SM4-mini positioned in a parabolic reflector

Nocmig with an SM4-mini

Earlier this autumn I bought a Wildlife Acoustics SM4-mini to use for a long-term nocmig project in North Norfolk. I set this up with a spare fibre glass parabolic reflector that was generously given to me, to improve the range of the microphone. The whole setup was fixed to a garden shed in a coastal village and angled slightly towards the coast and away from nearby trees.

The kit

The SM4-mini is pretty compact (approx. 5″ x 6.5″ x 1.5″), runs off 4 AA batteries and takes standard SD cards. The box is rugged and waterproof and has slots and holes for attaching it to things. The setup includes a single microphone but there is a second port to add an optional second microphone for stereo recordings.

One feature I really like is the mobile app used to configure the device. The device broadcasts a Bluetooth signal which provides basic info like how many recordings have been made to date. Open the box and press the pair button and you connect your mobile app to the device to control all the settings. It is highly customisable. For example, you can configure your own on/off cycles, or allow the devise to use sunset and sunrise times as the basis of the routine.

As you’d expect, you can control the sample rate, the length of files and the gain setting. You can also set a delayed start time. On my last deployment I knew the weather was going to be horrendous for a few days so I delated the start until after the bad weather to save on battery/card life.

Given the particular batteries, card and configuration you’ve selected, the app will estimate when the batteries will be depleted and when the memory card will be full.

The sounds

So what about the recording quality? I think it is pretty good as can be judged from the following recordings:

Common Scoter, Blakeney, 09/10/20 22:26. Unedited

Gadwall, calls of male and female, Blakeney, 29/09/20 01:23. Unedited

Great Bittern, Blakeney, 14/10/20 05:23. Unedited

Migrating thrushes and local Robins. Blakeney, 14/10/20 06:40. Low pass filter to reduce sound of central heating boiler

These are cherry picked interesting examples from my most recent deployment but I think they are representative of the quality the recorder provides. The recorder has run continuously since mid August with no failures or errors. I haven’t done any side-by-side tests of this (with or without the reflector) against other recorders.

Verdict

I’m very pleased with the device for the following key reasons:

  • it’s waterproof
  • it’s easily programmed via a nice mobile interface
  • it runs autonomously for 3 weeks (on my settings)
  • it produces nice clear nocmig recordings

When I retrieve the recorder I will try to make some side-by-side recordings with other equipment.

PS: for anyone considering mounting an SM4-mini in a parabolic reflector, getting the microphone element at the precise focal point of the dish is critical. Without removing the foam windshield of the mic it is hard to know where the mic element is located, but I was told by Wildlife Acoustics that it is 16mm (±1mm) from the outside of the case.

EM172 lapel mic, Zoom H4n Pro audio recorder, Audiomoth and mini USB mic (photo: Simon Gillings)

Equipment Comparison February 2020

On one of the few calm nights in February 2020 I tried to make simultaneous recordings of the same flight calls using a variety of equipment. I tested:

  1. A cheap USB microphone connected to a desktop PC. Mic level set to 90/100.
  2. An AudioMoth in a plastic bag. Gain set to Medium
  3. An AudioMoth in a homemade waterproof case. Gain set to Medium.
  4. A lapel microphone (EM172) with a digital audio recorder (Zoom H4n Pro; record level set to 80/100).
  5. A Dodotronic parabolic microphone with a digital audio recorder (Sound Devices MixPre 3; gain set to 25dB)

There was very little bird movement but I got useful comparisons for a Redwing and a Moorhen pass. Below is a recording of each call and a side-by-side comparison of the spectrograms.

Redwing

A single Redwing call at 2010. The call is a bit atypical but still conveys the quality of the recording using the different equipment. Without adjusting the gain on the USB recording this call is inaudible. It is visible and audible on all the other equipment setups but is stronger and more clearly defined on the final two recordings. The bird was not directly overhead, which explains why the lapel mic and parabolic mic have similar performance. Had the bird been directly above the parabolic dish that recording would have been much louder.

Cheap USB mic and computer:

AudioMoth in bag:

AudioMoth in case:

Digital audio recorder and EM172 lapel mic:

Digital audio recorder and parabolic reflector:

Spectrogram comparison
Redwing comparison 2

 

Moorhen

The Moorhen call was picked up and visible on all recorders, though it was still faint on the USB. On the two AudioMoth recordings only the first kek-kek-kek series is clearly visible whereas on the Lapel mic and Parabolic mic recordings the subsequent kek-kek notes are also clearly visible.

Cheap USB mic and computer:

AudioMoth in bag:

AudioMoth in case:

Digital audio recorder and EM172 lapel mic:

Digital audio recorder and parabolic reflector:

Spectrogram comparison
Moorhen comparison 2

 

Conclusion

Based on this very limited comparison the USB mic would miss some birds, though I should note that the mic used here is quite old and newer models might be more sensitive. More distant calls, calls of quiet birds, or calls where fine detail is needed for identification (e.g. flycatchers) might be easily missed or hard to identify with the AudioMoth. I did not run the AudioMoths on the high gain setting which might partly explain the slightly poorer detections. Previously I have found high gain generated too much white noise (hiss) in the recordings. Next time there is a break in the wind I will try again…

Postscript

The wind dropped again briefly and I was able to make a comparison of two AudioMoths, one with gain set to Medium and another with gain set to High. Again not many birds moving so not many comparisons possible. Of three Redwings detected by parabola: i) one was not detectable via either AudioMoth; ii) one was missed on Medium gain and barely visible on High gain; iii) one was picked up by both with negligible difference in sound quality. Of two Song Thrush calls detected by the parabola: i) one was missed by the Medium gain AudioMoth and just visible on the High gain one; ii) the second call was visible on all as can be seen in the following comparison:

ST call cf

Note this bird was not directly overhead, as evident from differences between the left and right channels of the parabolic recording. Without knowing these calls were present from the parabolic recording I may well have missed some of them in the AudioMoth recordings.

The lack of birds prevented a more detailed comparison but this does suggest the high gain setting might be better for the AudioMoths for nocmig.