Polk RTi A1 loudspeaker Measurements

Sidebar 3: Measurements

My estimate of the Polk RTi A1's voltage sensitivity on its tweeter axis was 90dB(B)/2.83V/m—slightly higher than the specified 89dB, and significantly higher than average. This speaker will work well with low-powered amplifiers. It is also a fairly easy load for the amplifier to drive. Though there is a combination of 5.3 ohms magnitude and –45° capacitive phase angle at 130Hz, and a minimum value of 3.8 ohms at 225Hz, the impedance remains above 6 ohms for much of the audioband (fig.1).

Fig.1 Polk RTi A1, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There is a significant discontinuity in the impedance traces between 250 and 300Hz. Investigating the cabinet's vibrational behavior with a plastic-tape accelerometer, I found a very strong resonant mode at 277Hz that was present on all surfaces (fig.2). I found this to be audible as a slight "hoot" on the half-step–spaced tonebursts on my Editor's Choice CD (Stereophile STPH016-2), but I note that Bob Reina didn't comment on any midrange coloration that might have resulted from this resonance. However, I suspect that it did underlie his feeling that the Polk's bass region sounded occasionally warm.

Fig.2 Polk RTi A1, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

There are other discontinuities in the RTi A1's impedance traces between 750 and 900Hz, but no vibrational problems in that region. However, as can be seen from the summed port outputs, measured in the nearfield (fig.3, red trace), two strong acoustic resonances are present. The lower-frequency mode is in the rear-facing port's output; the higher-frequency mode is in the front-facing port's output. Other than that, the two ports are very similar in their behaviors; both are tuned to 50Hz, for example, the frequency of the minimum-motion notch in the woofer's response (fig.3, green trace). As with Polk's earlier RTi4 speaker, which Bob Reina reviewed in October 2004, these resonances are due to Polk's Acoustic Resonance Control (ARC) port tuning, which is intended to minimize the effect of an internal standing-wave resonance by vibrating at the same frequency as the standing wave, but out of phase with it. These peaks therefore look more alarming in this graph than they might sound in reality. Nevertheless, as with the RTi4, there is a small peak in the RTi A1 woofer's farfield output in the same frequency region.

Fig.3 Polk RTi A1, acoustic crossover on tweeter axis at 50", with the tweeter (blue) and the nearfield responses of the woofer (green) and ports (red), plotted in the ratios of the square roots of their radiating areas below 350Hz and 1kHz, respectively.

Higher in frequency in this graph, the woofer appears to cross over to the tweeter (blue trace) just below the specified 1.9kHz, with asymmetric filter slopes. The tweeter rolls in with a steep fourth-order slope, while the woofer's rollout is closer to second-order in nature. While the woofer is quite flat within its passband, the tweeter shelves up by 5dB in its top octave. Fig.4 shows how these individual responses sum on the tweeter axis in the farfield. The slight hump in the upper bass is an artifact of the nearfield measurement technique; the Polk's low frequencies are actually flat to the point where they start to roll off, reaching –6dB at the port tuning frequency. The midrange and low-midtreble regions are impressively smooth, though a very slight rising trend is apparent, and again, the top octave is boosted by 5dB.

Fig.4 Polk RTi A1, anechoic response averaged across 30° horizontal window on tweeter axis at 50" and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

However, the RTi A1's lateral dispersion plot (fig.5) shows that the tweeter does get quite directional above 7kHz. In all but very small, acoustically very lively rooms, this will work against the on-axis boost in the same region, resulting in a more neutrally balanced treble. I do note, though, that Bob Reina commented that there was a "very subtle highlighting of the high frequencies," which suggests that the limited dispersion doesn't completely compensate for the top-octave boost, at least in his room. In the vertical plane (fig.6), the Polk RTi A1's balance doesn't change appreciably over quite a wide listening window, which will allow the speaker to be used on speaker stands of a wide range of height.

Fig.5 Polk RTi A1, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.6 Polk RTi A1, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

In the time domain, the Polk's step response on the tweeter axis (fig.7) reveals that both drive-units are connected with the same positive acoustic polarity. The speaker's cumulative spectral-decay plot (fig.8) is impressively clean, especially when you take into consideration the RTi A1's price of a nickel short of $350/pair.

Fig.7 Polk RTi A1, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Polk RTi A1, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Although I didn't like its lively cabinet, the Polk RTi A1 offers a lot of speaker engineering at a very affordable price.—John Atkinson

COMPANY INFO
Polk Audio
5602 Metro Drive
Baltimore, MD 21215
(800) 377-7655
ARTICLE CONTENTS
Share | |

X
Enter your Stereophile.com username.
Enter the password that accompanies your username.
Loading