Off-axis is the right axis for me!

Discussion in 'Room Acoustics and Treatments' started by Matthew J Poes, Mar 10, 2018.

  1. Matthew J Poes

    Matthew J Poes Staff Writer
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    I thought I would share a little response data to make a point about listening off axis. Most people believe that listening to a speaker on-axis, meaning that the center axis of the speaker is aimed at your ears, gives the best response. This is frequently not the case and in some cases, flaws in a speakers response that show up on-axis disappear off-axis. Most speakers do not have a crossover optimized for on-axis but rather for the power response (the average of the response across a 90 degree angle, at some set of increments, such as every 2.5 degrees, 5 degrees, etc.). This is because a speaker:
    1. May not be listened on axis, and
    2. Needs to reflect sound in the room with the same response shape as the listening axis (otherwise the reflections will sound different from the direct sound)
    Here are some example of how a response can change the directivity index, a line that represents how the response falls relative to the listening axis. The axial response, power response, and DI are in the upper box. The DI is of interest and is the white line toward the bottom of that graph.
    NS15 Response 0.PNG
    On axis, not flat and DI not ideal

    NS15 Response.PNG
    20 degree listening axis (how they should be listened to), note the flatter DI

    Kef Q15 0.PNG
    On axis, note the response and DI

    Kef Q15 26.PNG
    26 degrees, Ideal listening axis based on DI shape and flattest response

    As you can see, sometimes listening off of the central axis actually gives you a better response and better (smoother) response change as you move off of that listening axis. That means as your head moves the response doesn't really tonally shift. It also means that reflections such as first reflections will have the same response shape (just reduced in level) as the on-axis. That's good stuff!

    There is more value to this. What I've shared are the response of two speakers which have a flat response, well designed crossovers with good driver integration, and nearly ideal directivity. Most speakers do not look like this. However, you can use knowledge of this information to get a better sound from the speaker. My experience is that many speaker manufacturers either do not give a preferred listening axis or their recommendation doesn't match the response data for their speaker (meaning their recommendation is not ideal). Even measurements taken in room can give you the necessary information to know what angle provides the best and flattest response (though you won't be able to calculate the polar response or DI accurately this way, that isn't absolutely necessary for setup).

    Stay tuned for more details soon. I am finishing up a review where this became very helpful in getting the best out of the speakers. What I love about this tweak is that it can be a totally free fix that can have as big a benefit to sound as a fairly major change to the speaker. It can be like turning it into a totally new speaker, with a drastic tonal change for the better. This is primarily beneficial when a speaker has a bright tonal balance with a measurably elevated top end.

    Now as a quick primer, remember that on axis means the speakers are pointed at your head, not straight toward the wall behind you. 20 degree listening axis means rotated 20 degrees inward from the axis pointed at your head. A speaker's proper toe angle often looks very extreme.

    Listening Axis 1.PNG
    Above shows a speaker pointed directly toward the back wall. This is generally not the right way to position the speaker.


    Listening axis On.PNG
    This is with the speakers pointed on axis, pointed toward your head. This is 0 degrees.

    Listening Axis 20.PNG
    This is the speaker's pointed at 20 degrees from on axis, as you can see their on axis response actually crosses in front of the listener
     
  2. ddude003

    ddude003 Active Member

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    If the speakers pointed on axis, pointed toward your head, is 0 degrees then I have my ESLs pointed about 7.5 degrees outside of on axis... Crosses slightly behind the listening position...

    I agree that with REW and some time, speaker positioning is _the_ major factor in setting up a home stereo/theater...
     
  3. Matthew J Poes

    Matthew J Poes Staff Writer
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    ESL's are often highly directional so positioning can be even more critical for a good response.

    What ESL do you own?

    https://www.princeton.edu/3D3A/Directivity.html
    These guys have tested a bunch in their anechoic chamber. It turned out the flat ESL panels were by far the most directional speakers. While that may not be great in some scenarios, for 3D sound it is absolutely critical. It's also been noted as potentially critical for object based sound with many channels. It limits the sweet spot some, but improves pan quality when you start getting into like 20+ channels.
     
  4. ddude003

    ddude003 Active Member

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    These are Martin Logan ElectroMotion ESLs... They are a little finicky to get placed in the "best" location from walls and the listening sweet spot (read interface with the room)... And once you do they are amazing with their holographic 3D sound stage... Better than headphones... It is well worth the time and effort for anyone to get their speakers "tuned" to their room... Get the most from their investment in equipment... REW helped a lot in this process and a little acoustic treatment after that was the icing on the cake...
     
  5. Matthew J Poes

    Matthew J Poes Staff Writer
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    Glad to hear that REW helped you so and that things have worked out. I agree that this is can be really helpful. I might even say a lot of acoustic treatment can be even more icing, as I really do find it can be transformative in a lot of rooms. Of course given what I do on the side...I may be biased.

    Martin Logans use a curved panel. I've never seen their polar data as shown above, I'm not sure they have been measured in this way before and publicly displayed, but...my guess would be that the curved panel reduces their beaming drastically. A flat ESL is known to have VERY narrow dispersion, but the reason is because the panel radiates sound over a wide surface area and that controls the dispersion. In dynamic driver terms, its like having a line array with a lot of drivers both in height and width. Having multiple drivers in the horizontal pain will control sound in the horizontal plain. I don't consider it a good or bad thing, just something to understand when implemented.

    I have very little direct experience with large flat ESL's so I won't comment on the sound. I would surmise that they may have a narrow sweet spot and be finicky in setup to get the best tonal balance and soundstage, but that they would interact with the room more positively with less sidewall reflections and a more coherent sound. I'd love to spend time with some, but the only ones I've ever heard are Quad ESL's which are a little different.
     
  6. ddude003

    ddude003 Active Member

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    This hybrid speaker is what I would call a modified dipolar design ... The electrostatic transducer is curved 30 degrees and can reproduce all frequencies above 500Hz... The actual diaphragm is 0.0005" thick with a radiating area of 292 square inches... And a 8" woofer for below 500Hz... So this speaker has only one crossover at 500Hz... Its a pretty nice design and it has a sparkle and speed that sounds sublime...
     
  7. Matthew J Poes

    Matthew J Poes Staff Writer
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    Oh yes the speaker you own I am very well familiar with. They do have a magic all their own. Great bass and amazing imaging. While more a quality of line arrays than ESL's specifically, ML's are among the most attainable way of getting that kind of imaging magic. Very few speakers can equal that ability to present such a palpable sound stage. Most that can do what ML's do in the staging department cost a lot more money too.
     

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