Introduction: Concrete Transmission Line Bluetooth Speaker

Hi, I'm Ben and I like to make stuff. Today I'm going to show you how to make a Concrete Transmission Line Bluetooth Speaker.

I've been wanting to make a modern looking speaker for my room which is why I chose concrete for the case. I've had lots of practice with concrete and I love the industrial look it gives. Have a look at my other instructable on how to make a concrete Low Poly Mountain Planter. Concrete also improves the sound clarity because it is dense so reduces the vibrations and makes the sound less distorted.

I got inspiration from Pinterest by looking at pictures of speakers and was intrigued by Transmission line speakers which have a really cool look. Transmission line speakers have a maze-like a channel which aims to make the sound coming out of the port in phase with the movement of the speaker driver which is pushing the air. The other advantage is that the air in the transmission line loads the bass and lowers its resonant frequency which will affect the sound quality.

In this instructable, I will first show you how to make a universal speaker face which can be used for other speaker box designs and holds all of the electronics. I will show you how to solder the electronics which I mainly used from Diy Perks video on how to make a speaker as this was very affordable and easy to build. Then, I will show you how to make the mould to make the casing and pour the concrete. After we will build the sides out of plywood which will complete the build. I hope you enjoy my design and learn something new.

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Step 1: Tools and Parts

As I said previously, I used the components from DIY Perks speaker because I didn't want to spend more money than I had to on a DIY speaker kit. Although, I wanted mine to run off mains electricity and have a switch to turn it on. I have also added links to purchase the components. The electronic components are fairly cheap, however, the speaker drivers I bought were expensive but they sound amazing. If you are not willing to buy new drivers you could alternatively salvage them from an old speaker from a charity shop.

You will need:

Tools you need:

  • A soldering iron to solder the electronics.
  • Coping saw and hacksaw or bandsaw.
  • Power drill with an appropriately sized hole saw bit to drill out holes for the speaker drivers.
  • Stanley knife.
  • Sandpaper
  • Orbital sander to sand and remove air bubbles from the concrete.

Step 2: Universal Speaker Face

I first designed the speaker in SolidWorks so that I could easily adjust the design. Transmission line speakers are difficult to make because you need to be very precise to adjust the sound coming out of the speaker. I used a Transmission line speaker calculator to base my measurements off which allows you to enter the size of the drivers and other details, then outputs the correct measurements to use. After I had these values I designed my speaker. Attached are the files to the A4 sheets so that you can use my dimensions. I apologise that the main body looks very messy, there are too many dimensions that are vital when making the concrete mould.

The universal speaker face holds all of the electronics and the speaker drivers. It can be used on different speaker designs, for example, a simple plywood box case. I made mine out of a scrap beech sheet which came from a bread box.

First, draw out the design onto your 10mm plywood or other wood of choice using the dimensions on the sheet. Use a round object such as the end of your silicon tube to draw the curves or use a compass. Now it is best to drill the holes first, that way you can start again quickly if you mess up. Mark out a centre line onto the wood then use the dimension to mark the points of where to drill. I first drilled a pilot hole, then used my 55mm hole saw to drill the holes for the speaker drivers. Ensure that you clamp the piece down and have some scrap wood underneath to not burst through the wood too quickly as this will chip the wood. Then use a 5mm drill bit to drill the holes for the switch and the potentiometer.

You can now cut the design out using a coping saw or a bandsaw if you have one, and sand it down with an orbital sander or sandpaper to the desired smoothness.

After you can mount your speakers. Included with your speakers are foam rings to seal the speaker to the face, but if that is too small you can cut your own out. To effectively sand the edges of the foam, it was recommended by Bill Doran on Adam Savages Tested, to use one of those Dremel stone grinders which have no purpose to sand the foam.

Now you need to mount the speakers to the face. align the speakers so that they are centred, mark the mounting holes, then use a small drill bit to carefully make a shallow pilot hole for the screws. This process can be redone for the other speaker and the switch.

Step 3: Electronics

I have made a visual circuit diagram of how to wire up and solder the components because I want to make this project as simple as possible for beginners. I am not good at electronics either so I hope this will help you to understand how it goes together following the information on the chips. The 5V step-down board has been used because the USB Bluetooth board can only handle 5V instead of 12. I have used a 50K ohmic potentiometer which is connected in series to the amplifier to adjust the volume. If you purchase all of the items I linked at the start it is very easy and cheap to make.

To solder the wires to the Bluetooth board, I first removed the USB because this is not required, but I did keep the audio port in case I wanted to output the audio to another speaker later on. Also, you do not need to use long wires because the components will be close together, it will look messy if you do.

Step 4: Making the Mould

To make the mould I first drew the design out onto the chipboard sheet using my dimensions sheet provided. Then I began to cut the sided out using a bandsaw to the dimensions. To do the curves, I cut them out onto cardboard, added in flaps, added creases so that it could bend easier, then covered it in sellotape. I found that the tape works well to waterproof the card from the concrete, and leaves a smooth surface. You could also you foam board, but this will cost more money.

To glue the sides to the base I used silicon sealant to waterproof the edges so that the concrete could not leak.

Where the speaker face will go I cut out the outline of the speaker and glued it to the correct height. I added silicone sealant to the edges so that it would be easier to remove later.

Step 5: Pouring the Concrete

It is now time to pour the concrete. Using your ready mix concrete, mix a large amount of concrete with water following the instructions for your mix. Make sure you use a respirator and rubber gloves so that you do not burn yourself with the reacting concrete. Before you pour the concrete you need to cover the mould with oil, E.g. spray oil. This is so that it is easier to remove the mould. Furthermore, to reinforce the thin sections you will need to add so steel rods like some used tent pegs to strengthen it. Alternatively, you could make the whole thing using fibre reinforced concrete which is where you mix fibre strands into the mix so that the piece has better tensile strength in all direction.

When you pour the concrete you will want to do it slowly. Use an orbital sander or shake the mould to remove all of the air bubbles. Use a plastic sheet to cover the mould whilst it sets for 3 days to stop the concrete curing too quickly.

To remove the piece from the mould you need to use a spatula to carefully remove the panels. If you used oil and did not use too much adhesive to make the mould, this will be easy. Unfortunately, mine cracked whilst doing this so I had to mix up more concrete to bond it back together. So that you do not face the same issue I recommend using fibre reinforced concrete to strengthen it and use thinner chipboard for the mould so that it is easier to pry it apart.

Finally, I sanded all of the parts to make it cleaner.

Step 6: Side Pannels

To make the side panels I used plywood, but you can use any wood you have available. Using the dimension, I marked the width onto the plywood sheet and cut it out using my circular saw. As a makeshift edge guide, I clamp down a beam of plywood so that I can cut the wood straight. Then, I cut the ply to the correct length.

To join the concrete to the wood I will be using small sections of steel rods which will go through the wood to the concrete. Not only does this help to align the concrete pieces together, but it also adds a mechanical support to the concrete sections when it is standing upright. First, Drill several holes into the concrete using a hammer drill, then place a nail into the hole to align the plywood and mark a hole to drill. Use a wood bit to drill a shallow hole so that the steel rod fits snug. Repeat for both sides of the panel.

Finally, mark the curves and cut them out using a band saw or coping saw.

Step 7: Final Assembly and Testing

All of the parts can now be assembled. To bond the sides to the concrete I used a grip fill adhesive/ construction adhesive. I used caulk to seal the edges thoroughly so that the sound cannot leak out of the speaker. this can be repeated for the speaker's face as well. Then, I sanded all of the concrete and wood smooth. The finish I used was boiled linseed oil because this is a cheap, effective concrete sealer and finishes the wood as well.

Before you glue the other side panel you could add sound dampening to the speaker to lower the resonant frequency and dampen higher frequencies. It is recommended to use foam or polyfill, but I had some leftover carpet which I wanted to test how well it performed.


Overall I Think this was a very fun project to complete. It took me over a month and a half to design and make because I ran into several difficulties along the way like the concrete cracking. I really like the look of the speaker because it fits in well with the rest of my room. I have learnt lots of things during this project like how to do electronics which I previously have never done before. Some improvements I would like to make would be that I use some different coloured caulk because the white really stands out. Moreover, I could have used some bigger speaker drivers to further improve the sound because the speaker is very large.

I will post a video of me testing the speaker soon but unfortunately, my voltage step down board broke and the replacement has not arrived yet, but I want to enter this into the audio contest which closes tomorrow. If you could please vote for me in the Audio contest it would be much appreciated because I have worked very hard to make this. Thank you for reading, don't forget to Smash the like button!


Audio Contest 2018

Participated in the
Audio Contest 2018