[zybar]

George,

That sounds good.  For the next weekend, though, I'll be helping my friend get his house ready to be put on the market (he's moving to Minnesota -- couldn't find a job here in CT after months of trying).   After that, though, the weekends should be pretty clear (except Thanksgiving day).

But I probably won't be able to afford the traps (and 8th nerve stuff) for a while.  So, an initial analysis would be good (and could help me figure out a problem I may be having -- my imaging seems shifted t ...

If the image is shifted to the right, adjust the pots/putty to compensate.

I have the same issue here.

One side wall is concrete/earth and the other is stick frame.

George

[ctviggen]

George,

That's the same issue I have.  I was going to wait until Joe (or you) came over, as it'd be easier to have someone adjust the pots while another person listens.  I may do this myself, though; we'll see how much time I have (when all your weekend time goes toward helping someone else, it gets hard to do stuff).  Plus, it would be nice to do something "scientific" to analyze the situation.  What I might do is play some general tones through the system and disconnect one speaker then see what my RS meter says; then I'll play only the other speaker and see what my RS meter says.  I'll use this method for gross tuning of the pots/putty, then fine tune by ear.  Unfortunately, I gave my stereo test disk to someone, but I think I should be able to play tones using Avia.

[John Casler]

Non-modal peaks and nulls are caused by simple acoustic interference: A wave strikes a surface and reflects back into itself. At 1/4 wavelength from the boundary a null occurs. This is explained in several of the articles on my company's site, as well as in the video Non-modal Peaks and Nulls, also on our site.   .

Hi Ethan,

Just visited your site and have to say those are great videos.

But I do have some questions: While you state that "tones" are good ways to find "nulls", I wonder how effective or destructive the backwave is in a highly transient peice of music?

I also wonder why (or if) the "null" for dynamic music might transiently occur at various points in the room, as far as where it happens.

Let me explain:

Let's say we have a 30Hz burst leaving the speaker that reaches the listener, but has not reached the back wall for reflection?? Null or no null?

Now if a second (direct) burst leaves the speaker but the back wave (rebound) pressure has already passed the listener (moving forward now) and impacts the second burst say a foot in front of the listener??  What is the effect??

Looking at these possiblilities and the energy balance and exchange, I feel that the "interplay" between direct and rebound waves during dynamic music may be extremely complex.

Now of course, the answer is still to substantially retard the resulting "rebounding" waves as much as possible, but with the timing of direct wave release and the ability for these direct waves to be impacted by the rebound at other room positions other than the "continous tone" null, seems like an interesting consideration.  

I hope I have worded this properly to convey my question.

Very simply, the collision of opposing waves (direct vs rebound) in dynamic music, can occur at other points in the room.  How does this affect the direct wave, if it has been impacted by a rebound wave "prior" to reaching the listener?

Is the "energy balance" of specific frequency waves such that no matter where the "impact meeting"  occurs in the room, that they "cause" the energy nullification at the mathematical null whether dynamic bursts or steady state??

And how much affect do non-similar wave energys have on non-similar waves?  That is, if a 20Hz wave rebounds back into a 40Hz direct wave, what energy effect does it have on the 40Hz wave?

And one more question about the "energy return" of the rebound wave.  If a 20Hz wave rebounds off the back wall, obviously the return energy is "less" than the original wave, but is the frequency altered?  That is, does the frequency say "slow" a bit since the wall may flex, absorb and then send out the wave slower than it arrived such as 19Hz?

Obviously I know that I am talking in "absolute" terms and nothing is absolute.  

For example, a similar question would be that if the above "is" true, then might "corner rebound action" actually "raise" the frequency or energy?

This would be the result of the wave traveling on intersecting walls at the original speed and frequency.  As the energy of the waves traveling along each wall impacts the other in the corner the energy is "multiplied" and fired out at a higher frequency and energy.  Likely the frequency would be higher since the rebound/collision pressure would increase the speed of the leading edge, multiplying it.

All of these questions are simply mental meanderings, but would seem to be elementary physics.

Any ideas or comments?

[Ethan Winer]

John,

> how effective or destructive the backwave is in a highly transient peice of music? <

I know what you're asking, and I used to think the exact same thing - that it takes some time for peaks and nulls to "stabilize" in a room. But I have since learned this is not the case.

Yes, it does take a finite amount of time for sound to leave a speaker, bounce off the rear wall, and collide with waves still coming from the speaker. But here's the distinction: The difference in distance is only a few feet = a few milliseconds, and that's far faster than even the most transient low frequency burst you'll encounter in actual music.

Again, considering just the rear wall for simplicity: It doesn't matter how long it takes for the sound to get from the speaker to that wall. All that matters is what happens right in front of the wall, because a wave approaches the wall and then it reflects. So five feet from the wall it takes less than 10 milliseconds for the reflection to create a series of peaks and nulls.

> how much affect do non-similar wave energys have on non-similar waves? That is, if a 20Hz wave rebounds back into a 40Hz direct wave, what energy effect does it have on the 40Hz wave? <

Yes, different frequencies interact. But in practice the tones always sustain for much longer than a few milliseconds. Think about that. The lower in frequency you consider, the longer it takes to complete a full cycle before you can even tell which frequency it is. So the concept of "low frequency transients" is fundamentally flawed. If you were to strike an orchestral bass drum very hard then damp it immediately, the total time from start to -60 dB will be at least one second.

--Ethan

[ctviggen]

John,

Are you saying that when a 30Hz wave first hits the listener, there might not be a null?  I would say this would be true, but it'll only take a fraction of a second (the speed of sound is 1137 ft/second at 77F) to set up a null.  Your mind won't be able to determine that there's no null.  

(For the calculation:

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html)

So, I'd guess that while there is such a concept of impulse versus steady state (in electrical engineering terms) for the sound response of a room, you probably don't have to worry about impulse response until you get to really large rooms, like amphitheaters/stadiums.  For instance, my room is only about 27 feet deep, which means that nulls would occur in only 24/1000 of a second (24 milliseconds).  (Or maybe double that, to get two opposing waves:  one from the speaker and one from the back wall; still, that's only 50 milliseconds. Even if you quadruple it, to account for multiple waves going back and forth between the front and rear walls, that's less than 1/4 second)  I do agree that sound/room interaction is complex.

[ctviggen]

Dang, Ethan -- you're fast!  I wrote my response while you wrote yours.

[John Casler]

John,

Are you saying that when a 30Hz wave first hits the listener, there might not be a null?  I would say this would be true, but it'll only take a fraction of a second (the speed of sound is 1137 ft/second at 77F) to set up a null.  Your mind won't be able to determine that there's no null.  

First thanks Nathan for your comments.

Hi Bob,

I'm not really "saying" anything, but more "questioning" a few things, trying to get a "feel" for what actually happens.

While all these processes happen due to "simple" physics, the number of variables and complex interactions, make for some very heady concepts to wrap around.

The speed of sound, the dynamics and frequncy energys of music, and the dimensions and construction material properties, as well as the capabilities and set up of an audio system, provides some very interesting challenges to harvesting the "maximum sonic performance".

The energy direction, reflection, and interaction of sound as it is "released" into and then travels through a room is mind boggling.

I find it truly amazing that we are able to get "any" real asessments of how a speaker or component used in one environment, will sound in another.

Never the less (is that a phrase or did I just make that up   ) from what I can gather, and I have been gathering for too many years to count, reduction of room introduced sounds, and reactive energies is probably (or should be) a chief goal to better reproduction, if accuracy to the recorded information is the goal.

[ctviggen]

Hi John,

I totally agree with you -- sound interaction is very complex (which is why acoustic engineers still use part knowledge and part physics).  I didn't mean that you were asserting any position.  Here's an article about how sounds are created for virtual reality systems (simulating sound travels in rooms), and it gives me a feel for how complex the math is, especially since these are only approximations (they use the term "eigenfrequencies" for resonant frequencies -- an interesting choice of wording):

http://www.cg.tuwien.ac.at/studentwork/CESCG/CESCG97/miksicek/vrSound.html

I think people use approximations because you can't really do any serious analysis without resorting to computers.  See this for a funky eigenfrequency calculation of a room:

http://www.comsol.com/showroom/gallery/femlab/63.php

Also, on a side note, this has a pretty good overview of room acoustics and nulls:

http://www.meridian-audio.com/w_paper/Room_Correction_scr.pdf

[davejcb]

Here's a shot of my left corner trap, if anyone is interested...

[Red Dragon Audio]

you painted!  Looks good Dave.

oh and the traps look nice too...

So now with the absorbtion at center and the bass traps in the corners, how does it all sound?

[davejcb]

Well... I can't really say, because I had the Ack! dAck! before, not anymore... I think that sounstage and spacial depth are a lot better, but the sound itself isn't nearly as good, highs are not nearly as good than with the Ack! dAck!...

And yes, I painted!