B&O Tech: A day in the life

#17 in a series of articles about the technology behind Bang & Olufsen loudspeakers

 

This week, instead of talking about what is inside the loudspeakers, let’s talk about what I listen for when sound is coming out of them. Specifically, let’s talk about one spatial aspect of the mix – where instruments and voices are located in two-dimensional space. (This will be a short posting this week, because it includes homework…)

Step 1: Go out and buy a copy of Jennifer Warnes’s album called “Famous Blue Raincoat: The Songs of Leonard Cohen” and play track 2 – “Bird on a Wire”.

Step 2: Close your eyes and really concentrate on where the various voices and instruments are located in space relative to your loudspeakers. If you hear what I hear, you’ll hear something like what I’ve tried to represent on the map shown in the figure below.

A map of the locations of many of the instruments in Jennifer Warnes's recording of "Bird on a Wire".
A map of the locations of many of the instruments in Jennifer Warnes’s recording of “Bird on a Wire”.

I’ve used some colour coding, just to help keep things straight:

  • Voices are in Red
  • Drums are in blue
  • Metallic instruments (including cymbals) are in green
  • Bass is gray
  • Synth and Saxophone are in purple

Note that Jennifer sings her own backup vocals, so the “voice”, and the two “bk” (for backup – not Burger King) positions are all her. It also sounds like she’s singing in the “choir” on the left – but it’s hard for me to hear exactly where she is.

Whenever I’m listening to a pair of loudspeakers (or a car audio system, or the behaviour of an upmix algorithm) to determine the spatial properties, I use this map (which I normally keep in my head – not on paper…) to determine how things are behaving. The two big questions I’m trying to answer when considering a map like this revolve around the loudspeakers’ ability to deliver the (1) accuracy and (2) the precision I’m looking for. (Although many marketing claims will use these words interchangeably, they do not mean the same thing.)

The question of accuracy is one of whether the instruments are located in the correct places, both in terms of left and right, but also in terms of distance. For example, the tune starts with a hit on the centre tom-tom, followed immediately by the bigger tom-tom on the left of the mix. If I have to point at that second, deeper-pitched tom-tom – which direction am I pointing in? Is it far enough left-of-centre, but not hard over in the left loudspeaker? (This will be determined by how well the loudspeakers’ signals are matched at the listening position, as well as the location of the listening position.) Secondly, how far away does it sound, relative to other sound sources in the mix? (This will be influenced primarily by the mix itself.) Finally,  how far away does it sound from the listening position in the room? (This will be influenced not only by the mix, but by the directivity of the loudspeakers and the strength of sidewall reflections in the listening room. I talked about that in another blog posting once-upon-a-time.)

The question of precision can be thought of as a question of the size of the image. Is it a pin-point in space (both left/right and in distance)? Or is a cloud – a fuzzy location with indistinct edges? Typically, this characteristic is determined by the mix (for example, whether the panning was done using amplitude or delay differences between the two audio channels), but also by the loudspeaker matching across the frequency range and their directivity. For example, one of the experiments that we did here at B&O some years ago showed that a difference as small as 3 degrees in the phase response matching of a pair of loudspeakers could cause a centrally-located phantom image to lose precision and start to become fuzzy.

Some things I’ve left out of this map:

  • The locations of the individual voices in the “choir”
  • Extra cowbells at around 2:20
  • L/R panned cabasa (or shaker?) at about 2:59
  • Reveberation

Some additional notes:

  • The triangles on the right side happen around 2:12 in the tune. The ones on the left come in much earlier in the track.
  • The “synth-y fx around 2:20” might be a guitar with a weird modulation on it. I don’t want to get into an argument about exactly what instrument this is.
  • I’ve only identified the location of the bass in the choir. There are other singers, of course…

You might note that I used the term “two-dimensional space” in the beginning of this posting. In my head, the two dimensions are (1) angle to the source and (2) distance to the source. I don’t think in X-Y cartesian terms, but Polar terms.

An important thing to mention before I wrap up is that this aspect of a loudspeaker’s performance (accuracy and precision of phantom imaging) is only one quality of many. Of course, if you’re not sitting in the sweet spot, none of this can be heard, so it doesn’t matter. Also, if your loudspeakers are not positioned “correctly”  (±30 degrees of centre and equidistant from the listening position) then none of this can be heard, so it doesn’t matter. And so on and so on. The point I’m trying to make here is that phantom image representation is only one of the many things to listen for, not only in a recording but also when evaluating loudspeakers.

 

  1. What is the correct setup of BeoLab 8002. and have the choice of treble had an effect in relation to how consumers place their 8002.

  2. Hi,

    I don’t think that I understand your question. The 8002 should not be set up differently than any other floor-standing loudspeaker. So, they should be placed as two of the points in an equilateral triangle (where the listening position is the third point). This means that they are +/- 30 degrees from centre front, and both the same distance from the listening position. Personally, I would “toe them in” (meaning, rotate them so they are pointing at the listening position rather than straight ahead) since their on-axis response is directly in front of them – not 30 degrees to the side.

    Does this answer the first part of your question, or did I misunderstand you?

    I don’t know what you mean by “choice of treble” – do you mean the high frequency balance (relative to the mid’s and low’s) or do you mean the tweeter? If you meant either of these, then I don’t think that the 8002’s treble is so unique that it would demand a different placement. If you meant something else, then please let me know.

    Cheers
    -geoff

  3. Thanks Geoff,

    I’ve been looking for a while for information like that: a reference point to judge imaging. I was a bit confused with my setup since even if the singer is well centered, the image is wider to the left than to the right due to asymmetry in my room. The left tom images close to the speaker while the right one is where you locate it. I will try more toe-in from my left speaker (it will point straight at me while the other will be half-way), and with the balance towards the right to compensate and keep the center centered… Does this make sense?

    Also, could you point me to other reference material like that, maybe artificial sounds?

    Thanks again!

  4. Loudspeaker positioning should be done w respect to the room, not the listener. You specify this famous 30 degrees left and right but nothing about where it is in the most important factor, the reflecting surfaces near the speakers. If they are too near one or two surfaces, this will cause a clustering of images (the speakers and their virtual reflection images) that will destroy center focus. I use a pattern of 1/4 of the room width in from the side walls and an equal amount out from the front wall. This gives an equal spacing of the actual speakers and all of their virtual reflected images for a perfectly even, wide, spacious soundstage. This works for most speakers but is especially important for multidirectional speakers, which will have even more impressive depth and spaciousness from the reflected sound.

  5. Hi Gary,

    I agree that the loudspeaker placement in the room is a crucial issue as well – this is why this issue receives considerable attention in the Technical Sound Guide for the BeoLab 90. However, if you are sitting off-centre with respect to the loudspeaker pair (assuming 2.0 channel stereo) then the mismatch in the times of arrival of the two audio channels at the listening position (assuming that you haven’t done any compensation using delay and gain, at least as a start) will cause the phantom imaging to collapse to the nearest loudspeaker, regardless of location in the room.

    So, I would disagree that it’s the loudspeaker-room placement and not the listener-loudspeaker configuration that matters – both matter.

    With respect to rules-of-thumb like your 1/4 room width – this is very dependent on the frequency-dependent horizontal directivity of the loudspeakers, the reflective coefficients of the surfaces in the room, and the dominance of modal problems – as a start.

    Regarding the reliance on early sidewall reflections to enhance/exaggerate spaciousness, I’m afraid that we’ll have to agree to disagree on that one…

    Cheers
    -geoff

  6. Hi, Geoff. It was nice to meet you last week in Struer.

    If you’re in a constrained space – that is, I’ve got a room that’s 18’x15′ or so, where the only real place to put some 90s would be a 12′ wall. It’s open plan, so next to the left speaker would be open (with the last bit of the wall behind), and next to the right would be windows, running from about 1.5′ above the floor to 6′.

    So – reflective surfaces on one side. Nothing (for about 6+ feet) to the other. Not ideal, obviously, but you work with what you’ve got.

    Given that – will the room compensation be able to deal and give a good listening experience? Or is the space too compromised to work with?

    Thanks for any thoughts.

  7. Hi Dave,
    The Narrow Beam Width will help you out more than Active Room Compensation in this case. ARC will look after the low-frequency boundary effect differences and the room modes. However, the differences in your sidewall reflections will be reduced by using Narrow mode, since it will attenuate the reflection from the right-side wall, better matching the lack of reflection from the left loudspeaker.

    As for the size of the room. the bulk of the tuning of Beolab 90 was done in a room that is about 15′ wide by 18′ long. You can see a photo of that room in Figure 18 on this page.

    Cheers
    -geoff

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