Is this Monitor Statement Mod. OK ??

[randyc1]

This Mod is for a Monitor Statement , I hope Curt or Jim could give me some insight !

Same Vol (24L),...Same Distance between Drivers,...Same Width (10.5"),... Same Tunnel

Only Differences are: 2" less height , ...Depth,(tunnel is same)...and Port in Different place.

1. So can I put Port there ??
2. Height Diff matter ??

Thanks !

[Curt C]

randy,
Here is the issue with placing the port there: The acoustic energy off the back of the woofer has a direct path out of the enclosure.

This means instead of the port supplying output only around the tuning frequency of 34 Hz, all the frequencies of the woofer pass band will exit the port at high SPL levels. This obviously is not a good thing as the woofer output and the port outputs will combine destructively and constructively. the results will not be pleasing in the case of the monitor.

The rule of thumb is to never be able to see the woofer through the port. You do have several options however:
Port out the bottom of the enclosure
Place and elbow in the port so it points away from the woofer.
Use a slot port, say along the back panel that doesn't face the woofer.

I'm sure you will be able to find an option that will satisfy both your creativity and the physics involved...

C

[randyc1]

randy,
Here is the issue with placing the port there: The acoustic energy off the back of the woofer has a direct path out of the enclosure.

This means instead of the port supplying output only around the tuning frequency of 34 Hz, all the frequencies of the woofer pass band will exit the port at high SPL levels. This obviously is not a good thing as the woofer output and the port outputs will combine destructively and constructively. the results will not be pleasing in the case of the monitor.

The rule of thumb is to never be able to see the woofer through the port. You do have several options however:
Port out the bottom of the enclosure
Place and elbow in the port so it points away from the woofer.
Use a slot port, say along the back panel that doesn't face the woofer.

I'm sure you will be able to find an option that will satisfy both your creativity and the physics involved...

C

OK Curt , ...if I placed a slot port of 1"x 3.14" (same area as 2"round port) just above Tunnel and extending out flush with tunnel, like diagram in above post shows , ...Slot Port would be almost in same place as original .

If Area of slot (3.14) is same as Original 2" round port (3.14) ,would length be same to ??

[cjd]

I now see the cause of your question in the other thread.

Personally, I prefer front-porting - in this case, I might try to get a slot port below the woofer. However, I think where you have it here will also work. I would probably make the port width equal to the mid tunnel width and adjust the height accordingly (easier construction to me) but that's your call.

[randyc1]

I now see the cause of your question in the other thread.

Personally, I prefer front-porting - in this case, I might try to get a slot port below the woofer. However, I think where you have it here will also work. I would probably make the port width equal to the mid tunnel width and adjust the height accordingly (easier construction to me) but that's your call.

Yes ,same width as tunnel would be easier , but then would it be to thin ??

[cjd]

That tunnel is 6"? Not too thin, it would be a bit over 1/2" on the slot port, or you could recalculate for a more convenient dimension (say, a 3/4" port the full width) and find a new length.

C

[randyc1]

That tunnel is 6"? Not too thin, it would be a bit over 1/2" on the slot port, or you could recalculate for a more convenient dimension (say, a 3/4" port the full width) and find a new length.

C

Well that is part of my original question ,... Many combinations come to same area of 3.14 as a round 2" port , if i dont exceed 1:8 ratio ,do they all give me the same Port tune ,.. even if some ratio's calculate to a port length that's almost half the length as Original ????

[cjd]

You keep the cross-sectional area the same (i.e. 3.14sq/in) and the length the same.

So 1"x3.14" or 2"round or .5"x6.28" - all of those keep the same 8" port length! Where are you getting half the same port length? What 1:8 ratio are you talking about?

C

[randyc1]

You keep the cross-sectional area the same (i.e. 3.14sq/in) and the length the same.

So 1"x3.14" or 2"round or .5"x6.28" - all of those keep the same 8" port length! Where are you getting half the same port length? What 1:8 ratio are you talking about?

C

OK the 1:8 ratio is the height to length of Port Opening . they say not to exceed this ratio.

And when .5"x6.28" is entered in a Slot port length calculator ,...the length is now 4.11 inches ??much shorter that other ratio's ?? .... Different ratios have different lengths !!... That is what im questioning ??,

[cjd]

I have no idea what you are referencing, so I'm completely lost.

End correction is always fuzzy math, and I'm not really sure what the tool you're using has set in this regard. Probably other things in the mix too.

C

[randyc1]

I have no idea what you are referencing, so I'm completely lost.

End correction is always fuzzy math, and I'm not really sure what the tool you're using has set in this regard. Probably other things in the mix too.

C

Have u tried a Slot Port calculator ? ,....if yes have u also tried different ratio's of slots ??

[cjd]

I've never heard of "slot port calculator" - I use Jeff B's tools mostly, or check impedance.

I targeted ~28Hz on the Ansonica and impedance says I hit 29Hz. That was converting from a round port to a 1x7.5" port (using 3.1" diameter).

C

[randyc1]

I've never heard of "slot port calculator" - I use Jeff B's tools mostly, or check impedance.

I targeted ~28Hz on the Ansonica and impedance says I hit 29Hz. That was converting from a round port to a 1x7.5" port (using 3.1" diameter).

C

What diameter was the round port?

It's a port calculator ,but if you are using a slot it must be specified :http://www.mobileinformationlabs.com...0lenth%201.htm

[cjd]

What diameter was the round port?

It's a port calculator ,but if you are using a slot it must be specified :http://www.mobileinformationlabs.com...0lenth%201.htm

As I said, round equivalent to 1x7.5 is 3.1"

Tuning is right where it said it would, using the same method I'm suggesting.

I don't know what that calculator is doing, but I'm going to go ahead and state that it's just plain wrong in this instance. It says my port should be 4.5" shorter than it is. Impedance doesn't lie. Don't know what to tell you.

[PMazz]

WinSpeakerz gives me 7.25" length for a slot port of .5 x 6.28 tuned to 34Hz in 24 liter box.

[Curt C]

I can clear some of the fog here, I think.

The 1:8 ratio is arbitrary and probably stems from the increased port attenuation due to friction. A circular port will have the least amount of interior surface area, so there is minimal friction between the port walls and the air slug in the port. A wide, skinny port, with the same area and length will have the same fv, but more inner surface in contact with the air, and consequently more friction. While this friction will, in some cases cause audible attenuation, I doubt it will be a show stopper, and likely minimal in this case. The formula:

Fd = 0.5 * ρ * v^2 * Cd * A

where
Fd is the drag force
ρ is the mass density of the fluid, in this case air
v is the velocity of the object relative to the fluid
Cd is the drag coefficient
A is the internal surface area

Fd will be roughly proportional to attenuation, and also proportional to the surface area. It will be proportional to the square of the velocity, however. Double the velocity will result in 4 times the drag force.

One other observation: Since the inside end of the slot port will be supported by the side of the tunnel, the effective length of the port will be slightly longer than calculated. While this is interesting from a theoretical standpoint, in this case it will do little to change the tuning. I’d guess making the effective length perhaps 5% longer in this case.

C

[randyc1]

WinSpeakerz gives me 7.25" length for a slot port of .5 x 6.28 tuned to 34Hz in 24 liter box.

Wow that is a big difference from the (linked) Slot port calculator , which comes out to 4.11" long with a .5 x 6.28 slot?

http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL%20Port%20lenth%201.htm

[randyc1]

I can clear some of the fog here, I think.

The 1:8 ratio is arbitrary and probably stems from the increased port attenuation due to friction. A circular port will have the least amount of interior surface area, so there is minimal friction between the port walls and the air slug in the port. A wide, skinny port, with the same area and length will have the same fv, but more inner surface in contact with the air, and consequently more friction. While this friction will, in some cases cause audible attenuation, I doubt it will be a show stopper, and likely minimal in this case. The formula:

Fd = 0.5 * ρ * v^2 * Cd * A

where
Fd is the drag force
ρ is the mass density of the fluid, in this case air
v is the velocity of the object relative to the fluid
Cd is the drag coefficient
A is the internal surface area

Fd will be roughly proportional to attenuation, and also proportional to the surface area. It will be proportional to the square of the velocity, however. Double the velocity will result in 4 times the drag force.

One other observation: Since the inside end of the slot port will be supported by the side of the tunnel, the effective length of the port will be slightly longer than calculated. While this is interesting from a theoretical standpoint, in this case it will do little to change the tuning. I’d guess making the effective length perhaps 5% longer in this case.

C

Ok just to be clear this sentence.
:"Since the inside end of the slot port will be supported by the side of the tunnel, the effective length of the port will be slightly longer than calculated"

Does this mean that the Port ACTS longer than it really is?

[Curt C]

If we define the port length as that length that supports the air slug, on that side of the port next to the tunnel, it actually is longer. While the upper side terminates with an abrupt acoustic impedance change at the terminus of the slot, the lower side continues on unabated. Consider if A 2" port was designed such that it was flush with the rear panel, but at a 45 degree angle. The inner end is cut perpendicular as is the norm. If one would measure the port at the longest edge, compared to the shortest edge, one would measure a 2" difference in length. The accepted effective port length is assumed to be the average of the two lengths. The airflow along one side of a slot port adjacent to a panel is the same phenomena.

In the grand scheme of things I see exact port tuning as towards the bottom of the list of "Important things to consider in designing a speaker". When in doubt, I suggest erring towards a slightly lower tuning.

C

[randyc1]

randy,
Here is the issue with placing the port there: The acoustic energy off the back of the woofer has a direct path out of the enclosure.

This means instead of the port supplying output only around the tuning frequency of 34 Hz, all the frequencies of the woofer pass band will exit the port at high SPL levels. This obviously is not a good thing as the woofer output and the port outputs will combine destructively and constructively. the results will not be pleasing in the case of the monitor.

The rule of thumb is to never be able to see the woofer through the port. You do have several options however:
Port out the bottom of the enclosure
Place and elbow in the port so it points away from the woofer.
Use a slot port, say along the back panel that doesn't face the woofer.

I'm sure you will be able to find an option that will satisfy both your creativity and the physics involved...

C

OK so this is my new plan:

Changed Port placement with Slot Port ,with same Area (3.14) as Original port.

Same 24L Vol !

Same dist between drivers !

Same WIDTH 10.5"

Top Exit of Tunnel is 12" ,..bottom exit 13'' ...not a very big diff !

Totall box height is 2" shorter ( Drivers 1" from Bottom & Top edge instead of 2") ...will this affect sound ??

[Curt C]

My (admittedly cursory) examination indicates you plan is a go.

The results of a diffraction modeling program based on your drawing suggest the differences in diffraction will be minimal, with the delta about 0.5 dB maximum.

C

[randyc1]

My (admittedly cursory) examination indicates you plan is a go.

The results of a diffraction modeling program based on your drawing suggest the differences in diffraction will be minimal, with the delta about 0.5 dB maximum.

C

Great Curt !... did'nt want to change much of your Great design , glad the height does'nt change much ,...can't wait to start building !