- Written by Chris Groppi
- Published on 19 January 2009
On the Bench
I measured the CS3.7s using both a Smith and Larson Woofer Tester 2, and SpectraPlus FFT analyzer software with a Roland Edirol UA-101 24 bit 192 kHz balanced USB sound interface and an Earthworks M30BX calibrated microphone.
THIEL CS3.7 impedance magnitude and phase as measured with a Smith & Larson Woofer Tester 2.
THIEL CS3.7 impedance plotted on a Smith Chart normalized to 4 ohms. The green plus is the marker, at 103.7 Hz.
Impedance is shown in two formats, the traditional magnitude and phase plot, and the Smith chart style plot I have discussed before in our Technical Articles section. The results clearly show why a very powerful amplifier stable into low impedances is essential. From 100 Hz to 20 kHz, the impedance is around 3 Ohms. Minimum impedance of 2.3 Ohms occurs at about 120 Hz. Luckily, these low impedances are paired with very small phase angle (i.e. those low impedances are almost entirely resistive, with very little capacitance or inductance). Looking at the Smith chart, almost the entire frequency range is in a very small part of the plot near 3 Ohms real impedance. The green cross denotes 100 Hz. The rest of the frequency range from 100 Hz to 20 kHz is in the small loop just above the green cross. The rest of the trace below and to the right of the green cross is entirely at low frequencies from 20 Hz to 100 Hz.
THIEL CS3.7 THD spectrum of a 10 kHz sine wave.
THIEL CS3.7 THD spectrum of a 1 kHz sine wave.
THIEL CS3.7 THD spectrum of a 50 Hz sine wave.
Harmonic distortion was measured at 50 Hz on the woofer axis, and at 1 kHz and 10 kHz on the coaxial tweeter and midrange axis at a distance of 1 foot. Absolute SPL calibration was performed at 1 kHz using a Radio Shack SPL meter. THD+N was at approximately 0.25% at 10 kHz and 1 kHz, and climbed to 1% at 50 Hz. These are very low numbers, particularly at higher frequencies. At 50 Hz, the number is contaminated by background noise; I had to wait until my forced air heating cycled off to make the measurement, but there was still measurable noise from the street outside and from kitchen appliances etc. THD+N vs frequency was also measured in 1/3 octave bins. The rapid rise at low frequency is due to a lack of signal as the bass rolls off, not due to a real increase in THD. This shows a very flat spectrum around 1% for most of the audio band.
THIEL CS3.7 THD+N versus frequency from 30 Hz to 20 kHz.
Frequency response was measured at 1m and 2m distance from the speaker at listening height (about 6" below the coaxial midrange/tweeter). The response is admirably flat, given that the speaker was measured in a normal living room. Many of the room effects seen in the 1m trace are absent in the 2m trace.
In-room frequency response measured at a distance of 1m on the listening axis.
In-room frequency response measured at a distance of 2m on the listening axis.