Sound in the Lounge, Part 1

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.

It began when we bought a house that needed renovating. The house also needed a big shed… so that’s what it got first. But after the shed was built, and time passed, I decided I’d better get around to doing the work I’d long promised.

That’s work like putting-up new plasterboard on the walls of the dining room and kitchen, replacing all the kitchen cupboards and bench-tops, painting, and laying ceramic floor tiles.

This, you must understand, is not work I find particularly interesting. It’s OK – especially when you’re learning new skills, but after only a short time it starts to lose its glamour.  So, I thought while floor-tiling in the lounge room, what could I do to make this project a bit more interesting?

Hmm – how about built-in speakers? Like, subwoofers located in the crawl-space under the house, firing up through floor-mounted grilles? And then to complement them, speakers built into the walls?

In the past such ideas would have filled me with terror (what, tearing the floor and walls apart?) but these days, having become modestly proficient in flooring and plasterboarding, I reckoned it was all do-able.

Let’s start with the floor speakers.

Designs

My first thought was to mount a couple of drivers under the floor, with their enclosures being the whole of the atmosphere. That is, to use a genuine infinite baffle approach.

(If you don’t know anything about the ins and outs of speaker enclosure design, refer to the end of this story.)

If you look around the Web you’ll see some fantastic infinite baffle designs, where people have mounted drivers (often multiple, large drivers – like four 18 inch sub-woofers!) so that they work in under-floor or in-ceiling locations, pumping sound into the room through a short connecting duct.

The advantages of an infinite baffle design is that there is no speaker box to colour the sound, and with an infinite amount of air behind the cones, no stiffening of the driver’s suspension through the trapped volume of springy air.

The disadvantages are that without that trapped air, cone excursions can be large and so you need a lot of driver area (ie lots of big speakers) so that you can use less power and have smaller cone excursions - and yet still get adequate air movement.

In my application, I could think of two further disadvantages of the infinite baffle approach.

Firstly, mounted under the floor, the rear of the speaker is exposed to outside air – and so, during rain and fog, airborne moisture. With the crawl space well-ventilated, I could see the moisture degrading the speakers over time.

Secondly, and for me this one was the big negative, the subwoofer cones are not particularly good heat insulators (why should they be?) and so in winter, I could see my heating bill being even bigger than usual! After all, it’d be much the equivalent of having two big holes in the floor…

So how could these disadvantages – especially the heat loss one – be overcome? The answer is to use an enclosure – to box-in the rear of the speaker. After all, there’s plenty of room for a big box… or two.

Drivers

But before going much further, I needed to bite the bullet and source the drivers that I was going to use in this underfloor location. I decided to use two drivers (ie a stereo pair… I am not much into home theatre and so the system is predicated around two channel use) and started looking hard for suitable speakers.

I settled on some car subwoofers – the JBL GT5-15. These are 15 inches in diameter and have a continuous power handling of 300 watts. I bought new items privately for just AUD$190 a pair.

With the speakers in-hand, and with full Thiele-Small parameters available, I could start computer modelling different enclosure designs.  I’ve long used BassBox Lite – my copy is dated 2002 – and have had success designing subwoofer enclosures with the program. It’s one I trust.

So I fired-up BassBox and started modelling.

Models

The first design I modelled was the infinite baffle approach. Using the BassBox ‘sealed box’ program option (but with an inserted volume of 999999999 litres!) the software indicated a -3dB point of 33Hz. Pretty good – in the real world, as opposed to marketing literature, 33Hz is very low in note.

Using the software, I then trialled a sealed box – picking an arbitrary volume of 300 litres. The modelled response changed very little over the infinite baffle - after all, 300 litres is a pretty big box! In fact, when rounded, the -3dB point stayed at 33Hz.

But what about a ported box? The design of this is more complex, but by using a volume of 315 litres and a box tuned frequency of 20.5Hz (achieved in the software with a rectangular port 200 x 100 x 360mm long), the -3dB point could be stretched waaaaay down to 17.6Hz! That’s another whole octave lower than with the other designs – at 17Hz you wouldn’t be hearing the bass, you’ll be feeling the house shake.

This graph shows the modelled response of the three designs. Red is the infinite baffle, yellow is the sealed box of 300 litres, and green is the ported box of 315 litres. 

Note that these were never final designs – just indicative of what could be achieved with each approach.  And the trial modelling showed that a ported box was best.

Logistics

The next step was to work out how it was all going to be achieved. One of the biggest stumbling blocks was to source grilles suitable for floor use. I wanted them to be able to be trodden on, to be appropriately sized and to, as much as possible, not look weirdly out of place in a lounge room. They’d be inset in a neutral coloured, ceramic tiled floor. (The floor I was now tiling!)

I started by looking at ducted heating vents (they call them ‘registers’) but these were generally smaller than I liked. In fact, the biggest I could find with an acceptable style was only 14 x 6 inches – just half the area of the driver.

I then contacted grille manufacturer Rayson Industries in Melbourne (Australia) and found the company very helpful. It turned out they had some metal floor grilles that they were discontinued items – they were therefore cheaper than usual. These were available in a variety of sizes - I bought two 254 x 356mm grilles in a beige colour. These cost about AUD$70 each. The grilles are powder-coated steel, are very strong and look decent.

Next, how were the boxes to be got under the house? There is just a single entrance area to the crawlspace – a door about 550mm square. It would be possible - but very difficult - to build the enclosures under the house (there’s not even sitting-up room), so it would be best to make them outside and then insert them through the access door. That meant a maximum width/height of about 500mm.

So for a box volume of (say) 300 litres, the dimensions would be about 500 x 500 x 1200mm long. The overall speaker diameter was 390mm, and the required cut-out 365mm – so that worked OK with a face dimension of 500mm.

On paper that all seemed fine - but how would such an enclosure sit under the floor? What was the orientation and spacing of the floor joists? Observation and measurement showed that the spacing was too close (at 400mm) for the enclosure (and driver) to nestle between the joists. The box would therefore have to be positioned up against the underside of the joists, with an extension sealing the enclosure to the floor hole.

Hmm, now how to integrate the port? Typically, the port of a vented enclosure exits the baffle at a place other than where the main speaker is located. However, in my case, all the sound from each enclosure had to come through single 254 x 356mm floor grilles – there could not be a separate grille for the port. The vent would therefore need to work through the same floor opening as the driver.

So how to do that? I decided to have the port enter from the side, feeding air into the extension that connected the speaker to the floor grille.

Further design work resulted in this final configuration: 200 litres volume with two curved, right-angled 100mm diameter ports. Here’s a sneak preview of the top of one of the partially constructed boxes.

Next issue: building and testing the 15 inch underfloor speakers.

Speakers There is an amazing amount of misinformation around about how speakers and their boxes work. But get some straightforward ideas clear in your mind, and it all easily flows from there. How they work A speaker operates by moving its cone back and forth. When the cone pushes forward, it creates a high pressure wave which travels through the air, and when it moves back, it causes a low pressure wave - sometimes called a 'rarefaction'. The ear picks up these air pressure waves and turns them into sounds. The speed with which the cone moves determines the pitch of the sound heard by the ear. If the cone is vibrating slowly - like back and forth say 50 times a second - then a deep note will be heard. Higher pitched sounds are made by the cone vibrating more quickly - 1000 times a second (expressed as 1000 Hertz) is a squeal, 8,000Hz is a screech, and 15,000Hz is higher than many people can hear at all. Since we're interested here in bass, we'll stick to discussing the frequencies below 150Hz. A woofer - which is a large speaker designed to work at low frequencies - will sound lousy if it is not mounted correctly. Foremost is the need to separate the front and rear of the speaker. To see why this is needed, imagine a bare woofer sitting on a bench. The speaker is being driven by a powerful amp, and is running music with a lot of bass. The cone will visibly move forward and backwards with the music, but the bass will be poor. The reason for this is that when the cone moves forward, a 'proper' pressure wave in the air isn't created. Instead, the air simply moves around the edges of the speaker frame, filling up the low pressure areas created behind the cone. When the cone moves back again, the air flows the other way. Instead of pumping bass into the room, all the bare driver is doing is pumping air around its edges. So how to fix this? Use a speaker enclosure – often called simply a box. Different Designs Four major types of speaker boxes are used. The designs are: 1.     Sealed In this type of box the sound waves coming from the back of the cone are completely separated from those coming from the front. That’s good in preventing the air from simply being pumped back and forth around the edge of the driver – but it also means the rear sound waves are effectively wasted. That is, instead of contributing to the sound that moves you, they're dissipated inside the sealed box. However, while this type of box produces less Sound Pressure Level (ie they are less efficient on a watts per dB basis) they are easy to make, have only a gradual bass drop-off as the notes get lower, and can potentially handle more power than a ported enclosure. An infinite baffle design, referred to in the main body of the story, can be thought of as being a sealed enclosure of infinitely large dimensions. 2.     Ported A ported enclosure additionally makes use of the energy coming from the back of the cone. It does this by using a connecting port (or vent) that joins the inside of the box with the outside. The port diameter and length are carefully sized so that the plug of air contained within the vent is excited into back and forth motion, but its movements are delayed just enough that when the cone of the driver is moving forward, so is the plug of air inside the port. In this way, the two air movements complement each other. The advantages are twofold: firstly the efficiency of the system is greater (ie more SPL per watt of amp power), and secondly the bottom-end bass response of the system can be improved over the use of a sealed box. The downsides are that if the port isn't just right for the driver and box, boomy one-note bass can be the result, and even with well-designed ported boxes, at ultra-low frequencies the cone of the woofer becomes unloaded - which can cause it to be destroyed if you're not careful in how you set up the system. And while the bass response holds on to lower notes, once it does start to fall away, it does so more quickly than with a sealed enclosure. 3.     Passive Radiator A passive radiator design is a relatively rare. This type of design uses a passive radiator (a driver without a magnet) to act as a port - the cone and its suspension moves back and forth like the plug of air within a port, but at no times can the main driver become completely unloaded as is the case in a ported enclosure. The disadvantage is that passive radiators are rare - and finding the detailed specs on them which are needed to do good designs even rarer! 4.     Bandpass A bandpass design is a very tricky thing. Rather than producing frequencies from as high as the woofer can go - and then trailing them off at the other end as is determined by the enclosure design - a bandpass lets just a narrow spread of frequencies be emitted from the box. Because it is producing frequencies from only (say) 30 - 90Hz, it can be more efficient that the other box designs - it's concentrating all of its energies just in this narrow field of frequencies. All the sound comes from the ports, with the driver itself buried inside. There is a variety of bandpass designs - some mount the driver on the division between two boxes and vent just one volume, while others vent both boxes. Still others connect one box to the other by means of a vent and then have a further vent in the first box. Bandpass boxes need very tricky design indeed, and if you're not careful either the frequency spread (the range of bass notes produced by the sub) or the efficiency (how loud it is for a given input power) can drop right down. In addition, the bandpass boxes are much more complex to make than other designs - there tends to be lots of pieces inside that need to be airtight, and fitting the ports inside the box can be a drama. In the case of the underfloor speakers, the bandpass designs that were modelled gave no louder outputs but had the narrower frequency range. For the JBL 15 incher, the big ported enclosure was best.

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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