Snell Acoustics Type A Reference loudspeaker Measurements
My estimate of the Type A's B-weighted sensitivity was a little lower than the specification, 86dB/2.83V/m. Its impedance, howevermeasured with the Audio Precision System Onewas only moderately demanding, and then only in the treble, where the impedance dips below 4 ohms. Fig.1 shows the tower's impedance magnitude and phase with the tone control set to its "flat" position and the rear-firing tweeter switched off. Though the phase angle is moderate through the midrange, the magnitude is high. The peak at 46Hz indicates the sealed-box's tuning; this is high for the size of the enclosure, but remember that the tower is intended to be used with a matching subwoofer. With the tone control set to its maximum positions and the rear tweeter switched on, the impedance drops to 2.9 ohms at 3.7kHz; this should not be a problem for the kind of solid-state amplifier likely to be paired with the Type A Reference.
Fig.1 Snell Type A Reference, electrical impedance (solid) and phase (dashed) with the HF control set to its flat position and the rear tweeter switched off (2 ohms/vertical div.).
Fig.2 shows the electrical drive signals to the tower drive-units. The midrange units can be seen to be padded down a little compared with the woofer and tweeter, as is the rear-firing tweeter (the bottom trace above 4kHz). The resultant acoustic response of the individual drive-units, measured on an axis level with the tweeter 39" from the floor, using the DRA Labs MLSSA system, can be seen in fig.3. The woofers cross over to the midrange units at 300Hz, with the tweeter coming in above 3kHz. The rear tweeter is about 7dB down in maximum level and covers a bandpass of 618kHz.
Fig.2 Snell Type A Reference, individual electrical drive signals, measured with the external crossover loaded by the drive-units.
Fig.3 Snell Type A Reference, acoustic crossover on tweeter axis at 45", corrected for microphone response, with nearfield woofer and midrange responses plotted below 300Hz and rear tweeter response plotted above 2kHz.
Fig.4 Snell Type A Reference, anechoic response on tweeter axis at 45", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield midrange and woofer responses plotted below 200Hz.
The Type A's overall on-axis response is shown in fig.5. Impressively flat throughout the treble, the curve is marred by a bit of excess energy in the midrange. The lower midrange appears lean in this graph, but this may be a result of the difficulty in splicing the summed nearfield bass responses to the quasi-anechoic farfield response with such a physically large loudspeaker. In the bass, the restricted low-frequency extension can be seen, reaching 6dB at the lowest note of the four-string double bass, 42Hz. But this is sufficient to allow the external EC 200 electronic crossover to optimally integrate the towers with the SUB 1800 subwoofers.
Fig.5 Snell Type A Reference, effect of HF control set to its maximum and minimum positions, normalized to response on tweeter axis (5dB/vertical div.).
The effect of the HF control can be seen in fig.5. Just the differences between the on-axis response and the responses with the tone control set to its maximum and minimum positions are shown. These reach a sensible maximum treble cut of 2dB at 8kHz, and a maximum boost of 1.1dB at 10kHz.
Vertically (not shown), the Type A Reference's balance doesn't change to any significant extent as long as the listener's ears are somewhere between the two midrange units, a distance of between 30" and 45" from the floor. Laterally (fig.6), the Type A features the kind of even off-axis treble rolloff that has proved to correlate with excellent soundstaging.
Fig.6 Snell Type A Reference, horizontal response family at 45", 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 time domain, the speaker's step response (fig.7) indicates that all the drive-units are connected with the same acoustic polarity, while the associated cumulative spectral-decay or waterfall plot (fig.8) is relatively clean.John Atkinson
Fig.7 Snell Type A Reference, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).
Fig.8 Snell Type A Reference, cumulative spectral-decay plot at 45" (0.15ms risetime).