Micro Speaker for Wearable and Compact Electronic Devices
On a wearable device the acoustic budget is the part nobody sees and everybody feels. A watch or a compact device has no room for a cavity, no space for a port, and no tolerance for output that varies with the strap tension or the wrist position. What it has instead is a surface area that the product designer has already claimed, which makes the acoustic outlet a design negotiation rather than an engineering afterthought.
1. The Outlet Is a Design Negotiation
Short answer: In a wearable the sound outlet is decided before the driver is chosen, because the surface it needs has already been allocated to display, controls and brand.
A wearable device has no spare surface. The face carries the display, the controls and the brand; the edge carries the buttons, the charging contacts and the seals; the strap carries the mounting. The acoustic outlet is what remains, and what remains is usually a slot, a perforation pattern, or the gap between two housing parts.
This inverts the usual order of work. On a larger product the outlet is designed around the driver; on a wearable the driver is selected around the outlet. The consequence is that the acoustic design has to start from the geometry that the industrial design has already fixed, and the questions worth asking early are about that geometry rather than about the driver catalogue.
2. Front Cavity Height Is the Trade That Governs Wearable Output
Short answer: The height of the front cavity sets both how much airflow noise the outlet produces and how much bandwidth survives — raising it reduces noise and reduces bandwidth at the same time.
On a thin device the front cavity is the short passage between the driver's working side and the outlet. Its height determines how freely air can move, and that has a direct effect on what the user hears at the top of the band.
The trade is specific and measurable. A short front cavity, in the order of 1.5 mm, gives the airflow from the high-frequency content insufficient room to develop, and the result is a noise component in the upper band that is heard as a hiss or buzz rather than as tonal content. Increasing the height to around 5 mm gives the airflow room to become useful, and the noise component largely disappears. The cost is that a taller front cavity loads the system differently and reduces the effective bandwidth, which in one measured case fell from 6 kHz to 5 kHz.
This trade is worth stating in the specification rather than discovering during tuning. For a device carrying voice prompts, a reduction in bandwidth above 5 kHz is usually an acceptable exchange for the removal of audible noise, because speech intelligibility depends on the 2–4 kHz region rather than on the extreme top end. For a device expected to reproduce music, it is a different judgement entirely.
Table 1: Outlet strategies in thin and wearable enclosures
Outlet strategy | How it works | What it costs acoustically | Where it fits |
Slotted outlet along a housing seam | Sound exits through a manufactured slot | Acoustic behaviour depends on slot length and alignment; may leak rather than radiate | Products where the seam is already a design line |
Perforation pattern | A designed array of holes on the face | Loss depends on open-area ratio; small holes raise turbulence noise | Products where the face is available |
Gap between two housing parts | The parting line acts as the outlet | Uncontrolled; behaviour changes with assembly gap and venting destination | Not recommended as a primary strategy |
Side exit through a dedicated channel | Output routed to a controlled side face | Effective bandwidth becomes a function of the front cavity height | Devices where the face is fully committed |
Sealed driver with rear vent | The barrier sits inside the component | Requires a defined host cavity; moves the acoustic duty into the part | Exposed or wash-down devices |
3. Output That Changes With How the Device Is Worn
Short answer: A wearable is acoustically coupled to the body and the strap, so the level a user hears depends on fit rather than on the test fixture.
A device worn on the body is in an acoustically unusual position. The strap or the band partly seals the outlet against the skin, the body absorbs low frequencies, and the orientation of the outlet relative to the ear changes as the user moves. A measurement taken with the device held in the hand will not represent what the user hears.
The practical implication is that validation belongs on a wrist or in a fixture that reproduces a worn condition, and that the level target should be stated for that condition. This is also why a wearable product that sounds acceptable in a demo can disappoint in use: the demo is usually hand-held and open.
4. Driver Classes for Wearable and Compact Builds
Short answer: The usable window is set by depth: parts below 3 mm trade low-frequency content for thickness, and boxed platforms trade footprint for output when no cavity can be spared.
The classes below are read from one published sample catalogue and are illustrative of the available range rather than a standard. Values are stated at each part's published test condition, which is not the condition in a finished product. Thickness values follow the supplier's model coding, in which the final two digits express millimetres as integer plus decimal; where the coding and the specification string differ, the coding governs.
Table 2: Published drivers applicable to wearable and compact devices
Model | Format and published size | Published sensitivity and power | Published F0 | Compact-build fit |
HS150727H | Square magnetic, 15 × 7 × 2.5 mm, solder type | 91 dB at 2 kHz / 10 cm / 0.8 W | 1050 Hz ±15% | Listed for smart watches and surveillance cameras; the narrowest option in the range |
HS150827H | Square magnetic, 15 × 8 × 2.5 mm, leaf spring | 92 dB at 2 kHz / 10 cm / 0.8 W | 1050 Hz ±15% | Listed for smart watches; 1 dB above the narrower part for 1 mm more width |
HS160930H | Square magnetic, 16 × 9 × 3.0 mm, solder type | 93 dB at 2 kHz / 10 cm / 0.8 W | 1000 Hz ±15% | Smartphones and smart watches where 3 mm of depth is available |
HS151125H | Square magnetic, 15 × 11 × 2.5 mm, leaf spring | 95 dB at 2 kHz / 10 cm / 0.8 W | 900 Hz ±15% | A 3 dB gain over the 15 × 8 part for 3 mm more width, same depth |
HS151130H | Square magnetic, 15 × 11 × 3.0 mm, leaf spring | 95 dB at 2 kHz / 10 cm / 1.0 W | 900 Hz ±15% | Same footprint and level as above with more power headroom |
HS001342H42 | Round magnetic, φ13 × 4.2 mm, solder type | 88 dB at 2 kHz / 10 cm / 0.5 W | 1000 Hz ±15% | Digital cameras and close-range devices where a round format is preferred |
HS001534H39 | Round magnetic, φ15 × 3.4 mm, solder type | 90 dB at 2 kHz / 10 cm / 0.8 W | 800 Hz ±15% | Round format where the outlet is radial rather than flat |
HS204130H30 | Round magnetic, 20 × 14 × 3.0 mm | 90 dB at 2 kHz / 10 cm / 1.0 W | 900 Hz ±15% | A rectangular format where the cavity is wide rather than deep |
HS251233H | Square magnetic, 25 × 12 × 3.3 mm | 97 dB at 2 kHz / 10 cm / 2.0 W | 750 Hz ±15% | Where 3.3 mm of depth and a 4 Ω load are both available |
HS250926H | Square magnetic, 25 × 9 × 2.6 mm | 93 dB at 2 kHz / 10 cm / 2.0 W | 700 Hz ±15% | BOX construction — requires a defined host cavity, not a bare driver |
HS121722H | Square magnetic, 12 × 17 × 2.2 mm, solder type | 95 dB at 2 kHz / 10 cm / 1.0 W | 850 Hz ±15% | BOX construction — requires a defined host cavity, not a bare driver |
HS003021H | BOX platform, φ30, 21 mm height | 105 dB at 2 kHz / 10 cm / 2.0 W | 800 Hz ±15% | A compact device with no cavity to spare; the module supplies its own |
Two comparisons in that table are worth drawing out. First, between the 15 × 7 and 15 × 8 mm parts, the wider one delivers 1 dB more for 1 mm of width — which is a trade most designs can make in either direction. Second, between the 15 × 8 and 15 × 11 mm parts at the same depth, the gain is 3 dB for 3 mm of width, which is a materially better exchange than the first. Width is often the cheaper dimension in a wearable face.
5. Project Case: Desktop Companion Robot with a Side-Exit Driver
Project Case Study (Hongsheng)
A compact desktop companion robot needed clear speech reproduction for conversational use and enjoyable music playback, with a usable cavity of roughly 80 × 80 mm inside a 20 mm thickness limit. The customer's earlier attempts used bare drivers with composite diaphragms and none of them reached the required sound quality, while the parts that did were dimensionally incompatible with the structure. The diagnosis was straightforward: a bare driver cannot be sealed at the rear, so the acoustic duty was never going to be satisfied in that configuration.
Hongsheng's approach was to evaluate candidates against the amplifier rather than on paper. Seven or eight candidate drivers were brought to the customer's site together with two different amplifier boards, one tuned for speech and one for Bluetooth music, so that the customer could listen in the intended conditions and state which one was closer to the expectation and which fitted the structure. That produced a shortlist against real listening, and it established what had to be changed. The customer's amplifier was an 8 Ω design, so the voice coil was specified to match. A proprietary-platform part was then selected with four locating posts added on the structure, which satisfied both the acoustic requirement and the mechanical interface without any change to the customer's mould.
The side-exit front cavity was the part that required iteration. The initial front cavity height of 1.5 mm produced an audible noise in the high-frequency region, caused by the turbulence of airflow at that height. Raising the front wall height to 5 mm removed it. The improvement was not free: a taller front cavity reduced the effective bandwidth, which fell from 6 kHz to 5 kHz. The customer was taken through this trade and accepted it, since the primary use is speech. A second, separate saving was found in the connection, where a piercing wire was replaced with a standard 1.25 mm terminal wire, taking the wire cost from about RMB 0.25 to about RMB 0.07 per unit.
The measured result is a sensitivity of 105 ± 3 dB at 2 kHz referenced to 10 cm at 4.0 Vrms, a resonance of 500 Hz ±15%, rated power of 2.0 W and maximum power of 2.5 W. The final driver cost RMB 4.8, which suited a product whose own selling price was around RMB 1000. The programme is in mass production at 10k units. Hongsheng can supply private-platform drivers with added locating features where the structure is already fixed, and can review the front-cavity height against the noise and bandwidth requirements before a prototype is built.
Hongsheng reviewed the front-cavity height against the noise and bandwidth requirements before any prototype was built. That is the narrower and more useful position for this category: Hongsheng can supply private-platform drivers with added locating features where the structure is already fixed, and can review the outlet geometry before tooling rather than after.
One-line conclusion: the noise was a cavity-geometry problem, and the acceptance came from showing the bandwidth trade rather than hiding it.
6. Confirming a Wearable Driver with the Supplier
Short answer: The questions worth asking here concern the outlet geometry and the front-cavity height, because those determine the result before the driver does.
1. Ask what front-cavity height the supplier would specify for the planned outlet, and what effective bandwidth that height supports.
2. Ask what output level and noise component are expected with that height, and whether a measured example exists.
3. State the outlet geometry — slot, perforation or side channel — and ask whether it changes the recommendation.
4. Ask for sensitivity and resonance measured in a cavity of comparable volume to the one available behind the driver, not only in a test box.
5. State the measurement distance, applied voltage and cavity volume behind every published figure you compare.
6. Ask the supplier to evaluate candidates on site against the actual amplifier board, rather than quoting on datasheet figures.
7. Confirm the impedance the amplifier drives, and ask whether a custom voice coil is required to match it.
8. Confirm the mechanical interface — footprint, mounting depth, pads or connector, and any locating features the drawing assumes.
7. FAQ on Wearable and Compact Speakers
Q1: Why does a wearable device produce a hiss at high frequencies?
Most often because the front cavity is too short for the airflow to develop properly, so the outlet radiates turbulence rather than tonal content. Raising the front cavity height is the standard remedy and it works — at the cost of some effective bandwidth. Both halves of that trade should be stated to the customer before the prototype is built.
Q2: Does raising the front cavity height reduce the driver's output?
It changes the system rather than the driver. A taller front cavity loads the acoustic path differently, and the measurable consequence in recorded cases is a reduction in effective bandwidth. Output level may not fall, but the band over which it is available narrows, which is audible on music and largely irrelevant to speech.
Q3: How do I know how much bandwidth I actually need?
It follows from the content. For voice prompts and speech, the 2–4 kHz region carries the intelligibility, and bandwidth beyond roughly 5 kHz adds little. For music or any broadband content, the full audible range matters and a reduced bandwidth will be heard as thin. Stating which of the two the product carries is the first decision to make.
Q4: Is width or thickness the cheaper dimension to spend on?
In many wearable faces, width. Two parts at the same depth — 15 × 8 mm and 15 × 11 mm at 2.5 mm — differ by 3 dB, while two parts differing by 1 mm of width return 1 dB. Where the industrial design has left width available, it is usually the more productive dimension to spend.
Q5: Can a wearable use a bare driver, or does it need a cavity?
Where the housing cannot provide any sealed rear volume, a bare driver cannot meet a low-frequency target because the rear cannot be closed. Either the cavity has to be designed into the housing, or the driver has to bring its own — which is what a boxed platform does. Which route is preferable depends on whether the industrial design can still move.
Q6: Why does a wearable sound different when it is actually worn?
Because the strap partly seals the outlet against the skin, the body absorbs low frequencies, and the outlet orientation changes relative to the ear as the user moves. A hand-held open-air demo cannot reproduce any of those. Validation belongs on a wrist or in a fixture that reproduces the worn condition.
Q7: Should a wearable be validated at the highest volume or the nominal one?
Both, and for different reasons. The nominal level is what the user hears most of the time. The high level is where the front cavity and the outlet are most likely to produce audible noise, and where thermal and power limits show up. A device validated only at a low level can pass acceptance and still disappoint at maximum.
8. Summary: The Outlet Comes Before the Driver
Wearable and compact audio is decided by a surface area that has already been allocated, which makes the acoustic design a negotiation that should start with the outlet geometry rather than the driver catalogue. The relationship that governs the result is the front cavity height: a short cavity produces turbulence noise in the upper band, and a taller one removes it while narrowing the effective bandwidth. Because the trade is measurable and predictable, it belongs in the specification rather than in the tuning stage, and it should be stated as a trade — particularly where the product carries voice rather than music, since the intelligibility-critical region sits well below the frequency band that the trade gives up. On driver selection, the useful window is narrow and the comparisons that matter are often small: within the same depth, a few millimetres of width can be worth more than a change of part. And because a worn device is acoustically coupled to the strap and the body, final validation belongs in a worn condition rather than in a fixture, at both nominal and maximum level. Programmes that fix the outlet and the front cavity before selecting the driver are the ones that avoid a second prototype cycle.
Next step If you are evaluating a micro speaker for a wearable or compact device, the shortest route to a configuration worth testing is to state five things: the space available for the driver and its cavity, the impedance the amplifier will drive, the power the rail can supply, the target level at the intended listening position, and the outlet geometry the housing can provide. With those specified, our engineering team can recommend a suitable configuration for evaluation, or state plainly which part of the acoustic design has to change first.
More in This Series
· Cavity design as the variable that sets the result →https://www.hsdz-spk.com/news/572.html
· Sealed versus open construction for exposed products →https://www.hsdz-spk.com/news/573.html