).

• Polar behavior—Dispersion in horizontal and vertical planes.

• Power and other in-room responses.

• Nonlinear distortions of various kinds.

• Delayed acoustic resonances.

• Cabinet vibrational behavior.

It should be obvious that not one of the parameters in this second list appears to bear any direct correlation with one of the subjective attributes in the first list. If, for example, an engineer needs to measure a loudspeaker's perceived "transparency," there isn't any single two- or three-dimensional graph that can be plotted to show "objective" performance parameters that correlate with the subjective attribute. Everything a loudspeaker does affects it to some degree or other.

Of course, there are some performance parameters that correlate significantly—perceived bass extension with measured low-frequency extension, for example—but it is important to remember that there will always be other aspects of measured performance that also contribute. Anyone who looks at published measurements should never assume that one measurement—a frequency response, or an impedance curve, or a dispersion pattern—fully or even partially describes the sound that they will hear. It's only the totality of all possible measurements looked at simultaneously that will give the reader any idea of what's going on. What you hear always depends on more than one measurement. Ergo, no one measurement can tell the whole story.

And given half a chance, all measurements will tell lies. It's very easy to assume that if you get a piece of test gear, turn it on, and hook it up to the device-under-test, that the resultant graph is meaningful. It's never safe to assume that a) the graph is correctly plotted, or b) that you are actually measuring what you think you're measuring. You still need another source of data, much as in pre-calculator days someone using a slide-rule needed to know approximately what the answer would be before they did a calculation. When you measure a loudspeaker's complex impedance, for example, it is helpful to look at the waveform of the signal present at the speaker terminals with an oscilloscope and to listen to the speaker's acoustic output. The test set might still produce a nice-looking graph, even if the speaker isn't making a sound!

In the following sections of this three-part article are discussions of how I prefer to perform standard measurements and how they should be interpreted. How each measured area of performance affects the areas of subjective performance is examined, with particular attention paid to measured characteristics that appear to correlate strongly with very good or very poor perceived sound quality.