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Old 19th February 2009, 02:59 PM   #891
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by markus76


We discussed this earlier. Obviously you haven't learned anything from that discussion. There is no "transient onset" at low frequencies in an acoustically small room.
The transient onset of any type of percussion instrument always is a combination of higher and lower frequencies. It's this combined wave form that fully characterizes an instrument. Once additional signal such as room refelction and driver stored energy is mixed into this, the sonic chracteristic changes. Normally if both have a significant level not more than about 12db below the original signal, then it is not possible to currectly determine the effects of improvement in either area. If we are studying the sonic effects of imrpoving stored energy situation, then we need a faily dead room; if we are trying to understand the sonic effects of the room reflection and modes, then we need a speaker that has lower stored energy to prevent it from mixing into the source music. Normally, if the stored energy is low, and the room is fairly dead, then absolute polarity becomes more obvious.

Again, we should focus on the characteristics of the recorded instrument rather than just consider parial frequency range when trying to interpret listening impressions. Looking at individual frequencies is best referenced when trying to identify the source/cause of a problem to provide a precise fix.

And you are right, than nothing said the earlier discussion you refer to talks about anything I did not know before.
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Old 19th February 2009, 03:01 PM   #892
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Old 19th February 2009, 03:03 PM   #893
soongsc is offline soongsc  Taiwan
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Wrong exit.
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Old 19th February 2009, 03:29 PM   #894
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Default huh?

Quote:
Originally posted by soongsc

The transient onset of any type of percussion instrument always is a combination of higher and lower frequencies. It's this combined wave form that fully characterizes an instrument. Once additional signal such as room refelction and driver stored energy is mixed into this, the sonic chracteristic changes. Normally if both have a significant level not more than about 12db below the original signal, then it is not possible to currectly determine the effects of improvement in either area. If we are studying the sonic effects of imrpoving stored energy situation, then we need a faily dead room; if we are trying to understand the sonic effects of the room reflection and modes, then we need a speaker that has lower stored energy to prevent it from mixing into the source music. Normally, if the stored energy is low, and the room is fairly dead, then absolute polarity becomes more obvious.

Again, we should focus on the characteristics of the recorded instrument rather than just consider parial frequency range when trying to interpret listening impressions. Looking at individual frequencies is best referenced when trying to identify the source/cause of a problem to provide a precise fix.

And you are right, than nothing said the earlier discussion you refer to talks about anything I did not know before.
Seems to me you're confusing the reasons why one uses multiple limited bandwidth speakers, as opposed to single widerange drivers (hence the use of a subwoofer).

I also don't (usually) listen to sine waves. As you state, many percussive instrumernts have a sharp attack followed by a lower frequency component. I believe the purpose of crossover networks (active or passive) and multiple drivers in systems is to route the higher frequency components of said instruments to the appropriate (non subwoofer, for instance) drivers for faithful reproduction. You might want to do some reading on signal analysis to get a better grip on what's being presented here.

On another note, I've been moving subs around per Dr. Geddes methodology, and indeed by placing them more or less as defined (one in the corner, 2nd along adjacent wall ~ 1/2 way and out 4 feet, third near the listening location opposite wall from the second). I have a fourth sub, which I re-located to the second floor level midpoint above the listening position, and oh boy!, what a difference that made.... even with 4 subs running, bass sounds very smooth and articulate, and seems to come only from the main speakers (other than the building shaking infrasonics in HT and a few cd's that is), notwithstanding the widely differing positions of the subs

When I can find a helper, I'll try to make detailed measurements of the improvements and report back. So, thanks to Dr. Geddes his easily implemented practical guide to optimized subwoofer setup as well as his many other contributions here.

John L.
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Old 19th February 2009, 03:33 PM   #895
gedlee is offline gedlee  United States
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Quote:
Originally posted by MaVo

It seems that the harman folks with their symmetrical positioning try to cancel some low frequency modes, which seems to work quite well for some of the lowest modes. Earls random placement approach on the other hand builds upon engergizing the maximum amount of modes. If this is true, then his approach would have a very big advantage, since the modes give the signal a big boost in comparison to the "freefield" response one gets with canceled modes. Much smaller subs would be enough for the same spl then.

Is this correct?

As a sidenote... It seems Earls approach needs alot of low frequency damping, in order to make the sound smooth. What kind of non structural damping works best between about 20 and 100hz?
There is a difference in obtaining a smooth response at the highest LF output and obtaining it at a lower output. I don't think that it has ever been settled that "cancelling the modes" results in any smoother response than "exciting all of the modes", but I suspect that the answer depends very strongly on the amount of room absorption. A very reverberant room would appear to favor mode cancelling while a dead room would favor exciting the modes. There is a lot of variables to study and no one has looked at them all.

Exciting the modes will certainly result in a higher level of LF energy, but may result is a higher spatial and spectral variance in a room with low damping. But if the room has a lot of damping - like I "recommend" (not require as your post implies) then exciting the modes will help the LF loss that results from the damping.

In the end almost everything depends on the specific room and the available sub locations. Thats why there is so much argument here as everybody is assuming something different and, of course, we don't agree. But I will submit that a well damped room is going to beat a live room for low variance every time, and that exciting the modes is favored in this kind of room. That's what I have, that's what I build and that's probably why things have worked out so well for me. When you have a low damped symmetrical room - as all the models that have been discussed here HAVE TO assume - then this situation will tend to favor mode cancellation and specific symmetrical placement. But I look past this assumption to what an optimized real room will likely do - the answers are going to be different.

As to the best damping for very LFs, its clearly freely suspended walls with constrained layer damping. Not the easiest wall to put up, but I think that its certainly going to work the best.
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Old 19th February 2009, 03:38 PM   #896
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Quote:
Originally posted by soongsc
Wrong exit.
Only possible exit because I don't want to re-type everything that was written on what makes a piano a piano and a violin a violin. Hint: the difference is NOT in the low frequency region (<100Hz).
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Old 19th February 2009, 04:46 PM   #897
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Quote:
Originally posted by gedlee


In the end almost everything depends on the specific room and the available sub locations. Thats why there is so much argument here as everybody is assuming something different and, of course, we don't agree. But I will submit that a well damped room is going to beat a live room for low variance every time, and that exciting the modes is favored in this kind of room. That's what I have, that's what I build and that's probably why things have worked out so well for me. When you have a low damped symmetrical room - as all the models that have been discussed here HAVE TO assume - then this situation will tend to favor mode cancellation and specific symmetrical placement. But I look past this assumption to what an optimized real room will likely do - the answers are going to be different.

Generally agreed, but I'll point out (same as I did when you brought up this point re my AES paper) that it is likely the less damped rooms that are problematic. So, if you damp the room, there's less problem for your sub placement to solve. Ergo, the good results yo have gotten. But that may not be an option in all rooms.
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Old 19th February 2009, 05:09 PM   #898
gedlee is offline gedlee  United States
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Todd

From what I have seen most rooms are larger coupled spaces and not closed in at all, as all the sims assume. What happens in this case is not certain, but clearly the modal density will increase as well as the damping (more space to disipate LF energy if just through leakage). Hence, I am not sure that I agree that most or "typical" rooms will have low damping. That, I think, is an assumption that you are making that may not actually be correct.

But just as clearly, if someone is going to design and or build a room, such as a HT or listening room, then they would be remiss if they didn't design for higher damping at LFs. This then would appear to favor my approach, although, admittedly yours would work as well - its just a difference in cost.

There will be cases as you suggest, but as to its being "typical", that, I think, is an unanswered question.
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Old 19th February 2009, 05:17 PM   #899
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There is an element of optimization which I developed in my research and documented in my AES paper ("Low Frequency Optimization using multiple Subwoofers" - also in Floyd Toole's excellent book) that may be of some use. That is, if you are only using positional placement optimization, you can go one step further and take room dimensions into account. This would be most useful if you were building the room and could have some control of the dimensions. However, even for the built room you could still look up the dimensions and choose one of several basic sub placement configurations might be better for that particular room. Yes, it's based on a model of a rectangular room, and yes, it's better if you can incorporate measurements, but still it could be better than just automatically using the wall midpoint configuration for example. You can buy the AES paper for $5 on their website. I tried to upload the figure but it did not look good at the resolution that is allowed here.
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Old 19th February 2009, 05:23 PM   #900
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Quote:
Originally posted by gedlee
Todd

From what I have seen most rooms are larger coupled spaces and not closed in at all, as all the sims assume. What happens in this case is not certain, but clearly the modal density will increase as well as the damping (more space to disipate LF energy if just through leakage). Hence, I am not sure that I agree that most or "typical" rooms will have low damping. That, I think, is an assumption that you are making that may not actually be correct.

But just as clearly, if someone is going to design and or build a room, such as a HT or listening room, then they would be remiss if they didn't design for higher damping at LFs. This then would appear to favor my approach, although, admittedly yours would work as well - its just a difference in cost.

There will be cases as you suggest, but as to its being "typical", that, I think, is an unanswered question.
Earl, I agree, but please reread my post and tell me where I said anything about "typical". In any case, rectangular rooms with low damping are not exactly rare either.
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