Dynaudio Special 25 & Sub 500 surround speaker system Measurements
The Dynaudio Special 25's ported enclosure is tuned to approximately 40Hz, with a minimum impedance of 4.7ohms at 154Hz. I would judge this speaker to be relatively easy to drive, and rate its nominal impedance—conservatively—at 6ohms. The Special 25's sensitivity measured approximately 87dB/2.83V/m.
The horizontal front response of the Special 25 is shown in Fig.1 (violet). This is the pseudo-anechoic response averaged over a 30° forward horizontal angle (+/-15°) at tweeter height, combined with the nearfield responses of the woofer and port. The speaker's effective lower limit (–10dB) was 32Hz.
Fig.1: Dynaudio Special 25, pseudo-anechoic response off horizontal axis at 45° (red) and 60° (blue).
The Special 25's averaged front horizontal response falls into a very narrow window, producing the sort of smooth, even response you have a right to expect at this price but don't always get. However, JB was troubled by what he judged to be some excess midrange forwardness in his system. If you look closely at the averaged response (violet), the curve appears to show an elevated plateau of about +2dB from about 500Hz to 1.5kHz. Deviations of this degree over more than an octave can easily be audible, particularly if they fall anywhere near the region in which the ear is most sensitive (about 3kHz). If the deviations are small, they may well be audible even if they fall within the boundaries of an otherwise superbly engineered speaker—which is certainly the case here. In that event, the characteristic can be judged either a positive or a negative, depending on the listener, the program material, and the room.
The Special 25's wide off-axis responses are a little less forward, but, apart from the inevitable high-frequency rolloff, they are well matched to the averaged forward response. The vertical response curves shown in Fig.2 indicate that the Special 25 should sound best if the listener's ears are positioned on or below the tweeter axis.
Fig.2: Dynaudio Special 25, pseudo-anechoic response at 15° above (red) and 15° below (blue) tweeter.
To determine whether the 65Hz peak that JB heard from the Dynaudio Sub 500 originated in the subwoofer itself or in his room, we measured the Sub 500's nearfield response (not shown). It clearly showed a well-controlled response that held up strongly to 25Hz without peaks, and was down only a few dB at 20Hz. The deviation he reported was clearly in the room/subwoofer positioning.—Thomas J. Norton
All figures: Violet curve: pseudo-anechoic response on tweeter axis, averaged across a 30° horizontal window, combined with nearfield woofer and port responses. All measurements taken at 1m.