Inspired by the introduction of “La migration nocturne par le son” from Wroza & Rochefort (Delachaux et Niestlé 2021), I wanted to build myself a cheap parabolic microphone. I was always a bit lazy to do so; especially melting Plexiglas in the oven to shape your own parabolic dish was not my favourite idea. But when I found out that a squirrel dish could be used to construct your own parabolic mic, I started as soon as I could. You need of course to be able to solder (or know someone who does it for you) the capsule microphones to a 3.5mm audio jack.

Here below, you find some ideas on the material needed to build your own parabolic microphone (feel free to improve it). There are two versions, with a single pair of capsules and with a dual pair of capsules.

Franck Hollander (franck.hollander@gmail.com)

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A. Parabolic mic with 2×1 capsules

1. Parabolic dish

You find different “squirrel baffles” on the internet, most of them are 40 cm in diameter. That is a good size (the Dodotronic dish is over 50 cm, but it doesn’t amplify necessarily much more, see section D). A 40 cm dish is also easier to carry around. I bought the Audubon dish for 40 EUR (incl. delivery costs). Here is another good dish (50 EUR with delivery costs).

2. Capsule microphones

These very small microphones (used in the Dodotronic dishes) are extremely powerful, and probably the best option for NocMig recordings (not per se for the quality of the recording, but especially for its reach). The most powerful ones are the EM272 (sensitivity -28 dB, sold on micbooster.com), and even a bit more performant the Pui Audio AOM-5024 (sensitivity -24db, sold on mouser.com). I used the AOM-5024 model (you need to buy 20+ of them to avoid the high shipping costs).

To solder the EM272 capsules, a tutorial is found here.

To solder the AOM-5024 capsules, you can use this tutorial. See Figure 1 below for some of the different steps.

3. Audio jack

These very cheap audio jacks work well, and you can use them twice. If you want to buy other ones, think about buying some that are “double-wired” (each channel in a separate wire). On some modern 3.5mm audio jacks, you find both channels in one wire, and it sometimes brings problems (pers. comm.). Also, don’t forget you want stereo audio jack, so with two black bands on the plug (and not one, or three as for smartphones).

4. Solder material

If you don’t have solder material at home, you can buy for instance this one: it is cheap and works well. There is also a bit more expensive material, certainly worth if you want to solder more in the future (and not only for your parabolic microphone).

5. Handle and microphone holder

The Audubon dish has a central opening, where I fixed a 1 ½’’ sink strainer to it. It allows you to screw a 1 ½’’ siphon to it (straight or bended). I know, not all very glamorous material, but keep the low budget of this DIY dish in mind! 😉

I inverted the screw of the sink strainer (so that it goes inside the dish). I then screwed a “microphone holder” on it, which is a metal piece I found at home (it is probably originating from an IKEA furniture, very similar to this one). The interesting feature is that there is a vertical opening which allows to slice your microphones in or out of the parabolic dish. Having your capsules a bit outside the dish, increases the probability to capture migrating birds calling from the side. However, you also increase the number of local birds on your recordings. Having one set of microphones inside, and another set outside the dish might compromise this issue (see further).

After soldering the capsules in the audio jack, I fixed these to this “chair angle”, that I then fixed on the “slicing” microphone holder (see Fig. 2). You can simply tape the wires and capsules to the chair angle. After that, do not forget to protect the capsules with a plastic cover (I simply use cling film, very thin and still waterproof).

6. Wind shield

A wind shield is not only interesting for wind itself, but also to protect your microphones against water and humidity. I use a chair protection cover in polyester, and it works quite well. Even on a rainy night, the plastic film protecting the microphones were not even wet. What helps is to put a little “roof” (not too large of course) on top of the slicing microphone holder (any object you find), it again protects the microphones from rain, and is not giving such a sharp tension to the wind shield with the microphone holder. In addition to the wind shield, I also included some protecting foam close to the microphones, coated with some plastic cling film (see Figure 3).

The DIY dish is compact, easy to transport and not heavy, and looks in the end as in Figure 4.

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B. Parabolic mic with 2×2 capsules

The parabolic dish with 2 capsules gives excellent results and is comparable to the Dodotronic (see section D). A possible upgrade is to have 2×2 capsules separated from each other. This allows, for instance, to place a set of 2 capsules inside the dish, and another set of 2 capsules a bit outside the dish. The idea here is to enable the separation of a flying (migrating) bird above the dish, with a local bird calling from the side (note that this could also be done with a 2×1 setup). This could be of matter for species rarely calling in the middle of the night, and where it remains unsure if it concerns a local bird or an actively migrating bird (ex. Blackcap, Common chiffchaff). When migrating and calling above the dish, both channels should display a similar amplification. For a local bird calling from the side, or even better at a lower position when you place your dish at a higher location (ex. roof), calls on the channel outside the dish should be more amplified than those inside the dish. Alternatively, you can also place the 2×2 microphones all outside or inside the dish, it increases a bit the recording power, but probably also increases a bit the amount of background noise.

To build such a setup, you should solder a “bridge” between two capsules on the same channel (left or right), using again a piece of audio jack and the same soldering process as before by respecting the colour poles (+/-). Such a bridge is displayed on Figure 5.

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C. Costs for the DIY parabolic microphone

1. Parabolic dish: 40 EUR (incl. shipping)
2. Capsule microphones (4x AOM-5024): 31 EUR (incl. 20 EUR FedEx shipping)[1]
3. Handle and microphone holder (incl. screws, chair handle): 15 EUR
4. Wind shield: 10 EUR (incl. shipping)
5. Audio jack: 1 EUR (incl. shipping)
6. Shipping: 8 EUR (when 1, 2, 4 and 5 are ordered together on amazon)

TOTAL: 100 EUR

Extra: solder kit (20 EUR)

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D. Comparative tests

I compared my DIY parabolic dish with the Hi-Sound Mono dish (with 4 capsules) from Dodotronic (600 EUR inc. VAT and shipping) by recording three nights in October 2021. In a nutshell: results are very similar. Overall, both dishes have a comparable detectability (sometimes one dish captured better a far-calling bird than the other, probably depending from where the migrating bird was calling and/or the dish position). Four capsules increased a little bit the recording “power” compared with 2 capsules, but not significantly (so working with two capsules is perfect too). I had no local birds calling during the night, so this theory could not be tested. Here below some illustrations from the different recordings. Fig.6 illustrates birds at close range, and both dishes give comparable results. Fig. 7 and 8 show that some calls are more amplified on one or the other dish, probably depending on the angle from wish the bird called.

Fig. 9 illustrates a bird calling at mid-range, where it got more amplified on the Dodotronic compared to the DIY dish. Such cases occurred interchangeably between both dishes, merely depending on the position of the dish and/or angle of calling. Main result is that in almost all such cases, the species could always be identified on both dishes. So only the amplification of the recording varied depending on position, not on the material itself.

E. Stereo parabolic microphone with 2×1 capsules

The interest of separating the microphones physically from one another (with a shield between both capsules) lies in fact that one channel might amplify more than the other, thereby giving some information on the flight direction. This only works in rarer cases but might still be interesting in certain studies.

Again, with the idea to build such a system in the cheapest way, I recycled some materials I found at home: an old TV socket and the case of a very bad Christmas CD that I wanted to throw anyway 😉. I used part of the plastic CD cover as shield to separate both capsules, and the TV socket to hold the capsules. Again you can slice the microphones outside or inside the dish, and the capsules point well in the middle of the dish. Of course, many other inspirational ideas can reach (or improve) this setup, just try it! Here below some pictures.

E. Conclusions

Although the dish diameter being a bit smaller, the DIY dish showed comparative results to the Dodotronic dish. The advantages being the lower price, the compactness (for transport for instance) and the flexibility in mount type (straight or bended siphon, microphones inside or outside the dish). Given the lower price, you might also risk to leave your DIY dish for a longer period outside (indeed with a good weatherproof protection of the capsules) or in an interesting habitat. The disadvantage is that it does not look very professional (with a siphon and a sink strainer…) and is possibly a little less robust as the Dodotronic version (with more metallic parts).

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[1] The best is to buy 25 capsules, then you are just above 50 EUR and there are no shipping costs (so you are at around 2 EUR/capsule). You can for instance use the other capsules to record birds during the day in interesting habitats 😉

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.

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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:

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.

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

 

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

 

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:

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.