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Double check: Bass Reflex hole for 10L 80-3000hz top -> just 1,2cm length? Position?
Double check: Bass Reflex hole for 10L 80-3000hz top -> just 1,2cm length? Position?
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Old 15th April 2019, 12:07 PM   #1
HimG is offline HimG
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Default Double check: Bass Reflex hole for 10L 80-3000hz top -> just 1,2cm length? Position?

Greetings DIY Audio Freaks,

I'm building a 2 way top with the prv 250W 4 ohm unit and got a little bit unsure about my calculation results and wanted to double check them with you before i create some sawdust. I used WiniSD to find the Box size and tuning frequency suiting for my project and double checked it with BBox. A ended with a size of 10L and a tuning to 87hz (i'm using tapped horns below). Now i'm unsure about the bassreflex opening. I surfed the web to learn about the difference it makes if you put the port as a slice on the bottom or in corners and informed myself about the correction value k (eg. normal 0,7, for ports in corners 1,7 etc). That are my settings in bbox: Imgur: The magic of the Internet

The goal:
I want to make the box as small as possible in terms of high and width because i'm gone run & store it on a bicycle most of the time.
The bassreflex port has to be at the front. In order to make it as small as possible i thought about making a slice shape port of 1,5cm x 17cm at the bottom or a square 4cm x 6,5cm at the upper corner next to the tweeter. The last option would make the box 23,4cm tall, the option with the bottom slice would make it 25cm tall. So if the position would not matter, i would go with the port next to the tweeter.

Things i could not find out:

- BBox is suggesting me 19cm^2 as bassreflex opening. Because both options have the port near two walls, i'm calculating with a correction value k of 1,73. If i use a port with 19cm i'm comming out with a reflex lengh of 0cm or even -0,4cm. So i made the opening bigger to get at least the matrial thickness of 1,2mm. I read in the web that a port size of 1/3 or 1/4 of SD (154cm^2) would be ideal. Why BBox isn't suggesting me this? Or are this values just for subwoofers?

- Would it be ok to just put a big 4x6,5cm hole directly next to the speaker? I'm concerned that if it's to big the air can just free flow, thus giving me no real output as it has no real port length...?

- Does the position of the opening matters? I read it matters for top units, but i could not find out how. What would be the better one, a small slice at a centered location at to bottom or a uncentered port but with better dimension ratios in one corner?

At the moment i'm at the point of building both options and hear wich one is better or probably keep both if it's not a big difference. Just the port lengh of 1,2cm makes me aks myself if i should better chose a bigger opening to get at least a small port of 3cm or something. Or does it really not metter cause the tuning is so high (87hz)?

Hope someone can give me a few hints, thx!
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Old 15th April 2019, 11:44 PM   #2
Ron E is offline Ron E  United States
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I would use ~50cm^2 as an opening ~7.62cm dia x 12.5cm To get chuff free would require 70cm^2. I would not count on 1.73 as an end correction. Plan on a sharp HP filter at ~90Hz or so.

You might also consider smaller sealed boxes and running your horns higher if they can reach ~140Hz or so.
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Old 16th April 2019, 02:30 AM   #3
ICG is offline ICG  Germany
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Is this your driver?
The small VC does not take 250W rms, the listed 125W are much closer to the truth.

Well, TBH, that makes no sense. If you want to go as low as 80Hz, you'll lose a lot of spl, you'll likely get around ~84dB there, the driver gets the high spl only higher up. That means, you'd probably much better off with a different driver or ditch the 80Hz and cross it over higher, 120Hz should be fine and it can play a lot louder then. The 94dB will only be reached above 400 or 500Hz, that's the same with all 6" drivers.

A Qts of ~0,6 is not very well suited for a ported enclosure. It can be done but it's critical. If you want it ported anyway, the enclosure is too small. It hunches about 3dB in the upper bass and if you play at high spl the power compression (heat of the VC) will increase the already high Qt to ~1, maybe even 1,2 ,you'll get a much bigger hunch and will result in a bloated, muddy sound, you can't get that out with an EQ. To fix that, you'd have to increase the volume to ~16l. Did you add the serial resistance of the crossover coils in your simulation? If not, that would increase the Qt further.
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Old 16th April 2019, 06:27 AM   #4
mbrennwa is offline mbrennwa  Switzerland
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I like the idea of doing a speaker for a bicycle, but I don't quite know what your intended application will be. Anyway, since most bicycles live outdoors, I am assuming that's where you'll use it. Producing useful bass in such an open-space environment with a small speaker will be very challenging. Think again: do you really want to make a bass reflex system that will struggle to produce any bass? You might be better off to just forget about bass and make a small closed box focusing on lower midrange and up.
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Old 16th April 2019, 11:00 AM   #5
Ron E is offline Ron E  United States
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Here's a sim for you.

Realistic max input is ~50W RMS/8ohms to keep excursion below xmax above tuning. You can see port velocity is still 30m/s with the port I recommended above.
Attached Files
File Type: pdf boxmodel.pdf (123.5 KB, 9 views)
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Old 16th April 2019, 12:34 PM   #6
ICG is offline ICG  Germany
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Quote:
Originally Posted by Ron E View Post
Here's a sim for you.

Realistic max input is ~50W RMS/8ohms to keep excursion below xmax above tuning. You can see port velocity is still 30m/s with the port I recommended above.
Your simulation shows 97dB/120Hz at 2,83V. Even at 3 Ohm (2,6W) that's not possible with a 6" driver, you don't even get that with a 8" driver. Your simulation bases on the 94dB but that's wrong because the 94dB are only reached at ~400 Hz. On 150Hz you usually get ~82-84dB/1W on a 6" driver at best and the level from the port will not be higher than that. That means, your simulation is off by about 10dB but that's not all.

If >> THIS << is actually the planned driver, the 94dB aren't measured at 1W but instead at at 2V and the actual Xmax is (11-5)/2=3mm. And a simulation of the max spl is only valid if you introduce a very steep lowcut to prevent the cone doing huge excursions at even lower power figures because it still gets fed the low frequencies and - depending on the played music - below 80Hz is still a lot going on, otherwise it's even much less.

The piston excursion calculator gives a maximum of 103dB at 120Hz (where the maximum excursion above the port fs is) for a 6" driver at 3mm excursion. The difference to your simulation would be ~12dB. Did your simulation software use quarter space or even 1/8 space for the simulation? On a bike you don't have walls around you for roomgain.

Your simulation also predicts a power dissipation of 137W, wich is already over the 125W rms the 6MB250-NDY-4 data sheet states. Even with the fed (very optimistic) data the Vent sizer calculation of your simulation gives only 112.59105dB as max spl, not 124-125dB. Professional speakers with a 6" or 6,5" drivers don't list higher max spl than 108-110dB either (and that's usually peak).

Anyway, 103dB (or 106dB if you calculate with 4,3mm excursion) is still very loud.
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Last edited by ICG; 16th April 2019 at 12:36 PM.
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Old 16th April 2019, 09:40 PM   #7
mbrennwa is offline mbrennwa  Switzerland
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Quote:
Originally Posted by Ron E View Post
Realistic max input is ~50W...
I don't know how this speaker is going to be powered, but 50W from a bicycle would a A LOT. Typical power output of a cyclist is maybe 100 W. Adding 50 W just to drive a speaker would be very much.

A typical bicycle dynamo outputs about 6 V, and maybe 5 W. Using that to drive a speaker will require a very efficient driver. Given the size constraints of the "bicycle speaker", it will be impossible to get real bass out of that. No need to even think about bass reflex design details.
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Old 16th April 2019, 10:37 PM   #8
HimG is offline HimG
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Greetings to all and thanks for the discussion of my problem!
To clear some things up for everyone: Yes, my speaker is the 6MB250-NDY-4.
I'm using a mini dsp with 24db Butterworth filter for Lowcutting the tops / high cutting the tapped horn.
The Tapped horn is the volvotreter 38hz Tapped horn desing (link).
Each way (tapped horns, the pvr mid bass and Monacor DT-28N i chose as tweeters) get powered by an own TDA7498E wich is giving around 120w into 4 ohm per channel at 36V (half for the 8 ohm tweeter, according to this youtube test)
I power everything over a 6S (22v) 10Ah LiPo wich is feeding a 1800W (half is realistic i bet) step up converter to get the 36V.

So far i was using the 2 way boxes of my ca. 2004 phillips home stereo with 12 ohm 5" speakers and 40W rating for speaker + tweeter. I with the new tops i wanted to be able to get a little bit more max output but primary a tighter kick in the 100-120hz section (no house kick, kicks that are played over hd15 normally) while keeping size and wight at the minimum. I thought i would maybe eq down everything above the 200hz a bit because it would be to loud for the two tapped horn subs anyway and jus play the prv's in the kick section to the max.

Quote:
Originally Posted by Ron E View Post
I would use ~50cm^2 as an opening ~7.62cm dia x 12.5cm To get chuff free would require 70cm^2. I would not count on 1.73 as an end correction. Plan on a sharp HP filter at ~90Hz or so.

You might also consider smaller sealed boxes and running your horns higher if they can reach ~140Hz or so.
Sharp HP is in, but why you would not count an the 1,7 correction value? You suggest it'll be more?
With an oppening of 50cm^2 i would get around 5cm port length in bbox, so that would clear my confusion. ..but why is bbox suggesting 19^2 than ?¿? Programm fault?
Crossing the tapped horns higher is not an option. I crossed them 75hz with 24db but., still giving me response till 120hz because of the high distortion there (see link of them above). But i need something to support the kick in that section to make it tighter. Max kick should be 100-120hz like hd15 that have there max in that section too and don't play as los as well. So a 6 db drop of between 100-80 is ok, also because the tapped horns are still giving some output in this section too if played with 100w each.


Quote:
Originally Posted by ICG View Post
Is this your driver?
The small VC does not take 250W rms, the listed 125W are much closer to the truth.

Well, TBH, that makes no sense. If you want to go as low as 80Hz, you'll lose a lot of spl, you'll likely get around ~84dB there, the driver gets the high spl only higher up. That means, you'd probably much better off with a different driver or ditch the 80Hz and cross it over higher, 120Hz should be fine and it can play a lot louder then. The 94dB will only be reached above 400 or 500Hz, that's the same with all 6" drivers.

A Qts of ~0,6 is not very well suited for a ported enclosure. It can be done but it's critical. If you want it ported anyway, the enclosure is too small. It hunches about 3dB in the upper bass and if you play at high spl the power compression (heat of the VC) will increase the already high Qt to ~1, maybe even 1,2 ,you'll get a much bigger hunch and will result in a bloated, muddy sound, you can't get that out with an EQ. To fix that, you'd have to increase the volume to ~16l. Did you add the serial resistance of the crossover coils in your simulation? If not, that would increase the Qt further.
Yea, that's my dirver, i'm not expectim more than 125w from them anyway, just dropped the 250 because it's the name. I was looking for small Qts 4 ohm drivers and had the fital 6FE200, emmi Apha 6c and monacore sp 6 100 pro in considderation at the end dooing test sims too, but they where all around 2kg so i decided to make compromise on sound and efficency for 3kg less weight for two (i'm gonne tour 3 months with the half stack soon, so weight is important).

You are correct with the 2V / 1W thing, but do you think in terms on how to spl is going down at the lower frequencys the graph under "technical details" on the prv link you provided is correct?
According to the graph of the monacore sp 6 100 pro it has nearly the same spl fall of in this area even it has lower fs, so what you talking about is a 6,5" problem, or do you think drivers like these could get noticable more output in a 10L box than the pvr's?

Quote:
Originally Posted by mbrennwa View Post
I don't know how this speaker is going to be powered, but 50W from a bicycle would a A LOT. Typical power output of a cyclist is maybe 100 W. Adding 50 W just to drive a speaker would be very much.
I'm powering it from a LiPo battery, i just packed everything on a bicycle to hear music with friends while riding through nature. Dub and Steppas most, so i can't compromise on bass (; But i'm happy with my tapped horns, low bass is not the issue here.




Well, back to the problem. I've got the drivers now and gonne build the boxes with them. So would it be worth to extend the front size to get a 50^2cm bass reflex opening? Again they gonne be powerd with an amp that can put 2x100w into 4 ohm. Is the "port has to be 1/3 to 1/4 size of sd" rule more for subs or also important if xmax is only 0,4cm?
And has someone experience with the port position, as we didn't argued about the influence of different port positions for tops like these jet (slice at the bottom or square next to the tweeterat the upper corner)?

Thanks so far for the discussion, really appreciate it!
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Old 16th April 2019, 10:56 PM   #9
ICG is offline ICG  Germany
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He did not say its power is comming from the pedals, he just said he runs it on a bike, it could be a battery too.
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Old 16th April 2019, 11:03 PM   #10
Ron E is offline Ron E  United States
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Quote:
Originally Posted by ICG View Post
Your simulation...
I input parameters as given by the original poster. They are reasonably consistent. I don't have time to research a driver that someone did not identify.

Some things:
Quote:
Originally Posted by ICG View Post
Even at 3 Ohm (2,6W) that's not possible with a 6" driver, you don't even get that with a 8" driver.
Efficiency is not tied to driver size alone..and strongly tied to Fs.
eta=9.6e-10*fs^3*Vas/Qes =9.6e-10*109.4^3*4.8/0.62=0.009745 SPL(2.83)=112.1+10*log10(0.009745)+10*log10(8/3)=96.24

Quote:
Originally Posted by ICG View Post
On 150Hz you usually get ~82-84dB/1W on a 6" driver at best and the level from the port will not be higher than that.
See above.

Quote:
Originally Posted by ICG View Post
a simulation of the max spl is only valid if you introduce a very steep lowcut to prevent the cone doing huge excursions
wrong - the sim calculates excursion limited SPL. The SPL produced by the max input power chart. I mentioned using a highpass in my first post as I recall

Quote:
Originally Posted by ICG View Post
[something about] roomgain
The sim is halfspace, as most people expect.

Quote:
Originally Posted by ICG View Post
[something about] 124-125 dB
My (linear ) sim predicts what is in the vent sizer, roughly 113dB In free air, there will be spreading loss of ~6dB

To get the right numbers in baudlines calculator you need to enter 14/2.54/0.83 as driver diameter.
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