Free air resonance

Thank you @mijostyn ​​​​@mahgister

This is not about a subwoofer.

The reason I ask is that I have a pair of Altec Lansing 604Cs and I now, as of last night, was able to get a 604D to match the 604D that I already have.

I am currently using the 604Cs. The D was a backup in case one blew. Now that I have two D’s, should I switch?

As far as I can tell, however, the ONLY difference in the specs is the free air resonance.

I will be putting the cabinets on rubber feet designed to isolate them from the wood plank floor so that the building doesn’t become a de-facto loudspeaker. I’d like to close the port so as to make the midrange more detailed. This entails losing some bass, but …

I use a Velodyne ULD15, with an active crossover at 60hz for the bottom.

I’m thinking that a speaker with lower resonant frequency will sound better than the one with a higher resonant frequency.

But is the difference too small to even matter, or are the numbers a logarithmic progression instead of arithmetic and thus the difference bigger than one might think? And, does having an active sub crossover at 60hz make this question moot?

one other question: the cabinets are DIY, made of 3/4 ply with a veneer, geometry and materials to Altec specs standard in the day (1950s/60s). Would the bass benefit by stiffening them with another 3/4 layer of ply or solid wood on the interior? If so, do I stiffen all 6 sides, or would just the 4 perimeter sides (not front and back) suffice?

Also, someone told me that one mechanical difference between the two is that the stock C used a paper surround and the stock D used an accordion surround. As Cs are today almost always reconed with the accordion surround (mine are), and as the materials of the cone help determine the free air resonance, simply reconing the C allegedly makes it a D.

And yes, while I did once have a high math aptitude I’m these days inclined to look for layman’s answers to these questions, or as simple as possible.  

Thoughts?

Thank you again.

Ask erik_squires he is very helpful...

He know basic acoustic ...

He know a lot about speakers design, he created some,  and i know nothing useful for you  for this complex problem solution  ...

Thank you @mijostyn ​​​​@mahgister

This is not about a subwoofer.

The reason I ask is that I have a pair of Altec Lansing 604Cs and I now, as of last night, was able to get a 604D to match the 604D that I already have.

I am currently using the 604Cs. The D was a backup in case one blew. Now that I have two D’s, should I switch?

As far as I can tell, however, the ONLY difference in the specs is the free air resonance.

I will be putting the cabinets on rubber feet designed to isolate them from the wood plank floor so that the building doesn’t become a de-facto loudspeaker. I’d like to close the port so as to make the midrange more detailed. This entails losing some bass, but …

I use a Velodyne ULD15, with an active crossover at 60hz for the bottom.

I’m thinking that a speaker with lower resonant frequency will sound better than the one with a higher resonant frequency.

But is the difference too small to even matter, or are the numbers a logarithmic progression instead of arithmetic and thus the difference bigger than one might think? And, does having an active sub crossover at 60hz make this question moot?

one other question: the cabinets are DIY, made of 3/4 ply with a veneer, geometry and materials to Altec specs standard in the day (1950s/60s). Would the bass benefit by stiffening them with another 3/4 layer of ply or solid wood on the interior? If so, do I stiffen all 6 sides, or would just the 4 perimeter sides (not front and back) suffice?

Also, someone told me that one mechanical difference between the two is that the stock C used a paper surround and the stock D used an accordion surround. As Cs are today almost always reconed with the accordion surround (mine are), and as the materials of the cone help determine the free air resonance, simply reconing the C allegedly makes it a D.

And yes, while I did once have a high math aptitude I’m these days inclined to look for layman’s answers to these questions, or as simple as possible.

Thoughts?

Thank you again.

 

Hah, thanks @mahgister - the free air resonance is not, by itself, the most important thing in determining the use of a ported enclosure or not, and certainly nearly useless in determining in-room performance.

The questions of whether this is best as a ported or sealed cabinet is a different question than "what would work well in my room."

I’m thinking that a speaker with lower resonant frequency will sound better than the one with a higher resonant frequency.

I think this is not the right way to think about it. The question is driver to cabinet matching and then matching the total speaker (driver in a specific cabinet) to the room. There’s no best answer based on resonant frequency alone.

These questions are best asked in DIYaudio but you can also read the last paragraph here:

https://eminence.com/blogs/blog/sealed-vs-ported-enclosures

@unreceivedogma , Boy are you making like complicated.

The free air resonance of a driver is determined mostly by the mass of the construct (cone, suspension, voice coil and former) and the stiffness of the entire suspension. I doubt you will hear any difference between the drivers. Try them both. 

As for the enclosures, each surface has a resonance frequency. Every enclosure is a symphony of resonances. If you stiffen the walls by doubling up on the plywood you will raise the frequency of those resonances. There is no way to know if that will be better or worse than what you have now. Certainly a very stiff, heavy baffle (the surface to which the woofer is attached) is beneficial. It will limit the transfer of energy to the rest of the enclosure. I think I would leave everything else alone. 

If you really want to make them sing get another subwoofer and a two way crossover. Cross at 100 Hz or even higher. The woofers in that speaker carry a significant portion of the midrange which is being Doppler distorted by bass below 100 Hz. Taking those frequencies away from the Altecs will clean them up  noticeably, very noticeably at higher volumes. 

I was right about you  ... 😊

Your answer make sense for me and help me thanks... I hope it will helpful fot the OP...

Hah, thanks @mahgister - the free air resonance is not, by itself, the most important thing in determining the use of a ported enclosure or not, and certainly nearly useless in determining in-room performance.

The questions of whether this is best as a ported or sealed cabinet is a different question than "what would work well in my room."

I’m thinking that a speaker with lower resonant frequency will sound better than the one with a higher resonant frequency.

I think this is not the right way to think about it. The question is driver to cabinet matching and then matching the total speaker (driver in a specific cabinet) to the room. There’s no best answer based on resonant frequency alone.

 

These questions are best asked in DIYaudio but you can also read the last paragraph here:

 

https://eminence.com/blogs/blog/sealed-vs-ported-enclosures