How Hongsheng Solved an Insufficient SPL Problem in a Compact Electronic Device

How Hongsheng Solved an Insufficient SPL Problem in a Compact Electronic Device

  1. Why "Not Loud Enough" Is the Wrong First Conclusion

Short answer:  Because a level shortfall in a compact device has three candidate causes — driver capability, cavity, and mechanical limits — and the mechanical one is invisible in the specification.

A level complaint arrives as a single number: the device is not loud enough, or the sound is thin and tiring. The reflex is to look for a driver with a higher rating or a higher sensitivity figure. That reflex fails often enough to be worth pausing on, because a compact device's output is the product of the driver, the cavity behind it, the opening in front of it and the power the amplifier can actually deliver. A shortfall in any one of them looks identical from the outside.

There is a fourth cause that never appears in a specification at all: what the mechanical design allows. In this case it was the height of a single mould. The customer's product was assembled abroad and hand soldering of leads was not workable, so the electrical interface had to stay as spring contact. Only one mould provided that interface, and its height was not enough to carry a compound diaphragm — the construction that would have provided the low end the product needed. Every driver-side optimisation therefore ran into the same wall, whatever the numbers said.

The useful question is therefore not how to make the driver louder. It is which of the four contributors is actually short, and the answer has to be established before a part number is discussed.

  2. The Two Constraints That Were Not Acoustic

Short answer:  The assembly process fixed the contact method, and a single mould'shttps://www.hsdz-spk.com/news/594.html height fixed whether a compound diaphragm was possible. Both constraints sat outside the acoustic specification.

The first constraint came from manufacturing geography. The product is assembled outside the domestic market, and soldering leads there is not practical, so the design rule was that no wire-terminated driver could be used. Spring contact was mandatory. That single rule removed most of the catalogue, because it fixes the driver to whatever terminations the available moulds provide.

The second constraint followed from the first. Only one mould could provide the required spring contact, and its internal height was not sufficient to house a compound diaphragm, which needs more travel than a single membrane can use. The consequence is mechanical rather than acoustic: without a compound diaphragm there is no low end to recover, and no amount of resonance tuning substitutes for it. The customer's own work on the existing unit had already taken the resonance from 800 Hz down to 650 Hz without resolving the complaint, which is the clearest possible evidence that the resonance number was not the binding constraint.

This is the pattern worth recognising. Two decisions that have nothing to do with sound — where the product is assembled, and which mould happens to exist — between them determined whether the product could ever sound the way it needed to. A specification that lists only SPL and F0 cannot express either of them.

  3. What a Compact Device Can Be Given

Short answer:  Published compact configurations reach the mid-90s dB at 0.8–2.0 W, and the two thinnest 1217-class parts are BOX constructions that require a host cavity rather than a bare driver.

Table 1 lists published configurations in the compact segment. Two points matter for a shortfall investigation. First, the sensitivity figures in this size class sit in a narrow band — the mid-90s dB — regardless of whether the part is rated 0.8 W or 2.0 W, so a substitution inside the same footprint buys very little. Second, the two thinnest parts in the 12×17 mm class are BOX constructions: they require a defined host cavity and cannot be treated as bare drivers.

Table 1: Published configurations in the compact segment, at the stated test conditions.

Model	Outline	Impedance	Rated / max power	SPL	F0	Note
HS002038H	φ20 × 3.8 mm	8 Ω	0.8 / 1.0 W	93 dB	800 Hz	Round baseline in this size class
HS002045H	φ20 × 4.5 mm	8 Ω	0.8 / 1.0 W	94 dB	600 Hz	Lower resonance at the same rating
HS203045H45	20 × 30 × 4.5 mm	8 Ω	0.8 / 1.0 W	97 dB	800 Hz	Rectangular outline, larger radiating area
HS251233H	25 × 12 × 3.3 mm	4 Ω	2.0 / 2.5 W	97 dB	750 Hz	Higher rating, same sensitivity class
HS250926H	25 × 9 × 2.6 mm	4 Ω	2.0 / 2.5 W	93 dB	700 Hz	Thin square, lowest in the group
HS121722H	12 × 17 × 2.2 mm	7 Ω	1.0 / 1.2 W	95 dB	850 Hz	Requires a defined host cavity (BOX construction)
HS201623H	20 × 16 × 2.3 mm	7 Ω	1.0 / 1.2 W	96 dB	800 Hz	Requires a defined host cavity (BOX construction)
HS0023123H123	φ23 × 12.3 mm	4 Ω	2.0 / 2.5 W	95 dB	400 Hz	Pot-type, low resonance for its size

Read that spread against the complaint. If a product needs more low end rather than more mid-band level, the answer is not in this table at all, because none of these parts carries the construction that would provide it. That is the point at which the investigation has to move from selection to structure.

  4. Project Case Study — A Cavity, a Compound Diaphragm and a Preserved Contact Method

Project Case Study (Hongsheng) — A Cavity, a Compound Diaphragm and a Preserved Contact Method

The product was a 4G feature phone with multimedia playback, used for short video and audio over long sessions. The customer wanted a softer, less sharp sound at the level of a mid-range smart device, with targets of resonance below 850 Hz and sensitivity above 92 dB. The board was a single assembly and the original driver was a φ20 single-membrane unit with spring contact. Its resonance could be brought to 650 Hz, but the membrane's compliance was poor, there was no low end, and the sound was thin and tiring over long listening. Adjustments to that unit took the resonance from 800 Hz to 650 Hz and did not resolve the complaint, because the unit's construction could not provide the travel a compound diaphragm uses. Hongsheng's proposal was to change the structure rather than the driver: the rear housing was reworked to free cavity volume, which was a tooling change, and a 1217 cavity module with a compound diaphragm driver replaced the φ20 unit. Hongsheng preserved the electrical interface by mounting the spring contacts on a flexible printed circuit that connects to the driver, so the customer's contact method stayed exactly as it was. The delivered configuration measured 94 ± 3 dB at 2.83 Vrms, 10 cm and 2 kHz, with a resonance of 850 Hz ±15% and a rating of 1.0 W against a 1.2 W maximum, at a tax-inclusive unit price of 2.4 RMB on a product whose own cost is around 100 RMB. The project reached design freeze with the customer confirming the result met the requirement.

One-line conclusion: in a compact device the binding constraint is often the height a mould allows, and the fix is a cavity and a diaphragm change rather than a louder driver.

  5. How the Attribution Was Narrowed

Short answer:  Four assumptions were tested and three of them failed. The one that survived had nothing to do with the driver's published figures.

Table 2 records the attribution sequence. It is worth reproducing because the order matters: each assumption was tested against evidence before the next was considered, and the evidence was already in the customer's own trial history.

Table 2: Attribution of a level and low-end shortfall in a compact device.

Assumption tested	What it predicted	What the record showed	Outcome
The driver is too small; a larger one is needed	A larger outline would raise output	The structure accepted only one outline, fixed by the board and the housing	Rejected — the outline was not variable
The resonance is too high; tune it down	Lowering F0 would restore the low end	Adjustments took the resonance from 800 Hz to 650 Hz and the complaint remained	Rejected — the resonance number was not the constraint
The rating is too low; use a higher-rated part	More power would raise output in the band that matters	The delivered unit met the sensitivity target at 1.0 W once the structure allowed a compound diaphragm and a cavity	Rejected — rating was not the constraint
The construction cannot provide the travel	No driver-side change could add low end	Only one mould provided the required spring contact, and its height could not carry a compound diaphragm	Confirmed — the limit was mechanical, and the structure had to change

For products where the electrical interface and the internal height are both fixed, Hongsheng can review whether a cavity and diaphragm change can deliver the required band before any driver substitution is attempted. The review starts from the assembly constraint, because that is what determines which constructions are available at all.

  6. What the Physics Actually Allows at This Size

Short answer:  A compound diaphragm adds the mass and compliance that provide low end, and the cavity behind the driver raises the system resonance — so volume has to be freed before the resonance target can be met at all.

Two relationships govern what happened here. The first is construction: a compound diaphragm carries more moving mass and more compliance than a single membrane, which moves the resonance down and allows the excursion that produces low-frequency output. A single membrane in a constrained height cannot do this, which is why the mould height decided the outcome before any tuning was attempted.

The second is the cavity. Adding an enclosed volume behind the driver raises the system resonance relative to the driver's own value, because the trapped air acts as a spring in series with the suspension: the smaller the volume, the higher the resulting resonance. That is why freeing volume from the housing was a precondition rather than an optimisation — with the original volume the resonance target was unreachable regardless of which driver was fitted, and the reworked rear housing is what made the delivered 850 Hz ±15% possible on a target below 850 Hz.

Neither relationship is visible in a sensitivity figure. Both are invisible in a specification that lists only SPL and F0, which is precisely why a shortfall investigation has to ask about the cavity and the assembly constraints explicitly.

  7. What to Confirm Before Changing the Structure

Short answer:  Six questions separate a mechanical limit from an acoustic one — and the first two are about assembly, not sound.

1. How the product is assembled, and whether that rules out wire termination — the contact method is often the first constraint and it is rarely in the specification.

2. Which terminations the available moulds actually provide, and what internal height each one allows.

3. Whether the required low end needs a compound diaphragm, and if so whether the height budget can carry one.

4. How much sealed cavity volume the structure can give up, and whether that is a tooling change.

5. The measurement basis for the quoted sensitivity and resonance: voltage or power, distance, frequency point, and free-air or installed.

6. The target expressed as a band and a listening position rather than as a single figure, so that a pass can be judged against the real use case.

  8. FAQ: Insufficient SPL in Compact Devices

Q1: The device is not loud enough. Should I look for a higher-rated driver?

Not as a first step. In a compact device the output is set by the driver, the cavity, the opening and the available power together, and a higher rating often raises output in bands the product does not use. Establish which contributor is short before changing the part.

Q2: Why did tuning the resonance down not fix the sound?

Because the resonance number was not the binding constraint. The unit's construction could not provide the travel a compound diaphragm uses, so no resonance value could restore the missing low end. The limit was the mould height, not the tuning.

Q3: What does a compound diaphragm actually add?

More moving mass and more compliance than a single membrane. That moves the resonance down and allows the excursion that produces low-frequency output, which is why the height budget decides whether it can be used at all.

Q4: Why did freeing cavity volume matter if the driver was also changed?

Because the cavity raises the system resonance: the smaller the enclosed volume, the higher the resulting resonance. With the original volume the resonance target was unreachable with any driver, so freeing volume was a precondition rather than an optimisation.

Q5: The product is assembled abroad. Why does that affect the speaker?

Because hand soldering of leads is not practical there, so the contact method has to be mechanical. That fixes the driver to whatever terminations the available moulds provide, which in this case was one mould with a height limit.

Q6: How was the customer's contact method preserved?

The spring contacts were mounted on a flexible printed circuit that connects to the driver, so the interface the assembly line uses stayed unchanged while the driver and cavity behind it were replaced.

Q7: What result did the change deliver?

94 ± 3 dB at 2.83 Vrms, 10 cm and 2 kHz against a 92 dB target, a resonance of 850 Hz ±15% against a target below 850 Hz, and a 1.0 W rating, at a tax-inclusive unit price of 2.4 RMB. The project reached design freeze with the customer confirming the requirement was met.

  9. Summary: Fix the Structure Before the Driver

The case is a reminder that a compact device's sound is decided upstream of the driver. Two constraints that had nothing to do with acoustics — where the product is assembled, and the height of the one mould that could serve it — between them determined whether the product could produce low end at all. Until those were addressed, no driver substitution could have succeeded, and the customer's own trials had already demonstrated that.

The sequence that worked is worth keeping: establish the termination method, establish the height budget, establish the cavity volume the structure can give up, and only then choose the construction. In this programme that order produced a delivered sensitivity above target, a resonance inside its band, an unchanged electrical interface, and a unit price that is a small fraction of the product's own cost.

Next step  If you are evaluating a micro speaker for a 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 termination method the assembly process requires. 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 to Choose a Micro Speaker Based on Mechanical and Acoustic Constraints →https://www.hsdz-spk.com/news/594.html

· Micro Speaker Replacement Case: Improving Acoustic Performance in a Compact Device → https://www.hsdz-spk.com/news/596.html