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Dont want mr. Helmholtz in my sealed sub !
Dont want mr. Helmholtz in my sealed sub !
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Old 13th October 2005, 10:33 PM   #1
Coolin is offline Coolin  Netherlands
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Default Dont want mr. Helmholtz in my sealed sub !

Guys ive got a problem.

Ive got mr Helmholtz resonance bothering me in my smallish sub. And its a SEALED sub, no ports !!
Its around 90 Hz and its quite a sharp peak.
Using a parametric EQ i can get it out but i want to do it acousticly.
The peak is about 6-10 dB louder than the rest looking at the settings on the EQ. I'm crossing over at 80Hz btw
If you put your mouth in the box without the driver and "hum at 90Hz" you can hear it pretty well defined and loud. There arent any other peaks present as far as i can tell.

Its a fairly smallish 18mm MDF box with about 18 liters volume for a 8" driver. I'm thinking of using a couple of them to fight room interactions. Its the fourth box ive made to try to get the enclosure out of the equasion soundwise by making it largely non square internally. The results are great but this resonance peak is difficult....

Ive tried alot of damping and so far it has only widend the peak.
I could add more but i'm afraid this will make it less "quick"

Its a fairly lightweight paper cone and this is probably the reason for beaing able to hear it so well. If i lift the driver a little off the box its really obvious.

According to Win ISD the Qts should still be around 0.37 (tight sounding) and the box F around 50 Hz. I dont notice an extreme output here.

Havent done any measurements on this box yet.
How come ive never read about this before at DIYaudio ??

Anyone any suggestions ?

Regards,
Collin
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Old 13th October 2005, 10:48 PM   #2
454Casull is offline 454Casull  Canada
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It can't be a Helmholtz resonance unless the diaphragm acts as the port, which I doubt.
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Old 13th October 2005, 10:52 PM   #3
Coolin is offline Coolin  Netherlands
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Its the only way the sound can get out of the box...

Every volume of air has has its own resonance depending on "port"
sizing.

Why cant the driver act as an passive radiator instead of port ?
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Old 14th October 2005, 01:13 AM   #4
Rademakers is offline Rademakers  Netherlands
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Do you have bracing inside? Sometimes bracing is made that way, that it acts as a port (holes in a plate). Couldn't it be a standing wave btw?

Wkr Johan
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Old 14th October 2005, 02:35 AM   #5
Bose(o) is offline Bose(o)  Canada
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Technically it can't be Helmoltz resonance, but it can be resonance! What are the dimensions of the box? Also, how does the driver measure outside of the box and in free-air?

Are you leaking? I know that a "small" hole may have resonance issues most likely in the upper-upper range of a woofer's audible range, but it's an idea. With a Q of 0.37 you definitely have a severe resonance problem, not to sound like a broken record or anything, but WOW!

I can only think of standing waves. You mention that dampening widens the peak, well, this is exactly how dampening works and it is doing it's job--lowering the audibility of resonances.

Unfortunately it looks like you'll be building a new box, sorry man, but I'm certain that has got to be it.
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Old 14th October 2005, 02:56 AM   #6
RHosch is offline RHosch  United States
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Sorry, it's not the type of resonance issue you think it is. "Blowing" across the driver mount hole might give you a 90Hz hemholtz resonance, but when you mount the driver it is impossible for this to occur. Why? You don't have air moving sideways across the mounting hole with a driver mounted. Never. No way.

Not standing wave resonance either. 90Hz in a box small enough to actually move around? No chance. Way too low of a frequency to be the fundamental standing wave for any sane sized sub box.

Could be panel vibration. More likely an impedance resonance hump.
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Old 14th October 2005, 03:02 AM   #7
johninCR is offline johninCR  United States
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4th box? Sounds like a room problem. It's not a standing wave because the wavelength is too long at 3.8M . A 6-10db panel resonance on your box would be pretty obvious. Take it outside and give it a listen. Even money says it goes away. You probably have 2 or maybe all 3 room dimensions that are multiples of about 1.9m . If so, bass trap(s) are the only answer. You can EQ away the peak, but not unmuck the sound of a nasty room mode with that magnitude. Placement can help some or at least move the peak away from your listening position.
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Old 14th October 2005, 08:19 AM   #8
Coolin is offline Coolin  Netherlands
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Quote:
Originally posted by Rademakers
Do you have bracing inside? Sometimes bracing is made that way, that it acts as a port (holes in a plate). Couldn't it be a standing wave btw?

Wkr Johan
There are a few pockets of air but there is damping in there.
Im crossing over at 80 Hz so i doubt there are standing waves. The longest lenth is 40 cm.

Quote:
Originally posted by Bose(o)
Technically it can't be Helmoltz resonance, but it can be resonance! What are the dimensions of the box? Also, how does the driver measure outside of the box and in free-air?

Are you leaking? I know that a "small" hole may have resonance issues most likely in the upper-upper range of a woofer's audible range, but it's an idea. With a Q of 0.37 you definitely have a severe resonance problem, not to sound like a broken record or anything, but WOW!

I can only think of standing waves. You mention that dampening widens the peak, well, this is exactly how dampening works and it is doing it's job--lowering the audibility of resonances.

Unfortunately it looks like you'll be building a new box, sorry man, but I'm certain that has got to be it.
Size = about 400x420x26 externally.
Havent measured driver in free air. However i havent noticed this peak in a square sealed box.
Unfortionately i'm not liking the sound of all that damping !
And its not really working anyway


Quote:
Originally posted by RHosch
Sorry, it's not the type of resonance issue you think it is. "Blowing" across the driver mount hole might give you a 90Hz hemholtz resonance, but when you mount the driver it is impossible for this to occur. Why? You don't have air moving sideways across the mounting hole with a driver mounted. Never. No way.

Not standing wave resonance either. 90Hz in a box small enough to actually move around? No chance. Way too low of a frequency to be the fundamental standing wave for any sane sized sub box.

Could be panel vibration. More likely an impedance resonance hump.
The box is pretty stiff. It does vibrate though but not only in the middle of the panels, more like the whole box from the moving mass of the cone.
Regarding blowing over a bottle, you blow over the top but the air mass still vibrates in and out of the bottle.
I'm not blowing but humming with my mouth IN the hole where the driver should be.
I have just learned that a guitar also resonates with the air mass in the hole of te body at the lower frequencies..
thats a hole of about 100mm diameter and 3mm thick..... Due to end effects the total amount of air is much larger, something like 60-70% of the diameter on both sides of the hole.


Quote:
Originally posted by johninCR
4th box? Sounds like a room problem. It's not a standing wave because the wavelength is too long at 3.8M . A 6-10db panel resonance on your box would be pretty obvious. Take it outside and give it a listen. Even money says it goes away. You probably have 2 or maybe all 3 room dimensions that are multiples of about 1.9m . If so, bass trap(s) are the only answer. You can EQ away the peak, but not unmuck the sound of a nasty room mode with that magnitude. Placement can help some or at least move the peak away from your listening position.
Yeah 3.8 m wont fit in the box.
My room is fairly large and irregular and pretty resonance free. Only a few real low bass peaks nothing at 90Hz though.
I have tried a few different spots and its still there.



I must confess though there is an intentional volume of air in front of the woofer of maybe 9 liters volume.
This volume is tapered outwards and might be the cause of it all, im not sure at this point.


I was hoping there would be another explanation...

C
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Old 14th October 2005, 08:41 AM   #9
richie00boy is offline richie00boy  United Kingdom
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Dont want mr. Helmholtz in my sealed sub !
Quote:
Originally posted by Bose(o)
With a Q of 0.37 you definitely have a severe resonance problem, not to sound like a broken record or anything, but WOW!
If the box really is sealed (which it seems to be) then a Q of 0.37 is not a resonance problem, in fact it indicates very well damped indeed resonance.

Quote:
Originally posted by Bose(o)
I can only think of standing waves. You mention that dampening widens the peak, well, this is exactly how dampening works and it is doing it's job--lowering the audibility of resonances.
It can't be standing waves as the box is too small.

I'm pretty sure the room and/or positioning is the problem, especially as he has gone through so many boxes with similar problems.
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Old 14th October 2005, 11:35 AM   #10
Coolin is offline Coolin  Netherlands
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Its the fourth box because i want to explore the sound contributed by the box.

Winisd calculates the Q 0.37 for this box, i dont know if it is but is very tight and detailed sounding.
This was my goal and it sounds exceptional except for the peak.

I have not noticed this in the other boxes but this could be because it is masked by the "lesser" interior quality regarding reflextions etc.

Ik you think about it this should happen in every speaker but probably isnt noticable because its damped enough, especially compared to room resonances etc.
I'm listening to another sealed speaker right now here at work and it has a simular (but much smaller) broad peak in the lows.
Its a huge room so thats not the problem. The volume is also very low.

I can imagine that a long enclosure like a transmission line but sealed will have less of this resonance effect.

I will try to stuff the holes in the driver and also listen to another square enclosure..

CO
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