Sidebar 2: Measurements
I use a mixture of nearfield, in-room, and quasi-anechoic FFT techniques to investigate possible objective factors that might explain the sound heard. Readers wanting more detail on this regime should read pp.166–167 in the October 1989 issue (Vol.12 No.10).
The LS7t's response to a 25µs rectangular pulse (with the grille on) is shown in fig.1. The initial downward plunge is the tweeter, with then its overshoot on the other side of the time axis reinforced by the rise of the bass/midrange unit. This is in turn overlaid by ultrasonic ringing from the metal-dome tweeter, though this is mild compared with that, for example, of the SEAS unit found in the Thiel CS1.2 and Meridian D600, or the various Celestion aluminum-dome tweeters.
Fig.1 Rogers LS7t, impulse response on tweeter axis at 50" (5ms time window).
Repeating this impulse measurement across a ±15° lateral window, calculating the equivalent anechoic amplitude responses with the Fast Fourier Transform, and averaging the results gives a curve representing the spectrum of the direct sound from the speaker to reach the listener's ear. Plotted from 200Hz upward with 200Hz resolution, this is shown on the right of fig.2. Impressively smooth through the midrange, the response is upset by a degree of liveliness in the mid-treble and an overall shelved-up top octave. Repeating this measurement without the grille in place revealed the top-octave boost to become a little higher still, while the slight peaks at 3kHz and 5–6kHz were accentuated. The moral: for the smoothest treble from this speaker, leave the grille on. Fig.2 also reveals the tweeter resonance to lie at 25.4kHz, with a modest 5dB rise above the 1kHz reference level apparent.
Fig.2 Rogers LS7t, anechoic response on tweeter axis at 48", averaged across 30° horizontal window, and corrected for microphone response, with nearfield woofer response.
The left-hand curve in fig.2 shows the woofer's nearfield response. Reaching its half-power point at 53Hz, this does not take the output of the port into account. In-room, the bass extended to –6dB at 33Hz or so, though, as can be seen from the spatially averaged response (fig.3), plotted on a 1/3-octave basis, there is an excess of upper-bass energy in the room, this typical of a "traditionally" tuned reflex design. The overall response trend in-room, however, is pretty smooth through the midrange and treble, relieved only by a slight excess around 4–5kHz and in the top octave. "Excellent pair-matching," say my notes.
Fig.3 Rogers LS7t, spatially averaged, 1/3-octave in-room response.
As can be seen from the plot of impedance amplitude and phase (fig.4), the LS7t represents an easy load for an amplifier to drive, staying above 8 ohms over nearly all the audio band. The port tuning can be seen at 40Hz.—John Atkinson
Fig.4 Rogers LS7t, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.). Note that the phase scale is inverted, with inductive phase angles incorrectly shown as negative rather than positive.
