Measuring Loudspeakers, Part Three Page 6
Because the time data need to be truncated to eliminate room reflections, a farfield MLS-derived amplitude response is of little use in characterizing a loudspeaker's behavior at low frequencies. However, a classic 1974 paper by Don Keele  discusses the technique of taking a loudspeaker's response in the nearfield, with the microphone capsule placed very close to the radiating diaphragm. This appears to give a response that accurately reflects a loudspeaker's low-frequency output as assessed in the farfield, a conclusion more recently confirmed by Struck and Temme of B&K . If there are several diaphragms, the nearfield responses can be taken individually, then summed in the ratio of the square roots of their radiating diameters . Note that the sum needs to be complex, taking into account both acoustic phase and the phase shift associated with the different distances of each of the diaphragms to a nominal far-field listening position. The measured nearfield response is only valid while the wavelength of the sound is very much larger than the diaphragm size. However, as we are only interested in what happens below 300Hz or so, that is not a factor unless the speaker is very large—again measuring panel speakers proves problematic!
Fig.31 shows an example of a typical ported or reflex design measured in the nearfield. The individual responses of the woofer and port are shown, with their complex sum. The woofer output drops to a minimum at the port tuning or counter-resonance frequency. This graph has 1Hz resolution; with sufficient resolution, a true notch could be seen because when the port has its maximum output, there's so much back pressure on its diaphragm that the woofer can't move. At this point, almost all the speaker's output is coming from the port. Below that frequency, both woofer and port roll out with a second-order, 12dB/octave slope. However, as they are out of phase with one another, their summed response rolls out with an ultimate fourth-order 24dB/octave slope.
Fig.31 Nearfield low-frequency responses of typical reflex loudspeaker.
The effect of this on a speaker's in-room performance is interesting. Up to the middle of 1997, the vast majority of the 360 speakers I measured were reflex designs—300 models, or 83%—the designer using the port to extend the design's anechoic low-frequency performance. Yet in an actual listening room, the increased rate of low-frequency rolloff of a reflex design leads to less low-bass output than with an equivalent sealed-box design, with its 12dB/octave rollout. However, it is fair to point out that reflex loading doesn't just increase the anechoic LF extension but can also increase power handling and dynamic range in the bass. Similarly, having measured many speakers with exotic LF alignments, ranging from the so-called "transmission line" to multiported, multidriver monstrosities, it is my considered opinion that in almost every case, the same or better bass performance could be achieved with an equivalent sealed-box alignment.
For published graphs, the loudspeaker's nearfield response is spliced to the farfield response in the 300Hz region. However, as pointed out in the Keele paper, the nearfield response assumes a 2-pi or half-space loading for the drive-units—close coupling to the room boundaries. This results in an apparent low-frequency boost in the resultant composite graph compared with a true anechoic response made of the same speaker. Given that a loudspeaker's woofer and port are always within a fraction of a wavelength from one boundary—the floor—and almost always less than one wavelength from three other boundaries—the walls and ceiling—below 100Hz or so, my experience has been that this does give a truer representation of a loudspeaker's real low-frequency performance than the anechoic response in all but extremely large rooms. Certainly, the loudspeakers I have auditioned that have true, flat anechoic extension to very low frequencies sound as if they have a somewhat exaggerated bass response—which is how they appear with a nearfield measurement.