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SteveW
01-12-2006, 06:27 AM
For all you acoustical folks......

- When calculating/predicting boundary interference null/peaks, what is the appropriate reference location to use within the driver? RPG says assume it is at the dust cone. Another respectable source suggests it's further back - behind the voice coil in the magnet area.

- Would this location change when calculating rear boundary as opposed to side boundary?

- Does room mode frequency shift up or down with added absorption to a room? A speaker cabinet is just a small room. What happens there?

Thanks

Maron Horonzakz
01-12-2006, 07:03 AM
HUH? A speaker cabinet is just a small Room ? :blink:

SteveW
01-12-2006, 07:26 AM
Maron,

Both rooms and cabinets have a cavity volume enclosed by boundaries. Each has resonant frequencies. :)

Steve

Robh3606
01-12-2006, 08:38 AM
"Does room mode frequency shift up or down with added absorption to a room? A speaker cabinet is just a small room. What happens there?"

Your adding dampning. You can't change the primary modes without moving walls. The amount of padding in a speaker does shift things a bit because the apparent volume changes not the actual box boundry size, here you are tuning a box to a specific frequency and volume matters. Rooms are not tuned, well at least you hope the are not. You are trying to accomplish the complete opposite of a speaker enclosure.

"When calculating/predicting boundary interference null/peaks, what is the appropriate reference location to use within the driver? RPG says assume it is at the dust cone. Another respectable source suggests it's further back - behind the voice coil in the magnet area."

I would hazard the guess the acoustic center of the driver where ever that would be. You are talking inches in one plane where wavelengths are in feet down low. Does it really matter at 60Hz??? What about rear facing ports????? Your not going to know what's really going on until your speakers in there anyway.


"Would this location change when calculating rear boundary as opposed to side boundary?"

Side no rear???? Does it really matter??

I would get a good reference and use that as a basis for looking for a Professional Installer. You really need a Pro Installer to do this right. They will have the tools and know how.


Rob:)

Earl K
01-12-2006, 08:46 AM
Hi SteveW,

I suggest buying this book ( it'll offer many practical solutions to all sorts of room based acoustic problems ) ;

The Master Handbook of Acoustics ( 4th Edition )
( Author ) F. Alton Everest
( ISBN ) 0071360972
( Publisher ) McGraw-Hill ( previously published by Tab Books )


Here's the book (http://www.amazon.com/gp/product/0071360972/qid=1137077266/sr=1-1/ref=sr_1_1/002-6651239-9187220?s=books&v=glance&n=283155) available, online from Amazon books .

- You mentioned RPG / I assume you mean RPG Diffusor Systems Inc. (http://www.rpginc.com/)
- You'll find the author quite a fan of this company along with the intelligent implentation of its' products ( for all sorts of playback rooms ) . The author also indicates how to build a bunch of functionally similar products ( at least he does in the 2nd edition of this book / the version that I own ).


- When calculating/predicting boundary interference null/peaks, what is the appropriate reference location to use within the driver? RPG says assume it is at the dust cone. Another respectable source suggests it's further back - behind the voice coil in the magnet area.

- I believe that you're referring the "Acoustic" Center ( or the Apparent Apex ) of individual drivers . These "effects" need to be measured & established with FFT capable equipment ( with time windowing ) to reasonably establish where the acoustic wave appears to originate from .



- Would this location change when calculating rear boundary as opposed to side boundary?

- My "guess" is that one won't go too wrong by adding in series ( to your modal acoustic equation ) the electrical performance of your playback system ( these would be the realworld, frequency dependant , mechanically induced, propagation delays, as given in feet or inches ) .
- Though in the real world / I don't think these apparent ( added ) path-length differences contribute much of any significant change to the fundamental room modes that one must contend with.
- Why ? ( usually ) the speakers' one has on hand ( as well as the room to be used ) are facts of life for all but the most diehard listening enthusiast.
- ie ; so stated another way, put the speaker in your room & then measure all the offending modes ( interactive FR anamolies occuring between the playback system & the room ). Then look for some room treatment solutions ( this bypasses most of the math, unless your building a room from scratch ) .



- Does room mode frequency shift up or down with added absorption to a room?

- That's too simplistic a question. For meaningful answers , you'll need to study the book to learn ( some new questions ). All the different room modes involved are explained and solutions offerred to modal problems .
- But FWIW, one could theoretically increase the virtual volume of a room by adding in a sizable percentage of absorbing material ( say more than 5% volume ) .

A speaker cabinet is just a small room. What happens there?
- Yeh true,,, but IMHO, your thinking has just gone a bit too 3D to be useful without the proper backgrounding, ( as provided by the before mentioned book ) .

:)

Maron Horonzakz
01-13-2006, 07:20 AM
Read da book & loin something.;)

SteveW
01-13-2006, 12:36 PM
[QUOTE]- I believe that you're referring the "Acoustic" Center ( or the Apparent Apex ) of individual drivers . These "effects" need to be measured & established with FFT capable equipment ( with time windowing ) to reasonably establish where the acoustic wave appears to originate from .

Earl K, problem is I've looked in the books, etc. If the subject is even addressed at all, the answers vary. Hence my question here. I was hoping that someone here actually knew, or had the kind of information (on a 2235) that you referenced above.

[QUOTE]
- The amount of padding in a speaker does shift things a bit because the apparent volume changes not the actual box boundry size, here you are tuning a box to a specific frequency and volume matters.


Robh3606, perhaps I should ask the question a different way: What direction does the measured resonant frequency shift in a cabinet after adding dampening material? I'm assuming a larger enclosure (a room) would behave the same way in this regard. No? The analogy to the cabinet was my thinking there would be a larger pool of knowledge to comment.

There is a method to the madness here.... to maintain a specific desired resonant frequency in the room - after adding significant dampening, there is a floating wall that can be moved as required. With mimimal absorption in the room now, it is spot on. I just want to know which direction it's going to go when I add more dampening. An answer to the question will tell me.

Thanks to both Robh3606 and Earl K for the helpful comments.
Steve

Zilch
01-13-2006, 01:00 PM
MLSSA measures acoustic center distance directly, CLIO indirectly. See D'Appolito Testing Loudspeakers 7.5.5.

Earl K
01-13-2006, 01:33 PM
There is a method to the madness here.... to maintain a specific desired resonant frequency in the room - after adding significant dampening, there is a floating wall that can be moved as required. With mimimal absorption in the room now, it is spot on. I just want to know which direction it's going to go when I add more dampening. An answer to the question will tell me.


OK;

- Adding volume ( real or virtual ) to a sealed enclosure will lower it's resonant frequency .

:)

SteveW
01-13-2006, 01:38 PM
Thanks Zilch and Earl K. Ask the right question...get the right answer!

Steve

DRG
01-13-2006, 09:54 PM
For all you acoustical folks......

- When calculating/predicting boundary interference null/peaks, what is the appropriate reference location to use within the driver? RPG says assume it is at the dust cone. Another respectable source suggests it's further back - behind the voice coil in the magnet area.

- Would this location change when calculating rear boundary as opposed to side boundary?

- Does room mode frequency shift up or down with added absorption to a room? A speaker cabinet is just a small room. What happens there?

ThanksHere is Greg's answer to your original question:

In my opinion it is not going to be very critical. Mislocating the
speaker somewhat just shifts the calculated frequencies a little. We
actually never know the exact room size anyway because of wall flex
which makes the room slightly larger than it measures. I would use
either the baffle or dust dome for a location. The acoustic center of
the woofer is actually a little behind the motor, but time has nothing
to do with this. You need to know where the wave front actually
originates and that has to be on the cone or baffle of the speaker.

He would have posted himself but he is quite busy with the 60th Anniversary Statement System.

SteveW
01-13-2006, 10:56 PM
Doesn't get much better than that, thank you.

Greg......who's Greg? :applaud:

Thanks DRG