Dickcissel Night Flights | Home | Intro | Record & extract calls
Microphone design and building instructions
Updated: April 27, 2015
introduction
microphone circuit schematic
microphone circuit parts
building the circuit
construction of microphone housing
microphone installation
INTRODUCTION
The idea is to record vocalizations of night migrating birds. The technical challenge is to maximize the reception of sound from the airspace above a recording site and minimize the reception of sound from the terrain around the station (e.g., insects, frogs, wind noise in vegetation, road noise, etc.).
To achieve this goal, a microphone must have a directional sensitivity pattern and be aimed at the sky. Because the device will be exposed to weather, it must be waterproof. Windproofing is also an important consideration.
A microphone is a device that converts acoustic waves into electric waves. There are a number of techniques for accomplishing this but one of the most efficient is called the capacitor method. Two small charged plates are positioned very close to, but insulated from, each other. One of the plates is mounted so that it may be exposed to sound. This plate is designed mechanically to be very sensitive to small movements caused by the varying air pressure of incoming sound waves. As this plate moves, the electric charge it embodies also moves and this changing electric "field" motion is "sensed" and mirrored by the electric charge on the other plate. The result is an electronic signal that is essentially a reflection of the incoming acoustic signal. This signal can then be amplified and sent to a listening system (speakers or headphones), and/or an audio recording device, and/or a computer running software to automatically extract and archive nocturnal flight calls. Except for some individual species with distinctive calls (e.g., Dickcissel), automatic recognition software seems to be always on the horizon.
The device that converts acoustic waves to electric waves using the capacitor method is typically called a microphone element. The complete microphone then comes into being when a power supply and other electronics are added, and when specific housings are incorporated to modify the microphone element’s pick-up or sensitivity pattern, shielding it from untargeted terrestrial sounds and wind.
Microphone directionality is achieved by four means. Three of these methods involve using specific housings for the microphone element. Very simply, the first is the use of a parabolic reflector (dish) to focus (concentrate) incoming sound on the microphone element. The second is called a shotgun microphone. Here, the microphone element is placed within a tube that has equidistant slot openings on the sides. This results in producing sound wave interference inside the tube and wave cancellation of sounds arriving at the slots simultaneously, in other words, from 90 degrees right and left of wherever the microphone is aimed. In effect, this sound wave cancellation increases the relative sensitivity of the microphone in the direction that it is pointed. The third design is called a pressure-zone microphone (PZM). Here, the microphone element is placed very close to a rigid boundary. The incoming sound wave is received by the microphone element at almost the same time as its reflected wave from the boundary. The result is a doubling of sound pressure in the vicinity of the microphone element. The size of the rigid boundary determines the range of frequencies where the sound pressure is doubled. The boundary must be at least as broad as the wavelength of the sound to be doubled. Multiple boundaries can be used to increase the gain in sound pressure. An additional, but more complex method of achieving directionality is the use of a beam-forming microphone. Here, multiple microphone elements are positioned close together on a plane and the received signal of each element is processed mathematically and summed to determine the shape of the overall pickup pattern.
Simplicity and functionality are the two principles that have driven the design of the following pressure-zone microphone for recording avian night flight calls.
MICROPHONE CIRCUIT SCHEMATIC
MICROPHONE CIRCUIT PARTS
B1 – Standard 9V battery. 9V battery and snap connector with two leads.
C1 – 0.1 uF ceramic monolithic capacitor. This capacitor primarily functions to limited radio frequency (RF) interference. It should be mounted as close as possible to the microphone element terminals. The circuit will work without it but it will be more susceptible to RF.
C2 – 10 uF solid tantalum capacitor. This capacitor is used for decoupling the internal resistance of the battery - in other words, for smoothing out discontinuities in the power supply.
MICROPHONE ELEMENT – Any small electret condenser microphone element should work. Knowles Electronics, Inc. makes one (EK23029) with a frequency response well suited for picking up flight calls of migrant birds in the Americas. It has limited sensitivity below 2 kHz that is helpful for reducing wind, road, and aviation noise. In addition, this element has the C1 capacitor already built in.
R1 – 3.3k, ~1/4watt, carbon film resistor – This component functions to limit the flow of electricity (amperage) in the circuit. A standard 9V will typically be effective for 3+ months with the circuit design and components noted above.
AUDIO CABLE – Approximately 0.5-meter (two-foot) length of two-conductor cable. This will carry the ground and signal channels and will connect with a longer cable to carry the audio signal to your audio recording equipment. The length of the latter cable depends on how long a run it is between your microphone location and your recorder/computer/headphone station.There are many cable options of varying quality and price. One of the cheapest is four-conductor telephone wire. 100-ft sections can be purchased at retail outlets like Home Depot and Lowes for about $10 USD. You only need two of the four wires, one for the signal and the other for the ground. Audio cable of the brand name Canare, model L-4E6S, is a top of the line option that provides superior elimination of various electronic noise and is especially good when long runs of over 30 meters(100-feet) are necessary, and can help reduce electromagnetic interference. It is relatively expensive at over a $1 per meter. Searching Amazon.com often reveals good deals on specific lengths that already have connectors (see next section on XLR connectors). This brand is five conductor cable. Two pairs of conductors are twisted together and a braided shield forms the ground conductor. When using this brand in the above circuit, one of the twisted pairs (blue one) is connected to the ground conductor.
XLR CONNECTORS (male and female) – The male XLR connector is soldered onto one end of the short piece of audio cable; the other end connects to the circuit. The female XLR connector is soldered onto one end of the long piece of audio cable and a male XLR connector can be soldered on to the other end, if it is connecting to a preamp such as the USB Dual Pre.
How to get parts Web searching for the specific components is an expedient method to find and purchase products. Digi-key.com and Mouser.com are sites where one can buy all the electronic components in one purchase order. As noted above, deals on high-end audio cable and connectors can usually be found on Amazon.com. The Knowles EK23029 microphone element with leads can be purchased here.
CAVEAT This microphone when completed cannot typically be run directly into most recording devices. It will need to go to a preamp first. See discussion on preamps on the recording gear page.
BUILDING THE CIRCUIT
24 or 26 gauge wire and standard perforated circuit board are useful, but not necessary, for connecting components. Lead-free rosin-core solder is recommended for soldering the connections. At this point you can connect (solder) all the components of the circuit together except the microphone element and then proceed to the construction section below. Before you solder, twist the connecting wires together. Generally you want to get each connection soldered quickly so as not to risk damaging any of the components. The pictures below illustrates what the completed circuit might look like.
without circuit board with circuit board
CONSTRUCTION OF MICROPHONE HOUSING
Before the microphone element is connected to the power supply circuit, it is mounted on a plastic dinner plate. Rubbermaid makes a 20 cm (8 in) wide plastic dinner plate that can be purchased at large department stores. This plate has a flat surface of 16.5 cm (6.5 in) and the pressure zone microphone that is being formed will theoretically double received sound waves with a wavelength of 16.5 cm(6.5 in) or less. A 2 kHz wavelength is about 16 cm (6.6 in.) long so the microphone using this Rubbermaid plate will double the loudness of sounds with frequencies above 2 kHz. Most migrating birds in the Americas give flight calls that are above 2 kHz. Other barriers can be used instead of a plastic plate. A plastic cereal bowl could be used and may provide more gain, depending on the slope of its sides. A hollowed out pyramid or cone would provide additional gain. The audio frequencies augmented would be dependent on the size of the open aperture of the pyramid or cone. The Old Bird 21c uses a small acrylic pyramid along with a Knowles EK23132 microphone element.
PZM configuration using acrylic pyramid (with Knowles EK23029 mic element)
CONTINUATION OF CONSTRUCTION USING OLD DINNER PLATE FLOWERPOT DESIGN
A small hole (.2 cm/.1 in in diameter) is drilled about 3 cm (1.2 in) from the center of the plate. A power drill can be used but a utility knife (also called a carpet knife) with a razor sharp point can also be used manually to drill the hole. This latter route is actually more reliable as sometimes a power drill will crack a plastic plate. The microphone element is mounted on the surface of the plate using a piece of tape (black plastic electrical tape works well); the microphone element connector wires go through the hole and are connected with the microphone power supply circuit previously built. Several pieces of "duck" tape can be used to secure the circuit and the 9V battery near the center of the bottom of the plate.
A critical part of the microphone design is waterproofing. If water reaches the microphone element, it will be damaged and the microphone will produce a static noise. A simple way to waterproof the microphone element is to get a piece of plastic food wrap and pull it taut over the upturned edges of the plate. The excess wrap will typically cling to the underside of the plate and form a seal. If the wrap does not cling, it can be sealed on the underside of the plate with silicon sealant or any sticky substance (pine pitch or honey will work). Once the slack has been gently pulled out forming a drum-like tympanum, and the excess plastic wrap is folded under the plate, "duck" tape can be used to secure it further. Once the plastic wrap is in place, make sure that there are no small holes that water can enter. Most plastic food wrap in supermarkets is in the 12-13 inch wide range, barely wide enough but usable for most dinner plates. Wider plastic wrap can be obtained from restaurants or a restaurant supply store. Plastic food wrap will not impede the pickup of sound much (~3 dB at 8 kHz) and it will weather well through one migration season (4+ months). It would be wise to replace it before each season of use.
plastic food wrap pulling it taut
Once the waterproofing is finished, the plate unit is ready to be mounted on top of, and within, a combination of plastic flowerpots. Kmart has a line of flowerpots (Martha Stewart series) that work well for this purpose, but other models, and even other types of containers will certainly work. The 20-cm (8 in) terra plastic flowerpot from Kmart (item # 40108, ~ $2 US) is turned upside down and a good pair of scissors or a small saw is used to cut the bottom out. The bottom of the pot already has drain holes in it so you can cut fairly easily from hole to hole. This leaves a small lip left which is important because you will now put a fairly thick line of silicon sealant (pine pitch or bubble gum are natural alternatives) all the way around this rim. When this step is completed, take the plate unit and carefully set it down on top of the newly cut opening in the flowerpot, making sure that the cable goes down first and that you don’t damage the integrity of plastic wrap. Once it is resting on the pot, make sure the plate is centered over the pot and that a good seal has been made. It is important that this seal between the pot and the plate be 100% so that when rain eventually falls on the plate, it will not drain into the power supply circuit mounted on the bottom of the plate.
The final phase is to mount this latter plate and pot combination inside a larger pot, for example a large plastic flowerpot. If you have already connected the male XLR connector at the end of the microphone’s audio cable, then you will need to cut a 2 cm (~1 in) hole in the bottom of this pot. Otherwise, a hole just big enough for the cable can be cut and the XLR connector can be soldered on afterward. Either way, once the hole is cut, lower the previously constructed plate and pot combination down into the bigger pot making sure that the audio cable goes through the newly made hole. A few blobs of silicon glue or other similar sticky substance such as bubble gum can be stuck on to the bottom of the small flowerpot (its rim) in order to hold the plate and pot combination securely in place inside the large flowerpot.
The microphone is nearly complete. An option now is to line the inside of the flowerpot with some sort of sound absorption material so pickup of reflected sound is minimized. Many department stores sell a foam with egg-cartoon-like bumps. Insert a 125 cm x 35 cm (3.7-ft. x 14-in.) piece of this foam around the inside of the large pot (40-cm) and trim it even with the top of the pot. Whatever is used should not be so thick as to impinge on the plate inside the pot. It also must hold up after repeated rains. The bed foam mentioned holds up well in water but typically breaks down significantly within a few years of exposure to sunlight.
Once this material is in place all that remains is to obtain a piece of thin cotton or cotton/poly cloth to cover the opening of the big flowerpot. The object here is to keep debris, small hail, or heavy rain from rupturing the plastic wrap inside. You don't want it too thick or it will unnecessarily impede sound pickup. Jo Ann Fabric (a fabric chain in the US) has a good selection of thin but strong cloth. You want to look for material like a bedsheet or pillowcase, but preferably with a thinner weave that allows some light to pass through. This cloth cover can be cut to fit over the large flowerpot. Extra fabric should be left to drape over the sides about 4-cm (2 in). The cover can be secured tautly to the pot with "duck" tape or a bunch of clothespins (or other strong clips) will work. If "duck" tape is used, and the mike will be outdoors for several months or more, then it is wise to further seal the duck tape with a coating of 100% silicon sealant along the edges of the duck tape. The microphone is now ready for installation
.
MICROPHONE INSTALLATION
A typical location for this microphone is mounted on a roof with an open view of the sky above the din of insect or frog noise. A good way to mount the mike on the roof is to get a wood pallet, use a drill or utility knife to make holes in four sides of the lip of the plastic flowerpot, and connect bungy cords from these holes to the wood pallet tautly so it will stand solid and not move during wind buffeting. The audio cable is run into a building to audio recording equipment or a computer where the signal can be recorded or processed for bird calls.
