In this blog post we look at the Peerless TC9FD18-08 3-1/2" Full Range Driver.
I also look at the affect of a new diffusor design that gets installed onto the rear wall of the cabinet. This helps break up the rear wave coming off the driver.
I will conduct my initial testing without the diffusor, and then install the diffusor and re-test to see if there is any affect. If you would like to try the diffusor yourself I offer the 3D CAD files in various sizes here.
This TC9 is available for $20 USD each and represents to bottom of the price scale for this size category. It uses a patented PentaCut technology which is described as follows:
"Our patented PentaCut technology is a five-sided cut around the cone that prevents severe cone breakup and distributes cone breakup modes. By adding cuts to the cone, we are able to spread out the distortion modes, which helps us gain more control of the smoothness of the frequency response, while also achieving reduced distortion."
Measurements
I began by measuring the frequency response mounted in my small transmission line enclosure. The baffle measures 12cm wide and has no edge treatment. The anomaly at 2kHz is a rear wall reflection inside the cabinet which I hope the diffusor will help resolve.
Frequency Response
Off-Axis
Shown below is the 0,15,30 degrees off-axis results. The off-axis looks very well behaved for a driver of this size. It's not too far off what you would see for a tweeter.
Impedance Sweep
Most of what we are seeing here is caused by the transmission line enclosure. I've blocked the vent for this test. We can see the back wall reflection occurring at 2.1kHz.
Step Response
Burst Decay
Overall the burst decay is acceptably clean with the exception of the 2kHz resonance.
Cumulative Spectral Decay
This driver shows good time domain performance especially in the upper treble.
Harmonic Distortion
85dB test SPL is shown below. Distortion is high at nearly 1% in the bass and mid-bass region. The midrange and treble distortion remains low.
Increasing the test SPL to 90dB sees distortion jump in the mid-bass to 5%.
Increasing the test SPL to 95dB is shown below.
Intermodulation Distortion
Starting with a bass limited test signal at 75dB we see -60dB performance across the spectrum which is quite good.
Increasing the test SPL to 85dB is shown below. We some some increase in IMD at 2kHz and in the 10kHz region. 
Increasing to 95dB is shown below. We are now at -45dB IMD at 2kHz. 
Adding Rear Diffusor
I had the diffusor 3D printed at my local library for $5 dollars. I am now ready to install the diffusor. I designed the special shape using the ES curvature and pentagon shape. The idea is to create as many randomly angled surfaces. This should help reduce the amount of energy coming back through the driver cone.
Up until now all the tests were done with no diffusor. I installed the diffusor using a single wood screw as shown below.
I then retested with the diffusor in place.
Below is an overlay of the impedance sweep on the affect of the diffusor in blue. We can see some improvement in the 200Hz, 330Hz and 2kHz region.
Below is overlay on the affect of the diffusor in place (black).
We do see some flattening in the upper treble as well.
We see the energy at 2kHz dissipate by around 20 periods compared to 30 without the diffusor.
For quick reference the non-diffusor burst decay is shown below.
Below is the CSD with the diffusor.
Conclusion on TC9
The Peerless TC9FD18-08 represents a good performer especially for the cost. The driver is suitable for computer desktop applications requiring decent sound quality. I would not categorize the TC9 as audiophile grade, however it is free from any significant faults, however it does lack that last bit of clarity that comes with more premium products.
Conclusion on Diffusor
The diffusor has a measurable affect on improving the overall sound quality. Considering the small investment of $5 to have the diffusor 3D printed at my local library, it seems a worthy addition to even this low budget DIY project.