In this blog post I test the Purifi PTT8.0X04-NAB-02 8" Aluminum Cone Woofer and provide some listening impressions.
The PTT8.0X04-NAB-02 retails for around $700 USD currently. It sits squarely in the premium 8" woofer category.
Purifi sent me a pair of woofers to test and to develop a complete speaker plan. It's been over a year and I still have not been able to finish the plan due to other 'paying' projects getting in the way. I also introduced a design study in a previous blog post. So far I have a nice pair of 24mm thick baltic birch cabinets constructed with 'espresso' stain with a satin varnish finish, which is the cabinet I'll be using in this review. So in the interim, I've decided to publish the test data on the low frequency section at least.
Features
- Negligible Force Factor Modulation and Surround Radiation Distortion
- Low Magnetic Hysteresis Distortion
- “Real” long-stroke Performance
- Distortion remains low over full Excursion
- Uncompromised Midrange Performance
- Designed and Manufactured in Denmark
Measurements
I began my testing by measuring the impedance sweep. The driver is mounted in a 40L cabinet tuned to 28Hz.
Frequency Response
Measuring the response at 30cm yields the following result. The port overlay shows the 28Hz tuning with some port noise introduced into the response at 550Hz. I'm using a rear facing 3" flared port for this cabinet.
The response is very linear showing the typical baffle step rising response. The baffle is 30cm wide. The mechanical breakup is centered at 5500Hz. Bass extension is excellent with the driver's output strong down to about 35Hz where the driver's output is sharply reduced as it loads the port at 28Hz.
Time Domain
Looking at the time domain we can see that the burst decay is exceptionally clean.
The CSD plot shows a similar result within it's passband.
Distortion
Starting with harmonic at 85dB we see H2 at 0.17% for 100Hz. H3 is at 0.07% and H4 is at 0.11%.
95dB shows H2 at 0.15%, H3 at 0.12%, and H4 at 0.03%.
105dB shows H2 at 0.46%, H3 at 0.17%, and H4 at 0.05%. We do see some complaining at 40Hz with H2 peaking at 10%, however consider that 105dB is an extremely high SPL level especially for an 8".
Intermodulation Distortion
Switching over to the multi-tone signal, I used a multi-band test signal ranging from 50Hz to 5kHz spaced at 12 bands per octave.
Below is the test results for a 95dB test signal.
IMD is very low with the following results.
125Hz: 62dB
250Hz: 62dB
500Hz: 55dB
By 2kHz we see IMD reduced to -46dB. This correlates to my subjective listening where the woofer sounded best crossed below 1kHz.
These distortion numbers are the best I've measured for a woofer. I've tested a number of 15" woofers over the past few months and generally they all average around the -53dB mark for the 95dB test signal level. This includes frequency ranges from 125Hz, 250Hz, and 500Hz. We see that the Purifi exceeds this numbers by 9dB for the 125Hz and 250Hz region. It could very well be that the bump in IMD at 500Hz is related to port noise which we saw in the frequency response graph. A sealed enclosure may be better for maintaining clarity through this region or use the passive radiators offered by Purifi.
I was a bit conflicted when publishing this review because it begs the question on why one would consider a larger bass cabinet if the 8" achieves such great objective performance. There is a tactile element not reflected in the measurements here. Firstly, there is transient impact as you feel a wave of sound emanate with authority from the horn mouth which has a surface area of 6,000 cm^2, where the 8" has a radiating area of 235 cm^2. Secondly, the horn directs the energy towards you in a focused manner due to it's off-axis pattern control. This creates a pressure wave front that passes over the listener before interacting with the room. The room reflections then diffuse the pressure intensity of the wave front. This contrasts to the omnidirectional nature of the 8" direct radiator where room reflections arrive at the listener almost immediately after the direct sound, in almost proportional intensity. I say almost, since the sound is only attenuated by the physical distance traveled compared to the distance travelled by the direct sound. Our ear compensates for room acoustics (precedence effect, Hass effect etc.) however it doesn't compensate for the physical sensation we feel through our body, chair, floor, and physical vibration of the outer ear. Unfortunately I do not know how to capture this sensation with objective measurement, which can only be experienced subjectively. Having said all of this, it may be argued as to why not simply mount the 8" in a similar but scaled down version of the 1798 cabinet. In fact, this is likely a good solution. Even the new 10" Purifi may lend itself to a 50% scaled down version of the 1798 cabinet. This is something that's been on my mind for a few months now.
I went back and searched previous test data to see if there are other comparable woofers. The closest driver I could find was the Scanspeak 10" 25W/8565 which is shown below. It achieves the following IMD:
125Hz: -56dB
250Hz: -60dB
500Hz: -52dB
With the 25W/8565 costing less than half that of the Purifi, is it worthy compromise? Personally I would be happy with either, having used both of them myself. The 25W/8565 is flat in-room down to 25Hz using a 50L sealed enclosure. It also has a distinctly warm sound character while the Purifi does not really have any specific sound character.
Subjective Listening
The Purifi is really nice. A little soft on transient impact but otherwise a smooth performer. It has no specific sound characteristic and simply reproduces the signal, even complex material. It doesn’t complain at all until you reach its mechanical xmax limit…so very usable high SPL. It sounds clean up to a 1kHz crossover point. Overall, the Purifi 8" represents the best measuring woofer that I've had opportunity to test. I can speculate that the new 10" version is even better.