Where passive gain comes from
A horn adds no energy. Every decibel it delivers was already leaving the driver and being wasted. Two distinct mechanisms are usually reported as one number.
Impedance matching is the real mechanism
A small driver is a high-impedance source facing a low-impedance load. The diaphragm moves fast over a tiny area; the air in a room accepts energy slowly over a large one. Most of the driver's mechanical work never becomes sound — it is stored and returned, cycle after cycle.
A horn interposes a gradual transition between the two. The throat presents the driver with something close to the load it can drive efficiently; the expansion walks that load down to the open air. The same cone excursion then produces considerably more radiated power. This is genuine acoustic gain: more energy per unit of input, with no input added.
Directivity is the other half, and it is not the same thing
A horn also concentrates output into a narrower angle. A listener inside that angle measures more sound pressure, but this is redistribution, not conversion. Energy taken from the sides is delivered to the front.
Both effects raise the number on a meter held in front of the device, which is why they are easy to conflate. The distinction is practical: impedance gain survives when you move around the room, directivity gain does not. A device that owes most of its figure to narrowing will sound markedly weaker from a seat to one side.
What limits the whole exercise
Three ceilings apply, and the smallest one wins. The mouth area sets the lowest frequency that can be loaded. The path length sets how gradual the transition can be. And the driver itself cannot deliver more mechanical work than its excursion and motor allow, however well matched the load.
This is why passive cradles report gains in the region of several decibels rather than tens. The figure is real and audible — a doubling of perceived loudness is roughly 10 dB — but it is bounded by physics that no geometry evades.
| Item | Value |
|---|---|
| Mechanism 1 | Impedance matching — true acoustic gain |
| Mechanism 2 | Directivity — redistribution, not conversion |
| Low limit set by | Mouth area versus wavelength |
| Upper limit set by | Driver excursion and motor |
| Power required | None |
| Typical reported range | A few decibels; measure in your own setup |
What a few decibels sounds like
The number is smaller than it feels
Decibels are logarithmic, and intuition about them is poor. Roughly 10 dB reads as a doubling of loudness; 3 dB is the smallest change most listeners notice reliably in music. A passive cradle delivering several decibels is therefore doing something clearly audible but well short of twice as loud.
Most of the benefit is not loudness at all
A horn redirects output that was pointing at the desk and aims it at the listener. Even where the measured gain is modest, speech becomes easier to follow because the first thing reaching the ear is direct sound rather than a reflection off the surface the phone is lying on. Intelligibility improves more than the decibel figure suggests.
Where it will disappoint
Bass is unaffected. A mouth the width of a desk cradle cannot load wavelengths of a metre or more, so the low end stays exactly where the bare device left it. Anyone expecting a passive horn to add weight to music is expecting the one thing the geometry cannot provide.