Coffee table with speakers

Well Ben, you half remembered right!

You see, when two things love each other…

Yeah, so this thing I did back then was something I did because I just wanted to put speakers everywhere, and I did learn a lot about this when building it, but it’s been a very long time. So let me tell you what I remember.

What I built, was a solid wood coffee table, but I used some pretty decent A/B grade oak plywood (nice door stock basically). This is important, because when you do this project density of the material, and it’s rigidity are going to be key to success. Anyway, I basically built 2 cubes and ported them (to the outside of the coffee table back then, which was a mistake as my kid always put toys inside it, and they were a bitch to get out. Then, I built a table top, routed the edges and all that good stuff and put it on top of the cubes. However, with these two cubes firing “at” each other, the sound would have sucked balls… so I put a divider “free-floating” between the two cubes and terminating at the floor. Basically, it looks like this:

Anyway, with the top on it, it was a bit unconventional in terms of coffee table design, but I stained the whole thing special walnut and it started to look like furniture. Most people didn’t notice the speakers until they were on anyway.

So, here are a few things that aren’t apparent from my quick little drawings. One important thing is that the divider is “sunken” into the table top using a trench join, or dado joint. A few pocket holes and a lot of glue deadened this whole joint. Inside the boxes, I used fiberglass resin to coat the entire inside of the boxes, to both strengthen the joints and to further deaden the enclosures. I stapled fiberglass batting (like insulation) inside the flat surfaces of the box internals to both dampen sound reflection and help with the mass behind the driver. The ports were tuned for the resonance of the sub and a range that would also cover the music I wanted to listen to.

The glue and the fiberglass resin were super important in deadening the table. I could have an empty soda can, and a lot of loose change on top of the table and nothing rattled or moved. It was a really awesome project. I’d say the hard parts were knowing how much and where to deaden and strengthen, as I didn’t know too much about wood working back then and had no idea what characteristics this type of wood would have with regards to sound (since I’d worked only with MDF for sound prior to this project). Tuning the enclosure (size) and the port to give me the most sound and the least vibration was important as well.

If I had it to do over again, I’d actually build these inside of isolated boxes in the table. Basically, a tuned enclosure with dynamat all over it, and a box built around that with the table attached. Better isolation, even less noise.

Wait wait… you are going to port the “back” chamber to the “front” chamber, then port the front chamber to the outside? This would be called a ‘series-tuned’ enclosure as well, and it is a type of 6th order, but it may not do what you think it’s going to do. A bandpass enclosure simply means that you tune one or more of the cavities to let only desired frequencies escape. Some orders of bandpass involve the speaker being ‘inside’ an enclosure, but not all. While all examples of 6th I’ve seen do involve this, the different versions are done to very different effect.

I hope I’m not talking down to you here, but seeing what you have in mind, I feel like there are a few things I need to share with you. Let’s start with some basic definitions.

1st Order: Free-air, often called “infinite baffle”. Basically, you have a single divider between the front and back of the speaker, with the back not being sealed. Typically, this is for largish speakers that have a near flat response.

2nd Order: Sealed box. I think we all know what this is. This is the good all around performer, and you can usually get a lot out of both flat and speakers designed to peak in desired areas. The larger the driver, the more likely it is that it will perform better in a sealed enclosure… well generally anyway.

3rd Order: These are typically isobaric setups. This would be a sealed enclosure, with a driver mounted ‘normally’ in it, and another mounted facing toward it and wired backward. Or, it could be two subs wired together, one behind another each firing the same direction from a sealed enclosure. This is favorable for designs that wish to move more volume (air volume that is) without necessarily adding other parameters (like overall space). The effect is nowhere near 2x, but the incremental improvement can be desirable.

4th Order: This is a vented, ported box, or anything with a passive device. Any sealed box you see that has a port… 4th order. But also any home speaker or something you see that has what looks like an extra driver, but it doesn’t have magnet… also forth order. If you see an isobaric setup that’s vented… yep, still 4th order. These are

Here’s where people get confused quickly. If you put a box in a sealed enclosure, then seal the ‘front’ of the driver in a second enclosure and vent that, people like to call that a different order. But it’s not. That’s called a “single reflex” enclosure, and the order is defined by other characteristics. The example of two sealed boxes, with a driver in the shared wall, and the box in the ‘front’ of the box being vented… that’s still a 4th order design. It’s just called “4th order, single reflect bandpass”. And there are tons of variations of this too, all of which are still just 4th order. So you have 4th order with the driver inside and outside of a box.

This is also favorable for lots of types of speakers, but typically it’s good for when the needs of a driver in terms of box volume, either aren’t available and you need to “tune” the available space to enhance the frequencies that aren’t as well supported, or you have a driver that doesn’t perform as well in a frequency range that is desired, and you want to tune for that frequency to highlight it. These are generalities, but fundamentally these are the primary reasons for doing this. I’d say these are the most common types of enclosure for getting more sound out of drivers, and they are probably also the ones that are built wrong most often (designed for the space, not the driver).

5th Order: This is one of those weird ones too… because most of the examples above in the 4th order thing get called 5th, 6th and 7th order by mistake. 5th order is achieved by combining the effects of 1st and 4th order, or 2nd and 3rd. That’s all.

6th Order: One thing you were right about here, is that they are typically inside. However, the best way to think about 6th order is “dual reflex”. This means that two cavities are tuned for different characteristics. There is dual reflex bandpass, where there are two cavities, one on each side of the driver and both are vented to the “outside”. There are series tuned versions (which is what I think you are suggesting you want to do here) and that is venting the ‘back’ cavity to the front, and the front to the atmosphere. You have to be careful about how to tune these too as you are trying to bring out a particular frequency, usually a deficiency in the driver or a small box, and make it perform better. Then you are further tuning that to a resonance frequency that gives you the desired performance frequency range. It’s basically taking a ported box, putting the front into a box, so that you only hear what you want. I’ll explain why I would caution you about doing this in a moment.

There are other versions of 6th order where you have 2 drivers and to an isobaric setup like the above examples, or ones that you tune the back and have them fire into a common cavity and tune that. You can do it parallel, series or whatever. They are all still 6th order.

7th Order: Like 5th, this involves combining the above methods to create a more complex effect on the resonant air. I’ve heard many 6th order designs called 7th order (I often do this), but really it’s not 7th order until you limit the frequencies to the driver using a 1st order crossover.

8th Order: This should be defined like 7 and 5 as they sort of just combine techniques, but you do this in specific ways. Basically, 8th is a lot like 6th. You can vent a back cavity to atmosphere/outside, vent to the front chamber, then vent the front chamber to the outside also. –or you can vent the front and back to a common chamber and tune that new chamber with a vent.

The purpose here is that you’d tune two vents on the “back” differently, then tune the “front” to enhance the wide tune of the vent between the front and the back. Finally, you’d tune the resulting vent to only let the peak frequencies out.

Anyway, you have the internet too… and maybe you knew most or all of that stuff already, but here is why I bring it up. That 18 is a bad driver to do this with.

You’ve heard the term ‘slow and low’ applied to these, right? Well, it’s because they have slow motor/cone response because of the mass that is being moved, and the amount of power they require to do it (even in the days of the almighty Stroker 18″). They also reproduce low frequencies extremely well, and usually have a ‘bump’ at the low end when compared to most other drivers. So, what would you tune for in a vented enclosure?

Well, if the resonance frequency (Fs) of an 18″ is something like 32Hz you have no need to tune low. In fact, the driver is already going to play that bone rattling 40hz stuff really really well in a box. But let’s say for argument sake you really want to put it in a box that will make 40hz go insane. So, you design a box for that resonant frequency, and tune to 40 hz. It may be worth mentioning at this point, that the box is already the size of the finished coffee table given typical Thiele Small params. So, you could massively shrink the box, raising the resonance frequency of the box way way up… now you have a reason to tune low (like the 40 example). But, to do the series tune you talked about, you are going to need to tune for a wider frequency range and there is a lot of maths to do there. But, for argument sake, you’d probably end up with a wide shallow port tuned at 42Hz or some crap, and it’s all firing into another box. Now, to pull that frequency out as a resonant frequency… we are back to needing a big box. But, we can shrink it, the resonant freq goes up, and we tune lower. This will all work but there are a few things to note.

1. you are WAY outside the efficiencies of your speaker here, unless you found some crazy ass 18 that plays well in 1.2-1.5 cubic feet of space. You are going to have it play frequencies that it’s “muddy” at (120hz for example) and they are going to be very “boomy” compared to the driver’s natural frequency curve.
2. This thing will NEED a shit ton of power to do it. Not “can handle” but “will need”.
3. When you tune this much air in a small space you are going to get a ton of air ‘whiffing’… in other words you are going to hear air as much as sound.
4. If you are sure that you are right and my advice is based on old driver knowledge, make sure you add a low-pass filter to the whole setup, set it at like 80db with a 12db roll off and then adjust your terminology to 7th order dual-reflex bandpass.
5. The dampening material will only help you bring the resonant frequency of either cavity down a little tiny bit by adding mass. It’s not going to make a 1.2 cubic foot box sound like a 5 cubic foot box.

Me, I’d do it all with a monster model of a 10″ dual voice coil sub. Your kick drums will be awesome, car doors will sound sharply thunky and people won’t all sound like you are listening to them talk with your ear on their chest. That 18″ I’m afraid is going to result in a lot of heartache over something that seems like it should work better.

On the other hand, you could just mount the 18″ in a big ass box behind some furniture, set an aggressive cross-over on it, pump it full of tons of real power, and watch people jump off your couch when the bass slaps them in the chest. 🙂

One last thing… trying to get a 12volt amp to run in a 110 environment will be HARD to do. You can, but those setups are super expensive, and still usually have issues running for any length of time. Just ask your local car audio store. Most of those folks actually run their big crazy stuff on dry cell 12v batteries and a complex charging system to do it. Or at least that’s what we used to do back in the day. The converter you’d have to design to really do a good job of these, and survive the heavy pulls on the system during hard bass would be terrible. Because basically you are trying to convert voltage and current multiple times to arrive at the needs of the speaker. You’d be better off trying to find some Crown 1000 watt from the 80s or 90s in great condition and just hooking it up bridged mono to it.

Just as a point of interest, I built a 6th order dual reflex bandpass enclosure where I vented the front and the back to the outside, and the back to the front on the inside. I did this based on the recommendation of a little Florida speaker company called Oz audio for one of their 8″ subs. That damn thing was punchy, fairly low and not very power hungry. Most people who knew anything were convinced I had pair of 10s in there. Many thought I had a pair of 12s in there. The one thing I remember though, is how much work I had to do to get the math and my expectations to line up on where the front and rear really needed to be tuned for my listening habits. It was a ton of work, and not everything sounded clean on it. But it was damn powerful.

The 18 is a monster driver… I like them if you can get them loud and drop a ton of power on them. But they really work best in correct sealed enclosures.

I remember one time (I used to help a guy doing high-end home theater installs on the side while we worked in car audio) we mounted a sealed 18″ in the floor of a second floor media room. Holy crap, that was some insane loud.