SB Acoustics SB15SFCR39-8 / 5×8″ / paper racetrack midwoofer / 8 ohm

SB Acoustics SB15SFCR39-8 is a 5×8″ racetrack midwoofer with a hard paper cone, 8 ohm nominal impedance, 87 dB / 2.83 V / 1 m sensitivity and a free-air resonance of 32 Hz. It is designed for loudspeaker projects where a larger effective cone area, good low-frequency capability and easier integration into narrow or unconventional cabinets are required.

With its racetrack geometry, vented chassis, low damping rubber surround, symmetric suspension and optimized motor system, SB15SFCR39-8 is well suited for slim loudspeakers, lifestyle speaker designs, compact systems with extended cone area and ambitious DIY projects requiring an unusual but highly practical midwoofer format.

Product Description

SB Acoustics SB15SFCR39-8 is an unusual racetrack midwoofer with a nominal 5×8″ format. This oval geometry provides a larger effective radiating area than many round drivers with a similar front baffle width. As a result, the driver can be used in narrow, slim or non-standard loudspeaker cabinets where a conventional large round woofer would be difficult to fit.

The cone is made from hard paper produced in-house by SB Acoustics. This material gives the driver a natural sound character, good internal damping and a musical presentation through the bass and midrange. Compared with many metal cone drivers, this type of diaphragm is usually easier to tune and less aggressive in character.

The driver uses a vented chassis, low damping rubber surround and symmetric suspension to support stable cone movement. SB Acoustics also uses an optimized motor system and a non-conductive reinforced vented voice coil former, helping reduce compression and maintain clean performance at higher drive levels.

SB15SFCR39-8 is a very interesting choice for projects where a larger cone area is needed without using a conventional large round frame. The 8 ohm version is especially useful where an easier amplifier load is preferred or where multiple drivers may be combined in a more complex loudspeaker design.

Application

• slim floorstanding loudspeakers,
• unconventional two-way speaker designs,
• lifestyle and compact stereo systems,
• cabinets with limited front baffle width,
• midwoofer sections in multi-way loudspeakers,
• DIY loudspeakers using a racetrack driver format,
• bass-reflex and sealed enclosures,
• projects requiring a large cone area in a non-round shape.

Example Enclosure Use and Volume

Based on its Thiele/Small parameters, especially Fs 32 Hz, Vas 48.3 liters and Qts 0.42, the SB15SFCR39-8 can be considered for both sealed and bass-reflex enclosures.

For a sealed enclosure, a practical starting point for simulation is around 18–28 liters net volume per driver. This type of cabinet should provide a tight and controlled bass response, suitable for systems supported by a subwoofer or projects where speed and simple tuning are the main priorities.

For a bass-reflex enclosure, a sensible starting point is around 30–45 liters net volume, with tuning roughly in the 34–42 Hz range. This allows the large cone area and low resonance frequency of the driver to be used effectively, producing fuller bass in a slim or unconventional cabinet.

The final enclosure volume, port tuning, response correction and crossover frequency should always be confirmed through simulation and measurements of the finished loudspeaker. Due to the oval geometry and directivity of the driver, the front baffle layout and upper working range should be planned carefully.

Sound Character and Comparison

SB15SFCR39-8 offers the natural and musical character typically associated with a paper cone midwoofer. The hard paper diaphragm provides a good balance between control and smoothness, making the driver suitable for loudspeaker designs where a fuller, more organic sound is desired.

Compared with a conventional round 5″ driver, the racetrack design provides a significantly larger effective cone area, which can improve low-frequency ease and lower-midrange authority. Compared with a round 8″ woofer, it offers a similarly “larger” sense of movement while fitting into a much slimmer front baffle.

Compared with aluminium or carbon cone drivers, SB15SFCR39-8 will usually sound less technical and less sharply defined, but more natural and easier to integrate into musical loudspeaker projects. It is a good choice when the design requires unusual geometry, useful cone area and a pleasant paper cone sound.

Key Features

• 5×8″ racetrack midwoofer,
• hard paper cone made in-house by SB Acoustics,
• 8 ohm nominal impedance,
• 87 dB / 2.83 V / 1 m sensitivity,
• free-air resonance of 32 Hz,
• effective piston area of 178 cm²,
• vented chassis,
• low damping rubber surround,
• symmetric suspension,
• optimized motor system,
• non-conductive reinforced vented voice coil former,
• long-life silver lead wires,
• suitable for slim and unconventional loudspeaker designs.

Installation Notes

SB15SFCR39-8 has 4 mounting holes. According to the manufacturer’s drawing, the mounting holes are Ø5.3 mm, with Ø8.5 mm counterbores.

The outside front frame dimensions are 252.8 ±0.5 mm × 153.0 mm. The cutout shown in the drawing is 220.0 × 121.0 mm, and the magnet diameter is Ø100.0 mm.

The total depth is 95.8 mm, while the mounting depth from the front mounting plane is approximately 84.0 mm. The drawing also shows a 3.8 mm front lip / flange-related dimension.

With this model, it is especially important to check the dimensions with the physical driver before machining the baffle, because the oval racetrack shape requires more precise preparation than a conventional round driver. The driver should be mounted evenly and sealed properly to prevent air leaks between the chassis and the enclosure.

Safety Information

This product is intended for use in loudspeaker systems and DIY audio projects. Installation should be carried out with the amplifier switched off and disconnected from power.

Use an amplifier suitable for an 8 ohm loudspeaker load and do not exceed the rated power handling of the driver. Excessive power, amplifier clipping, incorrect crossover design or operation outside the intended frequency range may damage the loudspeaker.

The driver should be securely mounted in a suitable enclosure. Do not press the cone or surround by hand. This product is not a toy and should be kept away from children unless handled under adult supervision.