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Old 22nd June 2013, 09:17 PM   #1
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Default Sealed enclosure experiment

Hello all,

I am interested in this kind of loudspeaker enclosure design shown in the attachment. What are the pros/cons of this sort of loudspeaker cabinet?

I built a prototype enclosure with detachable divider panel between the chambers so that I could make experiments. The woofer used was a 6" unit from Peerless' old CSX-series (free-air resonance: 37 Hz). I measured the impedance of the loudspeaker with different divider panel configurations.

With the divider removed, the cabinet acts like a 34-litre sealed enclosure. There was an impedance peak around 56 Hz (42 Ω).

With a solid wood divider in place, the cabinet acts like a 12-litre sealed enclosure. The impedance peak was moved to around 74 Hz (35 Ω).

With some wadding in the divider panel the impedance peak rose to around 60 Hz (32 Ω).

I also tried placing stuffed 50x200mm and unstuffed 50x400mm cardboard tubes into the divider. It moved the impedance peak to a lower frequency, around 46-47 Hz (23 Ω) for the 200mm tube and 45 Hz (22 Ω) for the 400mm one.

It it beneficial anyway to lower the system resonance frequency in this way?
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File Type: jpg Acoustic dampning-SMALL_b.JPG (251.4 KB, 757 views)

Last edited by d1030180; 22nd June 2013 at 09:21 PM.
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Old 22nd June 2013, 09:37 PM   #2
Zero D is offline Zero D  United Kingdom
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Hi, well it's good to experiment, best way to learn

Looks like a KEF design from the 80's

Quote:
It it beneficial anyway to lower the system resonance frequency in this way?
It depends, if it works, it works !

If you use a design program, you can play around with All the various parameters to your hearts desire & see what makes a difference etc

I recommmend WinISD http://www.linearteam.org/download/winisd-07x.exe & HornResponse Hornresp
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Old 22nd June 2013, 09:47 PM   #3
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In your photos, what is the hole in the lower chamber at the back? Is that left open to the outside when you took your measurements?

The fact that the impedance peak is reduced when a cardboard tube is inserted into the divider suggests that Helmholtz resonance is occurring.

If your system is in fact sealed, then I believe that there would be a dip of SPL at the resonance frequency since your vent (the cardboard tube) connects the top chamber to the lower chamber only.

Regards,
Pete
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Old 22nd June 2013, 10:46 PM   #4
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Quote:
In your photos, what is the hole in the lower chamber at the back? Is that left open to the outside when you took your measurements?
Sorry, I forgot to mention that the hole on the back side panel is for the connection terminal. So it stays always closed.

Unfortunately I don't have any way to measure the frequency response of the system. At the university where I am studying and working we have a couple of Rion NL-20 and NL-22 sound pressure level meters. Those are used for measuring the SPL levels of concerts. Could those be used for hi-fi measurements too?

Rion Sound Level Meter NL-20 - Rion Sound Level Meter NL-20 Exporter, Importer, Manufacturer, Service Provider, Distributor, Supplier, Trading Company, Mumbai, India

Quote:
Zero D
Yes, I agree that experimentation is very interesting.
Actually I already modeled the sealed enclosure with WinISD Pro and found out that the simulated results do not differ much from my measurements.
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Old 22nd June 2013, 11:19 PM   #5
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You could measure the impedance versus frequency characteristic around the box resonance frequency. The shape of that curve would tell you whether or not you have devised a helmholtz resonator.

If helmholtz resonance is occurring, maybe you could use the damping effect of that resonance to counteract what otherwise would be a high Q such as 2.0 of the system with a solid wood divider?

Regards,
Pete
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Old 23rd June 2013, 05:07 PM   #6
4Torr is offline 4Torr  United States
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Dynaudio used to use a similar enclosure to damp its drivers, which had an overly high Qts and bass bump. They also employed a resistive port they called a Variovent. Despite their lofty claims of improving bass clarity, they were just a Band-Aid for boomy bass.
(I don't want to sound like I'm completely dissing Dynaudio, because they make some of the nicest drivers from the midbass up.)

Anyway, the point I'm trying to make is...
The more resistance you add to a bass reflex box, the higher your F3 regardless of any change of Fs. Any time you cause the sound energy from the driver to be consumed by fiber damping instead of by the damping effect of moving air and creating sound, you are throwing away some of the driver's efficiency at that frequency.

Damping material should only be added sparingly to the well-braced cabinet walls to reduce mid frequency standing waves. If you want the lowest f3, it would be difficult to beat Don Keele's formula, which yields a box 1.3 cubic feet, 34.1 hz port and 32.2 hz F3 for your driver.
http://www.xlrtechs.com/dbkeele.com/...estatement.pdf

Last edited by 4Torr; 23rd June 2013 at 05:24 PM.
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Old 26th June 2013, 04:52 AM   #7
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Quote:
Originally Posted by d1030180 View Post
Unfortunately I don't have any way to measure the frequency response of the system. At the university where I am studying and working we have a couple of Rion NL-20 and NL-22 sound pressure level meters. Those are used for measuring the SPL levels of concerts. Could those be used for hi-fi measurements too?
That sound level meter looks like it might be okay to measure frequency response. To measure the frequency response around the system resonance frequency (bass range) you could put the mike element of the meter as close as possible to the dust cover of the woofer without actually touching the dust cover. This is called nearfield measurement and it eliminates any effect on the response from room reflections.

Sorry I missed that in your first post you provide an impedance versus frequency graph. The tube in the panel definitely results in Helmholtz resonance.

It is generally desirable to lower the resonance frequency, but that is usually in the context of the size of the system. It would seem unlikely that the technique could yield a good ratio of bass cut-off frequency and corresponding size of the box. See 4Torr's post on the system devised by Keele.

Regards,
Pete
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Old 26th June 2013, 07:10 PM   #8
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Quote:
Originally Posted by 4Torr View Post
Dynaudio used to use a similar enclosure to damp its drivers, which had an overly high Qts and bass bump. They also employed a resistive port they called a Variovent. Despite their lofty claims of improving bass clarity, they were just a Band-Aid for boomy bass.
It would seem that maybe with such a system as the OP put together you could combine the lower box resonance frequency of a bass-reflex system with the more gradual roll-off of a sealed system below resonance. That is, at frequencies below the Helmholtz resonance frequency, the vent is just a passageway between the upper and lower chambers. Knowing how to go about it to get a good result wouldn't be easy I'm sure.
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Old 26th June 2013, 09:59 PM   #9
4Torr is offline 4Torr  United States
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Quote:
Originally Posted by cT equals piD View Post
It would seem that maybe with such a system as the OP put together you could combine the lower box resonance frequency of a bass-reflex system with the more gradual roll-off of a sealed system below resonance. That is, at frequencies below the Helmholtz resonance frequency, the vent is just a passageway between the upper and lower chambers. Knowing how to go about it to get a good result wouldn't be easy I'm sure.
When the tuned lower cabinet is off resonance and breathing in phase with the driver, both upper and lower cabinets are effectively joined and the response is similar to a sealed box. When the lower box and port are resonating however, they are operating in anti-phase with the driver and cancel each other at the mouth of the port. The upper chamber becomes the only compliance and sets the high pass slope. The result is more mid-bass efficiency and less low bass output.

The genius of a bass reflex is that the woofer, resonating chamber and port are arranged so that the front of the woofer and port output are in phase at chamber resonance. Driver excursion is reduced and sound pressure is optimally transferred to the surrounding air.

Last edited by 4Torr; 26th June 2013 at 10:04 PM.
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Old 27th June 2013, 09:40 PM   #10
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4Torr,
Is that pitting a high mass, low Qts, low Fs driver against a low mass, High Qts, high Fs, driver in such an enclosure?

Have considered doing the same as an experiment but instead of a dampening plate aka resistive port between the upper and lower, using an additional identical bass or midbass and using the current from the VC as a variable brake. Haven't done tho, thinking the phase shift induced would cause unwanted effects
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