We were interested in finding out whether 2 Audiomoths can replace 1 Song Meter. Financially and logistically, it would be very nice. For the Audiomoths, we used protective plastic cases with a hole drilled into them to let sound pass through an acoustic vent (GAW112 by Gore) and made a test outdoors in autumn of 2018. We strapped an Audiomoth to each side of our SM2Bat+, which used custom microphones (SPU0410LR5H-QB) that are at least more performant than SMX-U1 mics (in review). We recorded for 1 hour and 20 minutes 30 min after sunset near to a pond. This test is more fair than the one mentioned in the other forum because all microphones have equal protection levels (GAW 112 vents). The cases handled morning dew (there was plenty) well. They felt a bit damp inside tough, and it won't be easier to handle in tropical regions, so maybe little bags of silica gel could help (the kind you can put in an oven to recharge/dry out). The Audiomoths recorded sound, and every main bat pass (there were ~half a dozen, 2 morphocalls) that was detected by the corresponding Song Meter microphone was also recorded by the Audiomoth. These are good news. Bad news: The Audiomoths record distorted bat calls when they are strong (but before distortion): This confirms what we found in the post https://www.openacousticdevices.info/support/device-support/sound-transmission-with-and-without-cases-comparison-with-sm2bat). You see lots of additional harmonics, most above 100 kHz. I think Joe Chun-Chia Huang also highlighted that somewhere on the forum. We recommend the SPU0410LR5H-QB mic on the Audiomoth, it has been proven to be good and could be compatible. Signal-to-noise ratio looks better with the SPU0410LR5H-QB, and detection distances should correspondingly be better too: really faint calls would be missed by the Audiomoth, check screenshot. There are vertical bands of noise in the Audiomoths. UPDATE: according to Alex Rogers, these are due to the SD card writing and can be avoided by using a certain model. To conclude, 2 Audiomoths can replace a single higher-end dual channel recorder well, if you can live with small inconvenients (distorted ultrasound, avoidable noise ticks). It might be worthwhile to find out how to synchronize the recordings from 2 units accurately (also for triangulation and time-of-arrival measurements that might be useful to some).

We did some tests with a colleague who works with Audiomoths. I work with Song Meters, so we thought we could do useful comparisons. Materials We made a weatherproof case for the Audiomoth which consisted of a plastic box (Snack Box) of 9.5 x 5 x 2.5 cm, covered inside with synthetic foam for padding (see photos in Comparing 2 Audiomoths with 1 Song Meter). We drilled a hole in the plastic box on top of the microphone and covered it with a Gore GAW112 acoustic vent, which is used on almost all of the Wildlife acoustics microphones because it impedes ultrasound transmission only little. We attached another layer of foam to the lid of the box to prevent the PCB touching the lid and added silicone to the microphone corner of the Audiomoth to prevent contact of the PCB with the vent to reduce vibration transmission. We compared the sensitivity for ultrasound of the bare Audiomoth to the sensitivity of the Knowles SPU0410LR5H-QB microphone attached to the Song Meter SM2Bat+ (this is a custom microphone). Our Knowles microphone was inside a tubular microphone housing, and covered with a GAW112 acoustic vent and a wind screen from Wildlife acoustics. We programmed six Audiomoths to record continuously at a sample rate of 384 kHz, with a high gain, and we recorded at 192 kHz with the Song Meter. We emitted sounds of different frequencies with a portable loudspeaker (1, 2, and 6 kHz, SoundCore Anker) and the Wildlife Acoustics Ultrasound Calibrator which emits chirps at 40 kHz. Sounds were emitted at distances of 2, 4, 8, 16 and 32 m in front of the devices. We equalised the loudness of the SM recordings with the Audiomoth using the nearest unclipped recording of ultrasound by amplifying it. Comparison with SM2Bat+ We found that all our new Audiomoths record sound very similarly. The recordings were synchronized without gaps, and showed similar spectrograms. The sensitivity of the microphones and the ambient noise levels too were visually identical. This is good news, it shows the device quality is very consistent. The self-noise level of the bare Audiomoths was slightly lower than the self-noise of our packaged Knowles microphones: the 40 kHz chirps at 32 m to the front were barely visible on the SM recordings and clearly visible for the Audiomoth, but it is a slightly unfair comparison since the Knowles microphones we used had been used in the field, and were covered with a vent and windscreen. So: to be interpreted with caution. Audiomoths record several undesirable harmonic artifacts when sound level is high, but well before clipping. This is of relevance to bat researchers because fidelity of bat call recording will be low. We suggest that the other Knowles microphone we used might be better suited. Comparing different protection levels We repeated this test to compare the sensitivity of 2 bare Audiomoths (when used uncovered), 2 inside the plastic bag provided with the product, and 2 inside the case we built for the devices, at different audible and ultrasonic frequencies. We also tested the directivity of sound reception, by emitting sound in front and perpendicularly to the microphones. Unfortunately, the measurements for the bag setup are less precise as one unit did not record, so we had only one recording for that setup. Immediately apparent: For ultrasound (40 kHz) at 8 m, signals from bare Audiomoths were saturated, those inside cases were not, while signals from Audiomoths inside the plastic bags were not saturated already at 4 m. More in detail: We measured the mean sound amplitude for each unit (bare/plastic bag/ case) using ten identical, not saturated 40 kHz chirps. For audible sound, we used two 0.8 s selections for each frequency and device. We used the “Plot spectrum” tool in Audacity and averaged the measurements. For ultrasound (40 kHz): The plastic bag attenuated sound on average 22.15 dB compared to bare units. Our case attenuated sound at 40 kHz on average by 13.95 dB compared to bare units and were thus distinctly better than the plastic bags, which was not unexpected, but noticeably worse than the bare units. For audible sound: at 1 kHz, the bag did not attenuate sound noticeably, cases by 7 dB. At 2 kHz, the bag did not attenuate sound noticeably, cases attenuated sound by 5 dB. At 6 kHz, the bag attenuated sound by 16 dB, and the cases did not attenuate sound noticeably. This is the price we pay for weatherproofing with our simple design, and it leads to a reduction in detection range. At 40 kHz, with bare units, chirps at 32 m could all be detected (weak); with cases, very few chirps could be detected at 32 m and they could be easily detected at 16 m; within the bag, the signals were weak at 16 m and none could be detected at 32 m. (Plotting sound transmission curves would allow to interpolate the exact extinction distance for each design.) Audio signal shapes were visually looking relatively unaffected by the case, which is good news. The transmission loss caused by the case could probably be reduced if the gap between the case and the PCB was filled by a tube or better, with a horn. Directionality UPDATE: For more info about directionality, see this thread. We compared the amplitude of 4 chirps perpendicular to the Audiomoths against 4 chirps in front of them, for each unit, and took the average reduction in sound level to express directionality. Sound pickup at 40 kHz is quite directional. With bags, there is a 8 dB reduction from the front to the side (90 degrees). The bare units are actually attenuated more (12 dB reduction from front to side), maybe the plastic bags' uneven, slightly protruding surface could help in picking up sound from the side but there was only one unit and variability is high so I prefer not to speculate too much. The cases led to higher directionality (19 dB reduction). Sound pickup in the audible range was only checked visually with spectrograms, not measured (I can provide files). In theory, it should be less directional than in the ultrasound range anyway, and indeed, we did not see a difference between the bare units and those in bags or cases, but signals from the front are always visibly stronger than those from a perpendicular source, which is also to be expected. In order to obtain stereo recordings, more than one device facing opposed directions should be used (this is shown in the "Comparing 2 Audiomoths with 1 Song Meter" thread). To conclude, due to the high reduction in sound cased by the plastic bags, proper acoustic vents are highly recommended when you record bats, and due to the narrow sound pickup should be borne in mind. For recording birds, if sound quality is not critical, the plastic bags seem to do the job.