Snell Type K/II loudspeaker Measurements

Sidebar 3: Measurements

Driving the loudspeaker with a variable-frequency sinewave oscillator revealed a lively cabinet, the enclosure vibrating significantly. Many resonances were detected, the most severe of them at 100Hz and 270Hz, which produced audible cabinet contributions to the tone. Rapping the enclosure with my knuckles produced a fairly lively tone.

Looking at fig.1, the Type K/II's impedance magnitude (solid line) and phase angle (dotted line), the sealed-box tuning is evident from the single impedance peak which reaches 24.3 ohms at 63Hz. The impedance never drops below 4.5 ohms, thus should not present a too difficult load for an amplifier. However, an amplifier with some ability to drive current into low impedances is suggested.

Fig.1 Snell K/II, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Fig.2 shows the Type K/II's FFT-derived, anechoic frequency response measured on the tweeter axis with the loudspeaker's HF level control all the way up, spatially averaged across a 30° lateral window (footnote 1). The curve on the graph's left-hand side is a nearfield measurement made with the Audio Precision System One, with the B&K microphone almost touching the woofer's dustcap. A computer program JA wrote appends the nearfield measurement to the averaged MLSSA data, with approximate level matching between the two curves.

Fig.2 Snell K/II, anechoic response on tweeter axis at 48", with treble control full and spatially averaged across a 30° lateral window (red), and with the treble set to middle (blue), corrected for microphone response, with nearfield woofer response plotted below 300Hz (black).

Some irregularities between 500Hz and 3kHz can be seen, although the auditioning didn't suggest problems in this region. Above 3kHz, the response rises rapidly, with a very large increase in energy above 10kHz. This is typical of the Vifa tweeter used in the Type K/II. This curve corresponds to my impression of too much top-octave energy, rather than excessive upper-midrange/lower-treble brightness. The relatively gentle low-frequency rolloff typical of a sealed box can be seen to start below about 75Hz, confirming the Type K/II's specified –3dB point of 70Hz anechoic.

Looking at the time domain, the Type K/II's impulse response is shown in fig.3 as measured from 48" away on the tweeter axis. Very little ringing can be seen. The step response is shown in fig.4. Both drive-units are connected in the same negative acoustic polarity.

Fig.3 Snell K/II, impulse response on tweeter axis at 48" (5ms time window, 30kHz bandwidth).

Fig.4 Snell K/II, step response on tweeter axis at 48" (5ms time window, 30kHz bandwidth).

Fig.5 shows the Type K/II's cumulative spectral decay plot. The decay is quite rapid, but with a little bit of hash between 5 and 10kHz as well as in the low treble. (The dark ridge at 16kHz is the computer monitor's line-scanning frequency, and not part of the loudspeaker response.) The exaggerated treble can be expected to be less severe with the HF level control turned down, of course.—Robert Harley

Fig.5 Snell K/II, cumulative spectral-decay plot at 48" (0.15ms risetime).

Footnote 1: I performed all the MLSSA measurements on the K/II in Tom Norton's Los Angeles condo while I was visiting him to measure drive-units at sea level in connection with my investigation of the effect of altitude on loudspeaker performance.—John Atkinson
Snell Acoustics
300 Jubilee Drive, PO Box 3717
Peabody, MA 01961-3717
(978) 538-6262