Passive radiators in practice
A passive radiator can achieve low tuning without a long port, but it requires enough moving mass and substantially more excursion capability than the active driver around tuning.
A moving diaphragm without its own motor
A passive radiator is driven by pressure changes inside the enclosure, not by an electrical connection. Near system tuning it can supply a substantial part of the low-frequency radiation. It is a resonator coupled to the active driver and box air, not a second electrically driven woofer.
Why use it instead of a port?
A suitably large, deeply tuned port can become too long for a compact cabinet. A passive radiator avoids an open air channel and its airflow noise. In return it needs mounting area, adds cost and introduces a suspension and excursion limit of its own. It solves a specific construction problem rather than automatically improving every loudspeaker.
Mass changes tuning
Additional moving mass generally lowers resonance when effective stiffness is unchanged. The relationship is a square root, not a linear scale. A complete passive-radiator enclosure couples several masses and compliances, so a simple mass ratio is a teaching guide rather than a full tuning prediction. Record every mass change and check the actual alignment.
Area alone is insufficient
Near tuning, passive-radiator movement can be large even while active-cone movement is small. Effective area and usable one-way stroke determine displacement capability. A matching nominal frame diameter does not guarantee adequate reserve. Multiple radiators can help, but require recalculating the system; a universal “twice the diameter” rule is not a complete design method.
Watch below tuning and check the seals
Deep equalisation below the useful range can demand excessive active-driver excursion. A suitable high-pass may be needed, chosen from the specific design. Unintended leaks alter pressure coupling and can make noise. Check gaskets, cable passages, terminal plates and joints, and calculate net volume after subtracting internal parts.
Mounting and reaction forces matter
A moving mass exerts reaction forces on the cabinet. Opposed, suitably matched radiators can reduce some forces, but their behaviour and layout must fit the design. Gravity and suspension compliance affect static position; added weights can increase sag. Verify permitted orientation and leave clearance from grilles, furniture and the floor throughout the intended excursion.
Tune with measurements and secure hardware
Use low-level impedance and near-field measurements to investigate tuning before higher-level checks. Change one securely attached mass increment at a time, recording conditions and results. Do not treat loose washers or temporary adhesive as final mounting. Judge the completed system by response, distortion, limits and room integration, not the visual movement of an extra cone.
Work it out · answers unfold
Write down your prediction first, then your calculation. Open the answer only when you are ready to compare.
17 · An ideal 10 mH inductor carries 0.2 A. Calculate stored magnetic energy.
Show worked answer
- L = 0.01 H.
- Square current and multiply by L/2.
- E = 0.2 mJ.
18 · 10 mH and 100 Ω are in series across 5 V. Find τ and steady current; ideal inductor.
Show worked answer
- τ = L/R = 0.1 ms.
- The ideal inductor has zero steady DC voltage.
- Final I = 5/100 = 50 mA.
19 · Calculate inductor current in task 18 after 0.1 ms, starting at zero.
Show worked answer
- Time equals τ.
- Current rises to 63.21% of final value.
- I = 31.61 mA.
20 · A 10 mH inductor changes current by 0.2 A in 10 µs. What ideal voltage magnitude is needed? What does this imply at turn-off?
Show worked answer
- Slope: ΔI/Δt = 20000 A/s.
- U = L · ΔI/Δt = 200 V.
- A limiting current path, such as a suitable flyback diode, prevents uncontrolled spikes.
Further technical reading
Colloms & Darlington (2005): High Performance Loudspeakers, 6th ed. Chapter 4 (Low-frequency System Analysis), starting on printed page 117. Source information (COLLOMS-2005-6) →