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Just attach the drivers to timber panels and glue them on to concrete blocks!
C ON C R ET O
Speaker System
We were tempted – very tempted – to call these the greatest ‘ROCK’
speakers ever. But that pun would fall a bit flat because these speakers
are not rock – they’re concrete! More specifically, their ‘enclosures’ are
stock standard concrete building blocks – the type you'll find at very low
cost in just about every hardware store. Intrigued? Read on...
W
ant to make a cool pair of
sidered by many to be the ideal material
from which to make speaker enclosures.
speakers, but don’t have the
by
Allan
Linton-Smith
Concrete speakers have faded in
skills, tools or time to build
popularity since then, but are seeing a
proper boxes for them? No worries. We
have the solution for you! Just bung the drivers into some bit of a resurgence. Besides being practical, they also look
timber panels and glue them onto concrete blocks. It might pretty interesting, especially with nicely-finished, routed
sound like an odd thing to do, but you’d be surprised how timber front panels.
The concrete also helps to improve overall efficiency,
well it works.
This bookshelf speaker system gives punchy and clean transmitting less than 25% the amount of sound energy that
sound, and it’s a lot of fun to build, with excellent bass and a comparable wood or MDF enclosure would.
Many people prefer to have smaller speakers, but they oftreble out of one tiny full-range driver, plus a subwoofer or two.
For just a few dollars more, you can get a Class-D ampli- ten compromise on sound. These ones emit a solid bass and
fier module to drive both, with line inputs and Bluetooth have clarity which you will fall in love with immediately.
They also have a really smooth sound, partly because of the
wireless audio support.
Concrete is actually an excellent material to make loud- lack of resonance and partly because of our choice of drivers.
In keeping with the idea of simplicity and cheapness,
speaker enclosures from because it’s very stiff and it’s very
‘dead’ – you don’t have to worry about it resonating at all we’ve simply glued the front and rear panels onto the cheap
and ‘colouring’ the sound. As a bonus, concrete blocks (also concrete blocks with silicone sealant, and we’ve used a
known as concrete bricks, Besser blocks and breeze blocks) coaxial main driver so that no separate tweeter is required.
are cheap, readily available and have four square sides That also eliminates the need for a crossover network.
Another big advantage of using a single driver is its phase
already pre-assembled.
This is definitely not a new idea. Building speakers became coherence; that is, its ability to reproduce all frequencies
a bit of a fad in the 1950s. At the time, concrete was con- with mostly the same phase.
Features and
specifications
Frequency response: 90Hz-20kHz, ±6dB
Distortion:
<2%, 85Hz-2.7kHz
(0.8% <at> 1kHz)
Bookshelf efficiency: 91.5dB <at> 1W, 1m
Subwoofer efficiency: 88dB <at> 1W, 1m
The pair of Bookshelf speakers, housed in half-blocks. They’ll give a good
account of themselves ‘as is’ but team them up with the full-block Subwoofers
and you won’t believe how good they sound for such a tiny investment. Rock on!
30
Power handling:
2x15W (bookshelf)
2x50W (subwoofer)
Impedance:
8Ω (bookshelf)
6Ω (subwoofer)
Inexpensive
From under £100
Practical Electronics | June | 2021
and only have slightly less bass, slightly
higher distortion and slightly lower efficiency. In other words, they are almost
as good for roughly a quarter the price.
The Jaycar drivers are cheaper
again, but are much less efficient, so
given their relatively low 15W power
handling, you’d struggle to get decent
volume out of them. So that was why
we didn’t end up using them, and
didn’t bother measuring their actual
frequency response or distortion level.
However, we are using Jaycar 125mm
drivers for the Subwoofers, cat no
CW2192. They are also excellent value
given their 50W power handling, decent efficiency of 88dB <at> 1W, 1m and
low resonant frequency of 67.6Hz.
In fact, in the enclosure we’ve designed, they give some output down to
about 35Hz, which is impressive given
their small size.
Performance
The frequency responses of the two
speaker cabinets are shown in Fig.1.
Here, Bookshelf (the blue curve) refers
At 190 × 190 × 190mm, the half-block bookshelf speakers are exactly half the
to the smaller cube-shaped enclosures
size of the subwoofers because the latter are built in a standard 380 × 190 ×
which house the midrange drivers with
190mm concrete block. There are two huge advantages in using concrete blocks
coaxial tweeters, while Subwoofer (the
as enclosures: (a) they’re dirt cheap and (b) they cannot flex or move to colour
the sound in any way. You might say they’re as solid as a (ahem!) rock...
red curve) refers to the taller ported
enclosures with the larger woofers.
The Bookshelf speakers are pretty flat from 150Hz to
This yields a very realistic reproduction of the original
recording for voice, instruments or complex orchestration. 20kHz, with a moderate peak at 10kHz and a roll-off in
It is essential for accurate ‘soundstaging’ (positioning of response below about 180Hz. You can see why we decided
to build the subwoofers; they extend the bass response coneach instrument).
The small cone is very accurate in the upper and mid- siderably, down to around 65Hz, with a bit of a shelf from
range, but you will also get to hear pipe organ pedals and 40-65Hz. This means that really low bass won’t quite be
bass drum kicks if you build the extra subwoofers. They are ‘full’, but you’ll at least hear something down to about 35Hz.
Importantly, the system also provides low distortion
so cheap to make, why not build two for better bass?
sound, as shown in Fig.2. THD is well under 2% from 100Hz
to 2.5kHz and less than 0.8% at 1kHz. It is reasonably efDriver choice
We considered three different full-range drivers for the Book- ficient, delivering 91.5dB at 1W/1m/1kHz.
You may think that it is only suitable for small rooms,
shelf speakers (UK readers, see Parts List). Table 1 shows a
comparison of these units. We chose the Altronics C0626 on but they produced a considerable amount of sound in our
the basis that they are a lot cheaper than the Fostex drivers warehouse with only a few watts driving them.
+50
Concreto Frequency Response
Concreto THD vs Frequency, 1W <at> 1m
10/09/18 08:39:38
Total Harmonic Distortion (%)
5
+30
+20
+10
+0
-10
2
1
0.5
0.2
-20
-30
10
Subwoofer
Bookshelf
+40
Relative Amplitude (dBr)
10/09/18 08:34:12
20
50
100
200
500 1k
2k
Frequency (Hz)
5k
10k 20k
Fig.1: the Bookshelf speakers have a fairly flat response
above 150Hz; the Subwoofers fill in below 200Hz where
the bookshelf response drops off.
Practical Electronics | June | 2021
0.1
20
50
100
200
500 1k
2k
Frequency (Hz)
5k
10k 20k
Fig.2: distortion is low in the critical 100Hz-2kHz range
which contains a lot of human voice information as well as
many musical instruments.
31
Price per pair (approximate)
Impedance
Rated efficiency (dB <at> 1W, 1m)
Free-air resonance
Vas (litres)
Rated frequency response
Measured response (±5dB)
Measured THD+N (1kHz, 90dB)
Measured SPL <at> 1W, 1m
Fostex FE103En
£110
8Ω
89
83Hz
6
83Hz-22kHz
60Hz-15kHz
0.45%
92.5dB
Altronics C0626
£20
8Ω
95
120Hz
?
120Hz-20kHz
70Hz-15kHz
0.65%
90dB
Table 1: full-range driver
comparison. All three are
rated at 15W, employ a
ferrite magnet and suit a
93mm diameter hole cut-out.
Jaycar CS2310
£15
4Ω
83
122Hz
3.3
90Hz-18kHz
Fig.4 (below): dimensions
of the front and rear
baffles for both the
midrange and subwoofers.
We used 19mm DAR pine
but many other timbers
could be used.
Regardless, if you want good quality sound at reasonable
listening levels, these will not fail to impress.
A nice little Class-D amplifier module is ideal for driving
these speakers, for example, one of those I reviewed in the
August 2020 issue of PE.
They certainly could also be driven by one of our more
powerful/higher fidelity audio amplifiers, such as the SC200
(January-March 2018).
These Concreto speakers will provide many hours of
wonderful listening at a tiny fraction of the cost of a fully
built high-end hifi system. The sound is even more satisfying, knowing that you have built something a bit unusual!
Listening tests
We were amazed that these little speakers with 100mm (4inch) drivers could produce such a huge sound. Not only
that, but the realism, separation and positioning of the instruments and singers are truly first class. When using a quality
DAC and amplifier, the music was fabulously rich, and we
were able to pick out each instrument as if they were there.
But don’t believe us; you be the judge. Build it and enjoy
the rewards of listening to a concerto... err... Concreto. It will
be worth the effort!
Construction
Select your concrete bricks carefully. We selected the ones
with the smoothest surfaces and minimal cracks and chips.
You might want to spend a little extra and buy some spares,
because they are heavy and are easily dropped or bumped.
Fig.4
(guideline)
dimensions
of the front
and rear
baffles for
both the
midrange
and
subwoofers.
We used
19mm DAR
pine but
many other
timbers
could be
used. Your
blocks may
require
slightly
different
dimensions.
REAR
184
GRIND OR CHISEL 2 – 3 mm
FROM THIS AREA TO ALLOW
SOUND TO TRAVEL TO THE PORT
184
184
92
100
93mm
DIAM.
115mm
DIAM.
184
92
ALL DIMENSIONS IN MILLIMETRES
REAR
BAFFLE
92
SUBWOOFER
REAR BAFFLE
92
MIDRANGE FRONT BAFFLE
SUBWOOFER
FRONT BAFFLE
78mm DIAM. HOLE IF JAYCAR
PT3012 TERMINAL BLOCK USED,
OR 76.5mm DIAM. HOLE IF
ALTRONICS P2017 USED
78mm DIAM. HOLE IF JAYCAR
PT3012 TERMINAL BLOCK USED,
OR 76.5mm DIAM. HOLE IF
ALTRONICS P2017 USED
390
92
92
184
92
54mm DIAM. HOLE
IF BINDING POST
PLATE IS USED
32
FRONT
90
54mm DIAM.
92
54mm DIAM. HOLE
IF BINDING POST
PLATE IS USED
Practical Electronics | June | 2021
THESE HOLES
3.0mm DIAM.
(for one pair each of Bookshelf speakers and Subwoofers)
35
20
20
30
10
Parts list
10
THESE HOLES 8.0mm
IN DIAMETER
(70 x 70mm SQUARE OF BLANK PC BOARD)
10
10
ALL DIMENSIONS IN MILLIMETRES
Fig.5: here's the plate we made to house the speaker
terminals on the back panels. We used scraps of blank PCB
material; aluminium or other thin (rigid) plates would work.
2 full concrete blocks, 390 x 190 x 190mm
2 half concrete block, 190 x 190 x 190mm
2 DAR pine planks, 184mm x 1.8m x 19mm
2 100mm (4in) 15W 8Ω twin cone speakers [Altronics C0626]
2 5in (125mm) woofer/midrange speakers [Jaycar CW2192]
2 pairs of pre-mounted speaker terminals
[Altronics P2017, Jaycar PT3012] OR
2 pairs of red/black binding posts AND
2 70 x 70mm squares of fibreglass laminate (blank PCB
material)
16 20mm-long wood screws (eg, No.9/4.5mm thread)
1 3m length medium-duty speaker cable (figure-8)
1 pack of acrylic speaker damping material [eg Jaycar
AX3694 or from Parts Express; see text]
4 squares protective felt (or similar), ~170mm x 170mm
1 tube of neutral-cure clear silicone sealant
UK constructors
Make sure they are completely dry (especially if you take
them from your backyard); otherwise, the silicone sealant
won’t adhere too well.
For the subwoofer bricks, decide which side is to be the
front and the back, mark them with a pencil and then grind
or chisel 2-3mm from the back of the centre piece as shown
in Fig.3. We used an angle grinder fitted with a diamond
blade, but you can also use a masonry chisel.
Check how much you’re taking off with a straightedge,
and use a credit card as a feeler gauge. When finished, you
should be able to hold the straightedge across the front surface of the brick and slide the credit card between it and the
centre section, where you removed the material.
Remember that this project is more a set of guidelines and
suggestions rather than a specific recipe. Although the speakers
were specified with Australian readers/parts in mind, the designs
are easy to modify for UK-available parts. The key is to ensure
your concrete blocks of choice can accommodate your drivers.
For acoustic wadding, some good options are here:
http://bit.ly/pe-jun21-damp
The Fostex drivers are available in the UK, although the
FE103En has been replaced by model FE103Env. For cheaper
models, a search online for ‘4 Inch Coax 2 Way Car Speaker’
will give you lots of inexpensive options to experiment with.
For the Subwoofer, look for a ‘Woofer/Midrange Speaker
Driver, 5-Inch’, at around 50W; for example:
http://bit.ly/pe-jun21-woof
Timber panels
We used premium pine planks, dressed/planed all-round,
184mm × 1.8m × 19mm thick. You can then easily cut these
to 184mm and 390mm lengths to make the panels for the
Bookshelf and Subwoofer speakers respectively.
The hardware store may cut these pieces for you, and many
will do it for free. If they won’t, and you don’t want to do it
yourself, you could seek out a kitchen cabinet maker, who
would surely take on the job for a modest fee.
You can use any type of timber which is 19mm thick or
more, such as MDF or plywood, but we think pine looks nice.
Assembly is pretty easy, but do not rush it and allow plenty
of time between steps so that you don’t make any mistakes.
Once you’ve cut the panels to size, the next step is to
cut the holes, as shown in Fig.4. Ideally, use a hole saw for
the port holes in the Subwoofers, as they are fully exposed
You could use a jigsaw to cut the other holes, as long as
you don’t make them too rough, as the speaker surrounds
will cover the cuts. Hint: if you are using a jigsaw, cut from
the inside of the panels, so any ‘bruising’ is hidden.
One important thing to note is that the size of the holes
in the rear panels vary depending on which type of speaker
FRONT
BAFFLE
MIDRANGE
DRIVER
SEALED
MIDRANGE
ENCLOSURE
FRONT BAFFLE
PORT
SUBWOOFER
ACOUSTIC
WADDING
REAR
ACOUSTIC
WADDING
BAFFLE
BINDING POSTS
REFLEX
SUBWOOFER
ENCLOSURE
ACOUSTIC
WADDING
REAR BAFFLE
BINDING POSTS
2-3mm GAP FOR SOUND TO
REACH PORT CHAMBER, ALSO
WIRES TO REACH BINDING POSTS
Fig.6 (left): the sealed midrange enclosure is made from a ‘half block’ and midrange driver, as this semi-section shows.
Fig.7 (right): the ported subwoofer is made from a ‘full block’ and subwoofer driver with a tiny air gap between the halves.
Practical Electronics | June | 2021
33
Fig.8 (left): front side view showing how the ‘baffle’ is
secured to the concrete block. Make sure you use plenty of
silicone so the join between the block and panel is airtight.
PINE TOP/FRONT
plane. This makes the speaker look much better, although
it isn’t absolutely necessary.
Next, if you didn’t splurge on the pre-built speaker terminals, make up the connector plates from blank PCB material,
and standard binding posts mounted 30mm apart.
Make sure you’ve sanded away any imperfections in the
panels, then paint, stain or lacquer the panels. Allow them
to dry completely, and you are ready to assemble everything.
CONCRETE
C
ONCRETE
BLOCK
B LO CK
SILICONE BEAD
(NOT TOO
CLOSE TO
THE OUTER EDGE)
terminals you’ll be fitting. If you’re building the home-made
speaker terminal panels from a piece of blank PCB laminate
and a pair of binding posts, cut 54mm holes. If you’re using
the Jaycar PT3012 terminals instead, make the holes 78mm
diameter, or for Altronics P2017, 76.5mm diameter.
Once you’ve made all the cut-outs, sand the port holes
nice and smooth, and clean off any burrs from the other
holes. You might like to dress the edges with a router or
Fig.9 (below): here’s how the rear panel is attached to
the block. Make sure air can flow between the two block
halves, as explained in the text .
Assembly
This is pretty straightforward; you just need to proceed carefully, so you don’t damage anything; especially the delicate
speaker cones and surrounds.
Start with the backs of the enclosures. Dust off the concrete blocks, make sure they are clean and dry then apply a
3-5mm bead of silicone sealant (adhesive type) as shown in
Fig.9. Keep the bead close to the inside edge. This prevents
it from appearing on the exterior when you press the panel
into place.
Do not put any silicone on the centre piece at the back of
the Subwoofer bricks; otherwise, sound cannot travel from
the driver to the port! However, you do need to seal the centre
section at the front so as to direct all the sound to the back
and then around to the port.
Take your prepared rear timber panel and gently lay it
on the silicone bead. Once you are happy with its position,
slowly press it down onto the brick. If any silicone squirts
out the edges, quickly wipe it away with a damp cloth.
Add weight on top (eg, a pile of books) to keep the panel in
position and leave it to cure for at least 24 hours. Make sure
its position is correct because once it cures, you will not be
able to shift it!
Repeat this procedure with all the other enclosures. Then,
after 24 hours (or more, if you are in a cold climate), repeat
PINE REAR PANEL
CONCRETE
C
ONCRETE
BLOCK
B LO CK
34
DO NOT PUT
SILICONE BEAD
IN THIS AREA,
TO LEAVE A SMALL
GAP BETWEEN
UPPER & LOWER
CHAMBERS
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
Front view showing the baffle
secured to the half block and
the acoustic wadding inside.
The four screwheads could
be painted black so they don’t
stand out as much.
Practical Electronics | June | 2021
www.poscope.com/epe
The finished midrange speaker, here
shown from the under-side, with a 170
× 170mm piece of thin felt glued to
the block to ensure it doesn’t scratch
underneath surfaces.
this procedure with the front baffles
(see Fig.8). Just remember to add the
silicone across the centre of the subwoofers this time.
After another 24 hours, you are ready
to mount the drivers. But first, cut
60-70cm lengths of speaker wire and
solder them to each driver.
When the silicone on the cabinets
is completely cured, pack the subwoofers with acoustic wadding. We
used Acousta-Stuf Polyfill, available
from Jaycar.
Pack this in loosely behind the drivers. In a pinch, you can also use small
(dry) towels.
Push the speaker wires through so
that they are sticking out the hole in the
back panel, then mount the drivers using wood screws. You’ll get the neatest
result if you first mark and drill small
pilot holes, using the driver surrounds
as templates. Try to orient the drivers all
the same way; it generally looks best to
have the screws in the diagonal corners,
as shown in our photos.
Solder the wires sticking out the back
of the enclosures to the inside of the
speaker terminals, then mount those
terminals on the rear panels in a similar
manner. Your speakers are finished!
Note that you can stack the smaller
speakers on top of the subwoofers, or you
can locate them separately. The placement of the Subwoofers is not critical.
Depending on the surface your
speakers are going to be placed on, you
might like to glue a 190 x 190mm square
of felt or similar protective material on
the underside of each of the blocks.
Concrete scratches most other surfaces
quite nicely!
Driving them
The drivers in the smaller Bookshelf
speakers are full-range units, and the
Practical Electronics | June | 2021
Similarly, the completed subwoofer,
here seen from the back to show the
small (PCB) plate housing the terminals.
subwoofers only respond to bass frequencies, so you can drive each pair
from separate amplifiers. We suggest
that you don’t wire them in parallel as
the Subwoofers have lower sensitivity than the Bookshelf speakers, and
therefore require a slightly higher signal
level to get matching levels.
Two stereo amplifiers, each capable
of 25W into 8Ω or a bit more into 6Ω
should do the job. Our Subwoofer drivers can handle up to 50W each, so if you
like playing really bassy music, more
powerful amps are the way to go. But
you’re likely to get more power into
the Subwoofers anyway, given their
slightly lower impedance (6Ω vs 8Ω
for the Bookshelf speakers).
The Class-D modules we mentioned
earlier are suitable, as long as you
power the ones driving the Subwoofers
from a sufficiently high supply voltage
(eg, 20V+).
Connect the amplifier outputs to the
four sets of speaker terminals, then use
RCA Y-cables to connect the outputs
from your preamplifier to the left/left
and right/right pairs of power amplifier inputs.
You can then play some music and
adjust the individual amplifier volume
controls until the bass and treble levels
sound well-matched.
We allowed about two hours playing
various types of music at reasonable
volume to ‘run in’ the speaker drivers
before we took measurements; you
may find that your drivers are a little
stiff straight out of the box. You should
notice an improvement in the sound
with time, as you use them, especially
in the bass response.
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
- up to 256
- up to 32
microsteps
microsteps
- 50 V / 6 A
- 30 V / 2.5 A
- USB configuration
- Isolated
PoScope Mega1+
PoScope Mega50
- up to 50MS/s
- resolution up to 12bit
- Lowest power consumption
- Smallest and lightest
- 7 in 1: Oscilloscope, FFT, X/Y,
Recorder, Logic Analyzer, Protocol
decoder, Signal generator
35
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