How to measure Ganymede MEMS microspeakers
USound specifies and measures Ganymede MEMS speakers, Achelous and Adap, in both acoustic domains: in the baffle and the coupler (artificial ear). For both setups, proprietary measurement adapters are used to fit the standardised setup.
To measure in a baffle, the measurement adapter places the MEMS speaker with appropriate sealing inside the IEC baffle. The connection from the MEMS speaker diaphragm to the baffle front side is done with an opening with the same size as the cover inside. The ‘tube length’ is 3 mm with an additional gasket of 0.5 mm (compressed to 0.3 mm).
For the measurement in a coupler, the adapter was designed to bring the speaker close to the artificial ear without introducing additional resonances. This was obtained by a round tube with a size of Ø3.6 mm and a length of 1.5 mm with an additional gasket of 0.5mm (compressed to 0.3mm). With these dimensions, any emerging resonance is guaranteed to fall above the audible frequency range (>20kHz).
Measuring MEMS speakers with the speaker box Carme UJ-R1020
We designed a speaker box called Carme UJ-R1020, which adds a back volume of 100mm³ and seals the backside from the front side. It also feeds the Ganymede MEMS speaker’s electrical connections through a wire to either a Banana-plug, tiny-XLR or mini-jack (depending on the configuration). With this box, measurements in free-field are possible, as well as measurement on an artificial ear.
The box must be placed in a reflection-free space and pointed at the microphone. Since no baffle is used in this setup and a back volume is introduced, the frequency response will be different compared to the USound specification. Due to the missing baffle, the SPL at low frequencies is 6 dB lower in the box in free-field.
The Carme UJ-R1020 box can be directly placed on the base part of the coupler. To seal the air volume between the box and the coupler, some sealing material (gasket, putty) has to be applied between the two parts. Since the opening dimension is not equal to the one of the USound product adapter (data in the specification), some difference will emerge. Nevertheless, it is close enough to assess the MEMS speakers performance properly.