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Microphone design and building instructions
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 an audio recording device or processed to extract information.
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.
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. RadioShack sells 9V battery connectors with two leads. Part #270-324.
C1 – 0.1 uF ceramic monolithic capacitor mounted as close as possible to the microphone element terminals. This capacitor primarily functions to limited radio frequency (RF) interference. The circuit will work without it but it will be more susceptible to RF. Part # 272-135 at RadioShack; less than $2 US for a packet of 20.
C2 – 10 uF solid tantalum capacitor for decoupling the internal resistance of the battery - in other words, for smoothing out discontinuities in the power supply. Part #272-1436 at RadioShack; ~ $1.59 US.
MICROPHONE ELEMENT – Any small electret condenser microphone element should work. RadioShack sells one with a fairly uniform frequency response for about $4 US (Part # 270-092). Knowles Electronics, Inc. makes one (EK3029c) with a frequency response especially well suited for picking up flight calls of migrant birds in the Americas. It has reduced 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. Contact Old Bird (admin@oldbird.org) if you are interested in obtaining this Knowles microphone element.
R1 – 3.3k, ~1/4watt, carbon film resistor. This component functions to limit the flow of electricity (amperage) in the circuit. Part #271-1328 at RadioShack, less than a dollar for a pack of 5. A standard 9V will be effective for 3+ months if the microphone element draws .5mA or less like the elements mentioned above.
Audio cable – A .5 meter (two foot) piece 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. There are many cable options of varying quality and price. RadioShack sells basic audio cable (Part # 278-513 or 514). For $8 US you can get a 15 meter (50-foot) piece. 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. It is relatively expensive at over a $1.30 US per meter ($.40 US per foot). 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. One supplier, Hudson Audio Visual www.haveinc.com, will ship any length of Canare cable and will add audio connectors on the ends if requested. However, their minimum order is $75.
XLR connectors (male and female) – The male connector is soldered onto one end of the short piece of audio cable. The female connector is soldered onto one end of the long piece of audio cable. RadioShack sells several varieties of XLR connectors. They can also be ordered from Hudson Audio Visual. They run about $4 US a piece.
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 Kmart or other 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.
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. The 40-cm (16 in) terra pot at Kmart (item # 40116, ~$8 US) works well for this purpose. 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 utility knife to drill 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.
