FOSTEX FX120 BASS REFLEX SPEAKERS
Giovanni Militano 
INTRODUCTION
Having put together a couple of fullrange speakers in tower enclosures, I wanted to try a smaller enclosure like a bookshelf or studio monitor. With that in mind, I also wanted a very flat response from the speaker. The Fostex FX120 fullrange driver looked like a very good choice and generally seem to receive very positive comments from other DIYers. I am not skilled with woodworking, and since I wanted something that also looks good, I decided that prefabricated speaker enclosures would be a good idea. The curved speaker cabinets with a piano gloss finish from parts express
seemed like a good choice.
FOSTEX FX120 BASS REFLEX ENCLOSURE
The prefabricated enclosures are constructed out of MDF and have a volume of 0.38 ft3. For additional information about the enclosures, see my previous description of the Parts Express curved speaker cabinets.
Figure 1 shows the calculated frequency response for the FX120 in a 0.38 ft3 bass reflex cabinet tuned to 63 Hz. The frequency response was calculated using WinISD. The -3dB point is about 58 Hz.
Figure 1: Calculated Frequency Response - Fostex FX120 0.38 ft3 Bass Reflex Cabinets
The enclosure was tuned to about 63 Hz by using a 42 mm diameter port with a length of 65 mm. The photograph below shows the front baffle with the driver and port installed. The baffle is one inch thick, so to improve airflow there is a chamfer behind the driver which was created using a router.
Photograph 1: 1" Thick Prefabricated Baffle with Fostex FX120 and Port
The speaker cabinets are only lightly damped. I used 1/4" thick upholsterers felt in the front half of the cabinet and a little polyester fill in the back half. A stapler was used to hold the damping material in place. Glue could also be used.
Photograph 2: Speaker Cabinet Dampening
Once all that is complete, add binding posts, wire the speakers and screw the baffle on. Finished photographs of the speaker are shown below. Higher resolution photographs are available in the Photo Gallery: Fostex FX120 Bass Reflex Speaker.
Photograph 3: Fostex FX120 Bass Reflex Speakers - Rear View
Photograph 4: Fostex FX120 Bass Reflex Speakers - Front View
BAFFLE STEP CORRECTION CIRCUIT
Most people will likely prefer some baffle step correction / compensation (BSC). The amount of attenuation required will be room and amplifier dependent. About 3 to 5 dB of attenuation is a good starting point. You can use the component values shown in Figure 2 and tune the BSC to suit your room and amplifier.
Figure 2: Baffle Step Compensation Circuit for 8-inch Wide Baffle
I found that Rbsc=3R9 and Lbsc=1 mH (about 3.5 dB) worked well for my room. For those interested in a Zobel / Impedance Equalization Circuit for the FX120, I have measured the voice coil inductance (Le) of my drivers to be 0.11 and 0.12 mH.
MEASUREMENTS AND LISTENING IMPRESSIONS
Figure 3 below shows a sweep of the frequency response from about 150 Hz and up. The frequency response measurements were completed using a Radio Shack 33-2050 SPL Meter. The FX120 is a very smooth driver and I prefer it over the similar size (but inexpensive) Fostex FE127E driver. In a small to medium sized room, this speaker has great sonics and spectacular mids. I suspect this speaker would make a very good studio monitor.
Figure 3: Fostex FX120 Frequency Response Plot at 150 mm
The impedance sweep below was conducted using the Dayton Audio Woofer Tester 3 with no filters on the driver. The impedance sweep confirms the box tuning of about 63 Hz. A Zobel / Impedance Equalization Circuit can be used to tame the rising impedance response due to the voice coil inductance.
Figure 4: Fostex FX120 Bass Reflex Impedance Sweep