[Review] Passive smartphone amplifiers/boosters, do they really work?

Product: generic smartphone booster
Manufacturer: SDJZT, though available in dozens of similar versions under other brands - China
Cost: ±5€
Reviewer: Lucio Cadeddu - TNT Italy
Reviewed: June, 2023

Foreword

This is the first article of a hopefully short series - titled Spilling the beans - of reviews of components that fail to work as expected.

Being able to have better and more powerful audio from our smartphones, without having to connect cables, pair active Bluetooth speakers and similar gadgets is an interesting prospect: it would allow you to listen better in extreme simplicity. These devices exist and they are all more or less made in the same way: they act as a smartphone base that makes the phone's loudspeaker/s (usually placed in the base) communicate with a horn-loaded cavity. The effect of the acoustic horn is well known: it allows to increase the efficiency of the acoustic transduction.

The principle is the same as when you talk by bringing your hands to the sides of your mouth, to make yourself heard better from a distance, for example when we are angry in the middle of traffic ;-)
In the past it was an almost obligatory choice (think of gramophone horns) because the power available and the sensitivity of the loudspeakers were low. Horn loading economically compensated for these two problems, bringing with it a certain number of limitations, the first of which is a certain directivity: the sound is heard well and loudly on axis, much worse and much less off axis.

Our smartphones have an audio output which is necessarily limited, it would be nice to amplify them at no cost using the same principle. Thus, amplifiers for smartphones like the one you see here in the photo were born over the years. They differ in shape and materials, but the idea is always the same.

Intrigued, I wanted to try one. Having said that, and discarding fancy exotic products carved in very rare woods and with the cost of a good active Bluetooth speaker, I chose one of the many devices found on the net, on AliExpress or on Amazon and Ebay.

Such a device makes sense if it costs little, because - I repeat - with a couple of tens of euros you can buy a respectable Bluetooth speaker. For example, for around 15 euros you can buy a Dodocool DA150 which we reviewed some time ago and which is not bad at all. Moreover, it is rechargeable, so it can also be taken to areas where there is no power supply (e.g. in the garden).

I decided to call this device a "smartphone booster" and the one that I bought costs €5.70, including shipping from China. If you're lucky, looking through AliExpress's periodic offers, I think you can get half of this figure.

Horn loading smartphones

The model I bought has two slits that should coincide with the smartphone speakers. In the case of my OnePlus the positioning is perfect. I also tried with a Samsung and an iPhone. The recess where to place the smartphone is 8.8 cm wide and offers a fairly secure support. Under the device four non-slip rubber disks prevent accidental falls, however its featherweight can be a problem with heavy smartphones: a clumsy touch and...it flips over. The horn part is good looking imitation wood, the rest is real wood (it seems to be thin laminated wood). It measures 102x62x30mm and is also available in pink.

It is declared as Natural Amplifier for Smart Phone and cannot be blamed on the natural part of the description. As for the Amplifier... well. Directivity problems aside, which are negligible in this context, the amplifying effect of the horn is there, but only at certain frequencies. I took my trusty TASI 8151 sound level meter and a set of test tones (there are many on YouTube) and made a few measurements. I'll keep it short: at 250Hz you gain 1dB, at 1000Hz you gain about 3dB (which is a lot!) but at 5000Hz you lose at least 5dB. It doesn't make much sense to try upper or lower frequencies because the smartphone speaker obviously has limited capabilities.

The problem is that the amplifying effect heavily distorts the sound, boosting some portions of the audio range at the expense of others. This megaphone effect (you get the idea) can perhaps be useful with speech, which is however tonally distorted, but with music it is disastrous. The sound becomes cardboard-like and after a few seconds one prefers to listen to the smartphone without booster.

Conclusions

Experiment almost completely failed, I would say. These boosters inevitably amplify certain frequencies at the expense of others and the result is essentially unpleasant. Better, much better, get a small active Bluetooth speaker!

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Spilling the beans: Smartphone “Amplifier”

Horn-loading smartphones

Foreword

Horn loading smartphones

Conclusions