A handset receiver (earpiece) and a micro speaker are both moving-coil transducers built from the same parts: a magnet system, a voice coil and a diaphragm. The difference is that the receiver works pressed against the ear. Its diaphragm drives a small cavity of a few cubic centimetres formed by the ear canal and the pinna, not open air. That one fact explains why a receiver is loud on very little power, why its response has to be measured on an ear simulator, and why a call sounds thin when the phone is not held tightly to the ear. This article starts with the construction, then uses a pressure-field model and the Helmholtz resonator formula to quantify the low end, acoustic leakage and the high end, and finishes with the types of artificial ear and how receivers are measured.

• ≈80 dBEar cavity versus open air at 1 m
• 300 HzLow-frequency corner of a 0.3 mm² leak
• 8 NDefault 3GPP application force

1. Receivers and Speakers: Same Principle, Different Sound Fields

A phone usually contains two kinds of acoustic output device. The speaker at the bottom or side handles hands-free audio and ringtones, radiating into open space toward a listener tens of centimetres away. The receiver at the top handles calls: its sound port sits against the pinna and, together with the ear canal, forms an almost closed cavity. The speaker works in a free field; the receiver works in a pressure field. The two can be built in very similar ways, yet their design targets and test methods are entirely different.

Many recent phones also use the top receiver as one channel of a stereo speaker pair. The same part must then be smooth when held to the ear and still deliver enough level with low distortion in hands-free use, which makes the design considerably harder.

2. Construction: Magnet System, Diaphragm and the Two Cavities

Figure 1 shows a schematic cross-section of an inner-magnet receiver. A magnet carries a soft-iron top plate and sits inside a U-cup (yoke), leaving a narrow annular gap between the edge of the plate and the cup wall. The voice coil is bonded to the diaphragm and hangs in that gap. An audio current through the coil produces the force F = Bl · i, which moves the diaphragm back and forth.

• Diaphragm: usually a thermoformed composite film of PET, PEEK or similar materials, with a central dome and a surround. The surround geometry and film thickness set the compliance and hence the resonance frequency.
• Front cavity and sound port: the small space between the diaphragm and the front cover, opening toward the ear through the sound port. Its volume and the port dimensions form a Helmholtz resonator that sets the upper end of the response (Section 5).
• Back cavity and damping mesh: air behind the diaphragm passes through holes in the frame into a back cavity, which vents to the outside through a port covered with damping mesh. The trapped air acts as an extra spring, and the mesh adds acoustic resistance that flattens the resonance peak.
• Racetrack magnets: the space at the top of a phone is narrow and thin. Stretching the magnet into a long racetrack shape fits more magnet material and a longer effective coil into that width. Wire-lead and spring-contact versions simply differ in how the part is connected electrically.

Why does the back cavity change the resonance? The compliance of the enclosed air, referred to the diaphragm, acts as an additional spring with stiffness:

Example: a moving system with S_d = 40 mm², M_ms = 40 mg and K_ms = 1000 N/m resonates at about 796 Hz on its own. In a 0.5 cm³ back cavity, K_b ≈ 454 N/m and the resonance rises to about 960 Hz. Shrink the cavity to 0.1 cm³ and K_b ≈ 2270 N/m, more than twice the diaphragm's own stiffness, pushing the resonance to about 1440 Hz. The smaller the back cavity, the stiffer the air spring, which is one reason bass is hard to achieve in very thin phones.

3. The Pressure Field: Why Contact with the Ear Is Enough

When a cavity is much smaller than the wavelength (about 8.6 cm at 4 kHz, while the ear canal and pinna span only a few centimetres), the pressure is nearly uniform throughout and the air is simply compressed and expanded as a whole. A volume displacement ΔV = S_dx produces a pressure of:

For comparison, the same diaphragm mounted in an infinite baffle and radiating into half-space produces, at distance r:

Example: S_d = 40 mm² and a peak excursion of 5 μm. Pushed into a 2 cm³ cavity, this gives a peak pressure of about 14.2 Pa, roughly 10 Pa RMS or 114 dB SPL. The same excursion at 1 kHz radiating into half-space gives only about 34.5 dB at 1 m, or 54.5 dB at 10 cm. The ear-coupled case is about 80 dB louder, which is why a receiver needs so little power and excursion.

The two equations also explain why receiver and speaker responses run in opposite directions. Below its resonance f₀ a moving system is stiffness-controlled, so a constant force gives a roughly constant excursion; above f₀ it is mass-controlled, the excursion falls as f⁻² and the acceleration is constant. Therefore:

4. Leakage: When the Phone Is Not Sealed, the Bass Escapes

In real use there is almost always a gap between the phone and the pinna. The gap behaves like a short narrow tube: the air in it moves as a slug and acts as an acoustic mass, while viscous friction at the walls acts as an acoustic resistance. This leak sits in parallel with the compliance of the cavity. At low frequencies air has time to flow in and out through the gap, so no pressure builds up; at high frequencies the air slug cannot follow and the cavity behaves as if it were sealed again. The combination is a Helmholtz resonator with a corner frequency of:

Example: a 2 cm³ cavity with a leak equivalent to a tube 5 mm long. With a cross-section of only 0.3 mm², f_L is already about 300 Hz; at 3 mm² it is about 950 Hz. Figure 2 puts both leaks into a complete lumped-parameter model in which the moving system and the acoustic load are solved together:

• A leak has little effect above f_L but causes a steadily growing loss below it. When the leak is mainly resistive, the low end falls at about 6 dB per octave; when it is mainly mass-like, the slope approaches 12 dB per octave.
• Near f_L the leak mass resonates with the cavity compliance, so the pressure is actually slightly higher than in the sealed case.
• With the large leak, the level at 100 Hz is about 19 dB below the sealed case, heard as a thin, harsh voice. How tightly the phone is held varies from person to person and with grip, so the bass of the same phone varies a great deal between users.

There are two ways to deal with leakage. The first is leak-tolerant design: the receiver and the phone's acoustic structure deliberately include a controlled, fairly resistive leak path, so that the seal matters less for the low end. The price is that more volume displacement is needed to make up for that fixed leak. The second is electronic compensation: the equaliser in the voice path lifts the low end, and some solutions adjust the equalisation according to a real-time estimate of how well the phone is sealed.

5. Front Cavity and Sound Port: The High-Frequency Helmholtz Resonance

The small front cavity and the sound port form a second Helmholtz resonator. The formula is the same as in Section 4, with V replaced by the front-cavity volume V_f and A and l′ by the total area and effective length of the port. For a round hole the end corrections at both ends add up to about 1.7r (r being the hole radius), so in a thin cover the effective length is dominated by the end correction.

Example: with a 0.05 cm³ front cavity and a 2 mm² port 0.5 mm deep, the resonance is at about 8.0 kHz; enlarging the cavity to 0.2 cm³ lowers it to about 4.0 kHz. The resonance is normally placed near the upper edge of the voice band to compensate for the roll-off of the moving system above its own resonance. If the peak is too sharp, a dust mesh or damping material over the port adds resistance and flattens it.

6. Measurement: Ear Simulators and Artificial Ears

A receiver's sound pressure level is meaningful only under a defined acoustic load. Early ear simulators were simple fixed-volume couplers; later standards added an ear canal, an eardrum impedance and a pinna so that results come closer to a real ear. ITU-T P.57 defines several artificial ears for telephonometry:

The reference point must also be stated. The DRP (drum reference point) is where the microphone sits at the bottom of the ear simulator; the ERP (ear reference point) is at the entrance of the ear. The two differ by the transfer function of the ear canal, and ITU-T P.57 gives correction curves for converting between them. For complete phones, 3GPP TS 26.131 and TS 26.132 specify a Type 3.3 artificial ear on a head and torso simulator (ITU-T P.58); the receive response is measured at the DRP with a diffuse-field correction, and the default force pressing the phone against the ear is 8 ± 2 N, with 13 N specified for some tests. Less force means more leakage and less bass, so the application force must always be reported.

End-of-line testing of receivers is normally done in a dedicated coupler fixture, roughly as follows:

1. Impedance and polarity

   Measure DC resistance or impedance at 1 kHz to confirm the coil and the leads or spring contacts are connected; use a pulse or low-frequency signal to confirm the direction of diaphragm motion so that polarity is not reversed.

2. Response and sensitivity

   Sweep a sine at the specified voltage, record the SPL curve in the coupler and compare it with the reference curve and its upper and lower limit masks.

3. Distortion and abnormal sound

   From the same sweep, compute THD and analyse high-order harmonic energy to catch rub & buzz caused by coil rubbing, debris or excess adhesive.

4. Air-tightness and appearance

   Where required, check the sealing of the front and back cavities and that the sound-port mesh is intact and unblocked.

7. Design and Selection Notes

Impedance and drive

Receivers commonly use higher impedances such as 32 Ω and are driven by the earpiece amplifier in the audio codec. When the receiver doubles as a speaker, check power and maximum excursion against the voltage and load capability of that amplifier.

Back volume

A smaller back cavity means a higher resonance and weaker bass. Whether the phone leaves room for a back cavity, or lets the receiver use internal space as an extended back volume, directly affects the low end.

Sound port and mesh

Port area and depth set the front-cavity resonance, and mesh resistance sets the height of the peak. Dust and water meshes are acoustic components too; re-measure the response whenever one is changed.

Sealing

The foam or gasket between receiver and housing must be compressed evenly to avoid a front-to-back acoustic short and assembly variation.

Magnetic field and hearing aids

Stray magnetic field from the receiver is picked up by the telecoil in hearing aids, which is exactly what hearing-aid compatibility (HAC) testing looks at. The magnet system should also be kept away from the electronic compass and Hall sensors.