Simple Planar Magnetic Headphones

This instructable shows how to make simple headphones with decent sound quality completely from scratch. It takes four to five hours and costs about $10 to $20 (USD) (or less if you use scrap materials) per pair. No special materials or tools are needed. It is the simplest form of planar magnetic open-back headphones (I think). The headphones have only three components: the driver units (the parts that make sound) with hand-wound coils, earpads, and headband. The earpads directly mount on the driver units. The driver units attach to the headband using the drivers' magnets, so they can be easily repositioned along the headband or detached for transport and repair.

Making these headphones also teaches a lot about different physics concepts, so it is a good hands-on STEM project as well.

Notes before you begin:

• These headphones use strong neodymium magnets, which by their strong attraction forces can cause injuries if not handled with care. Don't bring magnets or a magnet and a metal tool in close proximity. The neodymium magnets are also brittle, corrosion-prone, and lose magnetization above room temperature. Learn more about neodymium magnet safety here, and please make your own at your own risk.
• These headphones are not as efficient as regular dynamic driver headphones, so if you plug them directly into your music player/phone/laptop, depending on the device, the size of magnets used, and your hearing, you might find the sound level too low even at the maximum volume setting. So if you like playing music loudly, this might not be for you. (You can use headphone amps to increase the sound level, but the impedance of the headphones is not that high, so I cannot guarantee that your headphone amp can tolerate that.)
• I'm not an audiophile, so the sound quality of the headphones might not be satisfying for audio enthusiasts. However, I think it's generally decently good for most ordinary consumers and certainly good for the price and simplicity.
• All the dimensions here are in SI/metric units (sorry if you love inches and pounds...).

Materials

• Solid plastic sheet, 2 mm thick (e.g. acrylic, polycarbonate, or PVC) - 14 cm x 12 cm for a pair of drivers, and about 50 cm x 3 cm (depending on your head size) for the headband. You can also 3D print the driver frame and driver magnet board using the STL files below (note: the STL file unit is millimeters). You can also 3D print the driver frame and magnet board as a single piece using the third STL file (add supports as necessary before printing). If you are 3D printing, I recommend extruding the magnet board to cover your magnets so that the magnets are not exposed. The STL model for the headband is also provided below for 3D printing, but I cannot guarantee that its size will work for everyone.
• Sponge or foam, about 1 cm thick - 1.5 cm x 44 cm for earpads, and about 3 cm x 30cm for a headband.
• Stretchable cloth - 7 cm x 14 cm for driver diaphragms, about 10 cm x 30 cm for earpads, and about 8 cm x 30 cm for a headband. You can use an old shirt or any other piece of cloth, but thick and strong cloth (e.g. denim) is not suitable for diaphragms.
• Stereo headphone/audio cable (or any three-core cable if you don't care much about sound quality) with a 3.5 mm jack. I recommend making it detachable using a 3.5 mm audio socket (or even better, separated 3.5 mm audio sockets for each channel) instead of directly wiring a 3.5 mm plug. You also need about 50 cm of mono or stereo audio cable for wiring from right to left drivers over the headband.
• Some 1 m long multi-strand copper wire/cable from which you will harvest thin copper wires in Step 2. I recommend 0.2 to 0.8 mm² (about 18 to 24 AWG) multi-strand wires as anything thicker is too stiff for the coils and anything thinner is too thin and fragile to wind manually.
• Two strong neodymium magnets - circular or rectangular, about 15 to 30 mm side/diameter and 5 to 10 mm thick. Larger magnets produce louder sound but require more careful handling and are more expensive. I use 20 mm x 20 mm x 10 mm rectangular magnets with surface magnetic field strengths of 380 mT, which work great but are so strong that it's almost impossible to pull them apart once they attract. I've also tried smaller circular neodymium magnets of 13 mm diameter and 3 mm thick, but I had to stack four of them together to get any audible sound. I think a way to start is to use whatever neodymium magnets you can obtain, and then stack more magnets later to increase magnetic field strength as needed.
• Plastic film / thin sheet (e.g. polystyrene, polypropylene, or polyethylene terephthalate) - 3 cm x 6 cm for a pair of coils. It should be thin enough (less than 0.5 mm) to easily punch holes using a needle but stiff enough to keep the wound copper strands in place in Step 3. Think of the thickness and stiffness of a disposable plastic party cup or a phone screen protector. Stiff plastic bags (e.g. chip bags) also work, but it is significantly harder to wind the copper strands around.
• If your hand-wound coils have a resistance below 10 Ω, I recommend placing a resistor of about 10 Ω rated at 1/4 W or greater in series with each coil. See the end of Step 1 and the beginning of Step 5 for details.

Tools

• Duct tape and double-sided tape - get strong ones if you can
• Glue - I like two-part epoxy, but any strong glue will do
• Soldering iron, solder, and soldering iron cleaner
• Multimeter or ohmmeter
• Plastic cutter for cutting out the 2 mm thick plastic sheet (not needed if you are 3D printing instead)
• Sewing pin or thin needle - for punching holes on plastic films in Step 3
• Other office items such as scissors, a permanent marker, a cutter, and a metric ruler

Shown above is the exploded view of a headphone driver unit (that makes the sound, i.e. a miniature loudspeaker) and the headphone assembly. The driver unit consists of a copper coil wound (weaved or stitched might be a more appropriate word) to a thin plastic sheet, a cloth diaphragm that allows the coil to move as it makes the sound, the frame that gives the diaphragm some tension, and the magnet board that holds the magnet on the back of the frame. An earpad mounts directly onto the driver frame, and the driver-earpad assembly attaches to a headband using the driver's magnet. You can play with the STL model of the assembled headphones provided at the bottom of this step to see how the parts come together. In theory, you can get rid of the magnet board by attaching the driver frames and magnets directly onto the headband, but I found this more difficult to work with and less comfortable to wear.

[The rest of the content in this step is somewhat technical, so you can skip it if you just want to make headphones. However, I think understanding the content here will help you make headphones safely and effectively. Also, I'm not an electrical engineer, so please correct me if I'm wrong.]

The coil is wired to an audio output (most likely your music player, phone, or laptop's headphone jack) that supplies an AC audio signal to the coil, which, in the magnetic field from the driver's magnet, experiences Lorentz force that accelerates the coil forward and backward, producing sound waves. The stronger the driver's magnet or the larger the magnitude of the AC audio signal, the greater the Lorentz force and hence the louder the sound produced.

When connected to the headphones, the audio output of your device effectively sees the total impedance (which is the opposition to the flow of current) Z consisting mainly of resistive and inductive terms¹, which are given by

Z² = R² + (Lω)²

where Z is the total impedance in Ω, R is the pure resistance of the coil in Ω, L is the inductance of the coil in H, and ω is the angular frequency of the audio signal in rad/s, which usually is between (2π times) 20 Hz and 20 kHz for audible range.

Since we are hand-making the coils, it is important to not short-circuit the audio port by ensuring that our headphone circuit (coils plus an audio cable) has at least the minimum impedance required for the port. Most audio devices don't have this value listed in their specs, but this Surfans F20, for example, has the acceptable earphone impedance range of 8 to 150 Ω, and judging from several online forum sites, impedance above 10 ohms generally don't instantly damage the audio port of most devices. However, I measured the resistance of all headphones and earphones I own, and they all had a pure DC resistance of 20 to 30 Ω.

In addition, the inductive reactance Lω of the coil we will wind is generally small. To see this, approximating the coil as a single-layer circular coil of 11 turns with an outer diameter of 30 mm and spacing between turns of 1 mm and by applying Wheeler's approximation, the inductance of the coil L turns out to be L ~ 120 μH, which at the frequency of ω = 1 kHz yields the inductive reactance of Lω ~ 0.74 Ω.

In conclusion, as explained in step 5, if your coil plus headphone cable has a resistance of less than 10 Ω, I recommend placing a resistor of approximately 10 Ω in series to protect your audio device. This will also be helpful if you mess up the coil winding and accidentally introduce a short.

(*¹ I think headphone cables and coils have some capacitance as well that modifies the sound feeling, but those parasitic capacitances are more of in parallel with the coils.)

Here begins the actual steps. First, you need to harvest two thin copper wires from a multi-strand copper wire. Obtain a 1 m long multi-strand copper wire of about 0.2 to 0.8 mm² (18 to 24 AWG), cut the insulation along the entire length of the wire, and pull the copper strands out. Then disentangle two wires out of the strands. The harvested wires do not have to be insulated. The harvested wires can be thick or thin, but thicker wires are easier to harvest and wind. On the other hand, thinner wires are more curly and fragile but can reproduce sound over a wider frequency range (or at least that's how I feel).

If you have a multimeter or an ohmmeter, measure and take note of the resistance across the 1 m copper wires you harvested. It should be between 1 to 20 Ω. If it is less than 1 Ω, it might be too thick to wind in the next step. If it is more than 20 Ω, it might be too thin and breaks often as you wind.

Cut out the plastic film / thin sheet into two 3 cm x 3 cm squares. Each of these square films will be used to make one coil. Using a sewing pin or thin needle, punch tiny holes spaced 1 mm apart along the entire diagonals of each of the square films. Placing a ruler on top of the film makes this process a bit easier. The holes don't have to be spaced exactly 1 mm apart, so don't take too much time trying to make perfect holes.

Once the holes are punched, start winding (maybe more of weaving or stitching) the copper wires you harvested in Step 2 following the pattern on the diagram above (The top right image). First, push in about 5 cm of the 1m copper wire into one of the outermost holes. Then, while keeping the shorter end fixed, push the other (longer) side of the wire to the adjacent outermost hole. Gently pull the longer end to give some tension to the section of the wire between the first and second holes. Then push the wire into the next outermost hole and gently pull to straighten. Following the pattern on the above diagram, repeat this process until you have about 5 cm of wire left on both ends. You should have made about 10 to 11 turns if you followed the pattern correctly.

If the wire breaks while winding (which happens for thinner wires or wires with a lot of twists), you can join them by twisting the broken ends together and putting a little bit of solder on the twisted ends.

Once winding is completed, measure the resistance across the coil. If you are using off-the-shelf enameled wires or wires harvested from certain audio cables, your wires might have an insulating coating on them. Remove the coating by sanding off the wire surface or applying some solder (which melts off the insulation). If the resistance is significantly less than the value you measured before winding, the wire may be in contact somewhere in the coil. Visually inspect the coil and gently push or pull the wire with a needle to keep even spacing between turns. Flexing the plastic film or putting some piece of transparent tape is also effective in keeping wound wire apart.

Follow the same processes to make one more coil.

Cut out two pieces of the driver frame and magnet board to the dimensions shown in the above drawing (the top right image) from a 2 mm thick plastic sheet. The dimensions don't have to be perfect. Alternatively, you can 3D print them using the files provided in the Supplies section. Cut out two pieces of stretchable cloth into the outer dimensions of the driver frame. These pieces of cloth will be the diaphragms.

Apply double-sided tape on one face of a coil you wound in Step 2, and stick the coil to the center of a diaphragm. Make sure that the end of the copper wire comes out (preferably from the same side of the coil) and that the end of the wire coming out of the innermost turn of the coil does not contact other parts of the coil as they come out.

To attach a diaphragm with a coil on to a driver frame, apply double-sided tape on a face of the frame. With the coil facing up, gently stretch a diaphragm while placing it onto the frame's double-sided tape so that there is some tension in the diaphragm when attached to the frame. With the frame-diaphragm-coil assembly on a flat surface with the coil facing up, push the center of the coil to see if it can travel back and forth freely. If it is already touching the flat surface beneath it, you need to re-attach the diaphragm with more tension. Cut out any excess diaphragm cloth so that nothing (except the ends of copper strands) sticks out of the driver frame. Follow the same steps to assemble the other driver.

Glue the neodymium magnets at the center of the magnet boards. It doesn't matter which pole of the magnets faces the magnet boards, but keep the polarity the same for both magnet boards (i.e if one magnet's S pole is facing the magnet board, the other magnet should also have the S pole facing the magnet board) so that when you assemble the headphones, the driver units repel, not attract, each other. As an option, you can increase the magnetic field strength (and hence sound volume) by cutting out a hole in the magnet board and embedding the magnet to make it flush with the magnet board surface. Do not attach the magnet board to the back of the driver frame yet to avoid any issues with the strong magnet (especially when soldering).

First, make sure that the magnets are out of the way so that your soldering iron doesn't get attracted to them. Then prepare a stereo cable with a 3.5 mm audio plug or socket for connecting to the audio output on your device, as well as about 50 cm of two or three-core audio cable for wiring to the left driver unit over the headband.

Check the resistance of the coils and the headphone cable. If the total resistance is below 10 Ω, I recommend placing a resistor of about 10 Ω rated for 1/4 W or above in series with each of the coils to make sure that you have a total impedance of at least 10 Ω. You might think this will reduce the sound volume significantly, but as far as I have experienced, it's not that much of a reduction. If you are placing resistors, place them for each channel separately and do not put one common resistor on the ground/common side, as this will induce serious channel crosstalk (discussed at the end of this step).

Wire the driver units as shown in the wiring diagram (the top right image). Do the driver units have polarity? I think the short answer is no because the audio signal is AC and the diaphragms are bi-directional. However, it seems² that the industry standard is to produce outward diaphragm motion when a positive voltage is applied to the positive terminal, so if you wire the other way around, your ears will feel pressure lower than the atmospheric pressure when the raw sound of your audio should have been higher and vise versa. Then is the reversed polarity audibly different? No. I tested all different polarity configurations but did not hear any difference whatsoever. Since, in headphones, sound waves from a driver hardly ever travel to the other driver to cause acoustic interference, it makes no difference whether the two driver units make constructive or destructive waves.

Note that the wire coloring for your audio plug socket might be different from that in the diagram, so check it with a multimeter before soldering. Usually, it is very difficult to put solder on a single thin copper wire because it can get oxidized very easily. Instead, twist the copper wire with the audio wire to be connected first, and then apply solder to the twisted part.

After soldering, attach the ends of the cables to the edge of the driver frames using some tape and/or glue. I recommend positioning the soldered locations above the driver frames so that they will be hidden by the earpads that will be placed on the frames.

Check the resistance across the audio plug/socket to make sure that the coils are properly connected.

Attach the driver magnet boards (with the magnets glued) to the back of the driver frames using double-sided tape or glue. If your diaphragm cloth is thin enough to see the coil from the other side, align the magnet with the coil as much as possible. After attaching the magnets, make sure not to bring the two driver units too close to each other, or they will attract each other.

Try testing the driver units by plugging into the audio output of your phone/laptop/music player and playing whatever audio you know well. You may need to crank up the volume all the way up and bring your ears very close to the driver units to hear any audible sound (do increase the volume slowly so that you don't instantly apply too much power on the coils). If you can't hear anything, double-check your circuit all the way from the jack to the coil. Note that the actual sound you get out of the completed headphones will be louder than what you hear in this step because there will be earpads that will seal air between the driver units and your ears, enabling more efficient sound transfer.

Here are some troubleshooting tips if you cannot hear at all or can hear but not as clearly as you should:

• If you hear a sharp noise resembling a metal string vibrating, your copper wires may be making contact with each other on the front side of the coil (the back side shouldn't matter because the wire is held in place by double-sided tapes). Visually inspect and gently separate the strands with a needle. Placing some thin transparent tape to cover the coil is sometimes effective we well.
• If you hear left and right channels mixing (this is evident if you can hear certain instruments well but not other instruments or vocal), then likely there is significant channel crosstalk, meaning the signal for the left channel gets applied to the right channel via common ground and vice versa. This is likely due to the fact that the resistance of the ground wire is high (5 Ω or more). To fix this, replace the ground wire (or the entire audio cable) with a better one or provide separate cables for each channel.
• Generally, if the diaphragm is in contact with the magnet board, the sound level is low and bass is weak. To fix this, insert something like a thin plastic ruler or ice cream stick through the opening on the back of the driver unit (there should be ~5 mm opening between the magnet board and the driver frame) and gently push the diaphragm away from the magnet board. Gently touch the coil from the front to see if it can travel back and force freely for about 1 mm.
• If you can't hear well, your connections at the headphone jack might be bad. Try re-plugging the headphone jack to see if it makes any difference.
• If you still can't hear, try putting more neodymium magnets on.

Note again that these headphones are inherently inefficient, so don't expect loud sound out of it without sufficiently strong magnets or a headphone amp.

(*²"How do I check polarity on loudspeaker?" in Electrical Engineering Stack Exchange.

"Polarity Conventions of JBL Professional Transducers and Systems," JBL Technical Note Volume 1, Number 12C.)

Cut two pieces of plastic sheet or hard paper to the same dimensions as the driver frame. Cut cushion/sponge/foam materials of your choice to four 1.5 cm x 7 cm x 1 cm strips and four 1.5 cm x 4 cm x 1 cm strips. These strips should form two square rings of 7 cm outer side, 4 cm inner side, and 1 cm height if arranged as shown in the second image (top right). Keeping this square ring arrangement, place the foam onto the plastic sheet or hard paper cut out to the size of the driver frame. This plastic sheet or hard paper will keep the foams together and provide some air gap between the earpad and the diaphragm.

Cut some cloth into two 5 cm x 29 cm strips. Using double-sided tape, wrap the cloth around the foam rings. Start wrapping the outer side first, and then pull the cloth to the inner side. Fasten the cloth at the back side of the rings (which is the side with the hard paper or plastic sheet) using double-sided tape or thread.

Attach the wrapped earpads to the edge of the diaphragms above the driver frames using double-sided tape, facing the side with the hard paper or plastic sheet down.

Cut a 2 mm thick plastic sheet to about 3 cm x 50 cm and bend it with heat to create a headband. Note that you might need a longer or shorter strip depending on the size of your head, so take a measurement with a tape measure before cutting the sheet. Note also that there are acrylic bending heaters intended for this purpose, so don't use a cooking burner like I did unless that's the only option you have. Adjust the bend until it fits your head. Alternatively, you can 3D print the headband using your own design or the STL file given in the Supplies section.

To attach the driver units to the headband, glue or tape pieces of ferromagnetic steel (e.g., stainless steel plates or washers) on the outer ends of the headband so that the driver units magnetically attach to them. I recommend attaching several pieces of steel so that you can reposition the drivers as needed to obtain the best comfortable driver locations.

As an option, you can add a cushion to the headband by sticking a 1 cm thick foam cut into about 3 cm x 20 cm and covering it with a sleeve made by sewing an 8 cm x 30 cm strip of cloth. Alternatively, you can make the sleeve detachable using snaps or velcro, and this way you can hide the audio cable from right to left channels inside the sleeve while still making the drivers detachable.

Magnetically attach the driver units to the headband. You can fine-adjust the location of the driver units by sliding them along the headband. If the driver units slide off or rotate too easily, put some duct tape or a thin rubber sheet on the inner side of the headband to provide some grip. As an option, you can keep the cable running from the right to left drivers tidy by attaching velcro or paracords along a few locations on the headband. Also as an option, you can cover the ends of the headband and sides of the neodymium magnets with some leather fabric to make them look nicer.

Now your pair of headphones is complete. You made it! Put it on and enjoy your favorite music with the world's only headphones made by and for you. If your headphones don't produce as clear sound as they should, you can try the troubleshooting tips at the end of Step 5.

Now that you know how to make them, you can change the materials or geometries to tune the appearance and sound. Try different ideas and find your favorite.

Lastly, here are some external sources that you may find (or at least I found) helpful and/or inspiring.

• Homebrew Headphones is a wonderful website by Shannon with headphone-building instructions and great resources for learning headphone basics.
• Ploopy, the planar magnetic headphones that I got a lot of inspiration from.
• Headphone Fundamentals in Audio Science Review is a very nice collection of headphone-related knowledge including headphone types, impedance, sensitivity etc.
• OpenStax University Physics Volume 1 Chapter 17 (Sound) and Volume 2 Part 2 (Electricity and Magnetism) are my go-to textbooks for basic physics.

Thank you for following this lengthy instructable all the way to the end. I hope you found the joy of making your own unique headphones. Feel free to give me feedback in the comment below. Also, If you measured the sensitivity or frequency response of the headphones or found a way to make the headphones louder without amps, please let me know.