I'm starting to learn about speaker renovation and have a query about internal cabinet dampening.
I have three speakers each with differing materials and placements
1, Lentek S4 is a sealed cabinet and has thick Wool Underlay type material but is positioned only on the top/bottom and sides leaving a large gap in the middle
2, Celestion DL10 has a small port at the back (no tube) and is stuffed from top to bottom with white poly material
3, Wharfedale Linto 2 again a sealed box, has loft insulation type wool, stuffed, like the Celestions, randomly, but with not as much actual material.
I know that some reasons for positioning are down to listening preference but is there a default for sealed and ported boxes?
I noticed that the stuffing covers the port on the Celestions, that doesn't seem right to me.
Cheers
I have three speakers each with differing materials and placements
1, Lentek S4 is a sealed cabinet and has thick Wool Underlay type material but is positioned only on the top/bottom and sides leaving a large gap in the middle
2, Celestion DL10 has a small port at the back (no tube) and is stuffed from top to bottom with white poly material
3, Wharfedale Linto 2 again a sealed box, has loft insulation type wool, stuffed, like the Celestions, randomly, but with not as much actual material.
I know that some reasons for positioning are down to listening preference but is there a default for sealed and ported boxes?
I noticed that the stuffing covers the port on the Celestions, that doesn't seem right to me.
Cheers
As a general rule, sealed enclosures are loosely filled with damping material, while ported enclosures have damping material affixed to the internal walls.
Some ported enclosures may have more damping material than stated above, but a clear air path between woofer and port should be maintained in order not to impede the reflex action.
Some ported enclosures may have more damping material than stated above, but a clear air path between woofer and port should be maintained in order not to impede the reflex action.
Yes what Galu said...
Acoustical damping is hard to do wrong. There are a lot of techniques and materials, and most of them work quite well.
Acoustical damping is hard to do wrong. There are a lot of techniques and materials, and most of them work quite well.
Regarding your Linton 2 renovation: Wharfedale used a minimal amount of damping material in my Denton 2 and Dovedale 3 speakers of the same era.
The optimum amount of damping material can be determined by the 'click test' described below:
The effectiveness of damping material is proportional to its density. So the light weight fluffy stuff does almost nothing while rockwool or densely packed lambs wool or fiberglass will absorb a lot of sound. Absorption declines below 300 Hz or so due to the typical size of the boxes used and thickness you can achieve. Damping works by impeding velocity. The velocity perpendicular to a wall is zero, so padding on a wall is less effective than that out in the box. I like to stuff enclosures with lots of damping to remove all box sound.
A sealed speaker will often have a bit more stuffing than a ported but it can depend on the desired tuning of the sealed and ported resonances w.r.t. frequency response, Q and effective size of internal volume.
With a ported speaker the stuffing is usually kept away from the port so that it doesen't restrict the slug of air flowing in and out of the port. However some ported designs and particularly older ones may have been designed to introduce significant damping into the port. Straws inside the port or a mesh across an end are examples of this. Stuffing isn't usually used for this purpose though.
Stuffing is less effective next to walls compared to away from the walls because the particle velocity at a wall is zero (the pressure variation in contrast is large) and hence the friction from air being squeezed between the fibres is low. This can sometimes say something about the competence of a design although there can be valid reasons to stick stuffing to walls if only a low level of damping is needed and cavity resonances don't fall within the passband of the driver.
A 2 way ported speaker with a midwoofer will suffer from high frequencies inside the cabinet leaving via the port (but not usually an issue with 3 way ported speakers). These high frequencies can be reduced by placing stuffing between the midwoofer and the port. It doesn't take much stuffing at high frequencies to damp significantly. For most port designs stuffing should be kept away from the high velocity region around the port mouth. It can also be wise to keep stuffing away from the coil gap. Again this can sometimes say something about the competence of a design.
Andy mentioned keeping stuffing away from the voice coil. This is a good practice.
I mostly use lambs wool for fiber damping. I have found that an excellent barrier material between the loose wool fibers and the driver motor is a single layer of thin polyester batting. It comes in a roll, and I buy the thinnest available at the fabric store, which is about 5mm uncompressed thickness. It is acoustically transparent, but it acts as a fine screen to keep loose fibers away from the motor.
I mostly use lambs wool for fiber damping. I have found that an excellent barrier material between the loose wool fibers and the driver motor is a single layer of thin polyester batting. It comes in a roll, and I buy the thinnest available at the fabric store, which is about 5mm uncompressed thickness. It is acoustically transparent, but it acts as a fine screen to keep loose fibers away from the motor.
Damping directly behind the driver is to lower the reflected sound back through the cone. The loose fill in a sealed cabinet also does this but it also breaks up some standing waves and reflections. It also has the added benefit of increasing the perceived volume of the box to the woofer. The effect is minor but it is there. I go for thicker rock wool damping on the walls about 1.5” thick and away from the port opening in a reflex cabinet. For sealed I use poly fill to loosely stuff the cabinet. Staple it lightly to the top and sides of the cabinet so it is still fluffy but won’t sag to the bottom of the box. Just grab a few fibers in the staple and it will hold up the rest, you don’t want to mat it down flat.
Some extra data, a coffee break 🙂
Vented boxes typically have low amount of damping material, because it dampens also the helmholz resonance, introduces loss even if the damping material is not between the driver and port.
Need for damping comes from sound wavelength being very different at low frequencies than highs, 50Hz doesn't fit inside most rooms, while 5000Hz is bit longer than average pinky finger. While any given physical size and shape loudspeaker box is particular size. Sound reacts with physical world objects differently depending wavelength. If wavelength is much longer than size of a physical object, the object has very little effect to the sound. This works both ways, physical objects making sound (drivers, resonating panels, pipes, anything), and sound interacting with physical world by reflecting, diffracting and transmitting. And leads to comprimises which in practice goes like so: if one optimizes a box for long wavelengths (bass) it basically gets worse for short wavelengths, and vice versa.
Resonances of a box walls, modes inside the box are usually not wanted so should be damped, while some resonances like the helmholz resonance of a ported box or driver main resonance which affects low frequency response are wanted. Effectiveness of damping material is wavelength dependent, as already mentioned by others, which we can utilize to dampen unwanted resonance while leaving a desired one with less effect. Any typical damping material is more effective on short wavelengths (high frequencies) than long (low frequencies).
So, for example a subwoofer box doesn't necessarily need any if all resonances inside the box and with the walls are way above pass band. In such case a reflex box would just lose low frequency output with damping material so there is ko point using any. On a closed box one might shape response of the woofer by reducing the driver main resonance with the damping, in other words lower system resonance, Qtc, so sometimes it coudl be good. Wall resonances could be damped without damping driver or helmholz resonance with CLD technique for example.
On a multiway speaker woofer box, whose bandwidth extend from low bass all the way through midrange one likely needs damping of everything above the very lows, because most resonances land on passband and would be audible, contributing to the "box sound". Speaker box walls have much more surface area than the woofer, so even small resonance is likely quite loud acoustically, altjough difficult to analyze and find. Thus good old rules of thumbs for damping. But, there is no reason not to think about it so that one can make what ever fits in any particular situation.
There are many forms of damping and reasons to use any, also tricks to avoid things altogether. This is why I wanted to expand a little. Hopefully anyone are now able to reason with it. Have fun!🙂
Vented boxes typically have low amount of damping material, because it dampens also the helmholz resonance, introduces loss even if the damping material is not between the driver and port.
Need for damping comes from sound wavelength being very different at low frequencies than highs, 50Hz doesn't fit inside most rooms, while 5000Hz is bit longer than average pinky finger. While any given physical size and shape loudspeaker box is particular size. Sound reacts with physical world objects differently depending wavelength. If wavelength is much longer than size of a physical object, the object has very little effect to the sound. This works both ways, physical objects making sound (drivers, resonating panels, pipes, anything), and sound interacting with physical world by reflecting, diffracting and transmitting. And leads to comprimises which in practice goes like so: if one optimizes a box for long wavelengths (bass) it basically gets worse for short wavelengths, and vice versa.
Resonances of a box walls, modes inside the box are usually not wanted so should be damped, while some resonances like the helmholz resonance of a ported box or driver main resonance which affects low frequency response are wanted. Effectiveness of damping material is wavelength dependent, as already mentioned by others, which we can utilize to dampen unwanted resonance while leaving a desired one with less effect. Any typical damping material is more effective on short wavelengths (high frequencies) than long (low frequencies).
So, for example a subwoofer box doesn't necessarily need any if all resonances inside the box and with the walls are way above pass band. In such case a reflex box would just lose low frequency output with damping material so there is ko point using any. On a closed box one might shape response of the woofer by reducing the driver main resonance with the damping, in other words lower system resonance, Qtc, so sometimes it coudl be good. Wall resonances could be damped without damping driver or helmholz resonance with CLD technique for example.
On a multiway speaker woofer box, whose bandwidth extend from low bass all the way through midrange one likely needs damping of everything above the very lows, because most resonances land on passband and would be audible, contributing to the "box sound". Speaker box walls have much more surface area than the woofer, so even small resonance is likely quite loud acoustically, altjough difficult to analyze and find. Thus good old rules of thumbs for damping. But, there is no reason not to think about it so that one can make what ever fits in any particular situation.
There are many forms of damping and reasons to use any, also tricks to avoid things altogether. This is why I wanted to expand a little. Hopefully anyone are now able to reason with it. Have fun!🙂
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