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Sound in the Lounge, Part 2

Building twin 15 inch subwoofers under the house floor

By Julian Edgar

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At a glance...

  • 4-part series
  • Designing, building and tuning a concealed house speaker system
  • Two ported 15 inch subwoofers beneath the floor
  • Two-way 8-inch ported enclosures in the walls
  • This story: building and installing the 15 inch subwoofers
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If you’re looking at the title to this story and wondering what, precisely, the topic has to do with cars – the answer is… nothing much.

But I am quite sure that a great many of you will find this series of interest – it has lots to do with using your hands, brains and skills to achieve a technically interesting outcome.

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Last issue in Part 1 of this series I’d settled on the use of two, 200 litre ported boxes for the 15 inch JBL subwoofers. Configured and driven as a stereo pair, these were to fit under the floor and fire into the room through two grilles inset in the floor.

This design was chosen because it:

·       gave extended low frequency response

·       fitted under the floor in the crawl-space

·       could be inserted through the small crawl-space access door

·       gave the drivers protection from the weather

·       reduced heat loss through the floor compared to (say) an infinite baffle design

Now – how to make them!

Materials and Construction

Because the boxes will be to an extent unprotected from the weather (the crawl-space is well ventilated and so exposed to fog and wind-borne rain), and also because of the low cost, I chose to construct the boxes from particle board flooring.

I used 19mm thick, “green tongue” sheets. This material has a mass of 13.1kg / square metre and is available in a variety of sheet sizes. It cuts easily with either a coarse-bladed electric jigsaw or a circular saw.

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Butt joints (rather than mitres) were used; however, full-length cleats were added to strengthen every butt join. These cleats were made from 40 x 20mm pine. Every joint was both glued and screwed, with the screws connecting to the cleats rather than to the particle board. Water clean-up Liquid Nails building adhesive was used. Lots of glue was placed at every joint so that it squeezed out as the panels were screwed together. A wet finger was then used to smooth this glue internally and externally along the seams, better sealing them. (The final panel, where internal access wasn’t possible, used an even greater quantity of glue!)

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The spacing of the floor joists meant that the main portion of the enclosure needed to butt against the underside of the joists, rather than sliding up between them. In turn this required that a narrower extension piece be used to connect the enclosure to the floor vent. The side parts of the extension are formed in-situ by the floor joists, while the ends of the extension comprise pieces attached to the box.

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Two ports enter one of the extension pieces from the side. Each port is 100mm in diameter and 330mm long. The ports are formed from curved plumbing fittings with extension pieces inserted in each end.

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Prior to insertion, each of the port extensions was formed into a bell-mouth. This was achieved by softening the end of the pipe with a heat gun before it was forced down onto an appropriately-sized inverted china bowl.

The port dimensions were specified by the BassBox software program used to design the enclosures. Extensive testing of the final boxes shows that the port diameters could have been made less than 100mm (with an appropriate change in lengths to match) as, at least in my use, air velocities in the ports are very low.

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To reduce panel vibration, two internals braces are used (arrowed). These connect the largest side panels, with one brace one-third of the way along the panel and the other two-thirds along the length. The braces were made from 25mm diameter, solid-cored bamboo broom handles cut to appropriate length – these are very stiff.

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All internal walls are covered with polyester quilt wadding, glued into place with Liquid Nails. In addition, two larger pieces of wadding are rolled and then inserted into the box, one at the end furthest from the driver and the other immediately below the driver.

I chose not to use a speaker terminal, the cable simply being run out of a hole in the box that was then sealed.

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The JBL driver is held in place by screws at each of the provided holes; in addition, a polymer sealant was applied under the lip of the frame.

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Thick foam rubber self-adhesive strip was applied to the top parts of the extension pieces to form an air-tight seal with the underside of the floor around the grille.

The BassBox design

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I used BassBox Lite software to design the enclosures.

Driver specs

Fs = 27.3Hz

Qms = 5.7

Vas = 106.5 litres

Xmax = 14.5mm

Sd = 855 sq cm

Qes = 0.76

Re = 3.63 ohms

Le = 2.42mH

Z = 4.3 ohms

BL = 16.53Tm

Pe = 300 watts

Qts = 0.67

2.83 V SPL = 92dB

Calculated box properties

Vb = 200 litres

Fb = 24.6Hz

Ql = 5

F3 = 20.7Hz

Two vents

Dv = 100mm

Lv = 330mm

Initial testing

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A frequency generator was used for initial testing. I used a standalone instrument I already had, but if you don’t have such an instrument, free and/or low cost downloads are available to turn a PC sound card or Apple i-device into a frequency generator.

The frequency generator should be connected to an amplifier and the amplifier used to drive the speaker under test. Be careful when driving speakers in this way: the power levels should be kept relatively low.

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Initial testing with the subwoofer enclosure positioned in the lounge room (as opposed to under the floor at this stage) showed that there was audible bass down to 25Hz, and strong bass from about 35hz. The response was smooth up to at least 500Hz – high enough for the in-wall speakers to take over the load. This is also the time to look for air leaks – but primarily because lots of glue was used during construction, there weren’t any air leaks in my boxes.

You can also use the frequency generator to ‘run in’ the driver, using a variety of low frequencies at different power levels.

Under the floor

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To provide an opening through which the drivers could fire, holes were cut in the floorboards. Because the floor was to be tiled, cement sheets had already been laid on top of the timber – so the holes were cut through both the sheets and the floorboards. Tiles were later laid around the holes.

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Steel floor grilles were then placed over the holes.

To be honest, I wasn’t looking forward to getting the enclosures mounted under the floor – the space is limited and the boxes heavy and unwieldy. However, with lots of help from my 8 year old son Alexander (who loves being under the floor!), things went pretty smoothly.

This was the sequence of events for each box:

1.     We slid the enclosure along the ground on a long, narrow piece of scrap particle board, until the enclosure was located directly under the floor grille.

2.     Step by step, we then lifted each end of the enclosure, placing bricks under each end as we went.

3.     A brick was then nestled into the dirt under the middle of the enclosure, a piece of strong timber placed under the enclosure and then a surplus scissors-type car jack placed between the brick and the added timber support.

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4.     The jack was then wound up, lifting the enclosure into place. The enclosure was raised until the rubber seal of the extension piece contacted the underside of the floor, and then adjusted up another 5mm or so to give a positive contact. (Don’t wind the jack too hard: it will crack the floor!) Some additional spacers of particle board at the opposite end of the box to the speaker kept the enclosure levelled.

The jacks stay in place: with heavily greased threads, they will be useable should the enclosures ever have to be lowered for repairs or replacement. (Scissor jacks are available from wreckers and the shops at municipal tips for very little cost.)

The sound

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So how do the speakers sound? Well, at this stage it’s impossible to tell – the rest of the speaker system (the in-wall 8 inch two-ways) needs to be in place, and, even more importantly, the system has to be set up with the right balance between all the drivers.

But I don’t want to leave you dangling. Here’s a sneak preview: with these speakers, the biggest problem is stopping the room's aluminium window frames from rattling!

Next issue: building and testing the 8-inch two way in-wall speakers.

Building boxes

You can use a variety of methods to cut out speaker box panels.

Easiest is to use a power circular saw, but you can also use an electric jigsaw or - as a last resort - a hand saw.

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When using the power tools, it's important that you don't cut free-hand - it's almost impossible to cut straight lines by just following a pencil line. Instead, clamp a long piece of (straight!) timber to the panel to guide the edge of the power tool along the desired path. Don't forget that this straight-edge will need to be clamped the correct distance from the actual cut line.

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The speaker hole is best cut out slightly undersize with the jigsaw. Sand the hole to the finished size with coarse sandpaper wrapped around a large diameter former (like a 1 litre paint tin). The approach will allow you to sand away any wriggles in the jigsaw work.

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Cut out tweeter and vent holes with a suitable hole-saw.

One of the advantages of building speaker boxes that will then be hidden is that you don't need to provide a nice finish. So rather than carpet, or more usual for household speaker boxes, a polished or lacquered finish, you can just leave the particle board bare. That makes your job a LOT easier!

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Before starting the build, buy plenty of glue cartridges - the large speaker boxes have many metres of joins that need to be well-sealed. As a guide, I used five glue cartridges on each box.

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In ported enclosures don't be tempted to use fibreglass insulation as the fill. If you use fibreglass, the speaker can pump small glass-fibre particles into the listening room through the port. Polyester quilt wadding is cheap, benign and readily available.

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Remember that the drivers in the underfloor woofers are vulnerable to damage. Should anything fall on the cone (easily occurring during construction or when the floor grille is removed), it's likely the cone will be irreparably damaged and you'll have to buy another speaker.

The parts in this series:

Part 1 - the ins and outs of building speakers into your walls and floor

Part 2 - building twin 15 inch woofers under the floor

Part 3 - building 8 inch two-way speakers into the walls

Part 4 - tuning the completed speaker system

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