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  • Geddes' Multisub






    The larger subject always worth some further discussion: Engineering high quality sound reproduction, with consideration of pscychoacoustic perception of low frequencies in small rooms, and the physical acoustics of low frequencies in small rooms in the modal domain around and below the Schroeder frequency.

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  • #2
    thanks - I"m very interested in this topic.

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    • #3
      Utter nonsense!

      The man is an idiot!

      ​It took weeks to explain room gain to him a number of years ago right here on the Tech Talk board (may have been Madisound back then but I believe it was here).

      ​I showed him my room gain measurements and he thought they weren't real.

      ​I explained the process and it turned out that he didn't even understand the concept of close mike measurement (loudspeaker measurement 101).

      ​John K. worked on him for several weeks after that until he finally gave in and said he thought there was no room gain because of an anomaly with the software he was using(what?).

      Now he's back in his own reality.......
      Last edited by daryl; 01-16-2017, 12:44 AM.

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      • #4
        I'm surprised, because I actually had a lot of respect for Geddes for his numerous work, but this is going too far. Room gain is an easily verifiable phenomenon in most rooms and it has a simple explanation. Granted, certain rooms do not have room gain (I suspect he has a room like that), and I don't have an explanation for that, but to dismiss room gain is simply wrong.

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        • #5
          Here's a link to some more information on the subject. There's also a link on the page that references a paper done by Harman.
          http://hometheaterhifi.com/technical...-kevin-voecks/

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          • #6
            That nonsense about "large rooms are better for bass"!

            What is he thinking?

            ​Large rooms have bass echo's which make bass unintelligible.

            ​Small rooms are less problematic only having modes to deal with.

            ​I worked in large ballrooms for 17 years and you cannot get the kind of sound that you can get at home.

            Much more damping material is required to control sound in large rooms also.

            ​Not just because you have more area to treat but because you have problems at lower frequencies which require much larger damping materials.

            ​Then he threw out the ridiculous notion that you need to gradually increase bass level as frequency gets lower because the decay is so fast in small rooms.

            ​He gave a value of 3db - 6db from 200hz down to 20hz which is barely perceptible with a gradual change over a wide bandwidth because of your relative insensitivity to the level of very low frequencies ([email protected], [email protected], [email protected], [email protected], [email protected]).

            ​A fast decay is desirable and gives more intelligible sound quality.

            ​If you want long bass reflections you could put them on your recording.
            Last edited by daryl; 01-16-2017, 12:03 AM.

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            • #7
              I remember that discussion with Geddes and room gain. I could not believe he didn't understand the concept. A room is an enclosure and it begins to pressurize as a whole below a certain frequency defined by the room dimensions. How much it pressurizes depends on how leaky it is. And like a sealed speaker, that gain has a 2nd order rising response and breaks where leakage overcomes the pressurization.

              The easiest place to hear it is in a car. Cabin gain is obvious to even a casual listener. Close the doors and roll up the windows and the added low end extension is obvious compared to having the doors open. A sealed sub in a closed car can produce some pretty amazing bass. So much for the notion that large rooms are better for bass.
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              • #8
                Originally posted by rickcraig View Post
                Here's a link to some more information on the subject. There's also a link on the page that references a paper done by Harman.
                http://hometheaterhifi.com/technical...-kevin-voecks/
                ​Of course I'm not contesting the idea of multiple subs or that you can optimize a signal for each one but this has been known for a long time and it was not originated from Mr. Geddes.

                ​As far as target response that's another thing.

                ​The only acceptable target response is flat so that your system reproduces the signal exactly how it is encoded on the medium.

                ​The test subjects certainly had an EQ preference.

                ​Though he said they're preferences varied considerably and that they're preferences were averaged to get to the curves shown in the chart.

                ​You can see that the averaged preferred headphone response is never more than 2db different from the averaged preferred in room loudspeaker response so you could easily split the differenced since they are so close.

                ​If this is the consensus then the information needs to get to producers and recording engineers to record with the proper tonal balance and not that people should be creating systems with 'target response curves'.

                ​The notion is ridiculous!

                ​Flat is the reference!

                ​You put a system in the studio that is also flat!

                Then you properly record the record so that it sounds as it should on a system properly aligned to a flat EQ.

                ​Simple as that!

                ​What, should I expect to see picture frames with 'target color correction curves' to compensate for the artists not using the correct colors?

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                • #9
                  Hi Pete!

                  ​Folks like to talk about room pressurization below it's lowest mode but actually there is considerable room gain at higher frequencies as you can see on my room gain chart.

                  ​The reason for this room gain is that speakers see increased acoustic impedance in room as frequency goes down which has only a small effect upon cone travel and therefore they produce more sound energy.

                  ​Reflections off room boundaries reinforce the SPL at the speakers diaphragm causing increased acoustic impedance if the boundary is within about 1/8 wavelength (14'@10hz, 7'@20hz, 3.5'@40hz).

                  ​As frequency goes lower more and more reinforcement occurs and your speakers frequency response in room exhibits a bass emphasis compared to it's free field response.
                  Click image for larger version

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                  • #10
                    Because of the noise floor and resolution in the lowest frequencies measurements can be unreliable. Proximity effect also can skew the results with line arrays. I once observed a guy claim low 20's extension on an array with drivers that measured -3dB in the mid 50's nearfield.

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                    • #11
                      Scary: He states his PhD thesis was on the sound field in a small room in the modal domain.

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                      • #12
                        I wonder why he doesn't come around here.

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                        • #13
                          Originally posted by rickcraig View Post
                          Because of the noise floor and resolution in the lowest frequencies measurements can be unreliable. Proximity effect also can skew the results with line arrays. I once observed a guy claim low 20's extension on an array with drivers that measured -3dB in the mid 50's nearfield.
                          ​People often don't understand that if you select a 20mS window for a time domain measurement system that every measurement point has a bandwidth of 50hz and is useless for bass measurements.

                          ​The short duration of the measurement sequence can result signal to noise issues as well since there is so little total energy in the measurement signal.

                          ​My measurement however is done with a high resolution sine wave system using sweeps of several minutes each (lots of total energy in the measurement).

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                          • #14
                            Originally posted by daryl View Post
                            Utter nonsense!

                            The man is an idiot!

                            ​It took weeks to explain room gain to him a number of years ago right here on the Tech Talk board (may have been Madisound back then but I believe it was here).

                            ​I showed him my room gain measurements and he thought they weren't real.

                            ​I explained the process and it turned out that he didn't even understand the concept of close mike measurement (loudspeaker measurement 101).

                            ​John K. worked on him for several weeks after that until he finally gave in and said he thought there was no room gain because of an anomaly with the software he was using(what?).

                            Now he's back in his own reality.......
                            You made me laugh when I read that.

                            I had a similar experience with him when I discussed adding cabinet diffraction to nearfield measurements before blending them. I ended up having a discussion with him on diffraction that we shouldn't have had to have given his background. It seemed quite silly to me.
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                            • #15
                              Originally posted by djg View Post
                              I wonder why he doesn't come around here.
                              DIYAudio is his forum of preference -- at least it was the last time I checked in over there, which was a few years ago. He has a group of steadfast admirers over there. I always felt that he formed opinions based on insufficient facts, then ignored any other facts that contradicted his opinions.
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