Preprint No. 2998
Presented at the 89th AES Convention
1990 September 21-25
Los Angeles
Continued from page 1 of 2
6 NEWSCAST, FILM
Dubbing, retakes, voice synch and voice over are not without impact
on the continuity of the audio track. Both off and on camera vocal
tracks are highly sensitive to room acoustic colorations. The variations
range from outdoor shots to vocal tracks taken in a closet. One
of the audio engineer's jobs is to maintain consistency between
the audio and video tracks.
A good example of consistency control is found in the production
of a television news story. Here the local personality, holding
a mic comments on some disaster which is to be seen across their
shoulder. The camera pans to another view and the narration continues.
The off camera vocal track is not an on-site recording. It was composed
and produced back at the TV studio. The voice over simply does not
sound the same as that recorded in the field. The life like sound
of the omni mic in the free field highly contrasts with the hyper
mic used in the small, semi-dead voice over booth.
The first major TV station to use QSF acoustics is KTVU, Channel
2 in Oakland, California. It has an award winning news show. Part
of their formula for success is the QSF vocal booth technique. For
three years (since 1987) they have been using the rapid diffusion/decay
rates of a QSF booth where they open up an omni mic in a tiny room
4' x 6' in size and get a voice over mixed with the background sounds
that almost perfectly matches full recordings in the field. |
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7 FLYING GOBOS
A example of another notable application of the QSF technique involves
the very well known show biz voice of Ed McMahon. Remote shoots
of his commercials caused a variance in the timbre of his voice.
This was unacceptable to the producers as his close mic'd voice
was too well known. One of two choices remained, either voice sync
everything or stabilize the acoustics of his shots.
It's expensive and time consuming to voice sync. Instead, the crew
rigged an acoustic cloud using the QSF gobo format, flying just
out of camera shot. The boom mic was just below the QSF cloud and
the track sounded great. The flying gobo essentially blocked the
intrusion of the overhead reverberant sound and provided quite a
few early reflections to help smooth over the table and floor bounce
effects.
This technique is also valuable in the high bay studios. A flying
cloud over the free-standing QSF gobo has not only lateral isolation
and enhancement but adds in the vertical component for even better
isolation and diffusion. |
Fig. 9 - Flying Gobo |
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8 REMOTE RECORDING
Recordings in a large reverberant space can be beautiful but can
also be terrible. The engineer tends to move back from the instrument
trying for a more natural instrument sound but too quickly runs
into the strength of the reverberant field. The QSF gobo method
eases the fit by enhancing the direct signal strength while softening
the level of the reverberation.
The mic is often high overhead. The traps are still set in a semicircle
pattern. The variables are the height of the instrument, the height
of the traps and the height of the mic. The higher the traps are
off the ground, the more energy leaks directly to the reverberant
field. The higher the mic is, above and outside the traps, the more
it hears the reverb.
The reflections of the talent back into the mic produce the diffusive
group of early reflections. An additional feature is that the reverberant
field is more quickly diffusive due to the spoked nature of the
sound source after it passes through the gobo. |
Fig. 10 - Reverberant |
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9 PIANO GOBO
The piano mic is often inside the open lid of the piano top. Unfortunately
this lid/sound board is not only good for projecting sound out,
but also good at collecting sound from the outside which the mic
also picks up. Often, the piano is draped in moving blankets. A
gobo can also be used to increase the signal at the mic and reduce
the intrusion of noise and echos.
The QSF gobo for a piano sets traps along the open side of the
piano with the reflectors inwards. Interior sounds are multiple
reflected while sound from the outside is attenuated. Mic placement
can be eased away from the strings without degrading the recording.
An aside: Piano sound boards are essentially parallel to the floor.
This allows the setting up of standing waves. Very irregular loudness
effects in the middle C octave and above are directly attributable
to this effect. About 1/5 of professional piano practice setups
include some sound damping material under the piano. Best results
are developed with acoustic materials that scatter mids and highs
but damp 200 to 400 Hz. A couple of traps used for gobo purposes
can be placed on the floor below the piano, with reflectors up for
best results. |
Fig. 11 - Piano Gobo |
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10 SPEECH TRAINING GOBO
This experiment was performed with the speech therapy department
at the University of California, San Diego. A speech training table
is typically set out in a larger room. It is the size of a card
table. Four Traps were added with the reflectors facing inward.
Speech training ensued with the hearing impaired.
The single bounce off the table creates a comb filter effect. By
adding 4 additional reflectors the comb coloration effects reduced.
The listener tries to imitate sounds. To mimic comb filtered speech
is inappropriate and would be judged incorrect by the teacher. The
multiple reflected signal is a more honest and accurate signal to
imitate.
When speaking, the student hears the sound of their own voice better
due to backscattering off the reflectors. Acoustic feedback increases
the rate of learning. The teacher and student both speak through
the same "chamber". The teacher can also better hear detail
in the speech of the student due to the enhanced acoustic coupling
by this set up.
Hearing impaired seem to hear better with one ear oriented directly
toward the speaker. In general, they are also very susceptible to
distraction by sounds coming in from the side. In this configuration,
sound from the side is blocked and replaced with a reflection of
the speaker's voice. Traps to the left and right of the speaker
also block room noise and help the speaker's voice stand out more
distinctly. Additionally, the traps near the speaker help to block
directionally competing sound. Not only does the gobo signal enhance
speech but it reduces distraction.
An interesting aside is that these Traps form somewhat of a "blinder"
for the student. Hearing impaired are easily visually distracted,
as are the learning disabled. This table top gobo provides a substantial
degree of visual barrier effect allowing the student and teacher
to be in better contact. |
Fig. 12 - Listening Function
Fig. 13 - Monitor Function
Fig. 14 - Isolation Function |
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11 INTELLIGIBLE LISTENING
The dynamic level of listening for the hearing impaired is limited
and compressed. For example at 50 dB,A, sound might really be at
the threshold of audibility for some frequency range yet at 80 dB,A
the weak frequency range would sound as loud as with any unimpaired
ear. In addition to spectral threshold and dynamic range problems
the hearing impaired very often lose the ability to discriminate
one sound above others in a crowded, noisy sound field. Echolocation
is the ability to corrolate the sounds from some particular direction
using signals into both ears. Current practice in hearing aids is
limited only to bandwidth level controls. Phase and time alignments
are not yet available.
The saturated sound fusion acoustic space provides remarkable listening
benefit for the hearing impaired. This is most especially evident
in contrast to the hearing aid and the mic/headset options presently
in common use.
All three factors found in the QSF acoustic space are an aid to
listening.
1. The RT-60 is fast, Both external and internal room noise is
rapidly attenuated so as to not be competitive with the signal.
2. The sound fusion window is saturated with diffuse ambience so
the direct sound is enhanced not in level but over time.
3. The acoustic space is wide and smooth so the received signal
remains consistent despite variations in listening and speaking
position.
Because the hearing impaired are quite vulnerable to room noise
masking of the intended signal, a quiet and non-reverberant room
is always recommended for better listening results. Intelligibility
tests show an inverse correlation between intelligibility scores
and RT-60. This leads to the conclusion that an anechoic space is
the best space for listening.
Although the tendency for better listening performance lies in
the anechoic direction, it is agreed by researchers that some ambience
is better for listening than none. This leads to the curious conclusion
that a less than "perfect" intelligibility rating might
actually be the more intelligible for listening. Not only has this
come up in the field of speech intelligibility- but in the present
work.
The reflections that could enhance intelligibility should not be
echo effects, outside the time windows of 50 ms. They would have
to be very early reflections that are corrolated with the direct
signal. Again, reflections used in correlation processes must be
coherent, not random phase type reflections. In intelligibility
testing, for example with Techron, % Alcons work, the direct/reverb
ratio must be established. The D/R ratio varies depending on how
many milliseconds after the direct signal that the cursor for calculation
is located. It is generally agreed that +50 to 70 ms is a good location
for D/R ratio calculations. |
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12 HIGH INTELLIGIBILITY LISTENING ROOM
Beyond the technical aspect of intelligibility measurements is
the autonomic response characteristics of the human listener. In
the dry anechoic room, without reflections, sound levels vary almost
violently. This is evidenced by the continual contraction of inner
ear muscles. Loud sounds cause an autonomic flinch reaction by the
muscles of the inner pap that protect the ear from damage due to
further loud sounds. The Fletcher Munsion curves are ample evidence
of this limiter action. However, the reaction time for this process
is 1/10 second. The staccato of speech in a very dry room produces
a rapid sequence of 70 dB level changes and the listener is plagued
with a distracting and tiring flinch reaction.
The saturated sound fusion window of 50 to 40 ms provides just
enough ambience to reduce the autonomic flinch responses to a minimal
level. But even here the lack of echoic ambience results in some
dynamic level flinch, a loudness suppression reaction. For non-recording
purposes the diffusive ambience probably should be stretched to
100 ms~ lightly into the echo region.
The statistical sound field of a diffusive space allows the listener
and/or speaker to move positions and the received sound to remain
the same. An inexperienced speaker using a microphone that is hardwired
or IR coupled into headsets injects a new problem into communication
- mic position relative to the speaker. For the teacher of a group
of hearing impaired to wear headphones inhibits the ability to listen
to the students' response. The students cannot be mic'd and so the
teacher does not wear the headset.
Headphones are promoted for learning speech because they reduce
the intrusion of unwanted and distracting signal. But they exclude
the airborne sound of the student's voice to oneself. Ultimately,
this will be the primary feedback mechanism one has aside from bone
conduction.
In the coherent diffusive sound field of the QSF space, each student
can wear hearing aids and hear the teacher relatively equally, irrespective
of student or teacher position. Conversely, the teacher hears a
student's speech as best as possible, so does each student. This
is a direct consequence of the wide, open statistically diffuse,
ambient sound field of the QSF technique.
The student has to practice speech in a space that allows them
to hear themselves on their own hearing aids. It is the only feedback
system they will carry with them into the "real world".
Hearing aids pick up a lot of room ambience in a regular room. There
is no ambience in a dry room. The Quick Sound Field technique is
an acoustic space that minimizes room noise and maximizes acoustic
feedback. Not only in promise but in practice the QSF type sound
field is a significant contribution towards improving the quality
of life for the hearing impaired. |
Fig. 15 - Listening Room |
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13 HISTORY
The very first QSF space was assembled spontaneously by one recording
engineer in 1984 who happened to have a lot of modular acoustic
units at hand in the studio. Since then many spontaneous QSF gobo
applications have been reported by users in the field. The only
common ingredient is that a variety of modular acoustic units were
available and the engineer's ear leads the way.
The formal development of the QSF method took place in 1987 when
a sampling booth design was needed. The traditional dry room gave
unacceptable signal for this level of processing. The acoustic signature
of the first booth was successful and a lighter weight version of
the booth continues in its place. The original booth has found a
home in a west coast recording school.
Within months after the initial QSF sampling booth was produced,
a number of studio projects included this booth technique. Each
time, the vocal talent in the studio discovered this new acoustic
space, they insisted on recording in it. Often, a second smaller
vocal booth had to be built to put the sampling room back on line
with its intended use. The most notable QSF sampling room was picked
up by musician Pete Townsend of the WHO. He was so impressed with
this new acoustic space that he endorsed it, without remuneration
to help encourage other engineers to try it. His room remains at
Eel Pie Studios just outside of London.
Because of the early pioneering efforts of forward thinking, recording
engineers, the Quick Sound Field has developed into a dependable
recording technique. |
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14 CONCLUSION
The acoustic signature of colorless "ambience" has been
presented. Acoustic systems to produce this effect have been developed,
with five years of experience and testing in a wide variety of applications.
The consensus is that this Quick Sound Field method is a major improvement
in mic technique.
The Quick Sound Field method uses the Haas Effect - this time involving
statistical diffuse reflections instead of discrete reflections.
The QSF establishes a fundamental distinction between coherent and
incoherent reflections. While random phase, incoherent reflections
may be acceptable in the realm of echo control, they produce masking
within the Haas sound fusion time period.
Historically, an inordinant amount of technical expertise and effort
has been focused on the control room acoustic. Now, attention turns
to the mic. The Quick Sound Field is the next logical step in comprehensive
development of the studio acoustic.
© 2009 Acoustic Sciences Corporation.
All Rights Reserved. |