In this blog post I test the Viawave SRT-7 on Horn No.1671 which is specifically designed to work with this driver. This horn features a 2kHz Fc along with the ES horn flare curvature for low diffraction and colouration.
I conducted my measurements with a 6.8uF capacitor to protect the driver. The resulting response is shown below at 0,15,30,45,60 off-axis. Pattern control is maintained well down to 2kHz.
Removing the protective capacitor results in the the measurements shown below. This shows that we have good pattern control even down to 1.5kHz.
With the microphone placed 30cm away from the mouth of the horn, I conducted burst decay measurements which are shown below. We can see this tweeter is virtually free from any resonances.
Looking at spectral decay within the ARTA test software, shows clean results (see below).
The step response shown below shows nearly perfect initial rise and decay with no lagging overhang.
The impedance sweep shown below shows only a 2ohm inductive rise through the driver's passband.
I looked closely at the driver's nonlinear function by testing the driver at various SPL levels and crossover frequencies. My test signal is a six band (tone) per octave multitone signal from 1kHz to 20kHz. The microphone used is a Shure SM57 which has a noise floor down to -85dB regardless of SPL level at the mic.
I began the test at a low SPL level of only 60dB (@1m) as shown below. IMD is below the noise floor of the measurement system.
IMD remains below the measurement system even after raising the test SPL to 65dB. (see below)
Increasing the SPL to 70dB still reveals no nonlinear distortion. (see below)
Increasing to 80dB we start to see some distortion products at 2kHz -60dB down from the fundamental. (see below)
Increasing the test SPL to 85dB we see distortion worsen to -52dB at 2kHz. (see below)
By 90dB we see distortion rise quickly especially in the 10kHz region. Distortion rose +10dB in this region compared to 85dB. (see below)
Adding a 4kHz 4th order high pass filter calms things down again. Distortion has now returned to a respectable -70dB at 10kHz, even at our test SPL of 90dB. (see below)
Increasing the crossover point from 4kHz to 6kHz results in a further improvement. Distortion is now only -77dB at 10kHz and barely reads on my measurement system.(see below)
Increasing the crossover point again from 6kHz to 8kHz reduces the distortion to below the threshold of the measurement system. This means that the distortion performance may be even better than -77dB.
Increasing the test SPL from 90dB to 95dB reveals the distortion again at -70dB. (see below)
Changing the high pass filter from 8kHz to 10kHz sees no change in the performance. (see below)
Increasing the test SPL from 95dB to 100dB results in -68dB. (see below)
Increasing the test SPL from 100dB to 105dB sees distortion remain low at -65dB. (see below).
It was very obvious during crossover setup that this tweeter needed to be crossed above 4kHz for acceptable clarity if reproducing SPL levels above 80dB. This is clearly demonstrated in the preceding tests. The Viawave exhibits excellent clarity and smoothness. I could find no faults other than it did seem to lack some expressiveness. In other words there was a slight lack in dynamics, however this would not be detectable if one didn’t directly compare to other alternatives. For example, it did not sound as realistic on electric and acoustic guitar as the Fostex T96A. This may be related to the driver’s tendency to quickly run out of headroom at 90dB in the 10kHz region. Musical peaks can easily exceed +20dB in crest, which may be why I can detect some subtle dynamic compression on the leading transients when the driver attempts to reproduce instruments such as saxophone and the snare drum. Regardless of this shortfall, within the driver’s linear SPL range, it exhibits world class performance. At the time of writing (May 2022), availability on this driver is uncertain.
Please also check out Hifi Compass’s review of the SRT-7 in stock form (No Horn) for comparison data. I could not help but notice different conclusions than mine on the maximum output capability. Hifi Compass indicates that this driver is capable of 104dB with a 1st order 2kHz crossover. I suspect this is due to the test method of using a single test tone sweep to evaluate harmonic distortion. This simple test tone lacks the modulation tones in the lower part of the frequency spectrum that alter the sound quality in the upper portion of the spectrum. In other words, a simple frequency sweep does not fully reveal the true distortion seen in music. So care should be taken when comparing data to observe what type of test signal was used. I would estimate that you loose -15dB of headroom when comparing distortion figures between a sine sweep and a multitone test signal. This also does not account for additional loses due to the transient peaks found in music.