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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 …

128x128unreceivedogma
mijostyn's avatar
mijostyn
8,537 posts
 

@erik_squires I agree entirely up until the last comment. This argument between wide dispersion and limited dispersion has been going on since I was knee high to a grasshopper.

Obviously, wide dispersion bounces sound all over the room creating more and louder reflections. Very limited dispersion, like a flat panel ESL is very annoying but sounds terrific if you lock your head in a vice. Controlled dispersion on the order of 45 degrees like you might get from a horn system or curves panel ESLs is best in terms of image and comfort. Dipole line sources are even better because of  strong attenuation to the sides, floor and ceiling.

I think it helps to think of sound like light. If you shine a flashlight at an object only things in line with the object will light up but switch on a naked bulb and the entire room lights up.  Omnidirectional speakers "light up the entire room." This usually makes a system very bright which some people like especially at low volumes. You may note that cymbals are poorly localized and there is a tendency towards sibilance with female voices and violins. A system with controlled dispersion usually sounds dull at first and people will think there is no high end. if you pay attention you will note that the cymbals are well defined and the high frequencies are there but now they are coming from the cymbal and not the entire room. Such a system is smooth and effortless without any sibilance. With loads of sound deadening you can make an omnidirectional system sound more like a controlled dispersion one at some expense and nail holes. 

@unreceivedogma , What is rock wool? I have not heard that term. Sounds like you have a nicely dampened room which is great because Altecs  have a tendency to get shouty. 

I think you are overthinking things. Use the pair of drivers you think are best constructed and listen to the results. Stiffening the front baffle of the cabinets makes sense but doing it right would mean removing the old one or using screws every 6 inches. You can not place clamps over such a large structure. My approach to the problem would be to make entirely new enclosures. I would remove the ports, decrease the volume depending on the math and cross to subwoofers at 80 to 100 Hz. That would be killer! 

erik_squires
16,021 posts
 

@mijostyn

 

There’s no argument about wide vs. limited dispersion in my writing, so I’m not really sure if you are replying to anything I wrote, except obliquely. That paragraph was in response to the idea of an anechoic room being ideal.

Acoustics and room treatments do go hand in hand though. The more controllable the dispersion the less room treatments are needed, but I know of no case outside of a measurement lab where a truly anechoic experience is a good thing.

I used to be a rep for Roger Sander’s speakers, and so I know the head-in-vise experience of a flat ESL very well. My statements as I made them, not as interpreted, stand.

While tight dispersion may give you the feeling of having your ear right up against the speaker they also do a poor job of communicating the illusion of space behind and to the sides of speakers, but a good solution when you have zero control over the room.  OTOH, if this is your ideal maybe headphones are a better solution for you. Far cheaper and requiring less fuss.

A room with controlled bass modes, a good mix of dispersion and absorption (including the ceiling) will outperform any attempt of recreating even the old Live-End, Dead-End experience, let alone anything approaching fully anechoic.

unreceivedogma's avatar
unreceivedogma OP
967 posts
 

@mijostyn @erik_squires

 

Thank you both, bunches. I’m learning a lot already.

Eric, the room is semi-anechoic, not anechoic. The floor would have to be treated as well, which it most certainly is not.

In my last home, I did the room this way and it worked very well. The room doubled as a library, and as I added books to the walls, the room became brighter: the books were the way to control room acoustics.

I started out building my own Dyna 70 and Dyna PAS when I was 14 years old, but since then I’ve otherwise never given too much technical thought to how to design a better system other than to substitute new components.

I’ve had the components I have now for 30 years on average, and with the new home (the wife and I leave this home feet first only: I came close 8 weeks ago, I got double bypass surgery after the discovery of a completely blocked Widowmaker, 😂. The surgeon says that after I complete recovery, I will be better than new! 🙂) and new listening room, and with my amp engineer who has worked on many home audio systems saying that he has heard only one other system that sounds as good as this one, I’m not gonna change the components, my thoughts have turned to how to get more out of what I have, so that means returning to thinking more about the science of these things. The weakness of the Altecs and of the Futtermans are convincing extension at the bottom. They are really good right now, don’t get me wrong but something does nag at me in this area. The cabinets are solid for their day, but there is some resonance there. They do allow for port size adjustments, but it’s just unscrewing a block of wood and sliding it up or down! Maybe painting the interior with a material that will stiffen the walls? I don’t know, and then it has to be reversible because what if it doesn’t work. A friend built beautiful new cabs for his Tannoys but he is top shelf carpenter, works for Broadway, I can’t afford him.

Maybe I need to change the subwoofer. But I don’t want to spend a lot of money either. The current sub crosses at 70. And maybe I just never found the right spot for that sub. In the new room, there will be lots of room to fool with placement.

configuration is

Cartridge: Koetsu Onyx

Tonearm: Sumiko MMT

Table: VPI HW MK IV with SAM

Step-Up Transformer: SHURE A86A

Preamp: Beard P505

Phase Alignment: BBE282ri sonic maximizer

Subwoofer: Velodyne ULD-15

Amps: Julius Futterman OTL3s, converted to triode by Jon Specter

Crossovers: Mastering Lab

Speakers: Altec Lansing 604C coaxial studio monitors

Semi-anechoic room design and treatment: by architect C.B. Wayne

cables: whatever

https://www.theaudioatticvinylsundays.com/about


Oh, and as for rock wool, it is used in a lot of commercial and audio acoustic panels. I use it to insulate my home because

- it’s the best thermal barrier

- it’s far and away the most fire resistant. Fiberglass, fire-resistant cellulose, etc burns by the time you hit 375 degrees This stuff doesn’t melt until you hit 2,000 degrees. An important consideration in historic buildings, of which all the buildings we have restored to date are

- it has excellent sound absorbing properties

My architect had done a few recording studios. He said that in my audio room, don’t sheet rock the walls Just cover the rock wool with burlap and then I will have the equivalent of a high quality recording studio for 1/20 of the cost, for the amount I was spending to make the thermal barrier anyway. Safe health wise also.

 

I don’t like using headphones  

Thank you again.

mijostyn's avatar
mijostyn
8,537 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
967 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.

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