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Author Topic: Reflections PDF  (Read 14557 times)

L_Tofastrud

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Reflections PDF
« on: September 05, 2006, 05:37:30 pm »

Below is a link to a document I started to write some time back (and haven't been able to find the time to actually finish).

http://www.fmdesign.com/support/ETC_article.pdf

It deals with our perception of recorded sounds in a listening environment.

Comments are welcome!

Regards
Lars Tofastrud
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franman

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Re: Reflections PDF
« Reply #1 on: September 05, 2006, 10:59:35 pm »

Thanks Lars... hey guys this is the real deal.. read it, live it, love it... my man knows of what he speaks!!
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Teddy G.

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Re: Reflections PDF
« Reply #2 on: September 05, 2006, 11:00:20 pm »

It has been a long day, which started early this morning with a 120+ mile round trip, via Amtrak, to Philadelphia for a VO session and ended - late this evening - with work, in my home studio for a TV show in DC, followed, of course, by "checking in" to several audio forums.

I say all this just so you understand why I'm not "getting it all" - at least not at the moment. However, THANKYOU for the post! For gosh sakes, FINISH IT! PLEASE!!! We need stuff like this to help us understand what is actually happening in our seriously under-treated rooms!!!!!!

Anyway, the "two rooms" description seems to make sense, even to my foggy brain. Thankx!


I wonder if one records in the same room one listens(Mixes) in, even from the same "position"(Sitting at the desk, speaking into a mic, recording, then listening back to the speakers, from same position) whether there might be 4 rooms? I'm tired...


Thankx again!

Teddy G.
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franman

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Re: Reflections PDF
« Reply #3 on: September 05, 2006, 11:08:24 pm »

and if you listen to the recording or your playing back the same recording that you listened to at the mix position, it's kinda like looking at yourself in the TV with the camera pointed at the TV, right.... (yeah, I'm beat too!)
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Ethan Winer

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Re: Reflections PDF
« Reply #4 on: September 06, 2006, 03:01:48 pm »

Lars,

> Comments are welcome! <

Nice job.

The more I've thought about this lately, the more convinced I am that comb filtering is the main problem caused by early reflections. You mentioned in your article deviations of +/- 10 dB or more, and my experience is that "more" is typical. It's often put forth that the phase shift and time delays from early reflections cause poor imaging and localization. That might be part of it, but my guess is that imaging is damaged mainly because the comb filtered response at each ear is so different. So you hear the sound as coming from each speaker rather than that as a true stereo image.

--Ethan

L_Tofastrud

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Re: Reflections PDF
« Reply #5 on: September 06, 2006, 06:41:00 pm »

Ethan,

This is really just one issue.  If there are comb filters there will be reflections that again will cause the distortions I talk about in the PDF.  You can't have one without the other.

What is the best way to determine if we have destructive reflections:  A frequency curve or an energy/time curve?

The reason why I prefer to look at an ETC curve is because we don't need to know what frequency we're dealing with.  It becomes a simple matter of reflection level in relationship to the level of the direct sound.  We also know that if the reflections have a level that is insignificant on the ETC they will not be able to create any comb filtering.

We see the effect of the reflections in a frequency curve as comb filtering but then again if you don't know the true anechoic response of the loudspeaker it might be difficult to know for sure if the variations in the frequency response are caused by reflections.  The frequency graph also leaves it wide open as to what is causing the comb filter while an ETC curve will tell you the distance to the surface that is reflecting the sound.

Looking at the frequency curve alone can also be "dangerous" since it might be easy to reach for an equalizer but comb filters can't be equalized since a time shifted/delayed copy of the original sound causes it.  If you move the measurement mic a couple of inches the comb filtering will look different.  The only way to deal with comb filters is to reduce the level of the reflected sound.  This is why I say that if we deal with the reflections we automatically get the frequency response of the loudspeaker.

Regards
Lars Tofastrud
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L_Tofastrud

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Re: Reflections PDF
« Reply #6 on: September 06, 2006, 07:44:25 pm »

Teddy G,

Thanks for your comments Smile

I think one of the reasons I haven't finished it is because I find myself going off on a rant about how horrible new CD's often sound and why people should record in stereo. Surprised

If you want to read about it let me know Smile  It could be a new thread since it is all related to the understanding of acoustics and psychoacoustics.

Regards
Lars Tofastrud
Senior Acoustician - www.fmdesign.com
Director - www.griffinaudiousa.com
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Teddy G.

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Re: Reflections PDF
« Reply #7 on: September 07, 2006, 08:35:12 am »

Not a "new thread", a "new" forum title, period! Hell, an entire book, a series of books, a televison series, a 3 picture movie deal!!!!

We are talking, here, of room accoustics, but the rail car full of worms, ready for opening, that would be "pseudo-stereo"(To put it kindly), that is most recordings for the last 40+ years, would be a wonderful topic for discussion all on it's own!

Indeed, the column heading might be:

"What Stereo?"

Putting lead guitar "left" and rhythm guitar "right" is not stereo - it is two mono tracks left and right.

What I have coveted, ever since I've seen them(Quality? I don't know, it's the portability and ease of use that I want.) is the Rode stereo mic(The one with the cool looking head) and a digital portable recorder, so I could go around and "record things", ANY things, just as they are. Sound, music, whatever. I am drawn to the combo(Or similar) as I think my brain is tired of mixing, or re-mixxing, or hearing others M or RM, as we try to "duplicate" something that was never really there. In reality, the guitar players each can come in seperately, stand in the same position, be recorded, then "placed" on a "sound stage". How bogus is that???

The first record I ever bought was "The Beach Boys -- Oh my God... NO that wasn't the name of the album, I just can't remember! Something about surfing, I believe??? Or girls??? Or girls surfing??? Anyway, on the back of the jacket was a picture of the 4 boys singing into ONE mic(I believe a Neuman?) all at one time. Did they really do that? All at once? HORRORS!!! Had to have been a "staged" photo. NO ONE could really do that!!! How would the engineer and producer "mix" them??? How could you put the fella's in different places on the sound stage? Had to be fake. Truth to tell, to hear what "the pros" DO do under such circumstances, all you have to do is buy a Beach Boys CD(Greatest Hits Volume 1) and hear! Then cry.

For instance. I have been looking to purchase a "test CD", to audition my new speakers. A CD that is "properly" made, using the latest methods - completely digital at least, to say nothing of 'real' stereo, of ANY type(Rock, country, classical, whatever.). I ask for suggestions, say on the forums, and I get answers like "Dark Side Of The Moon" or some such. I'm not sure anyone is listening? I'm not looking for the music, I'm looking(Listening) for the recording, itself - you know, like what WE do! To hear how far the science has progressed, not to listen for the latest "loudness tricks" or a particuarly fine guitar riff. I suggest we start there. Coming up with a list of real recording, done right. Some examples. Good talent? OK, but, good room, good use of room, good recording technique, good mastering, good pressing, so I can play it in my room and listen to it in my phones and hear 'good'. And, as I improve my room the good should get even gooder. Nothing like a good example to get one off on the right ear.

Enough. Sorry.

TG

Maybe: "Where stereo"(The Mel Brooks version.)

OH! "Surfin' Safari" - I'm tired and old, too...
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Ian Visible

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Re: Reflections PDF
« Reply #8 on: September 07, 2006, 08:58:45 am »

I want to read about it!

Please, please!

AndreasN

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Re: Reflections PDF
« Reply #9 on: September 07, 2006, 11:01:03 am »

Hei Lars!

Cool article! Think it may perhaps be easier for the novice to visualise if you include a picture of a typical listening room; with some arrows to show where the first reflection occurs physically and how that translates into delayed impulses in the ETC.

And please do go ahead with the pseudo stereo rant. Way too many folks in the industry are not even aware of the issue!

Your name seems to be Scandinavian, btw. Any relation to these parts of the world? =)

Teddy G. wrote on Thu, 07 September 2006 14:35


For instance. I have been looking to purchase a "test CD", to audition my new speakers. A CD that is "properly" made, using the latest methods - completely digital at least, to say nothing of 'real' stereo, of ANY type(Rock, country, classical, whatever.).


The ones I usually refer is the three 'best of Chesky Jazz and Classical, and audiophile test disk'. Chesky records, recorded and (not so much processed) by Bob Katz. They're rather old, ten plus years, but they still sound fabulously great. Haven't dug into the rest of the Chesky catalogue, but my understanding is that the label is dedicated to the sort of sound you're looking for.


Cheers,

Andreas Nordenstam
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Ethan Winer

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Re: Reflections PDF
« Reply #10 on: September 07, 2006, 03:47:30 pm »

Lars,

> The reason why I prefer to look at an ETC curve is because we don't need to know what frequency we're dealing with. It becomes a simple matter of reflection level in relationship to the level of the direct sound. <

I agree with all of that completely for all the reasons you gave. I was just saying that it seems to me the main damage caused by early reflections is the comb filter frequency response. As opposed to phase shift or what's vaguely referred to as "time smear" which usually get the blame.

--Ethan

franman

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Re: Reflections PDF
« Reply #11 on: September 07, 2006, 08:33:18 pm »

I think what Lars is trying to get across is that this is ALL ABOUT TIME... Frequency response is just a transform of the Impulse (time) response... It's all represented in the ETC and impulse response... People need to realize the "Cause and Affect" relationship here. It's the reflections (time smearing) that causes the comb filters (freq anomolies).

It's an important concept.... understanding that our entire business isn't really about frequency response, it's about time and impulse response... something we (Lars and I) are always trying to teach folks...  Very Happy
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jimmyjazz

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Re: Reflections PDF
« Reply #12 on: September 07, 2006, 09:53:20 pm »

If this starts into Fourier vs. LaPlace, I'm outta here.  Smile
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L_Tofastrud

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Re: Reflections PDF
« Reply #13 on: September 15, 2006, 04:05:11 pm »

To let people know:  I will post my "rant" soon.

Regards
Lars T
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jfrigo

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Re: Reflections PDF
« Reply #14 on: September 16, 2006, 02:50:05 am »

franman wrote on Thu, 07 September 2006 17:33

It's an important concept.... understanding that our entire business isn't really about frequency response, it's about time and impulse response...


I forget who originally said it, but I think I saw it over on Glenn's mastering board with regard to room treatment vs. monitor equalization: you can't fix a time domain problem in the frequency domain. It seemed a good way to "sound byte" it.
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Kendrix

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Re: Reflections PDF
« Reply #15 on: September 18, 2006, 06:35:11 pm »

Good article.

One conclusion you took from this data is that the larger recording room dominates our reflections-based perception on playback and that the smaller listening room effectively gets lost in that sonic picture.   (From looking at the red and blue curves.)

However, that is not always the case- correct?
I'm thinking about when the recording space is smaller/deader than the listening space.
In this case it seems the listening room will play a major role influncing our perception of the soundfield.
In this case it seems a dry recording is best- so you avoid the room-in-a-room effect during playback.  

I also note that stereo micing presents its own issues.  If you dont get things just right then phasing can harm the sound.  If you have numerous stereo tracks you have to pan intelligently or it can all turn to mush.  

In a world of  loud guitar amps, keyboards etc. that each need their own close miced tracks (to avoid excessive bleed) how would you simultaneously record them all in stereo?  

Seems to me in practice the best we can do is pick 1-2 key sounds to try and track properly in stereo.
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Ken Favata

L_Tofastrud

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Re: Reflections PDF
« Reply #16 on: September 29, 2006, 04:47:34 pm »

Ken,

Sorry for the very delayed reply

Your observations are correct.  I’ll try to address them one at a time:

You said “In this case it seems a dry recording is best- so you avoid the room-in-a-room effect during playback.”  While your conclusion that very dry recordings would have reflections lower in level than even the driest control room is correct the problem with this is that it’s not all music that is listenable if it is very dry.  Experimental recordings have been made in anechoic rooms and they are mainly used for evaluation and research purposes.  I can think of some Lyle Lovett songs that are very dry but even they have a signature of early reflections that will reveal itself in a properly treated room.  But you are right:  Such recordings will be influenced by the listening room and becomes a part of the sonic signature of the music.  At this point it’s actually not a “control room” any more but a “music reproduction room” and this is often the case for “hi-fi” listening where speakers with dipole or even a 360 degree radiating pattern are used in an often relatively live room.  In these instances the listening room will overpower the information in the recording and create its own spaciousness, maybe with exception of large classical pieces.  This works for a lot of people but it is not an accurate representation of what was recorded and this should in my opinion never be the situation in a control room.

If you’re asking if I would rather see recordings from anechoic chambers and then have live listening rooms with 360 degree dispersion loudspeakers the answer is no.  The creative use of space and placement in recordings is a very important part of the musical whole in my opinion.  Leaving the reverb up to the listeners room would not work IMO –it wouldn’t work with headphones.

In most instances with very dry recordings reverb with or without early reflections will be added during mixing.  The issue I have with this is that whatever “early reflections” and reverb is added it’s never comparable to the real thing.  It can sometimes create a pretty good illusion (but I can’t think of more than a handful of CD’s in my collection that I would include in this group) but I haven’t heard any recording where it actually gives the same sonic image as that of an actual stereo recording.  Just move your head a little off axis and the whole soundstage collapses.  One reason for this is the relationship between early reflections and reverb isn’t like in a real room and that there are so many more correlated and uncorrelated reflections present in the real thing than in a DSP box.

It doesn’t always take much to enhance the listening experience a lot.  You say “Seems to me in practice the best we can do is pick 1-2 key sounds to try and track properly in stereo.” And I agree with this – some stereo is a lot better than none at all.  One engineer that has done this with great results is Bruce Swedien.

The problem with bleed is that there are too many mic’s Wink  Even if individual instruments are recorded it seems like the mic technique is the same as that on a stage or in a studio full of loud instruments.  I’m sure comfort and the fact that decisions about sounds can be delayed has something to do with this.  A decision has to be made about what that recording will be in the final mix if you’re to record in stereo and this requires some experience and also, I’m sure, a very comfortable relationship between everyone involved (producers, musicians and engineers).  What makes me sad is that this all seems to be possible in jazz and classical music while the music that I prefer to listen to most of the time “suffers” from microphone techniques that might be necessary on a stage but shouldn't really be needed in a studio where the situation is so much more controllable.

Now, I do understand that there are creative reasons to use close mic techniques in a studio as well but it seems to me that these techniques are chosen more as a default and a habit rather than an effort to capture tone and an instruments sonic qualities *so that they fit into the musical whole that makes a song/final mix*.  In classical music there are notations as to how loud a sound should be in the “mix”- in pop/rock now it seemingly all has to be as loud as it can get.  In too many CD’s now it’s all directly on top of each other and it becomes an unlistenable mush of distortion (because of +dBFS signals).  Everything seems to be recorded at 1” and the only “soundstage” is left to right.  Not being an engineer I can’t explain what engineers do different now or where in the production chain the sound reaches these levels of horrible sound quality, but I know from what I hear (no rule without exception) that pretty much all of the CD’s I have that are good or excellent sounding are more than 5 years old.  So what is done differently? How can far superior recording equipment mean worse sound?

Next:  One of the reasons why layers of stereo sounds mushy are because there are “too many rooms” recorded when different stereo mic setups are used.  Psycho acoustically the brain simply can’t make any sense out of it because it would never occur in any natural setting.

My suggested solution would be to record stereo as few times as possible – record more instruments at once into the stereo mic: it doesn’t need to be live: re-amp all the guitar parts and/or synth parts for example.  Move amps around, play with levels and effects until you get the desired placement and room interaction.

...and don’t compress everything to the top of the digital scale.  It really kills everything that is enjoyable about sound when, seemingly every single track is compressed and limited to death before and during the mix and then one more time during the mastering.  It's not like resolution is an issue if you're down at -20dBFS but resolution and _severe_ distortion artifacts will be an issue when you approach 0dBFS.

Comments are always welcome!

Regards
Lars Tofastrud
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Kendrix

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Re: Reflections PDF
« Reply #17 on: October 02, 2006, 11:26:56 am »

Lars,

Thanks for the comprehensive and thoughtful reply.

I had never thought about the  "too many rooms at the same time" issue with multiple, spacially disparate stereo tracks being mixed together.  This is a very interesting point as it relates to confusing the sonic decoding mechanism of the brain.

Are you aware of any recordings of loud, amplified sources (such as in the pop/rock genre) that recorded in stereo as you suggest with mimimal close micing.  Are there any that also maintaining sufficient intimacy of the sounds?

The very few that come to my mind tend to sufer from a "too-distant" feel.
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franman

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Re: Reflections PDF
« Reply #18 on: September 06, 2007, 08:39:09 pm »

I'm surprised that none of you other guys have chimed in on Lars' stereo recording technique outlined above... This is one of his favorite "Soap Boxes".. He loves to talk about this subject and how it can (should) be implemented in pop and contemporary recordings. He has a lot of great ideas and although he is not an engineer by training, he has thought about this and done a lot of experimenting with the psycho acoustics involved in capturing and recreating "true stereo" recordings for non-classical applications... He's just foaming at the mouth to hook up with some of you guys who have the time and tools to work with him on his ideas to try and create 'better' stereo recordings. Him concept is an relatively simple one once you understand the principal... Talk to him and he'll give you the straight poop and his stereo recording concepts.. I think this reflection thread has a lot of potential not only in helping people understand the effects of early reflections in reproduction of sound (control rooms) but also how to capture and produce better stereo recordings for almost any genre... trust me, just crank him up and you'll be amazed at how much he has to say about this. At the end of the day, what we do for a living is all about making is easier and more enjoyable to produce better sounding records!! That is what it's all about (assuming you can find some decent music to record, that is!)... Enjoy!
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Nick Sevilla

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Re: Reflections PDF
« Reply #19 on: January 09, 2009, 08:06:02 pm »

L_Tofastrud wrote on Tue, 05 September 2006 14:37

Below is a link to a document I started to write some time back (and haven't been able to find the time to actually finish).

http://www.fmdesign.com/support/ETC_article.pdf

It deals with our perception of recorded sounds in a listening environment.

Comments are welcome!

Regards
Lars Tofastrud


Really excellent article, Lars!

Now I can point some of my skeptical clients, when they wonder why I sometimes don't treat all the instruments in a mix with reverb nor delay...

To me, putting an effect on an individual mix element "just because" is not good. Usually I do listen to mixes as I start them, without the time-based effects, in other rooms and spaces, to determine what the "dry" recording sounds like when added in a real listening environment.

Sometimes there is enough original room along with the instrument, that adding more reverb / delay, will just cloud it up. Especially the lead or dominant instrument.

Your article just confirms what I've been doing for a while.

Thanks,

Nick
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