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04-23-2015

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maxx
Posts 9
Joined on 04-23-2015

Post #:	1
Post ID:	21639
Reply to:	21639

Back chamber

Hi all!

I have build few horn projects. And i can't figure out how i can get rid off back chamber nasty sound (resonances) (300-500hz region).
Horn is 150Hz conical with JBL 2020H driver in 14L cilinder.

09-19-2015

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noviygera

Chicago, IL
Posts 177
Joined on 06-12-2009

Post #:	2
Post ID:	22003
Reply to:	21639

The role of the back chamber

I would like to ask for your thoughts on the importance of back chamber shape and construction. This topic became of interest when I learned that the back chamber of my midrange horn (250hz - 2khz) is noticeably resonating when I put my hand on it, around 500-600hz region. As you can see the original design, it's not big chamber and seems to be ok volume, creating a 190hz knee below which the freq response starts to fall off as this horn is specified to be 200hz and up.
However I am thinking it must be very important to forming a good bandwith limit AND at the same time, not adding own sound to the driver above that limit by resonating at all.
Does it need to be as rigid as possible? Does it's internal shape matter and are standing waves as issue in a small chamber? Should all volume be stuffed with damping felt?
So I start playing with improving this thing and a few things come to mind such as: 1. making smaller internal volume to raise the low freq knee (because I use crossover of 300hz) 2. increasing all external walls strength of this plastic chamber with wood frame or fiberglass epoxy 3. damping internal walls with either cork or aluminum butyl sheets (dynamat) 4. adding or subtracting damping material What are good ideas to try and what is the final goal if you learned that your chamber is flimsy and resonating?

Gera

09-19-2015

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anthony
Posts 338
Joined on 08-18-2014

Post #:	3
Post ID:	22004
Reply to:	22003

Start here...

http://goodsoundclub.com/Forums/ShowPost.aspx?PageIndex=1&postID=6011#6011

The back chamber is very important...about half of the drivers energy goes back there so it needs to be strong if you don't want it to resonate.

09-19-2015

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rowuk

Germany
Posts 454
Joined on 07-05-2012

Post #:	4
Post ID:	22005
Reply to:	22004

Resonance is not resonance

Being a musician, I deal with constructive resonance every day. Tubes resonate as a function of length, boxes resonate as a function of the internal length, width, diagonals. Irregular shapes resonate as a function of volume, horns resonate as a function of taper and length.

To start any discussion about how to deal with resonance, we need to know which ones are the problem. A speaker driver has a free air resonance. It also has diaphragm resonances. In a small closed box, those factors can also change.

The easiest way to eliminate box resonances, is to eliminate the box OR make it so big or so small that the desired pass band is not affected. If the problem is the diaphragm or motor, nothing that you do to the box can fix that.

That all being said, I know of no real "no bullshit" study on the effect of back chamber resonance on sound. I know of a lot of audiophile BS about what they think that they hear, or by inaccurate measuring they extrapolate bad sonics.

What Romy mentioned with "half of the drivers energy" really has NOTHING to do with the resonance per say. It is merely saying: make the enclosure solid and inert. That would only prevent IT from vibrating, but not change the effects of internal box resonance going back through the driver and distorting the front wave.

So, to discuss resonance intelligently, we need to know what you hear, what you measured and how loudly you must be playing for enough energy at 600Hz to vibrate a box attached to a horn. I mean 200 Hz I could understand, but 600? If you are playing extremely loudly, the question is if that resonance is even coming from the driver/box.

Whenever I feel blue, I start breathing again.

09-19-2015

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Paul S
San Diego, California, USA
Posts 2,656
Joined on 10-12-2006

Post #:	5
Post ID:	22006
Reply to:	22003

Trial and Error

The usual basic takes are: 1) Try to prevent resonance by using inert materials and/or construction. 2) Change the resonance frequency. 3) Spread it around. Basically, you won't know how something will affect your sound until you play with it, to get a better sense of how various treatments actually affect the sound from your driver in the situation it's in. You might try a heavier and/or more rigid back chamber, larger or smaller, and you might try lining and/or stuffing it with different materials, and tighteing and loosening connections, with softer or harder gaskets... the list goes on and on. At some point you will find the best compromise version of what you want to hear from the driver. On the other hand, can you actually hear a "bad influence" from the resonance that you mention here? Anyway, if you doggedly target precisely the part that bothers you most now you will at least find out sooner the price you will pay to address that problem in that way, and you can go from there, trial and error. Yippee.

Best regards,
Paul S

09-20-2015

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Romy the Cat

Boston, MA
Posts 10,156
Joined on 05-28-2004

Post #:	6
Post ID:	22008
Reply to:	22005

Do not be confused

rowuk wrote:
Being a musician, I deal with constructive resonance every day.

I told many time that conversations with musicians about resonances is a waste of time when audio is under discussion. Even in audio and instruments playing we call and understand the very same physical process but application of that physical possess are VERY different in audio and music playing. In music playing human awareness involved and a musician after learning all resonances consequences use different playing techniques to avoid or most frequently to use resonances creatively and with accordance of own expressive needs. In audio we do not have it and the resonances are brainless subject of only frequency and acoustic pressure.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

09-20-2015

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noviygera

Chicago, IL
Posts 177
Joined on 06-12-2009

Post #:	7
Post ID:	22009
Reply to:	22004

Further steps

anthony wrote:

http://goodsoundclub.com/Forums/ShowPost.aspx?PageIndex=1&postID=6011#6011
The back chamber is very important...about half of the drivers energy goes back there so it needs to be strong if you don't want it to resonate.

Thanks for this link, I've studied Romy's guide and now have some more, hopefully, pertinent questions:
1. Let's say the my original back chamber from the manufacturer is a good "starting point". Is Romy saying that those last few hertz of the fine tuning of the chamber volume are very important and audible, in other words the manufacturers design may be roughly correct but in my room and with my speaker placement it's worth it to further adjust the volume, in which case the manufacturer's design may not be optimal?I am talking NOT about midbass horn here, I am talking about midrange horn 300 to 2Khz.2. Further, once above chamber volume is fine tuned, how important is the shape of this chamber?3. No mention of sound absorbing material in back chamber. Is it needed?
So far it seems like it's a strong empty box of a very precise volume...

09-21-2015

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Romy the Cat

Boston, MA
Posts 10,156
Joined on 05-28-2004

Post #:	8
Post ID:	22010
Reply to:	22009

I would not worry about it. Set it once and forget.

noviygera wrote:
1. Let's say the my original back chamber from the manufacturer is a good "starting point". Is Romy saying that those last few hertz of the fine tuning of the chamber volume are very important and audible, in other words the manufacturers design may be roughly correct but in my room and with my speaker placement it's worth it to further adjust the volume, in which case the manufacturer's design may not be optimal?I am talking NOT about midbass horn here, I am talking about midrange horn 300 to 2Khz.2. Further, once above chamber volume is fine tuned, how important is the shape of this chamber?3. No mention of sound absorbing material in back chamber. Is it needed?

Well, if we had “manufacturers” of the midbass horns then it is very true they will be very seldom “correct”. There is however no manufacturers and all midbass horns are home made. Still, even the home made horn are very seldom correct, including my own midbass horns. The temperature is change, the furniture is moved, the driver cone suspension become less stiff as the driver work mode, the magnet get demagnetized… All of it and a few other factors make the perfect size of the back chamber made with precision of 1 Hz of Fs to be a bit imperfect. Why? Well, do you change your chamber calibration when season change in case your back chamber expose to temperature fluctuation. Do you re-adjust your back chamber after I would say a year of use when you diaphragm got more broken-in? Do you adjust your back chamber after you change your output tube on your DSET and the plate impedance was 10% different then during the calibration? So, the point that I making is that the precision of back chamber is in a way always approximate and we just set it when it shall be precise but we do understand that it will run away and will not be precise. I do not feel that it is bad to revise the precision once a while but I do not think that many people do it out there. It is like the people who rotate the large woofers – we all know that we need to fight gravity but how many people in reality do it?

About the need to deal with precision of back chamber at 300Hz? Yes, I would do it but the impact would be much lower. The type of the lower know that you get at 300Hz will be to a great degree overridden by your upper midbass channel. Also, as you set up your back chamber at 300Hz once it will hardly move away from it. So, I would not be too anal about back chamber at 300Hz.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

09-21-2015

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noviygera

Chicago, IL
Posts 177
Joined on 06-12-2009

Post #:	9
Post ID:	22011
Reply to:	21639

Damping material

Thank you for your response. I do find myself in a better position to optimize. Last thing is damping inside the chamber. Are there good rules of thumb on this, just like with volume tuning?

thank you. Gera

09-22-2015

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anthony
Posts 338
Joined on 08-18-2014

Post #:	10
Post ID:	22012
Reply to:	22011

I would be surprised...

noviygera wrote:
Thank you for your response. I do find myself in a better position to optimize. Last thing is damping inside the chamber. Are there good rules of thumb on this, just like with volume tuning?

thank you. Gera

...if you had a lot of room for damping material in that backchamber. In a regular sealed box speaker damping material is often used to present to the driver a larger apparent cabinet size (so you can make a smaller box) and to a lesser extent to lose a little of that energy through friction. In this case you need to find the right internal volume and not necessarily reduce its external dimensions. Also, horn backchambers tend to be quite low volume and combined with the dust/residue/crud issue with the magnetic gap of the driver I doubt you could get any positive long-term result by using damping material. Do not let me dissuade you though, I am just giving a general opinion and not necessarily one pertinent to your horns.

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