How high is the power consumption of USound MEMS speakers?

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updated April 18, 2019

First, we should clarify how power consumption is typically measured with electro-dynamic speakers or BAs. Since the impedance of these speakers is roughly constant over frequency (typically 4, 8 or 32Ω), this impedance value is taken as “nominal”. With this, a very simple transformation from a result obtained with a constant voltage into a result with constant power can be done. It should be noted, that this is the standard procedure but far away from the electrical truth. Especially around resonance frequency and at high frequencies, this simplification is inaccurate due to the strongly changed impedance.

In the case of USound MEMS speakers, a simplification like in the conventional speakers’ measurement cannot be done at all, due to the strongly changing impedance over frequency. The electric behavior of USound MEMS speakers closely follows the behavior of an ideal capacitor, where the impedance is inverse proportional to the frequency (low frequencies – high impedance; high frequencies – low impedance).

This poses the problem of not being able to give a single value for the power consumption of the MEMS speakers. Furthermore, most of the power consumption is reactive, making a speaker-level comparison even more unclear.

So, as shown, a fair comparison in power consumption between USound MEMS speakers and conventional speakers must be well thought through. More attention must be paid to obtain meaningful results.

Conclusion

The best way to assess the power consumption of any speaker (and compare different systems), is a system level evaluation in combination with a representative sound signal at a defined output level. This means a speaker-appropriate amplifier needs to be included in the assessment. Furthermore, a test signal has to be used that is representative for music/speech and adjusted in level to reach a defined reference output SPL. For the reference, level 94dB (in case of in-ear applications) is taken. As sound signal, the so-called IEC-Noise (IEC 60268-1) is recommended. Due to simplicity, also other types of noise, like white and pink, can be used.

For Achelous, the average power consumption together with the Amplifier (TI LM48580 and PS61046 – like specified in the datasheet) is stated below:

Condition: Supply voltfage: 3.6 V; state: April 2019

Power consumption with IEC noise (60268-1) [mW] 13.6
Power consumption with white noise [mW] 19.8
Power consumption with pink noise [mW] 17.4
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