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shots fired across the bow! (engineers vs 'the hifi people')

Discussion in 'audio' started by Jonathan, Jan 4, 2019.

  1. ToTo Man

    ToTo Man the band not the dog

    I wouldn't say 4.2m x 3.8m x 3.25m was tiny, it's small compared to my previous room which was 5m x 8m x 3.25m, but it's still bigger than many UK rooms! As you can see from the graphs, my main speakers roll-off below 35Hz, so my subs are set to crossover at 30Hz @ 24dB/oct to extend the response to under 20Hz. The bay window is a PITA. Rotating so that the speakers fire towards the door would be most practical from a room usage perspective, but would place the bay window at the first reflection point for the left speaker, which I presume will mess up the stereo imaging, so I'd definitely need to put absorption panels there. I've tried three GIK Monster bass traps on the rear wall behind my listening seat, which covered part of the bay, but I didn't hear a huge improvement and it actually made the measured frequency response worse. Also, the window is south-facing and lets in sunlight all day, which is another reason why I've located the hifi on the opposite wall, so that I'm the one absorbing the heat and UV, not my equipment! ;)
     
  2. davidsrsb

    davidsrsb pfm Member

    I would not try to do 20Hz in a smallish UK room with lathe and plaster above and below. My own experiments found that my original living room 6m x 5m x 6m with a thin board ceiling to a massive roof void was measuring like the biggest bass trap you have ever seen. My floor is concrete, so not too bad. Installing a mezzanine floor above, bringing the ceiling down to 3m made a great improvement.
     
  3. Tony L

    Tony L Administrator

    It is a box full of components, switches etc in the signal path, it will likely be more audible than the thing it is attempting to disprove! I have no issue with the concept of null tests, I’d just never trust bespoke technology or props from people with such an entrenced agenda to form my opinion. There are similarities here to both the James Randi cable challenge and Alan Shaw’s Harbeth equivalent - those offering these things always seem to want to put a box of their own technology into the signal path somewhere! That rules the whole thing out as far as I am concerned. If I want to A B something I insist it is done with nothing more than physically swapping that component in the system, i.e. no other parameter at all is changed.
     
    iant likes this.
  4. h.g.

    h.g. pfm Member

    REW is an excellent back-of-the-envelope interactive room simulator for rectangular(ish) rooms. You could use if for example to check how large the improvement is likely to be if correctly setup subs were located with one at the centre of the front wall and one at the centre of the rear wall. However, it is inappropriate for addressing a range of other questions and situations such as:
    - it won't help you size and locate your passive absorbers which is a large and important task.
    - it assumes the speakers radiate uniformly in all directions but controlling directivity is crucial for high sound quality
    - it doesn't handle non-rectangular rooms
    - it doesn't really handle the room response above the subwoofers

    To address these sorts of questions you need 3D engineering software that solves the acoustic wave equation by either gridding all the surfaces (e.g. BEM) or the internal volume (e.g. FEM, FVM, FDM,...) and includes reasonable models for scattering, diffusing and dissipating the wave motion. It is the quality of the latter which tends to distinguish the software.

    There are a fair few free FEM solvers around that can solve the wave equation at low frequencies and a few ray-type methods than can solve the high frequencies. What distinguishes them is the quality of the models for scattering, diffusing and dissipating the wave motion. I am not familiar with most of them (still finding and assessing them) but I have a growing suspicion I will need to write suitable software to address this aspect and integrate it with existing software to do the rest. Good quality but expensive commercial software exists but the available free software I have found so far is limited in what it can do.

    The bay window will be doing good things in shifting sound out of the axial direction into the side-to-side direction (you will see large structures to do this in well designed studios) but it is not symmetrical. This is the first thing I would check and if it is the main reason for the asymmetry I would look at adding similar diffusing surfaces on the right hand side in the form of an enclosure with porous walls, filled with absorbing material and tuned to address a room mode that needs attention.

    If he is interested in sound quality then yes he needs the subs to control the room response below 80 Hz. He doesn't say why he is using them to fill in below 30-35 Hz which for me would be a typical setting for a rumble filter to remove low frequency rubbish. Perhaps, unlike myself, he likes music or films that have significant wanted content rather than rubbish at these frequencies?
     
    ToTo Man likes this.
  5. Dozey

    Dozey Air guitar member

    To do a null test with any validity you would need to compare the same music signals using a microphone at the listening position. Not test tones, and no electronics in the signal path.
     
    booja30 likes this.
  6. ToTo Man

    ToTo Man the band not the dog

    Indeed, the room below does receive quite a lot of my bass! Luckily it's a detached house and that room isn't used too much so I can get away with it. Before I got my subs I would never have dreamed I'd be able to get 20Hz bass as my previous experience with whatever speakers I've had in this room has been varying degrees of a sharp roll-off below 40Hz. My XXLS400 subs naturally extend down to 10Hz without much roll-off. I don't do vinyl so don't need to worry about subsonic rumbles etc.
     
  7. ToTo Man

    ToTo Man the band not the dog

    Lots of excellent information here for me to digest, thanks!

    I'm not intentionally using the subs to control the overall LF room response. It would be great if they did, but this would mean re-locating them and also setting them to crossover higher. I was aiming for the simplest setup possible without the need for extra components in the playback chain, so I'm running my main speakers full-range and the subs with their built-in crossovers set to around 30Hz with a 24dB/oct roll-off, and am using a digital parametric EQ plug-in in Audirvana+ to smooth the response at the listening position by notching the main axial mode at 45Hz by -5dB. 90% of my listening is done at the one listening position so I am not too concerned about the response at other positions. I appreciate I would very likely obtain better results with DSP that's able to time-align the subs with the mains, but I'm actually quite content with how things work at the moment.

    The subs aren't used exclusively with the Tannoys, I also use them with my Celestion Ditton 66 which roll-off slightly earlier and more steeply than the Tannoys. This is the stereo bass response of the Celestions plus Subs at the listening position with 1/48 octave smoothing applied:

    [​IMG]
     
    Last edited: Jan 8, 2019
  8. davidsrsb

    davidsrsb pfm Member

    Below any room mode resonance, you are driving the room with pressure. The joining rooms also form part of the system. Things like making the door double layer to get better sealing will make a big difference. Does your room have a chimney?
     
  9. ToTo Man

    ToTo Man the band not the dog

    There is a chimney in the left (external) wall but it has never been opened (it is bricked over). The cupboard at the bottom left corner of the room is closed off with a door. I don't use it because the GIK corner traps prevent access to it. I'd considered stuffing the cupboard with rockwool but wasn't sure how much affect this would have given that there would still be a 50mm thick wooden door in front of it, this would also probably affect the left speaker more than the right? Interesting comment you make regarding the entry door on the right side of the room, it's also 50mm thick and there is a measurable difference in bass response when it's closed vs open. When it's closed there is more bass overall, but when I leave it ajar a few inches one of the bass nulls actually improves, so I tend to leave it slightly open.
     
  10. davidsrsb

    davidsrsb pfm Member

    Even bricked off, the chimney can act as a resonator. Can you estimate how long the flue is? Does it open into another room
     
  11. ToTo Man

    ToTo Man the band not the dog

    The chimney runs the full height of the house but is not used and is bricked over in my room and the room below so the only place it opens I think is on the roof. It's probably between 10 and 12 metres long in total. There isn't anything I can do about it anyway.
     
  12. Purité Audio

    Purité Audio Trade: Purite Audio

    Where else would you position your speakers within that room, where they are at the moment they have a
    What does this mean?
    Keith
     
  13. davidsrsb

    davidsrsb pfm Member

    When the room is much smaller than a wavelength, you are going to get fairly uniform pressure changes everywhere. This will be modified by air leaks through the floor, doors etc. The most efficient way to generate this low frequency pressure is to mount speakers in the walls.
     
  14. Purité Audio

    Purité Audio Trade: Purite Audio

    So the low bass whooshes out through cracks in the floor?
    Keith
     
  15. booja30

    booja30 pfm Member

    Could you model the door/opening by adding a rectangular volume beyond, roughly the shape of the room behind it, and open to the room you are trying to model?
     
  16. Jonathan

    Jonathan pfm Member

    what - with a sine wave?
     
  17. Sue Pertwee-Tyr

    Sue Pertwee-Tyr pfm Member

    And presumably into a static load?
     
  18. John Phillips

    John Phillips pfm Member

    I can’t comment on the quality of Ethan Winer’s actual null test kit (not enough details) but a null test does not in principle need to be done with any specific waveform or with any specific load.

    I have not read it for quite a while but Stereophile’s Carver Challenge from as far back as 1985 seems to have been based on Bob Carver nulling the actual signal difference between two amplifiers, working finally into a difficult load, and asserting that if the signal nulled well enough the amplifiers would sound the same. This seems to have been achieved. From page 4:

    “After this last bit of tweaking, where Bob was able to reinstate his 70dB null while driving a very difficult load, we now had what sounded like two absolutely identical amplifiers. No matter what speakers we used, every "difference" we thought we had isolated turned out to be there, in equal quantity, when we swapped amplifiers.

    This time, the listening went on through the whole afternoon and much of the evening, until all of us were listened out. More leisurely listening, refreshed by a good night's sleep, failed to turn up anything. As far as we could determine, through careful comparisons and nit-picking criticisms, the two amplifiers were, in fact, sonically identical. It is a gross understatement to say that we were flabbergasted! …”

    The devil is in the details we don't have but I would not write off this null test idea without some more information.
     
    Sue Pertwee-Tyr likes this.
  19. Dozey

    Dozey Air guitar member

    This was using a microphone and music I think, as I mentioned above.
     
  20. ToTo Man

    ToTo Man the band not the dog

    Was that aimed at me, Keith? I think you forgot to finish the question. ;)
     

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