MEASUREMENTS
As the schedule didn’t allow sufficient time for Martin Colloms’s pair of Thrax Sirens to be shipped to me for measurements, Thrax arranged for a different loudspeaker to be sent from their factory in Bulgaria. This sample had the serial number 211123. I used DRA Labs’ MLSSA system with a calibrated DPA 4006 microphone to measure the speaker’s behavior in the farfield and an Earthworks QTC-40 mike for the nearfield responses.
According to the manual, the Siren “is best heard with the tweeters at, or just below, ear height.” I therefore started by examining the farfield frequency response on the tweeter axis and immediately ran into a problem. This sample had a severe suckout centered on 2.7kHz in its tweeteraxis response, while MC’s measurements indicated that his Sirens didn’t have this suckout. The loudspeaker’s step response indicated that both drive units were connected in positive acoustic polarity, so I asked Thrax designer Rumen Artarski via email what the correct polarities should be. “The bass unit is ‘normal’ phase, meaning the cone will move outwards with a positive voltage on the red connector.
For the tweeter it is the opposite,” he responded. My sample must therefore have had its tweeter miswired at the factory. I removed the Siren’s rear panel and internal stuffing and rewired its tweeter in the correct inverted polarity. (Much easier to write than do.)
I looked first at the repaired speaker’s sensitivity. My B-weighted estimate was a little lower than the specified 87dB/2.83V/m, at 85.6dB(B)/2.83V/m. (MC also estimated the sensitivity; his estimate was slightly higher than mine, at 86.5dB.) The speaker’s impedance is specified as 4 ohms. My sample’s impedance magnitude (fig.1, solid trace), examined with Dayton Audio’s DATS V2 system, lay between 4 ohms and 5.4 ohms for almost the entire audioband. While the minimum value is 4 ohms at 175Hz, the electrical phase angle (fig.1, dotted trace) is high in the bass and lower midrange, which will have an effect on the equivalent peak dissipation resistance, or EPDR.1 This lies below 3 ohms from 69Hz to 215Hz, with a minimum value of 1.76 ohms at 108Hz. Recorded music can 1 EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier’s output devices as the loudspeaker. See “Audio Power Amplifiers for Loudspeaker Loads,” JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/ index.html.
have high energy in this region, meaning that the Siren is a demanding load for the partnering amplifier.
The impedance traces are free from the discontinuities in the midrange that would imply the presence of cabinet resonances, and the enclosure’s panels did seem welldamped when I rapped them with my knuckles. Using a plastic-tape accelerometer, I found a single high-Q, low-level mode at 480Hz on the top and side panels (fig.2) and an even lower-level mode at 535Hz on the aluminum rear panel. It is safe to say that these modes will have no effect on sound quality.
The saddle centered on 36Hz lying between two low-frequency peaks in the impedance magnitude trace suggests that this is the tuning frequency of the Siren’s reflex port. Measured in the nearfield, the woofer’s response (fig.3, blue trace) has the expected minimum-motion notch at that frequency, the point at which the back pressure from the port resonance holds the diaphragm stationary. The port’s nearfield response (red trace) peaks between 25Hz and 60Hz, in textbook fashion. Its upper-frequency rolloff is clean, with some low-level peaks between 400Hz and 900Hz. The low level of these peaks and the fact that the port fires to the speaker’s rear will mitigate any audible consequences.
The complex sum of the Thrax’s woofer and port outputs (fig.3, black trace below 312Hz) doesn’t have the usual boost in the bass due to the nearfield measurement technique, which assumes that the drive units are placed on a baffle that extends to infinity in both vertical and horizontal planes. The Siren offers excellent lowfrequency extension for a relatively small speaker, with an overdamped alignment.
The black trace above 312Hz in fig.3 shows the Thrax’s farfield output averaged across a 30° horizontal window centered on the tweeter axis. There is a slight excess of energy in the upper midrange and a small suckout just above 1kHz, but the response is otherwise even. The speaker’s output rolls off rapidly above the small peak at 19kHz.
The Siren’s radiation pattern on the tweeter axis (fig.4) is well-controlled, which implies stable, well-defined stereo imaging. As usual with a horn-loaded tweeter, the top-octave output starts to roll off at angles higher than 15° to the speaker’s sides. In the vertical plane (fig.5), the tweeter-axis response is maintained 5° above the tweeter axis, as suggested by Rumen Artarski. A large suckout develops at 2.7kHz more than 10° below the tweeter axis and 15° above it.
In the time domain, the corrected
Siren’s step response (fig.6) indicates that the woofer is connected in positive acoustic polarity; the tweeter’s output is now in inverted polarity. The start of the woofer’s positive-going step arrives first at the microphone, followed by the start of the tweeter’s negative-going step. The positive-going overshoot of the tweeter’s step has a much higher amplitude than the start of its step, which nicely overlays the woofer’s step.
The Thrax’s cumulative spectral-decay, or waterfall, plot (fig.7) is perhaps the cleanest in the midrange and low-mid treble I have ever found!
With its very quiet enclosure, relatively even frequency response, extended, articulate lows, well-controlled horizontal dispersion, and that extraordinarily clean waterfall plot, the Thrax Siren’s measured behavior is exemplary.—John Atkinson