Image Concept 200 loudspeaker Measurements
The Image 200's frequency response was measured in the listening windowspatially averaged to minimize room standing-wave problemsusing a 1/3-octave warble-tone generator; the nearfield low-frequency response of each speaker was also measured with a sinewave sweep to get an idea of the true bass extension relative to the level at 100Hz. The change of impedance with frequency and the voltage sensitivity (using 1/3-octave pink noise centered on 1kHz) were also measured.
Fig.1, the plot of impedance with frequency, shows the Image 200 to be a reasonably constant load through the midrange, reaching a minimum of 4 ohms in the upper bass, but then varying little. A 6 ohm impedance overall would be nearer the mark than the specified 8 ohms, which, in conjunction with the low sensitivityI measured around 8586dB/W/m at 1kHzwill mean that owners of the Concept 200 should use beefy amplifiers (in current terms) with at least 50Wpc to hand. Of typical reflex shape, the curve reveals the port to be tuned to 31Hz, suggesting good bass extension.
Fig.1 Image Concept 200, electrical impedance (2 ohms/vertical div.).
Measured in the nearfield, output from the lower woofer dropped to 6dB at 44Hz, but this will be augmented by the output from the port on the speaker's rear. Fig.2, the spatially averaged in-room response, shows the LF to reach down into the nether regions, not reaching 6dB down with respect to the 1kHz level until 20Hz! The unevenness seen in the midbass is probably a room interaction effect, the slight peak at 40Hz being due to a room resonance. The curve is then smooth through the midrange, though there is a lack of energy in the octave below crossover, with then a little too much energy in the room in the lower part of the tweeter's passband.
Fig.2 Image Concept 200, spatially averaged, 1/3-octave response in JA's first Santa Fe listening room.
The actual shape of the curve in the treble is almost identical to that of the Camber 3.5A, despite the latter's very different tweeter. Perhaps this is the response to give the best result in the NRC's standardized listening room? I found the treble response to be actually flatter off the tweeter axis, suggesting that the Concept 200s will sound most neutral facing straight ahead rather than toed-in toward the listener. The tweeter also appears quite smooth, although there is a hint of "character" between 10kHz and 12.5kHz.John Atkinson