Sound in the Lounge, Part 3

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.

With the 15 inch speakers mounted in 200 litre enclosures under the floor (see Sound in the Lounge, Part 2) , it was time to develop the in-wall speakers.

Like the floor speakers, I wanted their intrusion into the room to be minimal.

Wall speakers

Many different in-wall speakers are commercially available.

The vast majority use a 'bare' design: that is, the speakers are mounted on a baffle that is placed over a hole cut in the wall plasterboard. The space that is present behind the speakers depends on the size of the wall cavity. This volume can vary from wall to wall, so to cater for this, the designers of these speakers specify a fairly stiff speaker suspension on the mid / bass driver - otherwise, when installed over big spaces, the speaker would bottom-out when driven hard. In turn, this stiff suspension results in a high resonant frequency and typically, poor bass.

A different approach is to use designs that incorporate an in-wall enclosure. This type of speaker will sound much the same in different walls – a major advantage. This approach also allows the use of tuned (eg ported) enclosures. For example, Bose, with their pictured 191 design, takes such an approach.

After testing the Bose 191 (see breakout box later in this story) I decided to build my own enclosures and use the bass/mids, tweeters and crossovers from a pair of older Wharfedale Atlantic speakers I already had.

In standard form, each Atlantic uses three 8 inch drivers and a tweeter. The two lower drivers are housed in their own ported enclosure, and the mid/bass unit in a separate upper enclosure that is also ported. (The ports are on the back of the box.) The upper enclosure is about 15 litres in volume – a pretty good size for a custom-built in-wall enclosure.

I removed from the Wharfedales the mid/bass units, tweeters and crossovers and built them into a pair of new 15 litre enclosures, sized to fit in the walls.

The general approach, which is easily adopted, therefore consisted of:

·       obtain a pair of compact, good-sounding speakers

·       copy the enclosure design (volume, port length and size)

·       use the drivers and crossovers in the new-shape enclosures

This method has lots of benefits. It's cheap (much cheaper than buying the individual drivers and crossovers), requires no further acoustic design, and allows (some) sound testing before tearing the walls apart!

New boxes

One of the problems with building full enclosures into the walls (as opposed to using the wall cavities as the enclosures) is that you are severely limited in the thickness of box material you can use. If the available wall depth is (typically) 100mm, then using thick particle board for the enclosure can easily reduce the available depth to just 70mm – too small to accommodate the depth of most mid/bass drivers. To maximise the internal depth, I chose to use relatively thin 9mm medium density particle board.

To provide an internal volume of about 15 litres, the box dimensions are about 540 x 380 x 100mm.  Note that the depth was dictated by the thickness of the wall framing (max 100mm) and the width by the distance between adjoining studs (400mm).

The boxes were assembled with butt joints, nailed and/or screwed into place. Pine cleats (40 x 20mm) were then placed at the corners of the box. Water clean-up Liquid Nails building adhesive was used on all joins.

A surplus of glue was used so that it was squeezed out of the joints as the panels were pushed 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!)

The ports of the Wharfedale donor speakers were 50mm in internal diameter and 85mm long. However, a port of this size in the in-wall enclosures would put the internal end of the ports too close to the back wall of the boxes.

To achieve the required clearance, I formed new ports (of the same diameter and length as the Wharfdale ports) from curved plumbing sections. The internal ends of the ports were bellmouthed by being heated and then forced down over a small inverted ceramic bowl. (Note that these plumbing pieces have a smooth internal bend, not an inner radius that has a sharp edge at the change in direction.) Part of the bell-mouth needed to be ground away to provide clearance to the back wall of the enclosure.

I wanted a paintable, professional looking metal grille and obtained it by buying the cheapest 8-inch in-wall speakers I could find on eBay.

When the speakers arrived, I removed the cheap drivers and crossovers, and then cut out the internal plastic panel with an electric jigsaw. This needs very careful work. The result is a plastic frame into which the metal grille securely fits.

To give the grille frame clearance, I used a recessed front panel to mount the Wharfedale woofer, tweeter and the new port. The speaker panel mounts behind this front ‘window’ panel.

Internal surfaces of the enclosure were covered in a single layer of polyester quilt wadding, held in place with Liquid Nails adhesive. 

Bose 191 The Bose 191 is a commercially available, in-wall speaker. Its design uses a ribbed, plastic enclosure that hangs below (or alternatively, is mounted above) the metal speaker grille. This enclosure fits entirely within the wall. The enclosure uses a complex tuned port design. Mounted behind the grille are two angled, small, full-range speakers and the rectangular port mouth. I bought and carefully sampled the 191. Given the size of the Bose speakers, the sound is very good. Absolutely – no doubt about it. But compared to the larger, two-way design covered here, the Bose sound is pedestrian. That's especially in the lack of highs and, to a lesser extent, the subdued bass. Unless you have an absolute need for an enclosure no bigger than the Bose 191, you can do a lot better at a lower cost by some DIY work.

Testing

A frequency generator was used for initial testing of the wall speakers. 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.

The frequency generator test showed that the new-shaped speaker had response that extended down to around 50Hz and was strong from about 70Hz up to beyond the range of my hearing – excellent as a mid / bass and tweeter combination.  

Working at this stage with just the first finished enclosure, I then connected the speaker to an amplifier and played music. To be honest, I was surprised at how good it sounded – it was as good as the original Wharfedale (when of course the Wharfedale was played with just the mid/bass and tweeter connected). That it sounded much the same makes sense – but it felt quite odd when you looked at the weirdly-shaped enclosure.

But then things started going wrong.

I’d decided that the speakers should be mounted in the walls about midway between the floor and the ceiling. On paper, these positions worked well - the left-hand speaker located below a wall-mounted TV, and the right-hand one at a height to match. And that's where I cut the holes in the wall plasterboard.

The problem occurred when I placed the first wall speaker enclosure in the newly-cut wall hole – it sounded terrible! The sound was thin and lacked warmth - the upper bass / lower midrange had gone. (A test with the frequency generator showed a new and distinct hole around 100 - 150Hz)

It wasn't the enclosure or its drivers, because when I placed the system back on the floor it sounded fine again.

So what was it?

I had an assistant move the speaker box around while I listened. On the floor in the middle of the room - it sounded great. Up in the air, about a metre above the ground - fine. Over near the wall, still OK.

The assistant then slid the flat enclosure up the wall.

As soon as it reached the mid-point, half-way between the floor and ceiling, the sound was horrible! I then had my assistant raise the speaker still higher. When the grille reached a distance of about 10cm below the ceiling cornice, the sound was great.

So what could I do? Move the hole in the plasterboard… I extended the hole vertically by about 60cm, inserted the speaker enclosure - and everything was again fine.

An amazing change… As an amateur, I have been building and testing speaker systems for about 40 years. (I started well before my teens!) So I have some familiarity with the idea of moving speakers around within a room to improve their sound. You know, put the speakers in the corners and the bass is stronger but often less defined; place them near drapes and the treble gets lost - and so on. But I was just blown away to find out how susceptible to position was the sound of the wall speakers. I don't know if it’s partly the fact that the speaker drivers are mounted flush with the wall - and so their diffraction pattern is unusual - or simply that with the greater air loading caused by the presence of the floor (or ceiling), the mechanical impedance match is better. But whatever it is, the effect of location on sound quality was absolutely dramatic with the in-wall speakers. Be careful where you mount your in-wall speakers! (And if you are using bare speakers that simply locate over a hole in the wall, I am not sure how you can even work out the best location before cutting the hole.....)

Installation

Each wall speaker was installed using the following procedure.

The existing wall plasterboard was cut back so that the joins between old and new plasterboard would be located over timber studs (verticals) and noggins (horizontals).

A new noggin was nailed between two of the existing studs - the enclosure was then placed on it, sitting on two packing pieces.

More chipboard packing (arrowed) was used between the studs and the box to firmly locate the enclosure laterally. Liquid Nails glue was liberally applied underneath and on both sides of the box, effectively gluing the box in place between the studs. The front face of the box was located flush with the wall surface.

Plasterboard was cut to size fill the gaps and glued in place using Gyprock Wet Area jointing compound (I have found this plastering adhesive to be particularly strong). Finishing plaster was then trowelled over all the joins and sanded smooth.

The inner baffle, the grille and the surround were painted and then….

… the speakers were reinstalled.

The grille and surround were glued into place and then the wall was painted.

Taking this approach givdes a very secure final result – the speaker enclosure is part of the wall rather than just sitting in or on it.

Incidentally, if you are worried about getting the wall dead flat and you are not well versed in plastering – don’t worry much. Walls aren’t flat – just go around your house with a long straight edge and see all the waves in the walls you didn’t know were there!

The sound

So how do they in-wall speakers sound? In a word – excellent. In fact, with the new enclosures integrated into the walls, a surprising discovery was made – the drivers now sound better than they did in the original boxes.

The bass from the in-wall speakers isn’t particularly strong (that’s what the underfloor woofers are for) - although note that the bass would still be more than adequate for (say) centre and rear speakers in a home theatre system, or general listening.

But it’s the treble and midrange that are outstanding – the speakers having a ‘transparency’ of sound that is nuanced and very effective.

Conclusion

So the sub-floor subwoofers are in place and the in-wall speakers are in the walls… now for one of the most difficult aspects of the whole process to get right – getting all the speakers working with the correct balance and phasing.  

Next issue: setting-up the system