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Free air resonance

Hello all,

Is a speaker with a free air resonance of 25 hz meaningfully different from one with a free air resonance of 38 hz?

Specifically: is the one at 25 hz low enough to be in a sealed enclosure, as opposed to the one at 38 hz which most likely/definitely should be in a ported enclosure? And why?

Thank you in advance …

unreceivedogma
mijostyn's avatar
mijostyn
8,570 posts
 

@unreceivedogma , AHH the Duplex. I was thinking Voice of the Theater. Nice trick with the burlap. Yes, I think a rug would be in order.  I beat you by one year. I built my ST 70 and PAS 3X when I was 13. A friends father gave me a pair of AR 2ax's which he used at a party and wanted to get rid of. He owned an electronics firm in Boston. 

Your woofer crosses to the horn at 2 kHz and cuts off at 60 Hz. I can not find the T-S parameters for the 604a,b or c. I would treat it like a Bozac woofer with a large sealed enclosure on the order of 8 cubic feet. They are crying for custom enclosures. IMHO you need two subwoofers one next to each main speaker crossing as high as possible to get the low bass out of the Duplexes. It will make the Duplexes so much cleaner. I have never heard the Futtermans but I use triode OTLs and I bet they are a great match for the Duplexes. They are another reason though to take the low bass somewhere else. Are you high passing the Duplexes now? You mentioned custom crossovers. 

unreceivedogma's avatar
unreceivedogma OP
1,085 posts
 

@mijostyn

- there will be three Moroccan rugs. 🙂

- I use Doug Sax’s Mastering Lab crossovers. http://www.rintelen.ch/download/604xover_SP.pdf I had Jon Specter replace all the caps with Jensen audio grade caps

- as for the current enclosures, they ARE custom DIY enclosures, built to Altec specs of the 1960s. https://www.hifiengine.com/manual_library/altec-lansing/620a.shtml I think there is room for improvement, but I don’t (yet) know the first thing about it, engineering-wise.

- Altec claims that the 604C and D go down to 30HZ

- As for the Velodyne UHL15, it seems to crossover at 85HZ, see https://www.stereophile.com/content/velodyne-uld-18-amp-uld-15-subwoofers-specifications and https://www.manualslib.com/manual/533050/Velodyne-Uld-15.html?page=10#manual I don’t know that it can be adjusted to 100.

- T-S parameters?

- I had Lafayette speakers to go with the Dyna. I couldn’t afford AR speakers.

 

see what J Frum says here:

https://hifihaven.org/index.php?threads/embiggening-the-altec-620-thoughts-experiences-with-9-cu-ft-cabs-for-altec-604-8g.7649/
 

Y’all need to get religion with regard to enclosure simulation. With T/S parameters, modeling a basic bass-reflex cabinet is easy and accurate. It’s a brave new world.

You’ll learn all about the delicate balancing act between the driver’s electromechanical properties, enclosure volume, and tuning. It’s a lot more complicated than “bigger = better”, and you’ll see the effect changes have on frequency response, power handling, and group delay.

I’ve modeled the 604-8G every which-way, and I can’t come up with anything that’s better overall than a 9 cu. ft. cabinet tuned to about 40 Hz. - essentially, a 620.

I’ve said it before, and I’ll say it again: It’s almost like those Altec engineers knew what they were doing.

mijostyn's avatar
mijostyn
8,570 posts
 

@unreceivedogma 

I could not afford them either. Just happened to be at the right place and time.

I read an Altec spec sheet on the Duplex and the 3dB down point was 60 hz which would be about right for a woofer of that type of construction. 

T-S parameters (Thiele/Small) are a set of parameters that define a driver's electromechanical behavior. From them you can calculate the volume of the enclosure you need to obtain a given type of performance. They help getting you in the ballpark when designing a loudspeaker.  https://en.wikipedia.org/wiki/Thiele/Small_parameters

In order for me to get a handle on your enclosures I need the interior dimensions and the material they are made of such as 3/4" plywood or 1" MDF. The type of enclosure, sealed or ported and the dimensions of the port or ports. In today's world of loudspeakers, with the advent of DSP you can get away with murder. I design and build my own subwoofers and rely on digital signal processing to get them under 20 hz at + 6 dB. I am about to shoot the finish on my new ones and will publish pictures of the entire process on Imgur when I am finished. The best way to handle subwoofers is with digital bass management which is antithetical to the type of system you have or rather the type of attitude you have. It certainly is more of a challenge and I understand the attraction. You can push the envelope a lot further. You can high pass your  Duplexes by putting the right value capacitor in series with the input of your amps. You have to know the input impedance to calculate the value. Then low pass the sub with the filter it has, the highest it will go without calling attention to itself. With two subs I would definitely plug the ports but I have to warn you that I really dislike ported speakers. I would rather push the speaker down with digital bass management and use subwoofers designed specifically to make low bass. What you have in the Duplex woofer is really a very large midrange driver. Middle C is 254 Hz. In it's day it was a very fine driver and still is. Back then subwoofers were nonexistent.  

 

unreceivedogma's avatar
unreceivedogma OP
1,085 posts
 

@mijostyn 

 

"....In order for me to get a handle on your enclosures I need the interior dimensions and the material they are made of such as 3/4" plywood or 1" MDF."

I am certain that they are plywood but I can tell you tomorrow. Altec specs from their brochure below: I will check to see if mine in actuality do match exactly.

 

SPECIFICATIONS

Type: Bass reflex enclosure

Dimensions:  40" (1 01 .6 em) H  26" (66.0 em) W  18" (45.7 em) D

Pressure Sensitivity: 100 dB SPL at 4' with ALTEC 604-8G loudspeaker when measured on axis with 1 watt input of band-limited pink noise from 100 to 10,000 Hz and calculated to 4' equivalent (Ref.: 0 dB == 0.0002 dyne/cm2)

Speaker Accommodations: 15", front-mounted

Internal Volume 9.0 cubic  ft

Weight: 104 pounds (47.2 kg), enclosure only

Finish: Hand-rubbed oil on rift-cut oak, charcoal brown grille cloth

Recommended ALTEC Speaker: 604-8G Duplex Loudspeaker

 

ARCHITECT'S AND ENGINEER'S SPECIFICATIONS

The speaker cabinet shall be the bass reflex type, and shall provide front mounting for 15" speakers only. The cabinet shall meet the following criteria. Grille assembly,snap-on. Grille cloth, charcoal brown. Internal volume, 9.0 fe. Dimensions, 40" H x 26"W x 18"0. Weight, 104 pounds. Finish, hand-rubbed oil on rift-cut oak.

The speaker cabinet shall be the ALTEC Model 620A.

timlub
2,043 posts
 

https://forum.audiogon.com/discussions/ported-versus-sealed-speakers-is-one-type-better/post?highlight=QTS&postid=1951168#1951168

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