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AES Journal Forum: Comment by Gary Eickmeier on "Reflecting on Reflections"

Title: Reflecting on Reflections
JAES Volume 62 Issue 6 pp. 454-455; June 2014
Comment by: Gary Eickmeier

There might be flutter echo if you were doing it in a bare room, but not in a room designed for audio - either production or reproduction. For example, my room is 20 ft wide by 30 ft long and has some diffusers in the back half on the side walls, plus absorbent furniture at the listening position and plush throw rugs on all floors, and a cathedral ceiling. The RT60 is about 500 ms, about right for this size room. The walls near the speakers at the front of the room are flat wallboard done purposely to create a huge lattice of direct and reflected image sources. What all this means is that the first reflections are specular and occur only once, then are absorbed normally, the same as any other system. All that changes is the spatial characteristic, leaving the temporal unaffected except for what is contained in the recording. Doing the spatial by reflection like this is the only way to be able to adjust the direct to reflected ratio and make it more like the live model. It's a whole different universe with different rules. Paper to follow - hopefully!

Posted on January 10, 2017 at 6:57:33 PM EST

AES Journal Forum: Comment by Robert Olhsson on "Reflecting on Reflections"

Title: Reflecting on Reflections
JAES Volume 62 Issue 6 pp. 454-455; June 2014
Comment by: Robert Olhsson

Flat walls don’t necessarily reflect a flat response and can cause flutter-echo which is a distraction. Acoustical treatment and diffusion can be used to flatten the reflection response.

Posted on January 9, 2017 at 2:12:56 PM EST

AES Journal Forum: Comment by Gary Eickmeier on "Reflecting on Reflections"

Title: Reflecting on Reflections
JAES Volume 62 Issue 6 pp. 454-455; June 2014
Comment by: Gary Eickmeier

Mr. Olhsson's comments are refreshingly correct. Dr. Toole's final 3 paragraphs are also correct about the 3 choices on what to do with off axis sound. The final choice, reflecting the sound from hard flat surfaces is the correct one, and agrees with Olhsson. Let me explain.

For the entire stereo era and especially with advice given to audiophiles and engineers today, stereo theory has been shamefully incorrect, leading to bad practices in speaker manufacture and advice on their use. The graphics at the bottom of Dr. Toole's article illustrate the problem.
The one on the left shows the live sound situation and the one on the right the reproduction. I agree that the multi channel illustration would be better than just two channel stereo, but the basic two channel part is where I am going. It seems that we have been fed a lot of information that the direct sound from the two speakers is primary and all important. We have been told to dampen or eliminate the reflected sound output from all speakers. This is based on an incorrect stereo theory that the idea is to direct the two recorded channels to our two ears, and then we will "hear" what is contained in the recording in all its glory.
What is forgotten in this simplistic theory is that the all important spatial nature of the live sound will then be forced to arrive at the hapless listener's ears from only those two points in space, and not the multiplicity of incident angles that were recorded. The spatial nature of the live sound has been changed to a high direct field from two points only.
My claim is that this "head related" theory is fundamentally incorrect. What we are doing in stereo is not this "two speakers, two ears" head related theory but rather a field-type system in which the object is to reproduce sound fields in rooms, not signals for the ears. You do this by bringing the speakers out from the reflecting surfaces and directing the majority of their output in the reflecting directions and just a small amount in the direct field from the speakers themselves. By using specular reflectivity from the walls we retain the same frequency response as the actual speakers and incur an appropriate delay of the reflected sound to remain within the fusion time, and reproduce the spatial nature of the original. 
The paradigm now becomes the image model of the live vs the reproduction sound fields. The focus is now how closely these two fields match. I call it an image model theory for stereophonic sound, which I would hope will replace the head related direct sound so-called "accuracy" theory of the past. I am doing it in my home and it gives a 3-dimensional model of what is contained in the recordings with great depth, spaciousness, and realism of imaging that is much superior to any direct sound system I have ever heard.

Posted on January 6, 2017 at 7:03:17 PM EST

AES Journal Forum: Comment by Andrew Horner on "The Effects of MP3 Compression on Perceived Emotional Characteristics in Musical Instruments"

Title: The Effects of MP3 Compression on Perceived Emotional Characteristics in Musical Instruments
JAES Volume 64 Issue 11 pp. 858-867; November 2016
Comment by: Andrew Horner

Thank you very much for your comments. Constant bit rates were used in our study. Since all instrument sounds were in mono, the bit rates we picked were per channel.


We agree that a better wording would be that "music providers such as Spotify use MP3 or related compression algorithms".


Apparently, a saxophone solo in the middle of a piece also contains some other instruments like the rhythm section. It is not clear how mp3 compression changes the perceived emotion of real world music. However, our results give an insight into how MP3 compression may change the emotional characteristics of a music piece. We also agree that testing MP3 compression on musical excerpts is a very promising area for further work.


Thank you so much for your suggestions.

Ron Mo and Andrew Horner

Posted on December 30, 2016 at 9:15:51 PM EST

AES Journal Forum: Comment by Scott Dorsey on "Noise Analysis of Transformer-Coupled Preamplifiers"

Title: Noise Analysis of Transformer-Coupled Preamplifiers
JAES Volume 40 Issue 1/2 pp. 3-11; February 1992
Comment by: Scott Dorsey

The title of this paper is kind of misleading, because this is actually a thorough noise analysis of a whole bunch of different input stage topologies.  The reader can tweak some of the numbers to model specific implementations as well, but this is a thorough summary of where noise comes from in preamplifiers and it was very much a milestone in the field when it came out.

Posted on December 25, 2016 at 1:45:37 PM EST

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