Multiple Small Subs - Geddes Approach

[soongsc]

Where's the problem? Use a RTA and you know what the noise floor is and what impact it has on your waterfall diagram. In REW it's just one click ?it doesn't get easier than this.

Heh. I'm afraid I don't want to go OT to discuss that in detail. I Don't believe the manual explains anything to that much detail to ensure it will provide data that I had explained. Of course I can't expect people to understand this (as usual).

 

[soongsc]

...
Simulations can only tell you so much about real rooms, because rooms can easily violate the assumptions. Rooms like mine will clearly violate the models and its exactly those violations that I would contend offer the greatest gain. Its those things that make a room NOT act like a simple room - a modelable room - that improve it the most. You don't learn this from a model.

If the real situation is better than the simulation, and simulation will show good results already, then it seems to be what we expect from simulations. This is what we look for in any engineering situation. If there is something measured that contradicts this, then we just see how simulation can be realistically improved. Really no big deal.

 

[gedlee]

I think you are just afraid that the analysis reflects the reality of you recommended setup, and thus providing more credibility to john k's analysis.

Soongsc - I try and be patient with you but if you are going to level insulting accusations at me then I'm going to say to learn what you are talking about first and then you can insult me. You usually get things wrong, which you did here too.

 

[gedlee]

ok. I thought we agreed that the "direct" term was a fudge, but acceptably small error. That is, assuming a (relatively low) order modal calc is made, then the direct is not included, therefor a simple additional direct term is ok.

Todd My point has always been that there is a right way and a wrong way to add in the near field "fudge". You and Walker do it wrong. If its a small effect then why include it at all, and if its worth including then its worth doing right.

 

[cap'n todd]

Todd My point has always been that there is a right way and a wrong way to add in the near field "fudge". You and Walker do it wrong. If its a small effect then why include it at all, and if its worth including then its worth doing right.

But Earl, how do I include it without using a very high order, which takes too long? I tried up to 35 and didn't see much evidence of direct sound appearing (at a distance of 4" fromt the source). I did read the other day that using an area source rather than a point makes the summation converge faster. I'm not sure how to do that, but...

Also, I'm interested in having a seperate direct term, so thus the fudge. In any case don't forget that this is all a fudge if/since the absorption is non 0! THe model a fudge anyway, and probably in other ways too if you go through the derivations with a fine tooth comb.

 

[soongsc]

Soongsc - I try and be patient with you but if you are going to level insulting accusations at me then I'm going to say to learn what you are talking about first and then you can insult me. You usually get things wrong, which you did here too.

Well, if you do not care to address the Green's function vs. FEA in SoundEasy part, I fully understand your position, which is that others whom don't agree with you are wrong regardless.

It's a simple fact that all the best analysis in the world cannot generate result that match real world situation. But we still use it to try to get as close as possible. I simply can't see why when john k is putting himself on the line to do the simulation/analysis but you are not willing to provide your reference setup basic information and compare simulation versus measured data. This just draws some speculation doesn't it.

 

[gedlee]

But Earl, how do I include it without using a very high order, which takes too long? I tried up to 35 and didn't see much evidence of direct sound appearing (at a distance of 4" fromt the source). I did read the other day that using an area source rather than a point makes the summation converge faster. I'm not sure how to do that, but...

Also, I'm interested in having a seperate direct term, so thus the fudge. In any case don't forget that this is all a fudge if/since the absorption is non 0! THe model a fudge anyway, and probably in other ways too if you go through the derivations with a fine tooth comb.

I thought that I went through this with you before. Since the series does contain the direct field, adding it again is wrong. You can subtract out the direct field as a free space Green's function if you want, BUT then you must modify the remaining series to account for the part that you have subtracted. Morse shows how this is done for a membrane, but the exact same procedure works for the three dimensional acoustics problem and the remaining series is the same as Morse shows for the membrane.

It is true that a larger sphere as a source converges faster, but this is not so simple to do. Basically what happens is that the source is accurate beyond the radius of a 1/4 wavelength for your highest terms. Closer than that and the series solution is wrong. The bigger problem is the fact that your solution is real and thus cannot account for energy flow. If you look at the pressure gradient anywhere in the room, you will not find any net energy flow from the source or into the walls from absortion. In the interior of the room this is not a big factor, but near the source, where ALL the energy comes from, it is.

I did a paper in the early 80's showing this. A real solution will not find any energy flow from the source or into a sink. However a complex solution will. You will see energy flow from the source, but for a given limit of the series terms it only starts to appear at about 1/4 wavelength out. You will also see energy flow into a sink. We had a small patch, like an open window at one end of the room and the source at the other. The complex modes and eigenvalues were calculated with FEA. the energy could be seen to flow from the source, circulate arround the room as a complex pattern of standing and traveling waves and then exit the room.

It was this complex simulation that proved to me that source directivity at LFs in a room is a misnomer. The sound energy does NOT flow directly from the source to the sink but travels in a very complex, circuitous and indirect way. No directivity was ever evident at all.

Just for the record, I was doing room FEA calculations back in the late 70's. I had to wrote the code at that time as none existed to do acoustics. ANYSIS later adapted my techniques for use in their code. They used to credit this to me in their manuals. In the 80's I was doing complex modes and energy flow in rooms with FEA (NASTRAN). I'm not a novice at this.

 

[Patrick Bateman]

I'm really happy with the use of multiple subs. I started with three, and ramped it up to eight, mostly because my mains are so efficient. I am personally using a bunch of DIY subs, but it would be a lot less hassle to just go and buy a bunch of them.

If anyone reading this thread wants to give this a whirl, and do it cheaply, this might be an opportunity to do so.

Newegg has a $250 Polk subwoofer marked down to $100 at the moment. At that price, it's hard to go wrong. Try it out for a few weeks, and if you don't like the sound, go sell them on Craigslist. I wouldn't be surprised if you could RE-sell them for more than $100 each. They are currently $200/each at Crutchfield, and $150/each at buy.com. It's hard to find a plate amp for under $100, much less a finished sub.

Here's the manual:
http://www.polkaudio.com/downloads/manuals/home/PSW10_12_Manual.pdf

Here's the sub:
Polk PSW10

The promo code to get it for $100 is EMCLNLP28.

 

[gedlee]

I recommended that sub recently. A few of those would do the trick, and very cheaply.

 

[cap'n todd]

I thought that I went through this with you before. Since the series does contain the direct field, adding it again is wrong. You can subtract out the direct field as a free space Green's function if you want, BUT then you must modify the remaining series to account for the part that you have subtracted. Morse shows how this is done for a membrane, but the exact same procedure works for the three dimensional acoustics problem and the remaining series is the same as Morse shows for the membrane.

Yes, I thought we agreed that the series does contain the direct sound field IF CALCULATED TO A VERY HIGH DEGREE. Since I'm only using the first few terms, it should not have much of the direct field in it, but still reasonalby accurate for modes (also I generally dont have source and receiver that close together). So then adding in a direct term would be appropriate. Of course its not mathematically absolutely correct, but unless your telling me that it's a significant error, I'm happy. Remember, in my work I often need to simulate a large number of rooms/configurations in one go. In other words the code needs to be fast, so a good approximation is necessary. For example I did a study of Bonello criteria where I simulated hundreds of different room dimensions, each with hundreds of receiver locations.

I hope you're not quibling about a theoretical mathematical issue which has no significant effect. I already agreed long ago that it's an approximation

 

[ro9397]

Earl
What is the ideal recommended f3 bass cutoff for the main speakers with correctly setup multiple subwoofers?

The answer to the above question could save money & enlarge one's list of potential main speakers (& also assist w/ the speaker-to-amp interface).

Incorporating your advice has been as close as I expect to ever find to the proverbial free gourmet lunch.

 

[gedlee]

Earl
What is the ideal recommended f3 bass cutoff for the main speakers with correctly setup multiple subwoofers?

The answer to the above question could save money & enlarge one's list of potential main speakers (& also assist w/ the speaker-to-amp interface).

Incorporating your advice has been as close as I expect to ever find to the proverbial free gourmet lunch.

There is no "ideal". You fit the subs to whatever response the mains provide. I've done this with several f3's and all of my speakers have f3's.

Yes, doing multiple subs the way that I suggest, seems to be a no-brainer. I remain unconvinced that doing anything more is required.

 

[gedlee]

I hope you're not quibling about a theoretical mathematical issue which has no significant effect.

Whether it is or isn't, you will be forever open to criticism if you continue to do it wrong.

 

[cap'n todd]

Whether it is or isn't, you will be forever open to criticism if you continue to do it wrong.

If there's another way thats more accurate that calculates as fast, I'm all ears.

 

[Bigun]

If anyone reading this thread wants to give this a whirl, and do it cheaply, this might be an opportunity to do so.

Newegg has a $250 Polk subwoofer marked down to $100 at the moment. At that price, it's hard to go wrong.

This makes the idea of a DIY sub somewhat silly - if the quality of sound is good. Anybody heard this thing ? - makes me a bit nervous that it's BR when I was looking for a sealed unit to avoid the 'boom' box problem.

 

[gedlee]

If there's another way thats more accurate that calculates as fast, I'm all ears.

Todd

Have you looked up the Morse reference that I am talking about? There is no speed of calculation penalty for doing it right. In fact, because the series converges faster, it is more efficient, fewer terms are required. If you stopped arguing that wrong was OK and looked into what I have been suggesting you would have seen that.

 

[gedlee]

makes me a bit nervous that it's BR when I was looking for a sealed unit to avoid the 'boom' box problem.

Just seal the ports!! I've done this lots of times. Add some mass to the cone to lower the resonance, a slightly different mass for each one. You will loose efficiency with the greater mass, but if you have three or four you don't need efficiency.

 

[cap'n todd]

Todd

Have you looked up the Morse reference that I am talking about? There is no speed of calculation penalty for doing it right. In fact, because the series converges faster, it is more efficient, fewer terms are required. If you stopped arguing that wrong was OK and looked into what I have been suggesting you would have seen that.

I'll check it out, though rewriting the code is prbably not in the works for me right now. I should rahter compare exising code to some known good FEM/BEM method and see how far off I am. I suspect its a very small error for the way I am using it.

 

[TRADERXFAN]

-Brought below over from another thread to which I posted by accident

Since my listening room has one large opening to the rest of the house should I be thinking in terms of arranging multiple subs throughout the ground floor or can I just think about the main room?

I use what was designed to be a dining room for my listening room.
Its approx 13 x 14x9 feet high.

The rear wall is open, with a 8x8' opening, so most of that wall is this opening. Beyond this, the space opens into a 7.5' wide 2 story foyer/hallway, and then directly behind this is a mirror image of the room described -so the actual rear wall is about 35' back.

Also there is an open doorway on the left on the listening room.

The rest of the ground floor is almost entirely wide open. The house is "normal sized" I guess, not a huge house by most standards. So I assume I am still in the realm of the small room schroeder frequency, but not sure how much you can open up a room to a larger room before it clouds the way you set this up.

Going with the thinking that at bass frequencies the listener hears the room not the source, what would be the proper way to think of a situation like this?

Also, I currently use a dipole servo controlled sub, and it seems to work pretty well. However, as most things in audio, you never seem to realize deficiency until its in comparison to something better.

My first attempt to add subs:
I tried adding a 2nd, sealed, sub to this but I had a hard time getting them to sound well together. Maybe dipoles and monopoles shouldn't be mixed?

I didn't have the proper tools to measure or do spacial averaging. By ear, I did try several various setting of the phase and gain and locations in the room. Having little more than 3 walls with the mains along one, didn't leave me too many options for placement. I did build a tall stand and tried with it above the centerline, too.

I follow the Linkwitz suggestion to keep the dipole sub angled towards the listener, along the side wall, equidistant to the listener as the mains, which is also near a corner in this case.

Thanks for help.

Tony

Tony

Highly couple spaces act like one room, but the subs should be kept closer to the listening space. Your coupled spaces act like one big space, but the energy can flow into one of the other spaces and then back again. This can be a negative effect if the nerby spaces are highly reverberant, but its generally not a problem.

Measurements are almost a must and they are not hard to get. I have even used a Creative Labs mic that was free with a sound card. Works fine at LFs. Software is free and any measurements are going to help.

I've used monopole and dipoles together. Its not an issue, but might be a little harder to set up.

Would you think a more enclosed room would be better for the bass performance? I chose this room mainly because I would have few issues with rear wall reflection. I wonder if it would be better to set up the listening room in a spare bedroom until I finish the basement.

Would damping still be important if it is this open? I assume so, but thought I would ask.

I do have a measurement mic, and a laptop connected up with that Room eq wizard program to do the measurments, but not sure how to go about spatial averaging. I can run a frequency sweep, that's about it. There is a real time analyzer but it doesn't capture -at least I don't think it does.

I must say that I am tempted to simplify my life and just get another dipole sub for the other side of the room to boost output and call it a day. It would be about $550 for 2 more drivers and amp. The H frame box construction is very simple. I think it would integrate well together.

Earl, I think you mentioned that dipole bass you heard was smooth like the multiple monopole sub setup but fell off 25-50 range. The negative was it costs more to do that, but maybe since I am already half way there I just get another one. Each sub uses two 12" drivers per side with the shelving circuit built in (and designed to accomodate 3 woofers per amp). The main reason I would desire to add to it is because I find that I can bottom them out with certain material.

-Tony

 

[john k...]

Todd,

Can you give me any information of where you specified the mic or listening position in you calculations? I can get results very similar to yours but only at very limited mic position. If I move the mic around I see wide, deep dips in the response.

I have checked my code against an FEM simulation and for similar setups I get very similar results. I do not include a term for the direct sound.