Sidebar 3: Measurements
Monitor Audio Gold 100 6G loudspeaker: measurement setup.
The Monitor Audio Gold 100 6G was measured in a large conference room—11'11" H × 25'3" L × 15'8" W—on a PT&D MDT turntable. I used the Listen SoundCheck 24 audio-test software; a GRAS 40BD ¼" microphone and a 26CB preamp, with a Listen SCM-2 ¼" measurement microphone and a seven-microphone array at 5° vertical increments; a Listen AmpConnect 621 audio interface and amplifier; and a B&K Type 5434-B-001 accelerometer. The system was calibrated with a B&K 4231 acoustic calibrator at an SPL of 94dB at 1kHz.
The Monitor Audio Gold 100 was measured at 1m, at tweeter height. From the center of the tweeter to the floor was approximately 68.5".
Fig.1 Monitor Audio Gold 100 6G, electrical impedance and phase, 20Hz–20kHz.
The Gold 100 6G is a ported, three-way standmount loudspeaker with an 8" woofer, a 3" midrange unit, and a 1.5" × 1.75" Micro Pleated Diaphragm III (MPD III) tweeter, a variant of the Heil Air Motion Transformer (AMT). The minimum impedance (fig.1) was 4.4 ohms, with a very small phase angle; the speaker should be an easy load for any reasonably powerful amplifier to drive.
Fig.2 Monitor Audio Gold 100 6G, quasi-anechoic response on tweeter axis at 1m, 2.83V, with nearfield and farfield responses spliced at 150Hz.
With the microphone pointing at the tweeter, the average sensitivity, measured between 100Hz and 10kHz, was 87dB/2.83V/m. The frequency response (fig.2) was ±3dB from 56Hz to 45kHz, with an ultrasonic peak at 35.5kHz, probably a resonance of the MPD III tweeter.
Fig.3 Monitor Audio Gold 100 6G, group delay on tweeter axis at 1m.
Fig.4 Monitor Audio Gold 100 6G, impulse response on tweeter axis at 1m.
The group delay (fig.3) was good because all the drivers are close together. The tweeter frequencies arrive first, followed by those of the midrange and then the woofer. The transient response (fig.4) was very good, though the tweeter did display some ultrasonic ringing.
Fig.5 Monitor Audio Gold 100 6G, cumulative spectral decay plot on tweeter axis at 1m.
The waterfall (cumulative spectral decay) plot (fig.5) revealed ringing from the tweeter at 36kHz and from the midrange below 1kHz.
Fig.6 Monitor Audio Gold 100 6G, horizontal directivity: off-axis responses at 1m, in 5° steps.
Fig.7 Monitor Audio Gold 100 6G, vertical directivity: responses from 15° above axis to 15° below, in 5° steps.
The horizontal directivity (fig.6) was fairly narrow, with beaming above 10kHz. There was also some off-axis lobing around the crossover frequencies of 700Hz and 2.6kHz. The vertical directivity (fig.7) looked good, again with some lobing around the 2.6kHz crossover frequency. These speakers should be pointed at the listening position.
Fig.8 Monitor Audio Gold 100 6G, harmonic distortion at an SPL of 90dB: second harmonic, third harmonic, and THD (distortion percentage, right-hand scale).
For most of the frequency range, the total harmonic distortion (fig.8) was below 2%, which is good.
Fig.9 Monitor Audio Gold 100 6G, cabinet vibration measured with an accelerometer at several positions on the enclosure (driving voltage, 2.83V).
Finally, an accelerometer was used to check for cabinet resonances that could impact the sound (fig.9). The cabinet is made of braced MDF finished with wood veneer, so it is fairly rigid. The accelerometer was placed at different positions on the panels, and the vibration was measured using the same sinewave sweep at 2.83V. The highest levels were detected on the back panel, at around 1kHz.—Steve Temme
