Polk Audio RT25i loudspeaker Measurements
Despite its diminutive size, the Polk RT25i plays quite loud, its B-weighted sensitivity weighing in at 88dB(B)/2.83V/m. Its plot of impedance magnitude and phase against frequency (fig.1) also reveals it to be only a moderately needy load for the partnering amplifier. The minimum impedance value is 3.9 ohms in the lower midrange, though the combination of 5 ohms magnitude and a capacitive phase angle of 45 degrees at 150Hz probably means that inexpensive receivers rated as 8 ohm models will not give of their best with this speaker.
The saddle in fig.1's magnitude trace at 50Hz indicates the tuning frequency of the rear-mounted port. The trace is also free from the usual wrinkles that indicate the presence of resonances. Fig.2, a cumulative spectral-decay plot calculated from the output of a simple plastic-tape accelerometer fastened to the center of the sidewall, reveals that while the small enclosure does appear to flex at 145Hz, almost all the other modes present lie above 500Hz, where their subjective effects will be minimal. (All things being equal, a well-designed small enclosure will always behave better, in vibrational terms, than a large enclosure.)
Fig.1 Polk RT25i, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.2 Polk RT25i, cumulative spectral-decay plot of accelerometer output fastened to center of side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
.3 shows the RT25i's on-axis response, averaged across a 30 degrees horizontal angle, spliced to the nearfield woofer and port responses and their complex sum, and weighted in the proportion of their radiating diameters. The notch at 50Hz in the woofer trace confirms that the port is tuned to 50Hz, and while the port's output features some midrange liveliness, the fact that the main vent faces away from the listener should minimize any effect on the speaker's perceived sound quality. The speaker's overall low-frequency response starts to roll off just above 100Hz, and is well down in level by 40Hz, around the fundamental frequency of acoustic and electric basses.
Fig.3 Polk RT25i, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer and port responses and their complex sum plotted below 300Hz, 1kHz, and 300Hz, respectively.
The overall balance is flat but marred by two features. First is the fact that the midrange and what there is of the bass region are shelved down by 2dB or so. This will not have subjective consequences if the speaker is placed close to the wall behind it, when this region will benefit from some boundary reinforcement; however, on stands in free space, it will sound leaner than might be expected. As Bob Reina pointed out, the speaker lacked "dynamic bloom in the bass frequencies during densely modulated passages."
Second, there is a sharp discontinuity in the response between 800Hz and 900Hz. When extreme, this kind of response anomaly is often associated with a nasal coloration. However, as BJR felt that the little Polk was remarkably free from coloration, I suspect that the effect of this small peak was to enhance the sense of recorded inner detail and the articulation of vocals rather than to add an identifiable color.
Bob commented on the Polk's superb imaging, which correlates nicely with the speaker's well-controlled horizontal dispersion (fig.4). The tweeter does become quite directional above 12kHz, which might lead to a loss of perceived high-frequency "air" in large rooms. But then, a speaker this small is not likely to be used in large rooms. In the vertical plane (fig.5), a large suckout appeared more than 5 degrees above the tweeter axis. The RT25i is therefore best placed on highish stands so that the listener's ears are level with or just below the tweeter axis.
Fig.4 Polk RT25i, lateral response family at 50", from back to front: responses 90 degrees-5 degrees off-axis, on-axis response, responses 5 degrees-90 degrees off-axis.
Fig.5 Polk RT25i, vertical response family at 50", from back to front: differences in response 45 degrees-5 degrees above tweeter axis, reference response, differences in response 5 degrees-45 degrees below tweeter axis.
In the time domain, the RT25i's step response (fig.6) indicates that tweeter and woofer are both connected with the same positive acoustic polarity, but the decay of the woofer step is broken up by some ringing with a period of just over 1ms. This results in a slight ridge of delayed energy at around 900Hz in the waterfall plot (fig.7), the frequency of the small peak in the on-axis response. This plot is fairly clean higher in frequency, suggesting that the tweeter is quite clean for what must be an inexpensive drive-unit.—John Atkinson
Fig.6 Polk RT25i, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Polk RT25i, cumulative spectral-decay plot at 50" (0.15ms risetime).