Free air resonance

@erik_squires

 

thank you Eric.

I am working on finishing the room.
All the walls are 5” rock wool.
The gable ceiling is 12” rock wool.
All covered with fire resistant burlap instead of sheet rock.
The floor is hard wood.

Essentially, a semi-anechoic room. Very dead: the difference in sound quality is immediately perceptible as you climb the stairs into the room.

I was told by the guy who built my Futterman OTL3s that stiffening lowers the resonant frequency, but he was also quick to offer that speaker cabinet design is not his area of expertise. What you said about stiffening the baffle only makes good sense, as I would like to decrease the energy flowing through the cabinet walls. 

I think you are really getting confused about how the resonant frequency of a speaker driver (a good thing) works and mixing it up with how cabinet panels resonate (which is completely undesirable). The resonant frequency of a driver is used to model the bass performance in a cabinet in terms of the -3dB point as well as the slope and optimal flatness (Q) of the low end. It has nothing to do with distortion introduced due to imperfect cabinet materials. Along with Qts, and equivalent air mass and other parameters the resonant frequency is something you plug into your cabinet design software.

Driver resonance frequency has zero to do with the materials used or bracing methods used to construct the cabinet itself. The driver resonant frequency also has zero to do with driver value or quality but it is an indicator of the type of driver you have on your hands. A driver with a resonant frequency of 300 Hz is probably not going to go lower than a midrange for instance.

Also, you want diffusion between and directly to the sides and often behind the speakers and listening area. Anything truly anechoic is going to sound too dead, and lack imaging and space.

@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! 

@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.

@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.