Surround sound is cool. Hearing sound effects and music coming from every angle is much more immersive than just having the sound come from your TV. However, it isn’t easy to fit so many speakers in to every type of room. People who are into hi-fi will tell you that you need to have your speakers a specific distance from the wall, and rotate them at a specific angle to get the best sonic experience in the seating position. And god forbid if you tell them that you have your sofa against the back wall… “The rear wall will enhance the bass at random frequencies, it will sound boomy and horrible”.
What do you do then, if your room isn’t the right size and shape for an ideal home cinema? What if you only have space to put a speaker on one side of the seats? Well, one thing you probably shouldn’t do is build the speakers yourself (but maybe you still should).
I wanted to install a 5.1 surround sound system, starting with a 3.0 system. If you didn’t know, the first number is the number of ‘surround speaker’, and the second number is the number of subwoofers. Typically in cinemas sounds bellow around 80Hz are played by subwoofers, and higher frequencies are played by the other speakers. 3.1 systems have a left and right speaker, and a centre channel as well as a subwoofer (the 0.1). 5.1 systems add two surround speakers what are approximately in line with the seats. 7.1 systems have another two speakers behind the seats and point towards the TV. In 2014 a new standard was created, Dolby Atmos, that adds hight channels to the sound on films. The best way to do this is to cut holes in the ceiling and place speakers pointing at the seats from the ceiling at specific locations. However, for people who can’t do this, there is also an option to have speakers pointed at the ceiling (typically on top of other speakers at ear level) and to bounce the sound off of them to the seats.
This is all very interesting, and probably sounds great. However, I didn’t want to cut holes in the ceiling, or to buy a new receiver that can decode Dolby Atmos, and I still couldn’t fit in even a 5.0 system. I then had the idea of using the surface bouncing idea to reflect the sound off of a wall and back to the seats so that the speaker near the door would have room.
With this idea in mind, I took some measurements of the distances and hight of the seating position, location of sofa, etc and put this geometry into Fusion 360 – a CAD software. This allowed for easy determination of the correct height and angle for the speaker driver to be located, such that a reflection off of the wall would arrive at approximately ear level.
A speaker driver was selected, meeting the criteria of being full range (not including frequencies that might be covered by a future subwoofer), relatively small, high sensitivity, 8ohm, and relatively inexpensive. Since I don’t have any test equipment (oscilloscope or calibrated microphone) I thought it would be best not to try for a two way speaker, since designing and testing the a crossover without being able to measure anything really isn’t engineering, it’s just guesswork. The sensitivity requirement was set at about 90dB/1m/1W since one of the speakers has a long and inefficient path including a wall reflection. Small drivers were required so that the speakers can remain compact. (In hindsight, a coaxial speaker might have been a good choice, as a I could probably find a sufficient crossover network that someone else has calculated.)
The speaker driver selected was the FaitalPro 3FE22. These are 8ohm, full-range speakers (~100-20k Hz) that have an RMS power handling of 20W and a sensitivity of 91dB/1m/1W. These would then be expected to play at up to 104dB/1m, which isn’t as loud as the front stage, but sure is enough to damage your hearing with extended play. Further, rear channels really don’t have much happening most of the time and so if slightly more power is pushed through them during the final blockbuster-explosion, then they probably won’t catch fire. Interestingly, in Dolby Pro Logic, the first one in the ’70s, the surround channels were mono and limited to 7kHz. Modern movie mixes use the rear channels more, but still most of the sound will always come from the front speakers, and lower quality rear speakers are a reasonable cost saving endeavour.
To design an enclosure you need to know how the speaker will perform. In the ’60s and ’70s Thiele and Small worked out a simple model of how speakers react in various boxes. Thiele-Small parameters are used to model speakers, although they only apply for low frequencies. Some speaker manufactures ‘fudge’ their numbers a little bit, so before I ordered the drivers I wrote a small C++ programme that checks the Thiele-Small parameters against one another (the parameters are not completely independent). The driver checked out and so I trusted the rest of their measurements.
In order to ensure that a solid 100Hz was playable through the speakers a ported box was designed in WinISD (a free speaker modelling software). WinISD takes the Thiele-Small parameters and plots the frequency response of the speaker in different enclosures. The TL;DR answer to speaker boxes is that larger boxes allow deeper bass notes to be played as the air behind the speaker is more easily compressed (just like a long piece of rubber band is more stretchy than a short piece). Ported boxes resonate, exactly like a mass-spring (or pendulum) system. Frequencies that are near to the resonate frequency of the port excite air in the port and cause it to oscillate, this boosts the volume at the port resonant frequency. If you align the ports resonant frequency to be about the same frequency where the speaker driver starts to loose efficiency (and so play quieter) then boosting the volumes extends the linear frequency range.
There are some disadvantages to ports, in that you are basically always listening to the driver as well as the delayed response from the port, these two pressure waves can interfere with one another, and can also extend the length of a note while the port is oscillating. Ports with a small area produce a chuffing sound as air rushes through them, and larger ports need to be very long to achieve the same resonant frequency, they also have a large mass of air that can cause them to be improperly damped. This being said, perfectly good speakers can be designed with ports – like anything in engineering there is a trade-off between whatever compromises you choose to make.
The compromise that I came up with was to have a large long port that slightly boosted the bass. Due to the method of construction, this very wide port was easier to make.
The final parameters for the ported box was a volume of 2L, a rectangular port of 4″x 1″ and 40.6cm long. With these parameters I then went back to Fusion 360 to design the boxes. I designed two completely different boxes that both have the same port length and volume. The first of which was a traditional bookshelf speaker form, with a front facing port. The driver height was set to be just above ear level. The second box was designed with the speaker driver angled at 5.7˚ above horizontal, and a plate such that it can be stabilised under a sofa. The second box was also much taller, just under the height of the arm of the sofa.
Designing a box of the correct volume is easy enough, working out the correct port length was a little more challenging, but for a folded port it is easy to use some algebra to work out how many turns to use. To make sure the speaker was stiff enough everything was made from 12mm MDF (although I understand that plywood would have been a better choice as it is stiffer for the same thickness), and the front baffle was made double thickness. The top of the angled speaker was also made twice as thick to reduce the radiated sound. The sharp angles in the port will cause turbulence and reduce the efficiency of the port – I assume it was also reduce the Q of the port resonance (which should increase the tolerance in port length).
Width of the baffle for both speakers was 4” (~102mm), I don’t have a table saw, and cutting many metres of straight 4” wood was never going to happen. Luckily my local hardware store was able to cut the wood for me. I planned the design around this so that my manual sawing would have the fewest opportunities to ruin the build. I only had to cut strips to length, and cut out the side panels.
I 3D printed a circle cutting jig for my router in order to cut the 3” hole for the driver; an M3 machine screw holds the jig to the wood and sets the radius. For small holes the order of operations is a little awkward as the screw head is under the body of the router. Rather disconcertingly when in use the last part of the circle is quite hard to cut as the wood is only joined by a thin section. I drilled the centre holes for the speaker cut outs with the two piece of wood taped together, however, the router bit was not long enough to cut through both piece of wood, so they were separated for the final hole. I cut the hole undersized for the driver, and, after gluing together both components, I widened the hole with a rotary tool to make room for the basket and terminals.
The second speaker was much like the first. However, there were a few unusual challenges such as the angled joining points at the top of the speaker.
Test fitting the driver revealed the ideal location for the holes to fasten it to the baffle. I used M4 T nuts and machine screws to attach the driver as MDF disintegrates if screws are driven in and out of the wood. The drivers would have been difficult to flush mount, and already had gaskets on them, so this process was easier than it could have been. The T nuts were hammered in to the back, and later on in the build I used a clamp to seat them deeper as the first test fitting pushed them out (from the screw threading into the wood).
Each of the components were glued on to one of the side panels. 3D printed spacer cubes were used to hold the components of the port the correct distance away from one another. These were strong enough to clamp over and were printed very slowly so as to ensure they were dimensionally accurate. Clamps and weights (including a blender full of water) were used to hold everything together. Only one or two parts of the speaker were glued at each step, other parts were dry fitted in place to ensure everything stayed in position. Aluminium foil was used to separate surfaces that shouldn’t be glued.
The last part to be glued on was the side panel. The binding posts on both speakers were on the side panel as they were both designed to sit right up against a rear surface. Heavy gauge wire was soldered to speaker terminals and the binding post rings. The 3” hole for the driver was just large enough for my hand to tighten the nuts after the side was glued on.
I lightly sanded various parts of the speakers for fitting. The 5.7˚ angle for the angled speaker baffle was sanded into the front panel. And other parts were sanded flat to remove saw marks before gluing. The dust from this was collected and mixed with glue to make a wood filler that was used to fill the gaps made by my imperfect joinery. This filler was used inside and out of both speakers before the side panel was finally attached.
The two speakers were then installed in the living room, connected to the receiver. I manually set the distances and levels by playing white noise thought the each of the speaker in turn and using an app on my phone to measure the level– although at some point I should try the auto adjustment. The two speakers sound a little thin at the moment, but otherwise are fine. I have them set up as ‘small’ on the receiver, it then sends all of the bass to the front three speakers that have much larger drivers (the main speakers play down to 44Hz). I don’t know if this is the best compromise. Maybe I’ll update this when I’ve lived with them for a little while.
Surround music sound really good on the system with Dolby Prologic II music. This algorithm takes normal stereo music and sends the common signal to the centre channel and the difference between the left and right to the rear channels with some filtering. You get some interesting ambient effects and it really feels like the music is surrounding you. I’m sure better rear speakers would make it sound even better, but I’m quite happy for the moment with these. (You can do a similar thing without any fancy processing with a spare speaker and any normal stereo. Just connect the spare speaker to two positive terminals of your amplifier i.e. the positive from the left speaker and the positive from the right speaker. Then run that speaker behind you. The stereo sound should be about the same, but ambient effects will play through the rear one (you can do the same with two rear speakers). This set up is called a Hafler circuit, the ambient sounds that you hear are sounds that are out of phase between the L/R speakers.)
Movies also sound really good too. The first film we watched with the speakers installed was Jurassic Park, incidentally the first film with a DTS sound track. I was particularly struck with the echoes in the entrance hall of the Jurassic park building. In a darkened room, you really feel like you are in a room the size of the one depicted, not the size of your own room. The fire work effects in Coco were also very involving as you can hear them all going off around you.
First on my list of things to finish off with them is to get some primer on. The MDF won’t last long without something to protect it. However, that will involve quite a bit more sanding and a few clear days when I can paint outside, and then the final colour can go on. After that, I’d like to measure the output of the speakers and see if I can improve the sound. I have some DSP capability on the amplifier, but I might also try to implement a baffle step correction circuit.
In short, if you don’t have room to put a speaker for a surround sound system. You really can bounce the sound off of a wall, and it sounds pretty good.