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Author Topic: Reflections PDF  (Read 15198 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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