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Thread: Upfront bass illusion - Follow up

  1. #31
    Senior Member
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    Oct 2010
    Durwood, that's pretty unexpected. To me, at least. I assume mucking with the phase was introducing ITD weirdness, but I had assumed there was a rather small range on which that cue operated. It's too late for math tonight.

  2. #32
    Founding Member
    Join Date
    Aug 2010
    There might have been some errors in my way of quickly experimenting, but I figured I would try it anyway. The nice thing about headphone testing is that they remove the room, and remove external noises, rattles, buzzes, etc. With IEM's, you can remove the pinna too.

    In the real world, a low frequency wavelength is > than the head diameter so head shadowing does not really reduce the level enough. However, the head is sensitive to azimuth angles because that is how ITD is determined. At 0/180deg azimuth, we know the ITD is zero, and at +/-90deg is at it's maximum. A source of sound has access to both ears in the real world, with headphones the ears only receive what is fed to them via the headphones so crosstalk is out. This is sort of why I also tried it with varying 4 channels combined into 2. Maybe I managed to introduce scenarios that don't exist in the real world (or we could label them uncorrelated) so my brain and hearing have a hard time interpreting the result.

    Things that have been studied before and have been shown:

    -We cannot locate with any great degree of accuracy <120hz, but that does not mean we cannot tell if there is a difference in phase between two different sources of sound. Some models suggest a distinction between ITD and IPD.
    By convention, "frequencies below 90Hz produce no interaural cues useful for spatial sound or localization." Yet some claim they are able to hear a difference between a single subwoofer channel (whether or not to more than one subwoofer) and two ("stereo bass"). Reported research supports the Jeffress model of interaural time difference (ITD) determination in brain structures, and extending the accepted lower frequency limit of interaural phase difference (IPD). Meanwhile, uncorrelated very low frequencies (VLF <100Hz) exist in nearly all existing multi-channel music and movie content. The audibility, recording, and reproduction of uncorrelated VLF are explored in theory and experiments.
    -We are better at locating the direction of short bursts and clicks vs steady signals such as sinewaves.

    neither one of these seems to say anything about not being able to tell a difference exists however, unless I missed this. Me wanting to turn my head to equalize it might be a reaction to it.
    Last edited by durwood; 04-08-2011 at 04:10 PM.

  3. #33
    Founding Member Subwoofery's Avatar
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    Sep 2010
    Real Name
    Here's Erin's post and why I started this thread:
    So much so that once I hear a buzz, it's almost impossible for me to move past and continue tuning. Hell, I knock down 40/50hz in most cars a couple dB just to get rid of that thump in the back which causes me to feel like the stage pulls rearward.
    In most car, from 40Hz to 50Hz are the freqs that usually excite a panel - creating a buzz or rattle = localization of the bass.


  4. #34
    Junior Member
    Join Date
    Jan 2011
    I believe the 40-50Hz bump is constructive interference from the smallest interior dimension being around the same or a little under the same length as that frequencies quarter wavelength. As the full wavelengths approach the longest interior dimension, they begin to load up until you reach a point where the quarter wavelength is able to load up as well at which the interior pressure (phase) peaks, and equalizes. The good news is that below that frequency, being able to identify phase should be damn near impossible and is why so many use a 50Hz crossover point.

    Sent from my iPhone using Tapatalk
    Last edited by trebor; 09-19-2013 at 06:59 PM.


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