Anyone else used this? 45 degree internal panels *PIC*

[Max_Andrews]

I've been working with Dori S. via email on a sealed RS125/seas TDFC MTM (8-ohm nominal), "tall and skinny" style design, for use with a subwoofer, and when fleshing out the bracing I had an idea. Normally the rear waves from a driver hit the back of the cabinet (which is usually a flat surface), and then get reflected right back at the driver, which can smear the sound and create a resonant peak. Heavy stuffing helps, but I'd imagine there is still a lot of reflected energy hitting the driver in a shallow depth box. So I thought about putting 45 degree deflection panels behind the rear of each driver, reflecting the sound downward along the long axis of the enclosure and into "traps" that would absorb and at the very least diffuse the sound. Effectively, the theory is that you are extending the distance the rear waves travel, allowing them to interact with more stuffing and be absorbed more effectively, and also removing any resonance peaks associated with a straight path between the driver and the rear baffle. Will this work? Why/why not?

[Wolf]

...but I don't know how effectively. Several people here and on the former MAD board have used bracing and such like this.
Later,
Wolf

[daryl]

Internal damping is the key.

100% fiberglass fill will damp all resonances regardless of enclosure shape.

The enclosure shown below should have a very strong resonance associated with the front to back distance.

The baffle is not yet attached but midbass cuttouts are the same size as the distance wall to wall and from top/bottom to shelf-brace.

With the 100% fiberglass fill no resonance is measured in spite of the enclosure shape.

[billschu]

Yes, it does work. I've used it for several projects but NOT to the extremes shown in the drawing. If it is a sealed enclosure, you can reduce those panels by about 50% and use fiberglass stuffing.

If it's a ported enclosure and is long and skinny like the one I brought to DIY NE last year, a labyrinth of sorts can be used internally. I did that with those speakers. The enclosure is 36" x 6" x 7" externally so you can guess that I had to deal with pipe resonance. This is not a transmission line but just a ported speaker with a labyrinth of sorts to break up sanding waves while also allowing air flow from the driver to the port seemingly unimpeded.

Bill

> I've been working with Dori S. via email on
> a sealed RS125/seas TDFC MTM (8-ohm
> nominal), "tall and skinny" style
> design, for use with a subwoofer, and when
> fleshing out the bracing I had an idea.
> Normally the rear waves from a driver hit
> the back of the cabinet (which is usually a
> flat surface), and then get reflected right
> back at the driver, which can smear the
> sound and create a resonant peak. Heavy
> stuffing helps, but I'd imagine there is
> still a lot of reflected energy hitting the
> driver in a shallow depth box. So I thought
> about putting 45 degree deflection panels
> behind the rear of each driver, reflecting
> the sound downward along the long axis of
> the enclosure and into "traps"
> that would absorb and at the very least
> diffuse the sound. Effectively, the theory
> is that you are extending the distance the
> rear waves travel, allowing them to interact
> with more stuffing and be absorbed more
> effectively, and also removing any resonance
> peaks associated with a straight path
> between the driver and the rear baffle. Will
> this work? Why/why not?

[billfitzmaurice]

It does improve matters, but not to the point that you don't still have to fully line the cabinet. You do. What changes is the path length that internal reflections may travel unimpeded, raising the frequency at which standing waves can develop. In smaller enclosures standing waves are totally damped by absorbtive lining, but those measures are only good down to 300 Hz or so. When you make a cab with internal dimensions that a approach 1/4 wavelength below 300 Hz, ie, .9 feet, and it's within the cab bandwidth you need to break up the internal dimensions. OTOH, unbraced panels longer than ten inches or so are an invitation for vibration, so shelf bracing should be in there anyway. Having the braces at angles to eliminate parallel walls is a good idea, and should be utilized whenever it's practical to do so.

[dori_s]

> Yes, it does work. I've used it for several
> projects but NOT to the extremes shown in
> the drawing. If it is a sealed enclosure,
> you can reduce those panels by about 50% and
> use fiberglass stuffing.

> If it's a ported enclosure and is long and
> skinny like the one I brought to DIY NE last
> year, a labyrinth of sorts can be used
> internally. I did that with those speakers.
> The enclosure is 36" x 6" x
> 7" externally so you can guess that I
> had to deal with pipe resonance. This is not
> a transmission line but just a ported
> speaker with a labyrinth of sorts to break
> up sanding waves while also allowing air
> flow from the driver to the port seemingly
> unimpeded.

> Bill

Thats about the same size 6"x7" but they will be a bit taller or about 42-44" and the cabinet for the RS125's will be .4 cu.ft sealed.

Best,
Dóri S.

[billschu]

n/t

> It does improve matters, but not to the
> point that you don't still have to fully
> line the cabinet. You do. What changes is
> the path length that internal reflections
> may travel unimpeded, raising the frequency
> at which standing waves can develop. In
> smaller enclosures standing waves are
> totally damped by absorbtive lining, but
> those measures are only good down to 300 Hz
> or so. When you make a cab with internal
> dimensions that a approach 1/4 wavelength
> below 300 Hz, ie, .9 feet, and it's within
> the cab bandwidth you need to break up the
> internal dimensions. OTOH, unbraced panels
> longer than ten inches or so are an
> invitation for vibration, so shelf bracing
> should be in there anyway. Having the braces
> at angles to eliminate parallel walls is a
> good idea, and should be utilized whenever
> it's practical to do so.

[daryl]

> In
> smaller enclosures standing waves are
> totally damped by absorbtive lining, but
> those measures are only good down to 300 Hz
> or so. When you make a cab with internal
> dimensions that a approach 1/4 wavelength
> below 300 Hz, ie, .9 feet, and it's within
> the cab bandwidth you need to break up the
> internal dimensions.

Not at all.

100% fiberglass fill will eliminate all resonances regardles of frequency.

[romanbednarek]

> Not at all.

> 100% fiberglass fill will eliminate all
> resonances regardles of frequency.

But what do you do in the case of a ported enclosure? Fill reduces the port output in a vented design.

[Æ]

[daryl]

100% fill ported boxes are doable.

You would have to increase Fb because of the additional loss.

You probably would want to use a PR rather than a vent or you could use acoustic foam and clear an area around the inside end of the vent so you don't have high velocity airflow being impeded by damping material.

If you don't want 100% fill then use shelf braces located in the high velocity regions of the first and second modes and use damping materials heavily only in the compartment directly behind the drivers.

Some experimentation might have to be done to determine the window size of the shelf braces in the high velocity regions but I would say 25% would be a good place to start.

> But what do you do in the case of a ported
> enclosure? Fill reduces the port output in a
> vented design.

[billfitzmaurice]

That resembles an old concept called an acoustic labyrinth, not to be confused with quarter-wave pipes that had a similar look, and used the same name. The AL varied the spacing between the plates, the idea being that each small chanber would resonate at a different frequency at low intensity, rather than at one frequency at a high intensity. The walls were also lined to damp the reflected waves. But it was a far more complicated method of achieving the same result as just angling the plates.

BTW, fiberglass does not damp all resonances. Packed with sufficient density it will greatly reduce them, but at longer wavelengths fiberglas is no more a barrier to soundwaves within an enclosure than it is to their passage through, for instance, the walls of an insulated house.

[daryl]

What nonsense Bill,

> BTW, fiberglass does not damp all
> resonances. Packed with sufficient density
> it will greatly reduce them,

100% fiberglass filled enclosures have no resonances.

> but at longer
> wavelengths fiberglas is no more a barrier
> to soundwaves within an enclosure than it is
> to their passage through, for instance, the
> walls of an insulated house.

Low frequencies passing through an insulated house?

I couldn't imagine a more foolish or meaningless example.

First the inside and outside walls of a house are rigidly coupled and will transmit sound regardless of the effectiveness of insulation inside them.

Second and more on topic is that the the depth of the fiberglass inside of walls is fixed at 5.5" regardless of frequency.

In a sealed enclosure the amount of fiberglass that must be traversed for a wave to reinforce itself is one wavelength at each of the cabinets primary modes and multiples of wavelengths at higher order modes (first mode occurs where a 1/2 wavelength is equal to a cabinet dimension and multiples there of).

That means 3 feet of fiberglass traversed for a 300hz mode and 30 feet of fiberglass for a 30hz mode if you happen to have a cabinet with a 15 foot dimension.

[billfitzmaurice]

I'll have to keep that in mind the next time I build a 15 foot long box. But that's not of much use in a 2 cu ft sub. Theoretical calculations are only useful when they can be used in practical applications.

[daryl]

> I'll have to keep that in mind the next time
> I build a 15 foot long box. But that's not
> of much use in a 2 cu ft sub. Theoretical
> calculations are only useful when they can
> be used in practical applications.

It works the same in every size of box.

Larger dimensions mean lower resonant modes and a proportionately longer length of fiberglass to traverse in order to reinforce.

Smaller dimensions mean higher resonant modes and a proportionately shorter length of fiberglass to traverse in order to reinforce.

A small enclosure driven at frequencies lower than it's lowest resonant mode has a very high internal acoustic impedance (air spring) much higher than the acoustic impedance of air.

The environment is high pressure/low velocity.

Fiberglass stuffing works by friction and the higher the air velocity the more energy is absorbed.

Thus little energy is absorbed by fiberglass stuffing below an enclosures lowest resonant mode.

You don't want any absorbtion below the lowest resonant mode anyway so this is good but also misleading as it appears fiberglass is ineffective at low frequency.

At a resonant mode however the acoustic impedance of air comes into play and you then have a corresponding region of high velocity/low pressure in between the enclosure boundries.

The fiberglass fill is very effective in this low acoustic impedance region of the cabinet.

So fiberglass is very effective where cabinet dimension are large enough to support a resonance.

Fiberglass is more absorbtive where you need it and less absorbtive where you don't.

[daryl]

> It works the same in every size of box.

Actually I should say that fiberglass fill is more effective in larger enclosures.

[WmAx]

> Thus little energy is absorbed by fiberglass
> stuffing below an enclosures lowest resonant
> mode.

> You don't want any absorbtion below the
> lowest resonant mode anyway so this is good
> but also misleading as it appears fiberglass
> is ineffective at low frequency.

It is interesting to note that when you move past standard density fiberglas(usually in the 1.5lb/ft^3 density range), that high density fiberglas and rockwool(such as 6lb/ft^3 OC 705 fiberglass and 8 lb/ft^3 mineral wool board), used in a substantial amount in an enclosure, appear to dampen the tuned resonant frequency of even a sealed enclosure system, causing a smooth gradual pre-mature roll off.

-Chris