Orpheus 808 loudspeaker Measurements

Sidebar 3: Measurements

The frequency response of each speaker was measured in the listening window—spatially averaged to minimize room standing-wave problems—using a 1/3-octave warble-tone generator, which is said to be a little more analytical than the filtered pink-noise signal I have used in the past; in addition, the nearfield low-frequency response of each speaker was measured with a sinewave sweep to get an idea of the true bass extension relative to the level at 100Hz.

Measured nearfield on the lower woofer, the –6dB point relative to 100Hz lay at a low 36Hz, though there was a 3dB rise in output between 60Hz and 80Hz. Note, however, that this doesn't take into account the contribution from the passive radiator which comes into play below 45Hz. In-room, the extension held up down to 30Hz. Checking the subwoofer on its own revealed that it did roll off above 100Hz, at a 12dB/octave slope. Measuring the nearfield response with the subwoofer disconnected revealed that the upper woofer is allowed to operate fullrange at low frequencies, its #150;6dB point lying at 39Hz.

The spatially averaged in-room response (fig.1) correlated with the listening impression gained with pink noise of a "double-bump" sonic signature. The entire mid- and upper-bass region (50–160Hz) was elevated by between 4 and 6dB, the midrange then remaining quite flat, apart from a broad lack of energy in the octave between 500Hz and 1kHz, up to 2kHz or so. A pronounced peak in the treble response between 3 and 6kHz, possibly due to a tweeter resonance—the intensification of the sound was too lively to be a simple equalization feature—was then followed by a gently rolled-off HF.


Fig.1 Orpheus 808, spatially averaged 1/3-octave response in JA's room.

The sensitivity at 1kHz, measured using 1/3-octave pink noise centered on 1kHz, was around 83dB/W/m, appeared very low for the cabinet size and was 3dB lower than spec. However, this discrepancy would be explained by the measured lack of energy in the 1kHz region. Looking at the impedance curve (fig.2), the well-damped main box resonance appears to lie at 41Hz, but there is no pronounced dip below that frequency to reveal the passive radiator tuning. The use of two 8 ohm drivers in parallel results in a very low impedance through the upper bass, dropping below 4 ohms from 60Hz to 115Hz and reaching a minimum of 3.1 ohms at 79Hz. Many rock recordings have significant energy in this octave: the 808's low impedance in the same region, coupled with the low sensitivity, means that power-amplifier choice will be critical if the listener desires high replay levels with this type of music.—John Atkinson


Fig.2 Orpheus 808, modulus of impedance in ohms vs frequency in Hz.
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