Dedmans 1x12 speaker cab

[wazkelly]

Sounds good Deddy and no doubt you will add your retro touches to it.

[Dedman]

cabinet done, ready for painting when I do the ES-2

[wokkaboy]

cabinet looks great Dedman, presume its pine frame and is the speaker mount plywood ?

how did you join the pine frame, nails and glue ?

is all the electronic side of the amp pre-built ?

[Dedman]

yup pine and ply, glued and screwed. Any amp can plug into it, it's a 20w Celestion Greenback in it. The speaker was in the first cab I built but I put a Jensen in there. Buggered if I know where I'm gonna put this LOL

[wokkaboy]

ah ok so this will run off an another amp ? Worry about where it will go after it's built, that's blokes sense isn't it ? haha

[DrNomis_44]

That 1 X 12 Cab is looking good Dedman, should sound pretty cool with that 20 Watt Celestion Greenback in it.

[wazkelly]

Looking good buddy and from those shots it appears more compact in size compared to your other speaker cabinet?

[Dedman]

yup much smaller, running out of space :P

[Simon Barden]

Fairly tall and upright looking and based on the size of the cut speaker hole the dimensions look to be about 600mm x 600mm x 300mm? Are you going to put any sound insulation stuff inside?

I did this sort of thing with making stereo speakers when I was a teenager and the sound insulation stuff was just like a thin fibreglass insulation batt. It made a little bit of difference but I was itchy for days after installing the stuff and reckon high density foam would be the better way to go nowadays. There is probably some science on it to be found on the internet but it will need something to stop the timber vibrations resonating out of control.

The 'sound insulation' stuff inside Hi-Fi speakers and a lot of PA speakers (and the occasional guitar speaker, though this is pretty rare) is there for absorbing some of the upper mids and high frequency sounds within the enclosure. This is an attempt to stop standing waves building up inside the enclosure at the cabinets' resonant frequencies, (as determined by the size of the enclosure) and colouring the sound produced by the speakers.

A small Hi-fi speaker may have internal dimensions say of 28cm (h) x 18cm(w) x 20cm(d). The fundamental frequency of a standing wave is determined by the distance between two parallel surfaces; For any standing wave, the fundamental wave and all its related harmonic frequencies have zero amplitude where the wave touches at the sides. Points of zero amplitude on a wave are called nodes.



This is exactly the same principle as fretting a guitar string, where the string can't move at the saddle or where it's fretted, but the rest of the string is free to vibrate.

And standing waves are like feedback on an electric guitar. If the volume/gain is enough, the sound will vibrate the strings, but frequencies close to, or the same as, the fundamental or harmonics of un-damped strings will cause those strings to vibrate in sympathy at those frequencies, resulting in a build up of sound that may or may not be wanted.

The worst type of enclosure for standing waves is a square box, with all three sets of enclosure walls parallel and the same distance apart, as all three sets of walls have the same resonant frequencies. With parallel sided enclosure construction, you really want all the distances between sides to be different and also not multiples of one another e.g. a 80cm x 40cm x 20cm enclosure would still have lots of resonance issues, despite the distances not being the same.

Ideally, all three sets of the enclosure walls would be non-parallel, but practically this is hard to achieve and be both aesthetically pleasing and sturdy. But it is why the majority of portable PA speakers these days have non-parallel sides and tops/bottoms, with only the front and back being parallel (although internally the faces are rarely smooth which all helps).

So if a parallel-sided speaker is reproducing a wide range of frequencies (i.e. playing back music), it's exciting the air inside the speaker enclosure and it will cause standing waves within the enclosure. These will reinforce the sound produced by the speaker at those particular frequencies, and so 'colour' the sound reproduction.

Because most h-fi speaker enclosures are relatively small, the wavelengths are small and the frequencies quite high. We can convert wavelength into frequency by dividing the speed of sound by the wavelength. The speed of sound varies with air temperature and pressure, but it's around 343m/s at 20°C at sea level.

So our example hi-fi speaker has it's longest wavelength at 0.28m, and 343/0.28 = 1.225kHz. That's a pretty high pitch to start with, and harmonics will be whole-number multiples of those wavelengths i.e. 2x = 2.450kHz and 3x = 3.785kHz. 1.225kHz is around a D#6 on a piano.

It takes a lot of mass to absorb or damp out low frequencies, but as the frequencies go up, it takes less and less mass to do so. That's why carpets, curtains and fabric-covered furniture are so effective at reducing the amount of higher frequency reflected sound in a room. It also helps that fabrics and carpets have a fairly open texture. Closed texture surfaces reflect sound. When it's open texture, the sound waves can pass through and bounce around, loosing energy with each internal reflection (turning it into heat).

So to absorb sound, you need an open textured material or open-cell foam, with lots of internal surface area. Because speakers can get hot, especially if pushed too hard, you also want it fireproof. This is why glass-fibre strands are so often used. Glass fibre (with no resin) is a very effective sound absorber, though for use within a room it needs to be enclosed within open-weave fabrics to stop the fibres getting out.

Because the amplitude of standing waves is zero at the very sides of the enclosure, Ideally you mount your absorbing material away from the sides so that a) the amplitude of the sound wave is greater, and so it can absorb more energy and b) the wave passes through the material (loosing energy as it does so), hits the side and reflects back through the material again, loosing even more energy. Practically, it's easier just to staple it to the sides and hope for the best.

It really isn't there to damp the enclosure wall vibrations, though by absorbing some of the sound energy that hits the walls, it does have some effect. To stop the enclosure walls resonating you really either need a) a lot of mass b) a lot of cross-bracing c) curved sides that have a structure that isn't prone to vibrating or d) a combination of two or more of a) to c).

A couple of of DIY hi-fi speaker chaps I know use very thick marine ply, along with cross bracing and line the insides with lead (high mass) to reduce cabinet vibrations .

Some of the studio monitor manufacturers (e.g. Genelec) now use moulded high-density resin-based materials to create internal enclosures with no parallel surfaces but use rounded sides with a constantly varying radius.

But when it comes to guitar cabs, really anything goes. It's not there to reproduce sound, it's there to create it. The floating baffle (loads of resonance) on the earlier Fender amps was as much a part of the overall sound as the amp circuitry. According to an old bass-playing friend (who also had a company sub-contracting in the amp build for H&H and some other long-gone UK manufacturers in the '70s), the older Trace Elliot bass cabs relied much on cabinet resonance for their sound. With guitar cabs, if it sounds good, it is good.

[wazkelly]

Wow, lots of useful information and explains why modular PA bins migrated to trapezoid shapes rather than rectangular boxes. Agree with comments about Bass Cabs as they need to be made from much thicker and stronger material and lots come with ports and possibly tweeters as is the case on my 75w Hartke combo.

For guitar my personal preference is fully enclosed as it seems to produce a fuller sound but that can also tend to make them a bit directional too. Open back helps if you want a bit more headroom before things get too loud out front. Suppose I was spoilt playing through a slant front 4 x 12" quad box for all my gigging years and find it hard to reproduce that sound in something smaller.