Speaker placement (part 1)

We probably all agree that the speaker placement in a room will affect their sound, one way or another. . . 

There are two main aspects:

  1. Low frequency sound, radiated by the speaker/s that spherically “wraps” right around the speaker cabinet and ends up being reflected back to the listener from nearby room boundaries.
  2. Low frequency room resonance/s, and whether a speaker is placed, in a 3 dimensional position within the room which excites them – or not.  (discussed in Part two)

Another, but usually less important aspect, is that of mid and high frequency reflections.

Less important as:

  • Generally, a domestic room with a typical standard of furnishings won’t have either an excessively long reverberation time or harmful level of discrete reflections.
  • Time (since the 1960’s in a “modern sense” and since the 5th century BC in “longer terms”) has pretty well proved that bass frequencies are the single most important spectral area to have “controlled”, and affect :
  1. musical rhythmic timing
  2. performance emotionality
  3. portability of recorded material – to name just a few
  4. “Tune” recognition at low frequency

There are exceptions, naturally, and an example of a domestic home cinema with a completely unworkably long mid and high frequency reverberation time can be seen here.

Spherical radiation.

Traditional speakers (those in enclosed cabinets) do it, electrostatics and “open baffle” (as well as direct rear radiation) do it.  Pretty well, the only common forms of speaker that doesn’t  is “soffit” mounted (common in recording studios) and “in wall” more common in domestic settings.
So, if you have speakers that “can be moved” around a room, as against “installed” in the room, you enter the world of spherical (or rear) radiation.

Doing the math.

Our software calculates an aggregate deviation score, relative to a flat response, for a given speaker system.
That is, once the speaker height is determined, based on the speaker itself and the preferred mounting method (stands, platforms Etc.) a calculation is performed based on the premiss that the speakers can actually be moved, forward and back, left and right.  This assumes that we start with the speakers placed where we want them for room layout and aesthetic purposes, and gives us an initial deviation score.
In this example, a pair of Tannoy’s “live”  in a 6.2 x 4.5 room.  Our positional staring point is with them quite close to the wall behind them – about 1m, and gets us an initial deviation score of a rather large 102 dB.  This means that if you take account of all the low frequency reflected energy from the room boundaries, add them to the sound actually coming from the speaker cone itself, there’s an aggregate deviation from a flat response of 102 dB. best speaker position calculation software screen shot    Software determining best speaker position.
The actual frequency response this equates to is nothing less than a “thing of nightmares”.  As reflected energy mixes with the direct from the speaker cone, a complex addition, partial addition, subtraction and partial subtraction of all the wave fronts takes place resulting in the speakers response being some 2 or 3 times less accurate than it would if, say, you plonked them in the middle of a field.

Needless to say, room layout and aesthetics do often limit or completely preclude “ideal” placement, which, generally does end up being not exactly “user friendly”. Best speaker placement software screen shot.jpg
In our example room, the best we can get is an aggregate deviation score of 44 dB.  That’s a whole 58 dB better than our preferred starting point, but, does require the speakers to be 1.85 m from the wall behind them.  These figures (best aggregate deviation around 40 & speaker distance from the wall around 2 m) are pretty common and in this particular example basically “eat up” around a third of the rooms length in what could easily be described as “dead space”.

This may not be, and arguably shouldn’t be a major issue in a dedicated listening room or cinema, who’s sole purpose is that of audio and / or AV, but a real issue in a more multi purpose “family” room.  Where dedicated rooms are concerned though, we can easily turn this otherwise negative into at least a positive if not a double or even triple positive !. media wall diagram.jpg
Particularly in home cinema’s, where there is a good chance of a deliberately added “media wall” housing the projection screen, but in audio only listening rooms too, a deliberately constructed additional phantom wall, a meter or so out into the room can cure a lot of pain.
While we still undeniably “eat up” real estate, the advantages are:

  • Restore visual “sense”
  • Reverse the appearance of speakers being “stranded in the middle of nowhere”
  • Offer space (in the void behind) for cabling, equipment and specialist bass absorption acoustic treatment.
  • Offer new and greater interior design opportunities (finish, texture, colour, integrated lighting Etc.)

Two “phantom walls”, both concealing around a meter of dead space behind the speakers

phantom wall photo.jpg  media wall photo.jpg

There is a less physically intrusive option: that of electronically filtering.  It’s not a cure all by any means, as a room can still have a strong enough affect to effectively overpower the filtering capability. In any event, depending on the system, so much filtering can be applied that there’s really not a great deal of the original signal left – something which those of us that hold signal path purity in high esteem would question.  While I’m personally in favor of “control” and see brilliant examples of just that, from air craft to heating systems performing way better with really clever intelligent control than without, there is no better way to deal with an issue, than at it’s source.  In this case, it’s a position in three dimensional space issue, in the physical domain, more correctly and efficiently dealt with …  in the physical domain.

The issue of “control” (by that I mean “electronic”) has been really well brought home with our recent dally into directly delivered low frequency by vibration.  The concept of “bass shakers” has been around for an age, but as I thought, provides a rather “fairground” effect – totally over the top and destructive to serious music listening.  However, proper control has proved that it can and does allow for a directly delivered vibration system to seriously enhance music, providing a whole new dimension, level of involvement and maybe even “believable realism”.   It’s also no coincidence that directly delivered vibrations, are not affected by any room positional or room resonance issues. For sure, you’d never think of subjecting an audio signal to the harsh filtering and other control techniques that you have to for vibration systems, but for vibration systems, that is not really a matter of “purity” or “unpurity” as we are simply replicating very similar vibration to that experienced in the real world.  Further, any directly delivered vibration, with it’s freedom from room effects could well allow for “turning down the bass a bit”, in turn reducing the negative room related physical effects on acoustically delivered sound.

Actually, a really good solution from all angles, is a mix of physical and electronic “treatment”.  That way the speakers intrusion into the room can be limited, without the need for really drastic electronic treatment, and, with the aid of calculations (as mentioned above) a far more acceptable room position can be “negotiated” with the minimum of detrimental effect on frequency response.

As with any system, in any sphere of life, an element of forward design, allowing for informed decision is somewhat more than just a good idea – definitely not a luxury !

 

* Part two:  Low frequency room resonance/s, and whether a speaker is placed, in a 3 dimensional position within the room which excites them – or not.  Coming soon !