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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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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.

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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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