How to measure Ganymede MEMS microspeakers
USound specifies and measures Ganymede MEMS speakers, Achelous and Adap, in both acoustic domains: in the Baffle and in the Coupler (artificial ear). For both setups, proprietary measurement adapters are used to fit the standardised setup.
In the case of the baffle, the measurement adapter places the MEMS speaker with appropriate sealing inside the IEC baffle. The connection from the MEMS speaker membrane to the baffle front side is done with an opening with the same size as the cover-inside. The ‘tube-length’ is 3mm with an additional gasket of 0.5mm (compressed to 0.3mm).
In the case of the coupler, the measurement adapter was designed to bring the speaker close to the artificial ear without introducing additional resonances. This was obtained by a round tube with the size of Ø3.6mm and a length of 1.5mm 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 Carme test box
USound designed a proprietary test box, called Carme, which adds a back volume of 100mm³ and seals the backside from the front side. It also feeds the electrical connections from Ganymede through a wire to either 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 in this setup no baffle is used 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 6dB lower in case of the box in free-field.
The box can be directly placed on the base part of the coupler. To seal the air volume between box and coupler, some sealing material (gasket, putty) has to be applied in between the two parts. Since the dimension of the opening 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.