Bracing VS Damping

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#41
Bmsluite said:
It is bottom ported with a slot base on the bottom like a Wharfendale
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That is a bad position for a port. Also the enclosure modes will be worse than any panel resonance.

Have a look:

Post in thread 'Investigating port resonance absorbers and port geometries'
stv

Post in thread 'Investigating port resonance absorbers and port geometries'

augerpro said:
Now @stv has to make a spreadsheet for best port placement!
Click to expand...
I had some time to think and simulate and I guess there is not even the need for a spreadsheet! At least for the very fundamental rules.
And I am sure I am not at all the first one to find this "solution".

General requirements:​

  • keep efficiency of the desired bass reflex resonator high (thus, as little stuffing material as possible)
  • generate as few resonances as possible
  • avoid resonances exiting through the port (and the driver, but the port usually better transmits resonances)
  • try to move...
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#42
This on eis better (love seeing the kind of bracing we used. The braces should be solid between the driver magnet and the back. Avoid even spacing.

1748823917707-png.1467792


The reason, from Tappan AES paper.

Tappan-braces.gif


dave
 
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#43
stv said:
That is a bad position for a port. Also the enclosure modes will be worse than any panel resonance.

Have a look:

Post in thread 'Investigating port resonance absorbers and port geometries'
stv

Post in thread 'Investigating port resonance absorbers and port geometries'

augerpro said:
Now @stv has to make a spreadsheet for best port placement!
Click to expand...
I had some time to think and simulate and I guess there is not even the need for a spreadsheet! At least for the very fundamental rules.
And I am sure I am not at all the first one to find this "solution".

General requirements:​

  • keep efficiency of the desired bass reflex resonator high (thus, as little stuffing material as possible)
  • generate as few resonances as possible
  • avoid resonances exiting through the port (and the driver, but the port usually better transmits resonances)
  • try to move...
  • Like
  • Thank You
Click to expand...
Interesting. I guess I never realized I could mess with port location in hornresp. I'll model this up tonight to find a better location.

However, the resonances should get trapped inside of the plate on the bottom. The plate will have felt on the top of it.
1748896513989.png
 
#45
andy19191 said:
Without damping none of the work put into the cabinet by the driver will be dissipated and the magnitude at resonance will keep getting larger and larger until the speaker breaks. Doesn't matter how heavy or stiff without damping the potential magnitude of resonances is infinity. However, for a resonance at a particular frequency a stiff heavy cabinet will have more intertia compared to a floppy light one and so the magnitude of resonances will increase towards infinity more slowly for the same driver pumping energy into the cabinet. (Resonant frequency scales with the square root of stiffness over mass).
Click to expand...

I did not say that I don't use damping. I use all 3 methods.
 
#47
Bmsluite said:
However, the resonances should get trapped inside of the plate on the bottom.
Click to expand...
Assuming an enclosure height of about 1 meter there will be a first mode with wavelength 2 m (170 Hz). To dampen this frequency your felt would need to be around 50 cm thick.

A port should never be too near an enclosure boundary, worst is one end of a long box. All modes have a pressure maximum at the enclosure boundaries. So I would suggest to put the port at the back and keep the internal port end away from the bottom of the speaker ...
 
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#49
Bmsluite said:
Found this little tidbit that looked interesting. This is a KEF speaker. They use a multimaterial solution. My guess is thats just rubber stripping. Interesting that it doesn't go across the entire brace but is only a few strips here and there
View attachment 1468088

I will try something like this
Click to expand...

Note the term "velocity" in the chart, instead of "sound pressure". This is a measurement via accelerometers. No correlation whatsoever has been established to acoustic behavior, especially in relation to the direct sound (from the membrane & port) in the far field (listening position).

Great marketing - but nothing of substance.
 
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#52
stoneeh said:
Note the term "velocity" in the chart, instead of "sound pressure". This is a measurement via accelerometers. No correlation whatsoever has been established to acoustic behavior, especially in relation to the direct sound (from the membrane & port) in the far field (listening position).

Great marketing - but nothing of substance.
Click to expand...

If you know the normal velocity over the complete surface of a body solving the Helmholtz-Kirchoff integral equation will give you the pressure over the surface. With the normal velocity and pressure over a body the pressure everywhere external to that body can be calculated. This is how the akabak software and other acoustic BEM codes work. It is also how the sound radiation from a speaker cabinet is typically measured with a laser vibrometer or accelerometer in the presence of sound from the drivers.
 
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#53
If you know the normal velocity over the complete surface of a body
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But we don't, don't we.

The most thorough accelerometer measurements of loudspeakers I've seen have been a dozen or so points distributed somewhat evenly over a side panel. Often, it's just a few, or even just 1 point. And even with many points, we usually get no information about phase, only velocity.

Acoustical differences between structures in the far field, corresponding to a typical listening position, is the relevant information. If it isn't provided voluntarly, it's very likely because it cannot be provided, because the differences become negligible in the far field.
 
#55
stoneeh said:
The most thorough accelerometer measurements of loudspeakers I've seen have been a dozen or so points distributed somewhat evenly over a side panel. Often, it's just a few, or even just 1 point. And even with many points, we usually get no information about phase, only velocity.
Click to expand...

A cheap, qualitative way to do this is to use a mechanics stethoscope, You can quickly check all over a panel and hear what is leaking out. I guess one could connect a mic at the hearing end and get a qusntitative measurement.

dave
 
#57
ron68 said:
You are right, increasing frequency (rigid bracing) is one thing and dumping is another.
Click to expand...

And a bit counter to one another. Damping… adding mass without increasing stiffness lowers resonances and decreases the Q, making them easier to hear.

At HF plywood is pretty well damped. The glue between each ply, and the alternating grain dump well.

dave
 
#59
stoneeh said:
Great marketing - but nothing of substance
Click to expand...
Well, the measurement compares bracing without the damping and that with it. Of course one velocity on one point on a boundary doesn’t say that much, but two, like here, do (a little) more. And of course we hope KEF did their homework properly and this was one measurement of many. They sure have (had) the tools for it.

All the same sound transmission through boundaries of speakers isn’t my biggest concern. In domestic appliances even panel resonances aren’t that bothersome.
 
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