Monitor Audio Studio 15 loudspeaker Measurements
The Studio 15 is a pretty easy speaker to drive, as revealed by its plot of impedance amplitude and phase (fig.1), which drops below 8 ohms only between 110Hz and 800Hz, and in the region of the port tuning frequency of 55Hz. The ultrasonic tweeter resonance reveals its existence by the blip at 25kHz, and there is a faint hint of a wrinkle in the amplitude plot at 500Hz, indicative of possible cabinet resonant problems in this region.
Fig.1 Monitor Audio Studio 15, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The acoustic crossover, shown in fig.2 (the tweeter's output is shown in red, the woofer's in blue), indicates the crossover to lie just above 3kHz, as specified, and also reveals the very strong tweeter resonance at 25kHz. While the tweeter rolls in fairly smoothly, the metal/ceramic woofer cone features a peak in its on-axis response nearly an octave above crossover that almost brings its output back up to that from the tweeter. This is presumably due to its first cone-breakup mode, which doesn't seem that well notched out (though it is less prominent than the similar resonance in Monitor Audio's Studio 20). More on that later, but note that the tweeter also seems to have a little peak at the same frequency. This peak can also be seen in the quasi-anechoic response averaged across a 30° window on the tweeter axis (fig.2, black trace), which breaks up an otherwise smooth, very slightly rising response trend.
Fig.2 Monitor Audio Studio 15, anechoic response on tweeter axis at 44" (black), corrected for microphone response, with individual responses of the tweeter (red), woofer (blue), and port (green), measured in the nearfield below 300Hz.
On the left of fig.2 are shown the individual responses of the woofer (blue) and port (green), measured in the nearfield. The level matching between these two plots and that to their right can only be approximate; nevertheless, they suggest the Studio 15 to have reasonably good bass extension, with no spurious midband tones coming from the port. The lack of the expected nearfield reponse hump in the upper bass suggests a rather overdamped, lean balance overall.
Looking at the manner in which the sound changes as the listener moves to the Studio 15's side (fig.3), the high treble progressively rolls off, though a slight notch develops off-axis, working against the on-axis peak. With a center of 5.3kHz, this is a little high in frequency to be a crossover artifact. Vertically, a significant suckout at crossover does appear once the listener's ears are above the tweeter (fig.4), which reinforces the need for suitable stands to be used with the Studio 15.
Fig.3 Monitor Audio Studio 15, lateral response family at 44", normalized to response on tweeter axis, from back to front: differences in response 90–15° off axis, reference response, differences in response 15–90° off axis.
Fig.4 Monitor Audio Studio 15, vertical response family at 44", normalized to response on tweeter axis, from back to front: differences in response 10–5° above axis, reference response, differences in response 5–10° below axis.
Averaging 20 individual 1/3-octave responses for each loudspeaker in a 72" by 24" grid centered on the listening seat in my room gives a curve (fig.5) that correlates quite nicely with the speaker's subjective balance. The peaks and dips below 300Hz are residual room modes that haven't been removed by the spatial averaging, but the overall response is very smooth, broken only by a slight excess of energy at the bottom of the range handled by the tweeter. This undoubtedly contributes to the speaker's rather unforgiving nature with CD sources.
Fig.5 Monitor Audio Studio 15, spatially averaged, 1/3-octave response in JA's Santa Fe listening room.
The Studio 15's impulse and step responses are shown in fig.6 and fig.7, respectively. The impulse response of the woofer alone can be seen in fig.8 These plots appear to indicate that the drive-units are connected in inverse polarity. This is actually due to XLR wiring differences between the two amplifiers used to make these measurements. The drivers are connected with the same electrical polarity. Both impulse resposnes feature out-of-band ringing from the drive-units; while that of the tweeter is beyond audibility, the woofer ringing falls in the mid-treble, as can be seen from its waterfall plot (fig.9), which reveals the frequency of the mode to be 5.5kHz.
Fig.6 Monitor Audio Studio 15, impulse response on tweeter axis at 44" (5ms time window, 30kHz bandwidth).
Fig.7 Monitor Audio Studio 15, step response on tweeter axis at 44" (5ms time window, 30kHz bandwidth).
Fig.8 Monitor Audio Studio 15, impulse response of woofer on tweeter axis at 44" (5ms time window, 30kHz bandwidth).
Fig.9 Monitor Audio Studio 15, cumulative spectral-decay plot of woofer at 44" (0.15ms risetime).
The woofer resonance can also be seen in the waterfall plot for the complete speaker (fig.10) at the cursor position. (This is the plot for the other speaker of the pair; the resonance can be seen to be a little higher in frequency for this sample, at 5.8kHz.) I would imagine that this woofer behavior corresponds to the occasional subjective problems heard in the mid-treble.
Fig.10 Monitor Audio Studio 15, cumulative spectral-decay plot at 44" (0.15ms risetime).
There is also something strange going on below 1kHz in fig.10, which might correlate with the congestion heard on piano sound. There is not enough resolution in this graph to be sure of the exact frequency of the problem. However, it could possibly be related to the behavior of the cabinet walls. Fig.11 shows the cumulative spectral-decay plot of the output of a plastic-strip accelerometer placed on the side of the enclosure; it reveals three strong resonant modes at 360, 420, and 504Hz, with the latter being the strongest. With pink noise playing, the latter mode was clearly audible as a whistle when I was listening to the cabinet with a stethoscope. In the postscript, I conjecture that a single, high-Q resonance will not be too subjectively harmful. The problem with the Studio 15's cabinet behavior, however, is that there appear to be three resonances within a half-octave, at least one of which is likely to be excited much of the time. The effect was a "hoot" heard on recorded piano.—John Atkinson
Fig.11 Monitor Audio Studio 15, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).