35FR

SPEAKER ENCLOSURE CALCULATION — 35FRX4-155
DRIVER IDENTIFICATION
  Code:              35FRX4-155
  Name:              35FR
  Category:          Full-Range
  Size:              3.5 inch
  Brand:             REDCATT

CONSTRUCTION
  Nominal Impedance: 4 Ω (Re = 3.5 Ω DC)
  AES Power:         25 W
  Continuous Power:  50 W
  Peak Power:        100 W
  Voice Coil:        25.4 mm diameter, copper wire, aluminum former
  Magnet:            Ferrite
  Basket:            Steel
  Cone:              Carbon fiber reinforced paper (Paper CF)
  Surround:          NBR rubber
  Spider:            Nomex
  Overall Dia.:      93 mm
  Depth:             51 mm
  Net Weight:        0.85 kg

THIELE-SMALL PARAMETERS (measured, broken-in)
  Fs   =  67.0 Hz          (free-air resonant frequency)
  Re   =   3.5 Ω           (DC voice coil resistance)
  Qes  =   0.37            (electrical Q)
  Qms  =   4.35            (mechanical Q)
  Qts  =   0.34            (total Q — vented enclosure required; borderline ≤0.35 threshold)
  Vas  =   1.60 L          (equivalent air volume)
  Sd   =  38.5 cm²         (effective cone area)
  Mms  =   7.5 g           (moving mass)
  Rms  =   0.73 kg/s       (mechanical resistance)
  Cms  =   0.75 mm/N       (compliance)
  Bl   =   5.46 T·m        (force factor)
  Le   =   0.14 mH         (voice coil inductance)
  Xmax =   0.55 mm         (one-way linear excursion)
  Xmech=   7.45 mm         (one-way mechanical limit)
  Xprot=  12.2 mm          (one-way protection limit)

SENSITIVITY
  Datasheet Sensitivity:  83.7 dB (2.83V/1m, per CSV Sensitivity field)
  1W/1m reference (Lm):  83.13 dB (per CSV Lm field — used for Max SPL calculations)
  NOTE: The CSV Sensitivity (83.7 dB) and Lm (83.13 dB) fields are not self-consistent
        with Re = 3.5 Ω per IEC convention (expected 2.83V/1m = 86.7 dB computed from Lm).
        The discrepancy suggests the datasheet Sensitivity may reflect a specific enclosure
        measurement condition or a different reference. Lm = 83.13 dB (1W/1m) is used as
        the primary reference for all SPL calculations below.

DERIVED PARAMETERS
  Volume displacement:    Vd = Sd × Xmax = 38.5 cm² × 0.055 cm = 2.12 cm³
  Inductance rolloff:     f_Le = Re / (2π × Le) = 3.5 / (2π × 0.00014) = 3,978 Hz
  Cone radius:            r_cone = √(Sd/π) = 3.50 cm
  Cone beaming onset:     f_beam = c / (π × r_cone) = 34400 / (π × 3.50) = 3,128 Hz
  Max SPL (AES, 25W):     1W/1m + 10×log10(25) = 83.13 + 14.0 = 97.1 dB at 1 m
  Zobel network:          Rz = 3.5 Ω; Cz = Le / Re² = 11.4 μF (series, across driver)
  Column array limit:     f_array = c / (2 × OD) = 34400 / (2 × 9.3) = 1,849 Hz ≈ 1.85 kHz

FREE-AIR ANALYSIS (Qts = 0.34 — NOT recommended as primary installation)
  With Qts = 0.34, the driver in free air behaves as an under-damped sealed system
  with Qtc = 0.34 — no Butterworth alignment is possible.
  Free-air resonant frequency: Fs = 67 Hz (unchanged)
  Free-air f-3dB: Ω = √u, u = (8.651 + 6.945)/2 = 6.80 → Ω = 2.61 → f-3dB ≈ 175 Hz
  The free-air f-3dB of 175 Hz is unusable for music reproduction.
  Vented enclosure is strongly recommended for all standalone applications.
ENCLOSURE RECOMMENDATIONS
PRIMARY: VENTED (PORTED)
  Qts = 0.34 falls at the vented-only boundary (≤0.35) and benefits maximally from
  a reflex alignment. Vented enclosures extend bass response to 70–80 Hz, well below
  the sealed or free-air alternatives. Port: 30 mm diameter (Ap = 7.07 cm²).
  End corrections: flanged end 12.8 mm + unflanged end 9.2 mm = 22 mm total.

  --- VENTED ALIGNMENTS (30 mm port) ---

  Alignment V1: Vb = 1.5 L  |  Fb = 80 Hz
    Effective port length:  221 mm  →  Physical port length: ~199 mm
    Port air velocity:      1.51 m/s peak  (well below 17 m/s limit)

  Alignment V2: Vb = 2.0 L  |  Fb = 75 Hz
    Effective port length:  188 mm  →  Physical port length: ~166 mm
    Port air velocity:      1.41 m/s peak

  Alignment V3: Vb = 2.5 L  |  Fb = 70 Hz
    Effective port length:  173 mm  →  Physical port length: ~151 mm
    Port air velocity:      1.32 m/s peak

  Note: Port physical lengths span 151–199 mm across all three tunings (48 mm range).
  Port velocities are very low due to the small Xmax (0.55 mm) and Vd (2.12 cm³).
  A passive radiator may substitute for the port (as noted by manufacturer) to eliminate
  port noise and allow a more compact baffle profile in column enclosures.

SECONDARY: SEALED (per manufacturer design intent)
  The manufacturer designates this driver for sealed enclosures in column, 2-way, and
  1-way speaker systems. With Qts = 0.34, sealed alignments produce high f-3dB values
  and are suitable for multi-way systems where a woofer or subwoofer handles bass below
  ~150 Hz, or for column speaker arrays crossed above the sealed rolloff frequency.
  Sealed volumes assume nominal net volume (driver chassis displacement ~15–20 mL
  should be subtracted from gross enclosure volume).

  --- SEALED ALIGNMENTS ---

  Qtc = 0.707 (Butterworth):  Vb = 0.48 L  |  fc = 139 Hz  |  f-3dB = 139 Hz
  Qtc = 1.00  (maximally flat): Vb = 0.21 L  |  fc = 197 Hz  |  f-3dB = 155 Hz
  Qtc = 1.20  (compact):       Vb = 0.14 L  |  fc = 237 Hz  |  f-3dB = 174 Hz

  Sealed Qtc = 0.707 alignment (0.48 L, f-3dB = 139 Hz) is the practical lower bound
  for standalone operation; for column speaker integration with a woofer crossed at
  200–300 Hz, Qtc = 1.00 or 1.20 (0.14–0.21 L) are the preferred compact options.
CROSSOVER AND SYSTEM DESIGN NOTES
  INDUCTANCE ROLLOFF (f_Le = 3,978 Hz — within audio band):
    Le = 0.14 mH places the inductance rolloff onset at ~4 kHz. Progressive HF output
    reduction will be audible above this point. For tweeter-less 1-way use, DSP treble
    equalization or compensation at the amplifier stage is required to maintain flat
    response above 4 kHz. In 2-way designs, the crossover should be placed below 4 kHz
    to avoid progressive HF droop in the overlap region.

  CONE BEAMING (f_beam = 3,128 Hz):
    Pistonic behavior transitions to directional beaming above 3,128 Hz. Designed
    crossover points to a tweeter should target 2,500–3,000 Hz or lower for wide
    horizontal dispersion. The aluminum voice coil former provides good thermal
    dissipation and is consistent with extended HF capability up to the beaming onset.

  COLUMN ARRAY DIRECTIVITY (f_array = 1.85 kHz for 93 mm OD):
    In a vertical column array, coherent addition of multiple 35FRX4 units limits
    to approximately 1.85 kHz — the lowest column array limit in the full-range family
    due to the larger 3.5-inch driver OD. Column array designs using this driver should
    restrict the driver's full-range operating band to below 1.85 kHz and rely on a
    co-located tweeter for upper HF coverage, or accept increasing vertical beam narrowing
    above this frequency.

  ZOBEL NETWORK:
    A Zobel network (3.5 Ω + 11.4 μF in series, placed across driver terminals) is
    recommended for passive crossover designs to flatten the rising impedance from Le.

  PASSIVE RADIATOR OPTION:
    The manufacturer notes that a passive radiator may be used in place of a port to
    extend LF response. A PR system offers advantages in compact column enclosures:
    no port turbulence, flexible placement, and potentially shorter effective length.
    Target a PR resonance at 65–80 Hz to achieve comparable bass extension to the
    vented alignments above. PR mass tuning: Fp ≈ Fb = 65–80 Hz; Sp (PR cone area)
    ≥ Sd (38.5 cm²) is recommended for adequate displacement capability.

  IMPEDANCE NOTE:
    Nominal 4 Ω load with Re = 3.5 Ω. At resonance (67 Hz in free air), impedance
    rises significantly. Amplifiers rated for 4 Ω loads are appropriate. In column
    arrays, multiple drivers wired in series-parallel combinations can achieve 8 Ω
    or 16 Ω nominal loads for compatibility with standard amplifier channels.
FULL-RANGE FAMILY COMPARISON (8 drivers)
  Code              Size  Ω   W AES  Qts   Motor      Fs   Enclosure             −3dB    1W/1m  MaxSPL  OD    f_array
  20NFRX4-345A      2"    4   10     0.52  Neo        185  Free-air / open-back  272 Hz  79.6   89.6    48 mm  3.58 kHz
  21NFRX16-393D     2"    16  10     0.68  Neo        157  Free-air / open-back  164 Hz  85.8   95.8    57 mm  3.00 kHz
  22NFRX8-462A      2"    8   20     0.85  Neo        255  Free-air / open-back  219 Hz  83.5   96.5    61 mm  2.84 kHz
  25NFRX8-279       2.5"  8   20     0.71  Neo ring   123  Free-air / open-back  123 Hz  82.3   95.4    65 mm  2.65 kHz
  30FFX8-264B       3"    8   20     0.78  Ferrite    119  Free-air / open-back  109 Hz  83.6   96.6    79 mm  2.18 kHz
  31NFRX8-229       3"    8   30     0.88  Neo        115  Free-air / open-back   97 Hz  83.5   98.3    79 mm  2.18 kHz
  32NFRX8-463A      3"    4†  30     0.32  Neo ring   118  Vented (75–115 Hz)     75 Hz  89.4  104.1    83 mm  2.07 kHz
  35FRX4-155        3.5"  4   25     0.34  Ferrite     67  Vented (70–80 Hz)      70 Hz  83.1   97.1    93 mm  1.85 kHz
  † Re = 5.5 Ω (nominal 8 Ω label; actual nominal is effectively 4–6 Ω class)
  Max SPL at AES rated power at 1 m; column array f_array at minimum driver pitch = OD.
  Free-air −3dB calculated at Qtc = Qts (infinite baffle equivalent).
  Vented −3dB shown as Fb range of alignments tabulated above.

  SELECTION NOTES:
  • 35FRX4-155 is the only 3.5-inch driver in the family, offering the lowest Fs (67 Hz)
    and the broadest bass extension potential via a vented enclosure (70–80 Hz Fb).
  • Along with 32NFRX8-463A (Qts=0.32), it is one of only two drivers in the family
    requiring a vented enclosure; all others prefer free-air or open-back mounting.
  • The 93 mm OD gives the lowest column array coherence limit (1.85 kHz) in the family
    — the bass extension advantage comes at the cost of reduced HF array coherence.
  • Ferrite motor (as in 30FFX8-264B) provides cost-effective performance; neodymium
    drivers (Neo, Neo ring) offer better sensitivity and weight reduction.
  • The 35FRX4-155 max SPL (97.1 dB) is competitive with the 3" ferrite and neodymium
    drivers despite lower power rating (25W AES), reflecting a favourable sensitivity.
  • Choose 35FRX4-155 where: bass extension to 70–80 Hz is required without a subwoofer;
    3.5-inch format is compatible with enclosure geometry; ferrite motor cost is a priority;
    and column array operation above 1.85 kHz will be supplemented by a co-located tweeter.
  • Choose 32NFRX8-463A (Qts=0.32) where: maximum SPL (104 dB) from a vented 3-inch
    driver is the priority and sensitivity efficiency outweighs bass extension depth.
  • Choose 31NFRX8-229 (Qts=0.88) where: free-air/open-back installation is required
    with the deepest bass extension (f-3dB ≈ 97 Hz free-air) in the 3-inch class.
AI SUMMARY
The 35FRX4-155 is a 3.5-inch full-range driver featuring a ferrite magnet motor, steel
basket, carbon fiber reinforced paper cone, NBR rubber surround, Nomex spider, and a
25.4mm copper voice coil wound on an aluminum former. It is the largest driver in the
full-range family and the only 3.5-inch variant, with an overall diameter of 93mm and
51mm mounting depth. The aluminum former provides effective heat dissipation from the
voice coil gap, supporting 25W AES / 50W continuous power handling. The manufacturer
designates this driver for sealed enclosures in column speaker, 2-way, and 1-way
applications, with passive radiator extension noted as an option for extended bass.

Fs = 67 Hz, Qes = 0.37, Qms = 4.35, Qts = 0.34 — a motor-suspension design optimized
to maintain consistent performance at high drive levels, with controlled excursion and
reduced thermal compression during long-term operation. Vas = 1.60 L, Sd = 38.5 cm²,
Xmax = 0.55 mm (one-way), Xmech = 7.45 mm. Volume displacement Vd = 2.12 cm³. Sensitivity
is 83.13 dB at 1W/1m (Lm, per datasheet), with a maximum SPL of 97.1 dB at 1 metre
(25W AES). The voice coil inductance Le = 0.14 mH places the inductance rolloff onset at
3,978 Hz — within the audio band — causing progressive HF attenuation above this frequency;
DSP treble compensation or a 2-way crossover below 4 kHz is required for extended HF
coverage. Cone beaming onset at 3,128 Hz defines the upper boundary of wide-dispersion
pistonic operation. Zobel network: 3.5 Ω + 11.4 μF in series across driver terminals.

  Enclosure alignments:
  • Vented (primary) — 30 mm port:
      1.5 L / Fb = 80 Hz / ~199 mm physical port / 1.51 m/s peak port velocity
      2.0 L / Fb = 75 Hz / ~166 mm physical port / 1.41 m/s peak port velocity
      2.5 L / Fb = 70 Hz / ~151 mm physical port / 1.32 m/s peak port velocity
    (All port velocities well below the 17 m/s hi-fi limit; passive radiator is a viable
     port alternative per manufacturer guidance, targeting Fp = 65–80 Hz)
  • Sealed (secondary — per manufacturer intent, column / 2-way systems):
      0.48 L / Qtc = 0.707 / fc = 139 Hz / f-3dB = 139 Hz
      0.21 L / Qtc = 1.00  / fc = 197 Hz / f-3dB = 155 Hz
      0.14 L / Qtc = 1.20  / fc = 237 Hz / f-3dB = 174 Hz

In column array applications, the 93mm driver OD limits coherent vertical directivity to
approximately 1.85 kHz — the lowest array coherence limit in the full-range family. Column
designs using this driver require a co-located tweeter to maintain coverage above 1.85 kHz,
or must accept increasing vertical beam narrowing above this frequency. The low Qts (0.34)
means the driver generates minimal back-EMF braking at resonance, so excursion limiting
below Fb in a vented enclosure is essential; a DSP high-pass filter at Fb − 10 Hz or a
subsonic filter at 50–60 Hz is recommended. Multiple units in column arrays can be wired
in series-parallel to achieve 8Ω or 16Ω loads from the nominal 4Ω driver impedance.

The 35FRX4-155 is suited for bass-extended column speaker systems where a 3.5-inch vented
enclosure delivers 70–80 Hz bass without a subwoofer; 2-way and 1-way sealed enclosures in
column, surface-mount, or wall-mount speaker products where the manufacturer-recommended
sealed alignment (0.14–0.48 L) provides predictable, compact bass rolloff above 140 Hz;
multi-way installed sound systems where the driver covers the midrange and upper bass while
a dedicated woofer handles below 150–200 Hz; and cost-sensitive applications where ferrite
motor construction and a steel basket provide a practical alternative to neodymium drivers
without compromising the core acoustic performance of the 3.5-inch format.