Floor Loudspeaker Reviews

Sidebar 4: Measurements

The Concerto 3's voltage sensitivity is specified as a very high 95dB/W/m. My B-weighted estimate on the speaker's tweeter axis was a little lower than that, at 91dB(B)/2.83V/m, but this is still significantly higher than average. The impedance magnitude (fig.1, solid trace) remains above 7 ohms at almost all frequencies, and though the electrical phase angle (fig.1, dotted trace) is extreme on two occasions, both instances occur when the amplitude is 8 ohms or more, ameliorating the drive difficulty. With its high sensitivity and amplifier-friendly impedance, the Sonist speaker will be a good match for low-powered SET designs, particularly as the shape of the impedance curve will boost the speaker's mid-treble region, compensating for a measured lack of on-axis energy in that region (see later).

Fig.1 Sonist Concerto 3, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance traces have two small peaks evident, a fairly broad one at 150Hz and a narrower one at 273Hz. Investigating the vibrational behavior of the cabinet's panels with a simple plastic-tape accelerometer uncovered an extremely severe resonance at 273Hz. This was present on all surfaces but highest in level on the hardwood front baffle (fig.2). Driving the speaker with a signal generator feeding the amplifier (actually Faber Acoustical's excellent Signal Suite app running on an iPod Touch), the Concerto literally shook itself when the frequency was exactly 273Hz.

Fig.2 Sonist Concerto 3, cumulative spectral-decay plot calculated from output of an accelerometer fastened to front baffle 6" below woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

I must admit that I am surprised that Art Dudley didn't notice any midrange coloration that would have resulted from this resonance. In fact, he noted that "Singing voices were free from all of those colorations that appear to have been named in a doctor's office: nasal, chesty, hooty..." Perhaps AD missed hearing this problem (footnote 1)—though he did comment, enigmatically, that the Sonist "didn't shine with piano recordings in general," and piano recordings would indeed be affected by a panel resonance at this frequency. But it might be the case that the cabinet panel resonance is of a high-enough Q (Quality Factor), ie, covers a sufficiently narrow frequency band, that it might not be maximally excited with recordings made at modern Concert Pitch. A resonance needs to be stimulated with a tone lasting as long, in cycles, as the numeric value of its Q; ie, a resonance at 250Hz with a Q of 125 needs a 250Hz tone to last for half a second for it to be fully excited. The frequency of the Concerto's cabinet resonance, 273Hz, lies between the frequencies of the notes Middle C (261Hz) and C-sharp (277.2Hz), and it's possible that the resonance remains dormant much of the time. Even so, unpitched instruments like drums will still trigger the resonance, adding a "wolf tone" to their sounds.

Art wondered about the contribution of the rectangular port at the base of the baffle. The impedance plot reveals that the Sonist speaker is a reflex design with the port tuned to quite a high frequency for the size of the speaker, 42Hz (the lowest note of the 4-string bass guitar and double bass). The expected minimum-motion notch in the woofer's output occurs at 41Hz (fig.3, red trace), which is where the back pressure from the port resonance holds the woofer cone still. The port's output (fig.3, green trace) peaks slightly lower in frequency, but more significantly, there is a second peak centered on 150Hz, the frequency of the lower impedance anomaly seen in fig.1. This resonance is strong enough to produce a notch at the same frequency in the woofer's response and there are additional peaks, albeit at a lower level, at the harmonics of the resonance. Again, I would have expected this behavior to have audible results.

Fig.3 Sonist Concerto 3, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer (red) and port (green), plotted in the ratios of the square roots of their radiating areas below 350Hz and 750Hz, respectively.

Higher in frequency in fig.3, the woofer's response is fairly even below 2kHz, but there is a sharp spike apparent at 2.8kHz before it crosses over to the tweeter. The tweeter (fig.3, blue trace) is flat for its first two octaves, but then shelves up by 5dB above 15kHz.

Fig.4 shows how these individual responses sum in the farfield. The lows roll off below 40Hz with the 24dB/octave slope typical of a reflex design, while the higher-frequency port resonances result in some notches in the upper bass. The tweeter is almost 5dB too low in level in the octave above the crossover frequency—this will be compensated for when the speaker is driven by a tube amplifier having a high source impedance—but returns to full level above 6kHz and to an increased level above 15kHz. How this affects sound quality will also depend on the speaker's radiation pattern. The Sonist's horizontal dispersion, normalized to the quasi-anechoic response on its tweeter axis, is shown in fig.5. The off-axis behavior is surprisingly even, though a notch develops to the speaker's sides at the frequency of the on-axis peak at the top of the woofer's passband. There are also some off-axis peaks at the frequencies where the tweeter has some on-axis notches. All of this means that the room's reverberant field is not going to be as uneven or colored as you might expect from the on-axis response.

Fig.4 Sonist Concerto 3, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.

Fig.5 Sonist Concerto 3, 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.

In the vertical plane, fig.6 reveals that a sharp suckout develops at the frequency of the on-axis peak for axes above the tweeter, which is 37" from the floor with the speaker sitting on its carpet-piercing spikes. Sitting so your ears are above the tweeter axis might work against the audibility of the peak at the top of the woofer's passband, therefore.

Fig.6 Sonist Concerto 3, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–10° below axis.

The Concerto 3's step response on the tweeter axis is shown in fig.7. It reveals that while the tweeter (the sharp down/up spike just before the 4ms mark) is connected in inverted acoustic polarity, the decay of its step is smoothly integrated with the start of the woofer's positive-going step. The decay of the woofer's step, however, is overlaid with some higher-frequency ringing, which results in a significant ridge of delayed energy at the frequency of the on-axis peak in the cumulative spectral-decay plot (fig.8).

Fig.7 Sonist Concerto 3, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8, Sonist Concerto 3, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Sonist's Concerto 3 is easy to drive and will play loudly with very few watts but it suffers from some severe measured problems—that resonant enclosure, the strong upper-frequency resonances in the port's output, and that severe break-up mode at the top of the woofer's passband. All I can assume is that the balance of its virtues and faults has been carefully arranged by its designer to allow the music through relatively unscathed for much of the time, particularly when it is used with a tube amplifier having a high source impedance.—John Atkinson

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Footnote 1: I do note that Art preferred the sound with the Concertos 3s sitting on the floor rather than on spikes. Using the spikes allows cabinet resonances to develop to their fullest, whereas placing the speakers on the floor will damp the resonances somewhat.—JA