Monitor Audio Monitor 7 loudspeaker Measurements

Sidebar 3: Measurements

Regarding measurements, I use a mixture of nearfield, in-room, and quasi-anechoic FFT techniques (using the MLSSA system from DRA Labs) to investigate objective factors that might explain the sound heard. (Stereophile's measuring microphone is a calibrated B&K 4006.) The speakers' impedance phase and magnitude were measured using Stereophile's Audio Precision System One.

Looking first at the Monitor 7's impedance (fig.1), the port tuning is indicated by the minimum in the solid trace at 65Hz, which implies that the speaker will not offer much bass below this frequency. The impedance magnitude dips to 5.5 ohms in the lower midrange, but the speaker overall won't present inexpensive amplifiers with any drive problems. A B&K ST-140, with its loose bass, would probably be a good choice.

Fig.1 Monitor Audio Monitor 7, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.) Note that the phase angle is displayed with inductive angles at the bottom rather than the top.

The anechoic section of the 7's impulse response on the tweeter axis at a distance of 48" is shown in fig.2. Similar in overall shape to the Ambiance's impulse response, the decay of the tail is more complicated. The slight reflection just over 3ms after the impulse that I noted in the Ambiance review can also be seen here. 3ms is equivalent to a physical distance of approximately 34"; again, as there was nothing that close to the speaker during this measurement, I can only think that it is an artifact of the particular speaker stand that I used. Fig.3 shows the Monitor 7's step response. Absolutely conventional, it indicates that both drive-units are connected in positive acoustic polarity and that the tweeter's output arrives before the woofer's. Its step is smoothly integrated with that of the woofer, however, suggesting optimal crossover design.

Fig.2 Monitor Audio Monitor 7, impulse response on tweeter axis at 48" (5ms time window, 35kHz bandwidth).

Fig.3 Monitor Audio Monitor 7, step response on tweeter axis at 48" (5ms time window, 35kHz bandwidth).

Deriving the speaker's anechoic response above 200Hz with the Fast Fourier Transform gives the curve shown to the right of fig.4. The treble is broken up by a series of peaks, which probably correlate both with the life and sparkle that I noted in the speaker's treble and its tendency to "shout" in this region. Unlike other metal-dome tweeters that I have listened to and measured recently, the Monitor 7's unit has a well-suppressed ultrasonic resonance. It can be just seen at 31kHz, though the tweeter's overall output is well down at this frequency. (The measurement was made with a 35kHz bandwidth.)

Fig.4 Monitor Audio Monitor 7, anechoic response with grille on tweeter axis at 48", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz and 110Hz, respectively.

To the left of fig.4 can be seen the low-frequency responses of the woofer and port, measured in the nearfield. The woofer itself can be seen to roll off rapidly below 100Hz, its half-power point lying at a very high 82Hz. The rear-firing port covers the range from 35Hz to 110Hz, but assessing its contribution to the speaker's bass extension from these nearfield figures is fraught with difficulty. Fig.5 therefore shows the Monitor 7's spatially averaged response, measured on a 1/3-octave basis with the speakers in the same positions as the other two models reviewed this month. Compared with the midband, the bass starts to roll off below 100Hz in-room, with a slight rising trend discernible between 100Hz and 1kHz. There is useful output down to 60Hz or so, but the 7 has no real low bass to speak of, rolling off faster than the sealed-box Cambridge SoundWorks Ambiance, for example.

Fig.5 Monitor Audio Monitor 7, spatially averaged, 1/3-octave response in JA's Santa Fe listening room.

What bass it does have is also affected by high distortion levels. Table 1 shows the harmonic analysis of 50Hz and 100Hz tones at a microphone distance of 18". Distortion of the 50Hz tone was taken at a 76dB spl, approximately equivalent to a midband level of 87dB. Even at this moderate level, audible chuffing could be heard from the port, and attempting to raise the measured level at 50Hz to 86dB resulted in a sound that was unlistenable due to port wind noise. Even at the lower level, high levels of second, third, fourth, and fifth harmonic could be noted, suggesting that the woofer and the air in the reflex port are behaving in appreciably nonlinear manner here. Despite conventional wisdom holding that reflex loading reduces distortion in the bass, the Monitor 7 has appreciably higher distortion than the sealed-box Cambridge SoundWorks Ambiance at the same frequency and spl, despite its being more than 6dB more sensitive and thus having to work less hard. This is presumably due to the tiny port having to carry the entire burden at 50Hz. Things were much improved at 100Hz, and I carried out the distortion analysis at an indicated 86dB spl, equivalent to a midband level of around 89dB. The only significant harmonics now present were the third at –34.3dB (1.9%) and the fifth at –45.5dB (0.5%), the others dropping to the 0.1% level or lower.

Table 1: Distortion Levels

Harmonic 50Hz/76dB 100Hz/86dB
Second -25.2dB -58.7dB
Third -28.5dB -34.3dB
Fourth -37.3dB -62.8dB
Fifth -35.4dB -45.5dB
Sixth -40.4dB -60.5dB
Seventh -39.5dB -60.5dB

Returning to fig.5, the treble is reasonably smooth apart from an excess of energy in the presence region, this noticeable on all axes and audible as added brightness and detail. The very slight peak centered on the 12.5kHz band was accentuated on the tweeter axis, implying that not toeing the speakers all the way to the listening seat might be a good idea. The "waterfall" plot of cumulative spectral decay (fig.6), however, reveals that this peak is not due to a resonance. A series of resonances can be seen in the lower treble, persisting for some 4ms after the exciting impulse has been and gone. The presence-region peak in the anechoic response at 3kHz (the cursor indicates 2878Hz, but stating more than one significant figure here is spurious) is also due to a cluster of resonances, correlating with the accentuation of this region in the in-room plot (fig.5), while the lack of energy above it is probably due to crossover cancellation. This ragged behavior will contribute to the speaker's tendency toward treble hardness.—John Atkinson

Fig.6 Monitor Audio Monitor 7, cumulative spectral-decay plot at 50" (0.15ms risetime).

Monitor Audio Ltd.
US distributor: Kevro International Inc.
902 McKay Road, Suite 4
Pickering, Ontario L1W 3X8, Canada
(905) 428-2800