Micro Speaker for Portable Consumer Electronics

Micro Speaker for Portable Consumer Electronics

A portable audio product is judged in motion. The same device that sounds acceptable on a desk can be unusable in a pocket — because the housing moves, the back volume changes with the user's grip, and the noise floor of the environment is no longer the quiet of an office. Portability does not add a specification; it removes the assumptions that made an indoor selection work.

  1. Portability Is a Set of Assumptions Removed

Short answer:  A portable product does not need a louder speaker. It needs a speaker whose behaviour is predictable when the housing is held in a hand, sealed by a palm, and moved through air.

A desktop product is evaluated in a fixed condition: the housing rests on a surface, the cavity behind the driver is what the CAD said it was, and the room is quiet. Portable audio removes all three. The device is held, so the cavity behind the driver is partly closed by the user's hand. The device is carried, so the acoustic opening passes through moving air. The device is used outdoors or in transit, so the noise floor moves and the level that was comfortable becomes inadequate.

The consequence is that the datasheet figure becomes a weak predictor. A driver that produces a clean 95 dB in a 2 cc test box may produce noticeably less in a housing whose rear volume is interrupted by a battery, a PCB, or a hand. This is why portable products are among the most frequent sources of returns attributed to the speaker sounding thin when the part itself was not changed.

  2. Back Volume Is the Part You Cannot See

Short answer:  The rear volume behind a portable driver is usually whatever the packaging left over, and it is often the smallest volume in the product while also being the one that sets the in-cavity resonance.

In a portable product the space behind the driver is contested. A battery occupies the largest internal volume, the main board and its shield can take the space directly behind the acoustic opening, and the manufacturer generally reserves the remaining pocket for the driver and nothing else. The result is a cavity that is defined by packaging rather than by acoustics.

This matters because sealing a driver into a smaller cavity raises the in-cavity resonance rather than lowering it, following FC = Fs × √(1 + Vas / Vb). A driver with a published resonance of 350 Hz can therefore sit at 700 Hz or more in a handheld housing once the battery and board are in place. The audible result is thin output with no low-frequency content and a midrange that sounds strained — often reported as a volume problem when it is a loading problem.

Table 1: What changes about a portable speaker compared with a fixed-condition one

Factor	Fixed-condition product	Portable product	What it changes acoustically
Rear volume	Defined by CAD and unchanged	Set by packaging; often the smallest pocket in the product	Sets the in-cavity resonance, which sets how much low-frequency content survives
Airflow	Open, or vented by design	Closed by the user's hand for part of the session	Adds a variable parasitic load; level and timbre drift with grip
Noise floor	Quiet office, stable	Street, vehicle, public space; variable	Removes any level margin the design relied on
Orientation	Fixed face-up or face-forward	Changes constantly during use	Aimed output falls as the front face rotates away from the listener
Power source	Mains or a large internal battery	Small shared battery with other functions	Limits how much the amplifier can deliver at the rail
Environmental exposure	Indoor, protected	Rain, dust, temperature swings	Qualification follows the actual exposure, not an indoor assumption

  3. Reading the Test Box Line Correctly

Short answer:  A datasheet sensitivity measured in a 1 cc test box tells you what the part can do, not what the product will do; the two differ by whatever the enclosure takes away.

Portable products inherit a habit from larger audio categories: reading the published sensitivity as a product figure. The test box is a defined condition with a fixed cavity, a defined grille, and a defined mounting. A portable housing meets none of these conditions by default.

Two practices make the difference measurable. The first is to ask for the sensitivity measured in a cavity volume close to the volume actually available behind the driver in the product, not just the test box figure. The second is to require the measurement to be stated with its measurement distance, applied voltage and cavity volume, because a figure without those three cannot be compared with any other figure. Where a supplier publishes only the test-box line, treat it as an upper bound.

  4. Driver Classes for Portable Audio

Short answer:  The useful split is by available depth: sub-3 mm parts for thin slabs, 3–5 mm parts where a pocket exists, and boxed platforms where the product has no cavity to spare at all.

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; an in-housing measurement in a cavity of comparable volume remains the only transferable figure. Thickness values follow the supplier's model coding, in which the final two digits express millimetres as integer plus decimal.

Table 2: Published drivers applicable to portable audio designs

Model	Format and published size	Published sensitivity and power	Published F0	Portable build where it fits
HS003021H	BOX platform, φ30, 21 mm height	105 dB at 2 kHz / 10 cm / 2.0 W	800 Hz ±15%	A pocket too small for a cavity; the module brings its own volume
HS003058H	BOX platform, φ30, φ15.5 mm core	103 dB at 2 kHz / 10 cm / 2.0 W	800 Hz ±15%	Portable products requiring high level without a rear cavity
HS002628H28	BOX platform, φ26, 28 mm height	99 dB at 2 kHz / 10 cm / 2.0 W	500 Hz ±15%	A shallow pocket needing more low-frequency content than a bare part
HS003050H	Round magnetic, φ30 × 5.0 mm	97 dB at 2 kHz / 10 cm / 2.0 W	550 Hz ±15%	A device with a usable pocket; catalogue applications include Bluetooth speakers
HS002850H50	Iron frame, φ28 × 5.0 mm	97 dB at 2 kHz / 10 cm / 2.0 W	600 Hz ±15%	Same output class in a thinner frame where depth is the limit
HS003650H	Round magnetic, φ36 × 5.0 mm	97 dB at 2 kHz / 10 cm / 2.0 W	500 Hz ±15%	A portable speaker needing the lowest resonance in the round range
HS004550H	Round magnetic, φ45 × 5.0 mm	98 dB at 2 kHz / 10 cm / 2.0 W	500 Hz ±15%	Larger portable formats and Bluetooth speaker applications
HS0028110H110	Pot-type large magnetic, φ28 × 10.0 mm	96 dB at 2 kHz / 10 cm / 2.0 W	350 Hz ±15%	Portable products that genuinely need low-frequency extension and have depth
HS0034140H140	Pot-type large magnetic, φ34 × 9.0 mm	98 dB at 2 kHz / 10 cm / 2.0 W	300 Hz ±15%	Bluetooth speaker and telephone formats where bass extension matters
HS003058H / HS002628H28	BOX alternatives above	99–103 dB at 2 kHz / 10 cm / 2.0 W	500–800 Hz ±15%	Where the enclosure gives no cavity at all and a bare driver is not viable

One pattern in that table is worth stating on its own: the boxed platforms reach the highest published sensitivity while occupying no additional depth in the product, because the cavity is part of the part. Where a portable housing has no usable pocket behind the driver, that is often the only route that preserves any low-frequency content at all.

  5. Project Case: Portable Listening Device in a 13.5 mm Envelope

Project Case Study (Hongsheng)

A portable listening device for sleep, rest and short breaks was specified with three targets together: the lowest resonance the customer believed achievable, distortion below 1%, and a rated power of 2 W inside a 12.5 mm thickness limit. A leading global supplier's 20 × 38 mm driver was evaluated first and rejected: its published resonance of 120 Hz met the frequency target, but distortion exceeded the limit at 1 W, the level was too low, and the diaphragm was light enough that low-frequency impact was absent. Other suppliers had no suitable product at all, because the specification combined an application few of them served with a volume they would not customise.

Hongsheng's first proposal was an existing boxed platform with a resonance of 280 Hz. The customer accepted the sound in a listening comparison but the platform was 16.5 mm, over the thickness limit. The work then moved to the space actually available: with the customer's agreement the thickness went from 12.5 mm to 13.5 mm in exchange for the cavity volume that made the target reachable. A 40 mm diaphragm was customised for it, reaching a free-air resonance of 120 Hz and 230 Hz once assembled into its cavity, with distortion below 2% from the rubber diaphragm material.

Four samples were produced before the customer accepted the result. The first at 55 × 95 × 12.5 mm reached only 300 Hz. The second, enlarged to 60 × 100 × 13.5 mm, reached 280 Hz, so the diaphragm material was redesigned and the resonance came to 230 Hz. On the third, a passive radiator was added for more low-frequency output and the system resonance reached 120 Hz. On the fourth, the customer reported excessive enclosure vibration at low frequency, so the construction changed to a dual-diaphragm arrangement; a 60 Hz noise was then found, traced to the vibration itself, and reducing the diaphragm angle from 55° to 65° removed it. The project is at design freeze, with the customer evaluating other hardware before tooling.

Three points in that sequence are transferable. Negotiating thickness for cavity volume moved the requirement from an unachievable number to a testable one. The four-iteration record, each version changing one variable, is what established which dimension was actually binding. And the final acceptance was reached on measured low-frequency behaviour in a hand-held enclosure, not on the original specification sheet. Hongsheng can supply a boxed platform where the enclosure has no cavity to define, or a custom driver and cavity design where it does, and can review the cavity figures before a tooling commitment.

Hongsheng evaluated the enclosure space and the required cavity volume before selecting any driver. The positioning of this supplier is narrower than a general manufacturing claim and more useful: Hongsheng reviews the space actually available and the outlet geometry first, and only then proposes a boxed platform or a custom driver.

One-line conclusion: the specification was not met, and the programme was still won — by moving the conversation from parameters to measured behaviour.

  6. Confirming a Portable-Audio Driver with the Supplier

Short answer:  The questions that matter here are about the enclosure as it will actually be held, and about which of the five numbers the supplier is quoting to.

1. Ask for sensitivity and resonance measured in a cavity close to the volume actually available behind the driver in your product, not only in a 1 cc test box.

2. State the measurement distance, applied voltage and cavity volume behind every published figure you compare; without all three, figures are not comparable.

3. Confirm whether the device will be held in the hand during use, and ask what that does to the rear volume in the positions that matter.

4. Ask which cabinet or housing the part was evaluated in for any published low-frequency figure, and whether the figure survives a change of cavity.

5. For a boxed platform, ask whether the published acoustic volume belongs to the part or to a recommended host cavity, and what changes if the host differs.

6. Give the rail voltage available at the lowest battery state and ask for the level and distortion data at that condition.

7. State the environmental exposure the device will actually see, and ask which qualification applies to the driver itself under that exposure.

8. Ask what the supplier would change first if the low-frequency output were short of target in the assembled product.

  7. FAQ on Portable-Audio Speakers

Q1: Should a portable speaker simply be a more powerful version of the one used in a fixed product?

Not usually. Portability changes the enclosure more than it changes the level requirement. The cavity behind the driver is smaller or interrupted, the opening moves with the user's hand, and the noise floor is higher and variable. A more powerful part in a starved cavity raises the in-cavity resonance and can make the result thinner, not louder.

Q2: Why does my portable device sound fine on the bench and thin in the user's hand?

Because the hand closes part of the rear volume and changes the loading, while the noise floor of a real environment is higher than a test room. Both reduce perceived level, and the second one removes the margin that was assumed during selection. A bench measurement in a fixed fixture cannot reproduce either.

Q3: Is a boxed platform better than a bare driver for a portable product?

Where the enclosure has no usable cavity, yes — it is often the only option that retains any low-frequency content, because the cavity is part of the part rather than part of the packaging. Where the housing already has a controlled pocket, a bare driver of the same class is usually cheaper and adequate.

Q4: How much cavity volume does a portable speaker need?

Only as much as the low-frequency target requires, and the honest answer is that the requirement is set by the resonance target rather than by the driver. Sealing a driver into a smaller cavity raises the in-cavity resonance, following FC = Fs × √(1 + Vas / Vb). Where the product does not need low-frequency extension, a small cavity is perfectly workable.

Q5: What is the most common reason a portable audio product fails acoustically?

In our experience it is not an under-specified driver. It is a rear volume that was never designed, so the in-cavity resonance ends up well above the free-air figure and the product ships with less low-frequency content than the selection assumed. The second most common is an environmental qualification that does not match actual exposure.

Q6: Does a thicker housing always help?

Not for the acoustics directly. Thickness buys cavity volume, and cavity volume lowers the in-cavity resonance — but only if the volume is sealed and vented properly. A thicker housing with an open leak can perform no better than a thin one, and it costs space the product needed elsewhere. This is why thickness is a design lever rather than an acoustic remedy.

Q7: How should portable and indoor products share a driver platform?

Where the envelope allows it, sharing the platform is valuable for qualification cost and supply continuity. The caution is the cavity: a driver selected in a large fixed cavity and then reused in a small handheld one will not perform to the figure that justified the selection, so the reuse should be validated in the smaller housing rather than assumed.

  8. Summary: Portability Removes Assumptions Rather Than Adding Specs

What portability removes is more useful than what it adds. A portable audio product loses the defined cavity, the fixed orientation and the quiet room that an indoor selection depends on, and each of those changes the result in a way a datasheet cannot show. The practical sequence is to establish the cavity volume the product will actually have, require the sensitivity figure to be measured in a comparable volume, and treat the published test-box line as an upper bound rather than a promise. Where the housing has no cavity to spare, a boxed platform is usually the only route that keeps any low-frequency content; where it has a controlled pocket, a bare driver of the matching class remains adequate. Programmes that negotiate enclosure dimensions early — trading thickness for cavity volume where that is acceptable — reach a testable requirement faster than programmes that treat the original specification sheet as fixed. And because the user is a variable part of the acoustic circuit, the final acceptance belongs in a hand-held enclosure under real noise, not on a bench.

Next step  If you are evaluating a micro speaker for a portable audio product, 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 whether the device will be held in the hand during use. 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

· How a shallow housing changes the result, and when a BOX platform is the right route →https://www.hsdz-spk.com/news/573.html

· Sealed versus open builds in portable and exposed products → https://www.hsdz-spk.com/news/574.html