OSA Woofer Design

[James440]

Hello,

I'm very excited to finally figure out how to join the forums, as for many years I was not able to! 

I recently arrived at college and I'm very excited to have purchased my own pair of Smaller Advents! Unfortunately I got screwed and the original woofers are long gone . I'm looking to purchase a pair of original ones, if anyone happens to have a pair. But anyway, that's not what I wanted to ask in this thread.

I'm just very curious about the actual design behind these cabinets and woofers. I've read that they have amazing acoustics and sound about as good as the Large Advents. How is it possible that they could have produced the same sound from a smaller cabinet? I know the "spitwad" dust cap is mass damped which makes it very special. But what does this actually do for the woofer? I guess it increases the inertia of the cone which would allow it to extend further...? Am I correct with this theory or am I totally wrong?  I'd really appreciate any knowledge on this topic!

Thank you,

James

[Aadams]

Here are the results from a search.  Red Italics are a quote from the link.

The best example of this is the quite small The Smaller Advent Loudspeaker—AR-6 in size—about half the volume of full-size, 1.8 cu. ft. The Advent Loudspeaker, yet the small Advent has a fc of around 43-45 Hz, same as the large Advent, a third octave lower than competitive speakers such as the AR-6 or even the AR-2ax/AR-5, with -3dB at 48Hz.  This meant that the small Advent had a -3dB point of approximately 35 Hz, very low for such a small speaker system.  Henry Kloss wanted deep bass from a very small speaker, so he applied the so-called "Hofman's Iron Law" (simply a white paper quantifying, in simple terms, the well-known physics of sealed-box speaker design) and juggled the woofer's parameters, free-air resonance, cone diameter and damping, etc., to get a very low resonance. 

 

The trade-off in the Small Advent was very low efficiency (sensitivity) of the speaker, low power-handling capability and higher harmonic distortion.  I have a pair, and I speak from experience.  The speaker has very low efficiency and relatively low power-handling capability.  It was designed as a 4-ohm speaker to take advantage of modern solid-state amplifier's high current at low impedances, but it stressed some amplifiers.  It didn't sell particularly well with a sensitivity 2-3dB lower than the big Advent or other comparable speakers in its class, but it could go very low. 

 

The trade-offs weren't particularly appealing, and the speaker was not a sales success.  For the relatively few music-lovers who liked organ music played at fairly low volumes in an apartment or in a small room, this speaker did fine.  It's simply a matter of physics.  Compared with an AR-6, the Small Advent is significantly "punchier" and more potent-sounding with its lower fc, but it also sounds more "strained" than the similarly sized AR-6.  The latter has greater excursion capability and lower harmonic distortion, possibly accounting for perceived differences.

 

With a vented system, of course, the designer can use a combination of woofer parameters and vent size and length, tuning the system for the desired low frequency—down to a certain point, of course.  Below cutoff, of course, there is no protection for the excursion of the woofer cone, and vented systems with very low resonance frequencies are vulnerable to over-excursion, bottoming, excessive distortion or "chuffing" in the port, etc.  

[James440]

23 hours ago, Aadams said:

Here are the results from a search.  Red Italics are a quote from the link.

The best example of this is the quite small The Smaller Advent Loudspeaker—AR-6 in size—about half the volume of full-size, 1.8 cu. ft. The Advent Loudspeaker, yet the small Advent has a fc of around 43-45 Hz, same as the large Advent, a third octave lower than competitive speakers such as the AR-6 or even the AR-2ax/AR-5, with -3dB at 48Hz.  This meant that the small Advent had a -3dB point of approximately 35 Hz, very low for such a small speaker system.  Henry Kloss wanted deep bass from a very small speaker, so he applied the so-called "Hofman's Iron Law" (simply a white paper quantifying, in simple terms, the well-known physics of sealed-box speaker design) and juggled the woofer's parameters, free-air resonance, cone diameter and damping, etc., to get a very low resonance. 

 

The trade-off in the Small Advent was very low efficiency (sensitivity) of the speaker, low power-handling capability and higher harmonic distortion.  I have a pair, and I speak from experience.  The speaker has very low efficiency and relatively low power-handling capability.  It was designed as a 4-ohm speaker to take advantage of modern solid-state amplifier's high current at low impedances, but it stressed some amplifiers.  It didn't sell particularly well with a sensitivity 2-3dB lower than the big Advent or other comparable speakers in its class, but it could go very low. 

 

The trade-offs weren't particularly appealing, and the speaker was not a sales success.  For the relatively few music-lovers who liked organ music played at fairly low volumes in an apartment or in a small room, this speaker did fine.  It's simply a matter of physics.  Compared with an AR-6, the Small Advent is significantly "punchier" and more potent-sounding with its lower fc, but it also sounds more "strained" than the similarly sized AR-6.  The latter has greater excursion capability and lower harmonic distortion, possibly accounting for perceived differences.

 

With a vented system, of course, the designer can use a combination of woofer parameters and vent size and length, tuning the system for the desired low frequency—down to a certain point, of course.  Below cutoff, of course, there is no protection for the excursion of the woofer cone, and vented systems with very low resonance frequencies are vulnerable to over-excursion, bottoming, excessive distortion or "chuffing" in the port, etc.  

Thank you for the info, but I was mainly asking why is the dust cap weighted so much? What is the purpose of this? 

[Pete B]

This is really very simple.  Make the woofer suspension very loose, then the box air spring dominates

the compliance and you set the cone/dust cap mass to resonate with the air spring at about 42 Hz (Fc).

Give the voice coil enough overhang to get the displacement that you need and set the magnet size

to control the peak or lack thereof at Fc.

The tight air spring due to the small box requires high moving mass to obtain the low Fc, this is why

the moving mass is high.

They probably found that the big heavy dust cap was the best way to increase the moving mass without

causing increased cone breakup.

[James440]

On 3/23/2018 at 4:15 PM, Pete B said:

This is really very simple.  Make the woofer suspension very loose, then the box air spring dominates

the compliance and you set the cone/dust cap mass to resonate with the air spring at about 42 Hz (Fc).

Give the voice coil enough overhang to get the displacement that you need and set the magnet size

to control the peak or lack thereof at Fc.

The tight air spring due to the small box requires high moving mass to obtain the low Fc, this is why

the moving mass is high.

They probably found that the big heavy dust cap was the best way to increase the moving mass without

causing increased cone breakup.

Okay that's very interesting. Thank you for the info