Grace Design SB66 User Manual

The stereophonic zoom

The

stereophonic

By Michael

A variable

Williams

dual
microphone system for
stereophonic

sound

recording

Intr

oduc

tion

In the field of monophonic sound recording the
sound recording engineer has a large range of
different types of microphone from which to
choose his particular preference for recording
any specific sound source or sound environment.
All manner of frequency response curves can be
chosen either to modify the timbre of the sound
source, or, if desired, to reproduce it as faithfully
as possible. The directivity of the microphone and
its position in relation to the source, can also be
chosen to obtain, fairly easily, the desired
relationship of direct to reverberant sound.

Unfortunately we do not have the same flexibility
in the choice of microphone systems for stereo­phonic sound recording. The number of available
microphone systems and the types of micro­phones used, is relatively limited, and, almost
without exception, those that are available have
fixed characteristics. It is undeniable that each of
these systems may be optimum in one particular
situation, however none of them can be
considered as meeting all our needs for
stereophonic sound recording.

In no way can we consider our present stereo­phonic sound recording microphone systems as
enabling the listener to experience natural
perception of the original sound source. The
sound image as perceived by the listener is
conditioned by the choices imposed by the sound
recording engineer. Balance, sound perspective,
localisation, etc are determined by the sound
engineer and his skill in transmitting these
parameters to the listener will remain part of the
art of sound recording for many years to come.

zoom

It is unfortunately too often the case that the
position of the microphone system is a
compromise between a good stereophonic sound
image and the optimum ratio of direct to
reverberant sound. In more general terms, it is
evident that the different parameters that must be
taken into ac- count during the process of sound
recording are infinitely variable, each situation
encountered being in itself unique. The conditions
that are necessary to achieve optimum results in a
specific context with the present "tools of the
trade" are but rarely encountered.

Even in the face of this very limited choice, many
attempts have been made to establish which, of the
different systems available, could be considered as
having the best overall performance. The futility of
this type of approach becomes immediately evident
in the light of the fact that each specific system has
a unique set of characteristics and that the sound
recording sources and environment are infinitely
variable.

On the contrary, rather than reduce the choice of
systems, an effort must made to increase the
number of systems available. Each sound recording
engineer must have the largest possible se- lection
of systems to choose from, in order to solve the
specific problems presented by a particular
recording situation and, to express his own
personal interpretation, as freely as possible.

The ideal is of course an infinitely variable system
covering all situations. This document describes
just such a variable dual microphone system called
« The Stereophonic Zoom » and explains

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the basic theory of dual microphone systems in
general and how this has been interpreted to
develop this variable system.

The "Stereophonic Zoom" enables the sound
recording engineer to get much nearer to the
optimum result in the great majority of recording
environments. It uses any desired first order
microphone directivity with it's associated
frequency response curve, and enables
reasonably independent control of:

• Stereophonic Recording Angle,

• Angular or Geometric Distortion,

• Reverberation distribution,

• Early reflection localisation .

The analysis of the physics and psychoacoustics
involved in this type of system has already been
published in papers given by the author to various
Conventions of the Audio Engineering Society:

1984 - 75th AES Convention in London - Preprint
2072,
"Stereophonic Zoom ; a practical approach to
determining the characteristics of a spaced pair
of microphones"

1987 - 82nd AES Convention in London ­Preprint 2466,
"Unified Theory of Microphone Systems for
Stereophonic Sound Recording"

1990 - 88th AES Convention in Montreux ­Preprint 2931,
"Operational limits of the Variable M/S Stereo­phonic Microphone System"

1991 - 91st AES Convention in New York ­Preprint 3155,
"Early Reflections and Reverberant Field
Distribution in Dual Microphone Stereophonic
Sound Recording Systems"

The basic operational simplicity of this system is
described in this document. This should enable
the sound recording engineer quickly to use the
St Zoom System for everyday sound recording
with a minimum of study.

The stereophonic zoom

The operational

appr

oach

I. 1 Characteristics of the
standard listening configuration

The basic characteristics of a stereophonic sound
recording system are determined as a function of
the relative position of the loudspeakers in relation
to the listener during reproduction. It is almost
universally accepted as a Standard Listening
Configuration, that the listener must be placed at
the summit of an equilateral triangle, the
loudspeakers being positioned at each extremity of
the base of the triangle and directed towards the
listener (Figure

1).

Figure 1 – Standard listening configuration

It is essential to attenuate reflections from the
ceiling, floor and walls and in addition, symmetry
should be maintained in relation to the shape of the
listening room, so that any remaining reflections
affect equally the sound heard from the left and
right channels. Only then will the conditions be
adequate to hear with clarity and precision the
stereophonic image generated by the specific
microphone sys- tem used during the recording
session.

Further improvements can be made to the listening
environment by following the IEC recommendations
concerning the specification of a standard listening
room. Another interesting solution to the problems
in designing a listening room has been developed by
Bob Walker of the BBC Research Dept., and
published in Audio Engineering Society Preprints
3543, 3853 and 4645.

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I. 2 Localisation

Without going into all the details described in
Chapter II, one can say that the localisation of a
sound source between the loudspeakers is
obtained :

• by varying the intensity ratio between the two
loudspeakers,

• or by creating a time difference between them,

• or by a combination of both intensity and time
difference.

If the same sound is produced by each loud­speaker at the same level and at the same instant,
then one will get the impression that the sound
image is situated in the centre between the two
loud- speakers (0° in Figure 1). If on the other hand
the sound produced by the right channel is louder
than the left, then the localisation of the sound
image will be situated somewhere to the right
hand loud- speaker (between 0° and 30° in Figure

1). If on the other hand the sound intensity of both

channels is the same, but there exists a small time
difference (less than a millisecond) between each
channel then a similar effect is obtained i.e. if the
left channel is in advance of the right then the
sound is localised to the left and vice versa.

It is quite commonplace to create a variation of
the Intensity Ratio on a mixing desk, by means of a
simple potentiometer, normally called a "pan pot".
On the other hand it is quite exceptional to find a
variable delay line associated with a mixing desk to
create Time Difference information between left
and right channels. Both of these techniques
produce localised sound sources between the
loud- speakers, unfortunately without any
information necessary to produce natural
acoustic size and stereophonic acoustic
environment. This
French "Monophonie Dirigé", which translates to
"Directed Monophony" or "Positioned Monophony",
as opposed to Natural Stereophony. Even the word
‘”Natural” in this con- text could be considered as
a misnomer, as we are really involved in the
process of producing the “impression” of a
natural listening experience.

process is

very aptly called in

The stereophonic zoom

The intention in this document is to
a

variable dual microphone system that will
reproduce realistic stereophony, thereby
creating a good "impression" of relative
acoustic size for each sound source and
maintaining the continuity of the sound
environment. It is important again to
emphasise the word "impression" in this
context, as we are indeed only concerned by
impressions. For instance, in recording an
orchestra, we must create the impression of
depth and localisation of individual
instruments, as realistically as possible. The
acoustic signals, received by our ears in the
listening room coming from the loud­speakers, bear very little relation to the
acoustic information actually received by the
listener in the concert hall. In fact, it could be
said that, any recording and reproduction
sys- tem which is capable of giving this
"impression" is acceptable, no matter what
means are used to obtain it.

Intensity Difference (or Intensity Ratio) and
Time Difference information can be generated
by two spaced directional microphones. The
Intensity Difference information generated by
the microphones, is a function of the position
of the sound source and the angle between
the axes of the directivity patterns
microphones. The Time Difference
information, on the other hand, is a function of
the position of the sound source and the
distance between the microphones. To obtain
Intensity Difference information only, the
microphones must be "coincident", whilst
Time Difference information only, will be
obtained with spaced omnidirectional
microphones or parallel spaced directional
microphones.

describe

of the

I. 3. 1 Operational characteris­tics of the variable dual micro­phone system

A Variable Dual Microphone System is
basically two identical directional
microphones mounted in such a way as to be
able to modify both the distance between the
microphone capsules and the angle between
the axis of directivity (Figure 2).

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Figure 2 – Angle and distance between two microphones

The distance and angle between the microphones is
chosen according to the Sterophonic Recording
Angle that is desired. Figure 3 shows the various
combinations of distance and angle as a function of
the Stereophonic Recording Angle. The x and y axes
(abscissa and ordinate) represent the Distance and

Figure 3 – SRA

diagram

From now on, this diagram, which we will call the
"SRA diagram" (Stereophonic Recording Angle
diagram), is our basic guide to choosing a specific

The stereophonic zoom

Angle between the microphones, whereas the
individual curves represent the various
combinations of angle and distance needed to
obtain a specific stereophonic recording angle
(shown in circles as a ± value on the
diagrams).

for cardioid microphones

combination distance/angle corresponding to a
given Stereophonic Recording Angle. Certain
limitations to our choice of distance/angle will be
considered a little later in this chapter.

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The Stereophonic Recording Angle is defined as :

THAT SECTOR OF THE SOUND FIELD IN
FRONT OF THE MICROPHONE SYSTEM
WHICH WILL PRODUCE A VIRTUAL SOUND
IMAGE B

Let us consider as an example, the combination :

(20cm
an- gle between the

ETWEEN THE LOUDSPEAKERS

20cm /

90°

.

between the microphone capsules / 90°

directivity

axes

)

We can read off the SRA diagram (Figure 3) that
this combination has a S.R.A. of :

±

50°

Note : The Stereophonic Recording Angle is, by
convention in this publication, always specified as ±
an angle, '+' indicating clockwise measurement and
'-' indicating anticlockwise measurement. This
convention is adopted in order to avoid confusion
with the specification of the angle between the
microphones which is specified as the total angle. In
the above ex- ample this means that + 50° is
measured clockwise from the front centre axis of
the microphone sys- tem, and -50° measured
anticlockwise (a total stereophonic coverage angle
of 100°).

Two other possible combinations with the same

Stereophonic Recording Angle of ± 50° are :

10 cm / 130° and 30 cm /

But of course there are a multitude of combinations
possible for a specific SRA. Here are two examples
of other readings from the SRA diagram :

10cm / 60° has an SRA of ±

40cm / 110° has an SRA of ±

All sound sources within this angular sector will be
reproduced as virtual sound sources BETWEEN the
left and right loudspeakers. Any sound source out­side this sector will be reproduced ON either one or
the other loudspeaker. The operational procedure for
setting up the system consists basically of measuring
the sector occupied by the sound source and using
this as the required SRA of the microphone system.
The combination distance/angle can then be read
from the SRA diagram.

90°

30°

50°

The stereophonic zoom

It should be said however, that it is not obligatory
for the SRA to be equal to the angle occupied by
the sound source. Most sound recording
engineers in fact prefer the SRA to be slightly
larger than the sound source sector. This is
equivalent to leaving a little headroom in a
picture or more correctly in this case "sideroom"
in the sound image. The amount of sideroom is
obviously a matter of individual judgement, but is
rarely more than about 10° for a small group of
musicians. On the other hand, in the case of a
much larger orchestra it is quite often
necessary to do the opposite and place the limits
of the SRA within the orchestra (negative
sideroom), the left limit being within the first
violins, the right limit within the double basses.
This allows more space for the individual
instruments (flute, clarinet, oboe, etc…) in the
middle of the orchestra. But this is a question of
individual preference, and there are as many
different choices as there are sound recording
engineers!

I. 3. 2 Microphone position

How does one determine the position of the
microphone in the first place? This is not a
question of measurement or even following a set
of rules, but more one of individual preference.
However, it is simple enough to describe those
factors that are modified with a change in micro­phone position.

The variation of distance between the sound
source and the microphone will certainly change
the level of direct sound reaching the micro­phone, however this will be perceived as a
change in the ratio of direct to reverberant
sound. It is this ratio that is responsible almost
entirely for our perception of depth or sound
perspective within the sound image. Each
individual sound engineer will have his own
subjective appreciation of the optimum value of
this ratio in relation to the type of recording
being made. In the case of multiple sound
sources, as for in- stance with an orchestra, the
position of the microphone system must also
take into account the relative acoustic levels of
individual instruments or sections of
instruments, the aim being to obtain a good
“balance” between all the different sections of
the orchestra.

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Once the microphone position has been deter­mined, one has only to measure the angular
sector covered by the whole of the orchestra,
decide how much "sideroom" one wishes, and
then read off the corresponding combination of
distance and angle:

Stereophonic Recording Angle

Sound Source Sector +

Figure 4 – SRA

Figure 5 – SRA

diagram

diagram

=

"Sideroom"

for a hypocardioid

for a hypercardioid

The stereophonic zoom

I. 3. 3 Frequency response curve
and directivity

A major advantage using the « Stereo Zoom » or
"Variable Dual Microphone System" is that we do
not need to restrict the choice of directivity pat­terns to only cardioid microphones. In Figure 4,
the SRA Diagram is shown for a specific
Hypocardioid ("Wide Angled cardioid" or " Infra­carded") Microphone, and Figure 5 shows the
SRA Diagram for a specific Hypercardioid
Microphone.

microphone

microphone

(back attenuation 10 dB)

(back attenuation 10 dB)

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