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Lespaul1

1x8" simple cab: how low can it go?

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Posted (edited)

Hi everyone,

I recently replaced the speaker in my Fender Rumble 15W (sealed) with a LaVoce NBASS8. I now would like to take that speaker and put it in a small DIY vented cab. I created a project in WinISD and I'm satisfied with the results using a 20litres box (0.7ft3) and 52hz tuning frequency. This gives me an F3 of 50hz but the frequency range of the speaker is rated 55hz-3000hz. Does that mean that I'm not actually going to get the F3 that WinISD is showing? Thanks!

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Edited by Lespaul1

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Assuming you have put the data into Winisd correctly then what you end up with should be close to the predicted response. 

When manufacturers rate their speakers frequency responses it is only really ever a rough guide, the bottom end is affected by the cab they do the measurements in, often if you dig into the notes they will tell you how it is measured, They may even have just calculated a figure or used F3 as the lowest frequency. In your cab it will be different. At the top end it becomes subjective, The cone will break up at higher frequencies and the response becomes uneven, sometimes very uneven. Sometimes they give the -3dB point sometimes -10db, sometimes something else. This speaker will make a sound at 4khz and much higher too.

It's easy enough to make an 8" speaker with decent output at low frequencies. A heavier cone and soft suspension lowers the resonant frequency, what you lose is efficiency. Ultimately you can't have loud, deep and small. If you look at 8" PA cabs you'll find they come out at around 113db maximum output which is what winisd is telling you this will do. There's no reason this won't sound good but it's going to do so at practice levels.

Build your cab and enjoy :)

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10 hours ago, Bill Fitzmaurice said:

According to what?

Manufacturer specs

 

4 hours ago, Phil Starr said:

It's easy enough to make an 8" speaker with decent output at low frequencies.

Thanks Phil, useful info. that was the general feeling, it's easy to get this 8" driver to go low, lower than most 12s actually, but not as loud. Which makes it perfect for a bedroom / practice cab.

General question, am I correct assuming that I shouldn't be concerned about the excursion at 40hz? As I am at -9db there the driver won't probably cover it. But low E is at 41hz so should I look at the excursion at that point to define the max displacement?

Also, what's the general opinion about port placement at the back? From what I learned in subwoofer building is that the port at the back should be ok, as long as it's not in the middle (you can't see the speaker magnet, roughly).

Is it worth placing the speaker a bit offset in the front panel or make the cab a bit taller to avoid vibrations due to frequencies bouncing around? I'm thinking of an enclosure of 31cm (W) x 31cm (H) x 28cm (D) so front panel is a square.

 

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Off centre is better but of course there isn't much chance of that with a 20cm speaker in a 30cm cab. You could make the cab 20x25x40 or some such variation and putting the port directly in line won't be an option as it will probably long enough to touch the speaker. Port placement at the back isn't a problem.

For practice I wouldn't worry too much about excursion. You are likely to be at modest levels and because of the placing of the pickups on your bass there isn't a lot of fundamental there anyway.

I think you've misunderstood the computer model, the cone will be moving as in the excursion graph, it just won't be making much sound and will be 9dB down. That's a characteristic of all ported cabs. Below tuning they effectively just become an open box with nothing to stop the cone moving other than it's own suspension. Your amp may be limiting things at that frequency though and it's only sometimes a problem. In your own room rather than a gig you'll hear if the speaker is stressed. All this is normal

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50 minutes ago, Lespaul1 said:

Manufacturer specs

OK, I see that on the catalog page. It's wrong. The low frequency limit is determined by the combination of the T/S specs and the cabinet alignment, so you can't assign an arbitrary number to the low frequency response. Your model is accurate.

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Posted (edited)

It's been said before and no doubt it will be again - when you hit a low E, the main part of the sound that is both produced & what you hear is the Second Harmonic, i.e 82Hz. The fundamental down at 41Hz is small fraction of the total output. In your graphs that is -8dB down. If you have to relate that to any one factor, then, when running winISD & noting your input wattage, look at the cone excursion graph to where 41 crosses the line. In your third graph xmax is exceeded at approx 44Hz. It's not a problem; there's not a lot of sound energy down there and that sort of figure for bass speakers is very common..

The area in blue just exceed the "usefull for bass" overall frequency response on the Lavoce web-site. If the manufacturer says the frequency range is 55 - 3K  then even by their published graph in this case it ranges from about 80dB @ 50Hz to to 100dB @ 3K; so it's + or - 10!
See the penultimate sentence in Phil's second paragraph ( approx 10am post).


The elevated area of frequency response (green) is typical of many loudspeakers. It might help to "cut through"  in a mix of instruments but the cone is ceasing to behave like a stiff cone and is starting to flex & bend so the sound produced will have more distortion & roughness.

As for ports, placement and length become a problem usually the smaller the volume of the cabinet. If you have a rear port then if possble, keep the speaker cab a small distance away from the wall at the back; perhaps 4 - 6 inches.

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Edited by Balcro
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The chart on the LaVoce site is with the driver mounted in a wall. That's how raw driver response is measured. It doesn't reflect response when in an enclosure, which totally changes low frequency response. It should only be used to compare the half-space response of various drivers above 200Hz.

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Posted (edited)

Thanks everyone, there is some beefy information here. So the manufacturer response is measured in free air and doesn't imply that the driver can't go below that range.

The little guy is coming together, It's made of 12mm softwood ply, as one of the requirements of this project was to have a lightweight cab. It looks like a good size for my TC electronic amp.

IMG_20200404_122953.jpg

Will have to cut a second hole for the port, round the corners and proceed with the finish.

Re the finish, is there any particular treatment you would recommend to make the softwood more solid? I'm going to apply Duratex on the outside, but would it be worth applying on the inside a coat of 70/30 water and wood glue?

- Tuning: to get a 53hz tuning I would need a vent approx 67x140mm. I wanted to use a plastic pipe, but I found the Monacor BR-70 which is basically spot on what I'm looking for, 67x 139.5mm. Only problem is for some reason the cross area in cm2 is higher. Is it because of the flared end? Does that change my tuning frequency compared to a straight tube?

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Edited by Lespaul1
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You can get a liquid wood hardener or stabilizer. Its a watery consistency and just soaks in. Works a treat. Although I can't remember what make it is.

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I don't think there is a lot of point in coating the inside, the only reason to protect the wood is that being soft it may dent but the paint finish is fairly flexible. The Duratex should offer fair protection to the outside

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14 hours ago, Chopthebass said:

You can get a liquid wood hardener or stabilizer. Its a watery consistency and just soaks in. Works a treat. Although I can't remember what make it is.

Ronseal do one

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I'm working with a 12mm ply and it feels less sturdy / less solid than MDF. The main reason to coat the inside of the box is to provide some extra solidity to the wood, as it seems that the stiffer the surface the better it is to avoid resonance. 

I changed my mind since, and decided to try without any finish on the inside and will definitely keep the product on the outside to a minimum, as I am not really too sure the TuffCab will adhere to wood fillers or wood glue

"MDF and plywood will readily accept Tuff Cab. Some resin, epoxy, plastic and solvent-based wood fillers will impair adhesion over the filled area."

Has anyone ever applied DuraTex/TuffCab type of coating over Ronseal wood filler or wood glue? Any issues with adhesion? 

Update: last Saturday I got the VonHaus router (yay!) and rounded all the edges.  The neighbours probably hate me but the little cab starts to look nice... 

IMG_20200407_193434.thumb.jpg.26350ba90dd52267fc1cc21dbc799284.jpg

Currently glueing an extra piece of 6mm MDF the front baffle. This will make the front baffle 18mm thick.

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The type of ply you have chosen isn't the best choice for a cab, It's really made to be a cheap material for shuttering on a building site for forming poured concrete. You won't get significant stiffening by treating the surface, so why do it? I'd imagine the Tuffcab would take to the PVA with no problems but try it on a piece of scrap first. The Tuffcab would be better just applied straight to the wood though.

If you want to stiffen the panels then you'd be better of bracing the cab, the simplest way of doing that is to run pieces of timber across the cab gluing them in place so that the opposite faces are joined somewhere near the middle of the panels, slightly off centre is best and make sure the braces don't get in the way of the speaker or reflex port. 

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Posted (edited)

Thanks Phil. When you say it's not the best for a cab, does it mean it's very bad and that I'd be better off starting over again with MDF? 

I'm adding an extra panel of 6mm to the front and the back, can that make things a bit better?

Edited by Lespaul1

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I've made up all sorts of cabs with off-cuts, they all work and are a great basis for tweaking later on. This is an 8" cab too so the forces on it won't be excessive. At this stage I'd stick with what you have and see what it sounds like. 

If you turn up the cab really loud you can feel the panels vibrating with your fingers. Find the worst spots and put a length of dowel across the cab to damp the vibrations at that spot, by the time you have a couple of dowels in place you'll be pleasantly surprised at the damping you've achieved. You can go on doing this forever and there are other panel damping techniques too. The more wood you add the better it should get but you are adding weight all the time so there's a compromise.

For the record MDF is a great material acoustically but heavy and doesn't cope with damp, chipboard (particle board in the States) is pretty good, medium weight, but has to be the right grade and is difficult to finish well and isn't very tough. Hardwood plywood is pretty tough and lightweight so great for portable cabs. Softwood ply is, well soft, so you don't get the toughness you want. 

Your cab is looking fine to me so get it finished and see what it sounds like, we can talk you through damping when you get to having a working cab :)

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On 04/04/2020 at 12:59, Lespaul1 said:

 Tuning: to get a 53hz tuning I would need a vent approx 67x140mm. I wanted to use a plastic pipe, but I found the Monacor BR-70 which is basically spot on what I'm looking for, 67x 139.5mm. Only problem is for some reason the cross area in cm2 is higher. Is it because of the flared end? Does that change my tuning frequency compared to a straight tube?

port.png.cc52460f284974eaceb800241ab469bf.png

 

image.thumb.png.70984b97efa9226ce4c56f5ea1f0c5f2.png

 

 

The use of that trumpet shaped tube (Monacor) won't make a significant difference to the tuning. For tuning and air velocity purposes, I believe you only count the straight horizontal length of the tube, not the extra length added by the flared ends. LIkewise, work with smaller 67mm diameter at the inner end.

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