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Fostex FE103En Bass Reflex Bookshelf Speakers

 Gio Militano   Canadian Flag   To email Gio, type out the email address.

Fostex FE103En DIY Bass Reflex Bookshelf Speakers

Until recenty my desktop computer has always shared a room with a Hi-Fi setup so there has never been a need for using decent speakers with the computer. When the desktop computer was moved into it's own room (with no HiFi) I quickly realized that good speakers were now required for decent quality music playback. I had been using some low cost Radio Shack speakers with an inexpensive Tripath based amplifier and I was really itching for an upgrade. The speakers would be used primarily for background music while working on the computer and also for watching videos, gaming and some recording. As the speakers would be used on a desk, the size needed to be restricted to a small bookshelf / monitor sized speaker. Over the past several years I have grown quite fond of the sound from the various Fostex fullrange speaker drivers. A fullrange driver can be quite appealing as it offers point source sound with no crossover to mess with the phase so you also get great imaging. The lack of a crossover reduces the material costs and also greatly simplifies the design and construction of the speaker. For these speakers I decided to try out the Fostex FE103En fullrange driver ($44US, Apr 2013) in the "recommended" bass reflex enclosure from the datasheet. The bass reflex loudspeaker enclosure has a volume of 6 Liters (L) and is tuned to 95 Hz.

Fostex FE103En Fullrange Speaker Driver

Photograph 1: Fostex FE103En fullrange speaker driver.

Bass Reflex and Ported (Vented) Loudspeaker Enclosures

Bass Reflex and Ported (Vented) Loudspeaker Enclosure Cross-section Diagram A bass reflex (ported) speaker enclosure improves the low frequency response of the loudspeaker system by transmitting the energy from the rear diaphragm of the driver and the rear baffle through a tuned port to the listener. These phase-inverting type of loudspeaker enclosures are most commonly referred to as vented, ported or bass-reflex. The interior of the speaker boxes are typically lined with a thick damping material or stuffed with loose synthetic or wool fill. The fill is used to dampen the rear wave from the driver and minimize standing waves and reflections within the enclosure. The bass reflex type of loudspeaker enclosure is among the most common type used as it lends itself well to small size and reasonable bass output.

Fostex FE103En Recommended Bass Reflex Speaker Enclosure Plans

The loudspeaker box details are from the Fostex FE103En datasheet and recommended enclosure plans and are shown below with some additional build notes. The datasheet notes the enclosure volume as 6 L and tuned to 95 Hz. From the dimensions the gross volume is calculated to be 6.9 L and the box tuning frequency to be about 97 Hz.

Fostex FE103En Bass-Reflex Speaker Box Plan

Figure 1: Fostex FE103En recommended bass-reflex type speaker box plan

Figure 2 shows the calculated frequency response for the Fostex FE103En Recommended Bass-Reflex Speaker. The frequency response simulation was completed using WinISD speaker designing software. The orange line on the plot shows the calculated frequency response using the Thiele-Small parameters from the datasheet. The blue line shows the calculated response using the actual measured parameters of the drivers. See the Fostex FE103En fullrange driver page for a comparison of the Thiele-Small parameters. Despite the variation in the parameters, the net effect on the actual response is rather small. In the response plots there is a 2 dB hump around 100 Hz. I tend to find that I prefer this slightly warmer bass response as the speakers sound more punchy and it also helps balance the forward mid-range.

Simulated Frequency Response - Fostex FE103En Recommended Bass-Reflex Speaker

Figure 2: Simulated frequency response - Fostex FE103En recommended bass-reflex speaker

Building - Fostex FE103En Bass Reflex Speakers

The loudspeaker cabinets were built using 15 mm thick Baltic birch (multiplex) plywood. Baltic birch plywood is dense and strong, made up from 11 ply sheets. Baltic birch plywood is often amongst the most sought-after wood for producing loudspeaker cabinets. Part of the reason being that Birch has a natural resonance that peaks in the high and low frequencies, which are typically the most difficult for loudspeakers to reproduce.

Baltic Birch (Multiplex) Plywood for Speaker Cabinets

Photograph 2: Baltic birch (multiplex) plywood for speaker cabinets.

The speaker connection is a DB input cup located on the rear baffle. Be sure to use the supplied gasket and seal the speaker terminal input cup so there are no air leaks from the enclosure.

Fostex FE103En Bass-Reflex Speaker Box

Photograph 3: Fostex FE103En bass-reflex speaker box

For best results, it is important to properly dampen the speaker cabinet. Acousta-Stuf fiber fill is used inside the speaker cabinet to dampen the rear wave from the driver and to reduce standing waves and internal reflections. Fluff up about 110 to 170 g (4 to 6 oz) of Acousta-Stuf fiber and fill the speaker box. Try and keep the fill material away from the immediate rear of the speaker driver.

Acousta-Stuf Fiber fill for speakers

Photograph 4: Acousta-Stuf fiber fill for speakers.

Acousta-Stuf fill used to dampen speaker box

Photograph 5: Acousta-Stuf fiber used to dampen speaker box.

There are a couple of easy modifications that can be made to improve the FE103En drivers. Fill the open gap between the magnet and the speaker frame using duct seal. This will help to stiffen the motor assembly and dampen the frame against ringing. Also, place felt on the rear of the magnet to cover the reflective surface. For full details of the driver mods, see the information page on modifications and tweaks for the small Fostex fullrange speaker drivers.

Modified and tweaked Fostex FE103En fullrange speaker drivers

Photograph 6: Modified and tweaked Fostex FE103En fullrange speaker drivers.

The wire leads from the speaker terminal input cups were soldered directly to the driver contacts. Make sure that the driver gasket in place and attach the drivers to front speaker baffle using the screws and washers supplied with the FE103En driver. Once the speaker cabinet is all sealed up, check to make sure there are no air leaks at the driver, port and terminal cup.

Finished Fostex FE103En Bass Reflex Speakers

Photograph 7: Finished Fostex FE103En bass reflex speakers.

Baffle step compensation - Fostex FE103En bass reflex speakers.

Since the loudspeakers utilize a fullrange speaker driver, there is no need for a crossover dividing network. If you are planning on using the finished speakers as near-field monitors you can skip over this section on baffle step compensation. However, if you plan to listen to the speakers from a far-field listening position, a correction network will be necessary to account for the loudspeaker enclosure diffraction loss that occurs in the low frequency range. The high frequencies from the speaker radiate sound into "half-space" (only radiate sound forward). But at the low frequencies the speaker radiates sound both forward and to the rear ("full space"). The net result is a gradual shift of 6 dB from low to high frequency and this is what is refered to as the baffle step or enclosure diffraction loss. 6 dB is how much correction would be necessary in open space. However, that much attenuation will really reduce and suppress the dynamics of the fullrange driver. With wall loading the diffraction loss will be somewhat less than 6 dB. It is suggested to experiment with speaker placement in order to limit the required baffle step compensation to a maximum of about 2 - 3 dB. Pulling the speakers closer to the walls can help reduce how much correction is required. Also, if you toe the speakers inward it will help tame the shouty top end somewhat. You can use the online baffle step corection calculator to determine the value of inductor and resistor required for the circuit. Figure 3 shows typical values for this bass-reflex speaker enclosure. Note - If you will be using the speakers as near-field monitors, no baffle step compensation network is necessary.

Fostex FE103En Bass-Reflex Baffle Step Compensation Circuit

Figure 3: Baffle step compensation circuit - Fostex FE103En bass reflex speakers.

Since the speakers will be primarily used as nearfield (studio) monitors, the terminal cup binding posts were connected directly to the driver terminals. If you will be using a baffle step compensation circuit and prefer to house the correction circuit inside the enclosure you can use a bi-amp speaker terminal with one set to the baffle step circuit and the other set wired directly to the driver. This will leave you with the option of using the baffle step compensation circuit or by-passing for use as nearfield monitor speakers. Photograph 8 from Peter's Fostex FE127E DIY bass reflex bookshelf speaker build shows a bi-amp speaker terminal cup with a baffle step compensation circuit.

Baffle Step Compensation Circuit on bi-amp speaker terminal

Photograph 8: Baffle step compensation circuit on bi-amp speaker terminal.


For the first listening evaluations I tried the bookshelf speakers in the main TV room (Photograph 9). The amplifier is an inexpensive Denon A/V HDMI receiver. The player is a Oppo BDP-83 and the other component is a Panamax mains voltage regulator / filter / surge protector. Off to the right and not shown in the photo is a powered 10-inch (250 mm) diameter subwoofer in a sealed enclosure that is used to fill in the bottom end of the speaker system. The bookshelf speakers sit on VTI stands and are connected to an inexpensive Denon home theater receiver set to a 2.1 speaker configuration. Through the receiver settings, there is some control in selecting the subwoofer / speaker crossover point which has been set at 80 Hz. Removing the need for the small Fostex drivers to pass the deep bass tones greatly reduces distortion and improves the sonic performance. Alternatively, an active crossover can be used. Experiment with crossover points between about 80 and 120 Hz. Over about 100 Hz the bass will start to be locatable so don't be afraid to try a sub on each channel in close proximity to the speaker. The speakers work well in this setup and would also work as rear surround speakers.

Fostex FE103En Bookshelf Bass Reflex Speakers on Stands

Photograph 9: Fostex FE103En bookshelf bass reflex speakers on stands.

I have been using these little speakers with the desktop computer in the office for a few months now and I am really happy with the results. The speakers are powered from a Topping TP23 (25WPC class-T) amplifier with USB input. For playback I use foobar2000 and the source media is high resolution FLAC files. Since the speakers are used in the nearfield, there is no need for a baffle step compensation filter. With no filters the speakers deliver wonderful playback full with rich details. For nearfield use I don't have a need for a subwoofer, of course one could easily be integrated into the setup.

Fostex FE103En Nearfield Monitor Speakers

Photograph 10: Fostex FE103En Nearfield monitor speakers.

With the single driver you get point source sound with no crossover to mess with the phase and the result is great imaging. The drivers have a somewhat forward midrange and top end. If you toe the speakers inward (about 10 to 20 degrees off-axis) it will help tame the forward response. In the farfield, I found that with as little as 2 dB of BSC there is less of the detail and magic that draws us to these sorts of fullrange drivers in the first place. Overall, I am really pleased with these small speakers on my computer desk. Comments and questions about this project are welcome in the Fostex FE103En Bookshelf Bass Reflex Speakers thread.

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