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Speaker/Room Measurement Witchcraftery

Discussion in 'audio' started by S-Man, Apr 8, 2018.

  1. Tony L

    Tony L Administrator

    Certainly well worth flipping phase and listening/re-measureing. I don’t know enough about the Kefs to comment, but classic Tannoys (Reds, Golds etc) have the bass driver out of phase and the compression driver in phase, i.e. you’d want any subs to be phase inverted in that context.

    PS Those heretics who think Tannoys suck are actually 50% correct!
     
    Hipper likes this.
  2. S-Man

    S-Man Kinkless Tetrode Admirer

    No need to be afraid...
    I was going to get the measuring kit out to prove this once and for all, but decided just to invert the phase of both ZRBs (so that they are out of phase with the LS50s). It took about 2 seconds to hear that it sounds terrible. I don't need to measure that, believe me!!!
     
  3. S-Man

    S-Man Kinkless Tetrode Admirer

    Tony - just done the 1.5V cell test on the LS50s, they are most definitely not suckers!
     
  4. Ayya Khema

    Ayya Khema pfm Member

    what do you mean, sounds terrible? the worst effect it would have to have your sub and mains out of phase would be a reduction in bass output: that certainly would not sound terrible, simply less bass
     
  5. S-Man

    S-Man Kinkless Tetrode Admirer

    Going from great bass to no bass sounds terrible.
     
  6. cooky1257

    cooky1257 pfm Member

    None of this makes any sense, in order for there to be a phase cancellation, ie no bass, there would need to be significant output in the sub band from the Kefs.....
     
    Ayya Khema likes this.
  7. ToTo Man

    ToTo Man the band not the dog

    Agreed.

    Agreed.
     
  8. ToTo Man

    ToTo Man the band not the dog

    That's interesting. My subs have a continuously variable phase knob from 0 to 180 degrees. I tried 0 and 180 and saw no difference in the FR, but settings in between do make a difference (the peaks and dips shift in both frequency and magnitude. The adjustments were starting to make me neurotic so I ceased and desisted and left the subs set to 0 degrees and it sounds fine to my ears. But now I'm wondering if they should actually be set to 180 degrees? Damn you! :D
     
  9. Tony L

    Tony L Administrator

    Time/phase alignment gets way more complex with subs depending on where they are. If your Subs are *exactly* the same distance from you as the Tannoys then yes, 180 degrees should be correct assumung you have the sub connected at the amps output. It gets even more complex if you are driving the sub from a preamp output as some amps phase invert and others don’t. As an example if you were using a Quad 303 or 405 you’d need to flip phase if you were using a line out, not if not as these amps flip phase 180 degrees. I’d look at your system very carefully from a logical perspective, but trust your ears at the end, if it sounds right it likely is.

    PS I seem exceptionally sensitive to phase/time error and have yet to hear any sub/sat system that sounds right to me. I spent ages measuring and adjusting a pair of 400 Watt BK subs with my La Scalas (which are a time/phase nightmare on their own!) and eventually gave up and sold them as I couldn’t get it to hang together right. I remember spending an evening dialling them in (as ever to the point they were almost off), then listening to the system the next day and thinking it was the best I’d heard it, and later realising I’d not turned the subs on at all that day!
     
  10. S-Man

    S-Man Kinkless Tetrode Admirer

    Here's another possible way to integrate subs....
    You cannot defeat the laws of physics and a "perfect" signal into a real room will result in peaks and troughs... and to some extent that's what your ears expect! So adjust your subs for a measured response that most closely matches the simulated response (e.g. with the Room Sim feature in REW).

    Give this a try and see if it sounds better than the more conventional method of trying to integrate them for the flattest ampitude response.
     
  11. Ayya Khema

    Ayya Khema pfm Member

    a bad untreated room, or what you call "real room", will screw a "perfect signal". the laws of physics is that sound waves sent into a untreated room will not be absorbed fast enough and will mess with the original signal, via reflections, at the listening position.

    to be clear, all untreated room sucks bad compared to well treated room. I personally cannot enjoy a untreated room anymore and would rather listen over hifi headphone if I couldnt have a dedicated treated room.

    not at all impossible to have a close to ruler flat response at the listening position with lots of treatment (and not overly done as to not make your room overly dead) and careful LP and speaker placement.
     
  12. cooky1257

    cooky1257 pfm Member

    All the earlier graphs have disappeared.
     
  13. fatmarley

    fatmarley Registered User


    Whatever's loudest at your listening position Is the correct phase. If you have too much bass or a hump, adjust crossover frequency and/or level.
     
  14. ToTo Man

    ToTo Man the band not the dog

    Is there a way to tell correct phase of the subs from the SPL/Phase graph is REW?
     
  15. fatmarley

    fatmarley Registered User

    I haven't used REW, but I'd be surprised If there was a way to tell. I know that getting accurate measurements for loudspeaker design, In room, Is Impossible without taking a nearfield measurent of the woofer and splicing It with a farfield measurement. The lower In frequency you try to measure, the more the reflections mess up your result. Gating will only get you accurate results down to 200hz (If you have enough space) to 500hz In a typical room.

    Oh, and what I said earlier Isn't 100% true. An even order filter will give you a null when the polarity of one driver Is reversed, but the response will stay flat with an odd order filter. Thinking out loud (could be talking rubbish because this Isn't something I've had to deal with), I wonder If the best way to get an accurate summation of subwoofer to woofer would be to first reverse the polarity of one of the drivers and then adjust the phase dial to find the biggest null at the crossover frequency (assuming even order obviously) once you're happy, change the polarity of the driver back to how It was.

    Before that, I'd take a nearfield measurent of my woofer (assuming not ported) and find the -3 or -6db point (depending on what crossover you use) and set my subwoofer crossover to that. If everything above works out ok, In theory, all you would then need to do Is adjust the subwoofer volume level to match the mains.
     
  16. S-Man

    S-Man Kinkless Tetrode Admirer

    Yes very annoying, must be something to do with postimage. :(
     
  17. Ayya Khema

    Ayya Khema pfm Member

    speaker design, in room, is indeed impossible for obvious reasons. but to integrate a sub to mains in room, its quite different. you need to have the response of both the mains and the sub separately taken at the listening position due to room gain under 100hz. you cannot just take the xover setting on the sub for granted that it will for example, if you set it at 50hz to cut at 50hz, ect. so measurements at the listening position are very important and not that innacurate, simply easily changed by a change in mic placement. this is because in small rooms, if the listening position placement is near 25%/50%/75% of the room lenght, nodes and modes are very severe. placing the LP around 38% or 19% is recommended.

    as a example, in untreated rooms and bad LP placement, sometime measuring a speaker at a set point will show a dip of 10db at 70hz, then moving the mic 2 feet away from that location and the dip could be gone. its very important to udnerstand your room first. in my room, relative to my speaker palcement, I know exactly where I have a good lateral response even if I move my mic 2 feet here or there I still get more or less the same FR response.

    I feel op is in a good situation, but his sub probably 5db too high for my taste, but some prefer the sub frequencies boosted in such manners. im only concern about the 40hz dip...
     
  18. fatmarley

    fatmarley Registered User

    5db! If I turn up the bass by one 1db It's too much. I can't see the graphs. Where does the extra 5db start from?

    I think what makes integrating a sub easier than a tweeter to mid Is the size of the wavelengths. Integrating a woofer to mid Is easier than mid to tweet (although getting accurate measurements Is harder). As the wavelengths get longer you don't have to worry about relative offsets so much to get good phase integration.
     
  19. Purité Audio

    Purité Audio Trade: Purite Audio

    I did save one plot, I believe this was sub and LS50’s
    [​IMG]


    Keith
     
  20. S-Man

    S-Man Kinkless Tetrode Admirer

    I fear the honeymoon period with the LS50s is coming to an and.
    AK's "metallic colouration" and Elephantears' "slight upper mid nasal emphasis" might be the end to my hopes of long term bliss :(
     

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