DIRECT vs. REFLECTED
by Steve Deckert
I have to admit that since meeting Dan last
year and helping him in the development of his room treatment products,
I have been giving more thought than normal to room acoustics.
And one of the most interesting and on going arguments I've seen
in the audiophile community is the one about direct vs. reflected
energy. Bose gets a bad rap by many audiophiles because they
are always reflecting some energy off the walls etc.
are panel speakers that radiate in a bipolar pattern,
bipolar moving coil speakers, conventional "audiophile" direct radiating
speakers, spherically enclosed moving coil speakers, 4 sided pyramid
shaped speakers with moving coil speakers on all sides, and radial
speakers that use typically a bending wave cone, or bladder to radiate
sound in a 360 degree pattern.
is what I have found...
eliminate electronics from the equation and just focus on the relationship
between speakers and your room. It has been my experience
that in the categories of imaging, soundstage size & depth,
focus, and believability, the relationship between your room and
speakers is actually more of a factor than what type of speakers
we look at the most common type of loudspeaker, that being a cabinet
with the drivers in an array 90 degrees from the floor and at a
parallel angle with the walls as our sample. I will try to paint
a picture of what happens.
of all lets reduce our example to a single woofer that plays out
to between 1000 and 2000 cycles. Regardless of the size (diameter)
of the woofer, it will radiate the majority of itís sound in a beam
pattern of 30 degrees or less.
say the speaker is 90 dB efficient and in a 14 x 16 listening room.
(The exact size of the room is not a critical component in this
illustration). Lets only look at one speaker and assume it
is set up in the conventional place out away from the walls and
pointing at the listening chair centered in the room.
is considered a direct energy listening scenario. However
you will only hear direct energy in itís virgin state come from
the speaker for the first 10 to 30 milliseconds... after that the
bounce off The rear, side and finally front wall will blend across
your sound field creating a smearing effect. In this sense
there is no such thing really as direct energy unless perhaps you
hung your speakers and listening chair from a crane several hundred
feet in the air out in the middle of a corn field.
these three primary reflections are what destroy the focus of your
sound stage, it seems to have been assumed that reflections are
categorically bad. Dan has
made some real advancements in controlling these reflections and
absorbing excess energy to minimize this evil. This is done
by using quadratic theory diffusers at all the critical reflection
the beam of sound coming from your speakers at a sound pressure
of 100 dB. It has a cone shaped pattern of ever expanding
wavefronts at around 30 degrees. It wishes past your ear and
hits the back wall in a circular pattern of around 8 to 10 feet
in diameter. The pattern of sound when it wised past
your head was about 3 to 4 feet in diameter. Since The sound
at your ear was 100 dB at a concentrated diameter of 3 to 4 feet,
The reflection off The back wall will be reduced by 50% power at
a diameter of 8 to 10 feet. This means that reflection that
is now aiming for your side and front walls is at an energy level
of 93dB! Naturally you will hear this reflection as it finally
passes your head and it will be delayed in time enough to confuse
your brain and make it hard to focus or localize the instruments
in the recording.
using quadratic diffusers we are able to bust these reflections
down and change their pattern of radiation from a single specific
angle to a 180 degree horizontal fan. By changing the energy
from 30 degrees at the first reflection point to 180 degrees, you
can see where the amplitude of that reflected energy would be substantially
reduced by as much as 6 times, or 15 dB! At this amplitude
your brain hears the reflections as ambiance and does not try to
interpret them as part of the primary wave front. Result:
Focused perception of a clear image.
this situation has been modeled and the result is my current listening
room, I have come to the conclusion that listening to conventional
speakers in a room without this quadratic theory diffusion is to
listen in vain. No matter HOW MUCH you spend on gear and speakers
you will not achieve the clarity and holographic presentation possible
in your playback of the recording.
that have drivers pointing out to the sides or rear often accentuate
this problem by reflecting concentrated patterns of energy directly
off the nearby walls.
the illustration of a single speaker in a room playing around 1000
cycles there are three problem reflection points that have to be
dealt with to get good sound. As you can see it is an aggressive
and unnatural pattern.
look at natural wave fronts for a minute and compare them
to the wave fronts that come off a cone shaped speaker...
This is a shot of my listening room. Yes I know what live
music sounds like in there as you can see. Look at the angles
of the drum heads and guitar strings in relation to the room.
The white lines you see are sound wave fronts that I tried to draw
in. If you really observe this for awhile you will see that
almost no musical instrument radiates sound in a cone shaped pattern
directly at your forehead. The horn instruments with the exception
of a few always point somewhere other than directly at your face
unless that is the intended emphasis of the musician for a brief
moment. This would tend to indicate that a conventional speaker
can never totally get it right!
I stumbled on to a new d.i.y. idea that I am posting on The web
site called The "Radial Loudspeaker".
As a result I have started to realize that this may be a design
of loudspeaker that actually works WITH a room rather than against
it as we've clearly demonstrated in the above discussion about typical
loudspeakers. In this model we have a true 360 wavefront almost
like that coming off the drum kit.
the radial design, energy is even across a 360 degree plane and
180 degree hemisphere. If substituted for the conventional
speaker in our earlier illustrations, you can calculate the following...
wave front passes your head but this time it is already a diameter
exceeding your room size. The reflection point is changed
from a 30 degree 8 to 10 foot diameter standing reflection, to 180
degree graduated non-standing reflection, and hence 15 dB down at
the first reflection point. Also interesting to note is that
the room corner behind your speakers now becomes the first primary
reflection, but since it is graduated it folds to become time coherent
with the primary wave front as shown in blue.
a test, one of these speakers was placed in a room and compared
with a conventional speaker on the other channel. The fear was that
because your ear only gets 30 degrees or so of the primary wave
front, the energy of the speaker would have to be increased by a
considerable factor to match the loudness level of the direct radiating
speaker on the other channel.
this possible side effect could mean 30 / 180 or 6 times The energy
to match our conventional speaker. That would be 15 dB, and
that would raise the reflected energy in the room by 15 dB making
the advantage we thought we had disappear.
happened was rewarding. Since the room corner, i.e.. side
and rear wall near the speaker fold the wave fronts into coherently
timed graduated fronts, the listening chair actually gets the full
energy of the 360 degrees divided by losses and graduated reflection
is something like 360 / 1.4 = 257 degrees for a loss of 103 degrees
or 28 % which equals just over 1.5 dB.
was confirmed by listening as the conventional speaker had an efficiency
of 90.5 and the radial had an efficiency of 95.5. When measured
in the room against the 90.5 speaker, the difference was not 5 dB
but 3.5 dB! This confirmed the equation and eliminated the
fear of losing 15 dB. The only way that could happen is in
a very large room or hanging from a string over a corn field.
The past few days I've been listening to this radial speaker in
its unfinished state, and with all the room treatment removed I
have as good or better (actually better) performance than using
the conventional speakers with a full armament of room treatment.