Controlled Reflection Isolation Booth
Preprint No. 2531
Presented at the 83rd AES Convention
1987 October 16-19
Sampling room design is of current interest. This is due to the influx of sample processors into the studio. The goal is to catch samples that are musically realistic. The question arises as to how short the sampling time window can be, what controls it and what is in it. Independent of the sampling problem, a studio mic technique was developed over three years ago. It is characterized by a strong liveness quality being added to an otherwise quite dry sound. Over the last year, sampling rooms have been designed and built using this acoustic technique, providing very satisfactory results. This paper presents the design strategy and acoustic signatures of recording rooms that have this “quick” sound quality and presents a case for its suitability as a sampling room.
In the beginning, there is only one. Soon, the knowing few step forward, and eventually come the hordes. This is also the lifeline of each acoustical moment. We have the direct signal, soon followed by a set of early reflections, trailed by the multitudes in reverberation.
The direct signal is received as it is sent. Both the early reflections and the reverberation will have distinct characteristics that are a function of the reflecting surfaces that support them. Each of these two reflection groups can be weak or strong. They may have temper, or spectral band pass characteristics. They each will also have a temporal or time-wise signature that describes their density and distribution of discrete signals including the decay rate. A flow diagram can be drawn outlining the multiple signal path options between the sound source and receiver. This outline is loaded with vocabulary that describes the quality of the sound options.
0.1 Early Reflections
There are two sides to the control room window. The recording studio produces the signal which then is mixed in the control room. Recording studios are very carefully set up to produce the proper composite balance between the direct, strong early and weak late reflections (1). The goal is to get a natural, realistic and full instrument sound onto a track suitable for the mix.
Control room standards require the engineer to hear the set of early reflections produced in the studio without distraction by the set of early reflections that belong to the control room (2). Its ambience is allowed after an initial time delay of 30ms.
0.1A and 0.1B show, at the risk of oversimplification, a comparison of the ETC (Energy Time Curve) of these two rooms, and that the handling early reflections is their major difference. The recording studio (A) emphasizes early reflections. The control room (B) suppresses them.
0.2 Recording Studio Rooms
Large recording studios with strong diffusive surfaces and fast decay rates are prescribed for accurate instrumental recording. They provide plenty of early reflections. The vocal booth and drum booth are other types of source rooms in the studio that have opposite natures. They are small and usually quite dead. In spite of themselves, they are often used as isolation rooms for instrumental recording.
0.3 The Sampling Room
The acoustic environs appropriate to a sampling room are at present ill defined. The traditional vocal booth approach is over damped, too dry. There are not enough early reflections to collect sufficient signals to develop a realistic sample. The musician also needs to hear the full sound of the instrument; such feedback is necessary in order to fine-tune voicing detail.
The acoustically bright and large studio produces reflections that are strong, easy to play to, but mainly too time delayed and initially too sparse. If sampling occurs in a small bright room, the early reflections may be soon enough, but risk being too strong and too colored with small room resonances. Such small room mic work is extremely position dependent; setup and repeatability are difficult and time consuming.
When the concept of diffusion is introduced, the time frame of 10ms for early reflection signals is the basis (3). Triple tonguing trumpet players in a concert hall produce audible dynamic transients whose duration is between 10ms and 20ms (4). Strong, dense and early reflections are necessary to accurately track musical transitions.
The apparent goal is to establish a small sampling room that has very fast decay rates, as does the small vocal room and drum booth, yet it must have a measure of very early, neutral and diffuse ambience, reminiscent of the larger recording studios. Rapid decay with rapid diffusion may well define the timewise signature appropriate to the sampling room. This will have the quality of being acoustically “quick,” i.e. live yet dry at the same time (5).
1.0 The Quick Sound Gobo, the “Acoustic Island”
A gobo technique was developed over three years ago that reduced the sluggish presence of the large recording studio ambience, while increasing the density of early reflections. A sense of liveness is developed in the signal. The original “Acoustic Island” gobo technique remains in use in numerous studios and is present here (6).
1.1 The Setup
The Acoustic Island utilizes a grouping of cylindrical sound absorbers called TUBE TRAPS.* A description of them will develop an understanding as to the reason for their use. An interior air volume (C) is surrounded by a dense fiberglass wall (R). This is a lumped parameter design whose acoustic RC time constant helps to access the low frequencies (7,8). About one half of the surface of this patented (9) trap is covered by a “limp mass” (L); it reflects mid and high frequency sound yet passes the lows for absorption.
The setup for the Acoustic Island gobo is in the form of a horseshoe pattern. Two 3 foot sound trap cylinders are connected together to form a column. Typically, seven columns are placed on a 2 foot radius centered about the mic. The reflectors of the traps are directed inward.
This gobo system performs two acoustic functions at the same time. The absorptive side of each trap faces the room, intercepting the sound of the room. This acoustic shadow zone feature develops 5dB isolation from room ambience. The second feature is by the reflectors. The direct signal is immediately followed by a dense fill of diffuse signals, strong in the first 10ms, which provides a boost of 4.7dB in the nearfield ambience. This immediate, dense and diffuse backfill is the voice of the Quick Sound Field (QSF)* effect.
*TubeTrap and QSF are both registered trademarks of Acoustic Sciences Corp.
1.2 Gobo Testing
A typical vocal gobo was set up in a lightly treated, gyp board sound-testing room (8 x 14 x 18.5’). A “hot spot” speaker simulated the voice and 1/4” mic was positioned 2 feet away. The setup was 9 feet out from one corner with a patch of carpet below and some 1” fiberglass batt overhead.
1.2A and 1.2B (see below) are the ETC and waterfall taken in the room without a gobo. The ETC is 40ms, and the TEF (Time Energy Frequency) time ranges from 1 to 33ms. The ETC shows very few signals in the first 10ms compared to the second 10ms period. The direct to reverb energy ratio is 8.1dB with an early decay rate of 0.2 sec. Not the first 20ms has sparsely distributed returns. TEF waterfall (B) shows the room holding energy up through 5k, but notice the rapid shift from the full spectrum direct signal to the half spectrum set of room reflections.
1.2C and 1.2D (see below) show the gobo setup but with the reflectors positioned to the outside. The room reverberant field is weakened, dir/rev ratio is 13.1dB, as the direct sound is absorbed by the traps. TEF waterfall and ETC both show some increased density of early reflections, due to the impedence discontinuity of the absorbers.
1.2E and 1.2F (see below) show the correct setup, reflectors toward the mic. Note the ETC, tremendous early reflection backfill. Direct/reverb ratio is 8.85dB, with an early decay rate of 0.05 sec. Reflections from the gobo immediately follow the direct signal for 10ms. The ETC has the classic QUICK SOUND FIELD signature, an immediate and strong backfill of diffuse energy lies just behind the direct signal. This feature establishes the “quick” quality of sound, giving it a lifelike, snappy presence.
1.2G (left) shows the EFC (Energy Frequency Curve) of the early diffuse reflections, the backfill off of the reflections of the traps. A frequency sweep was taken at 5 1/3ms, only 3ms following the direct signal. This reflection is also visible in the waterfall of 1.2H. Frequency is linear in both. The neutral, broadband early diffuse reflections are clearly present.
A-F Acoustic Island Gobo Signatures
2.0 The Quick Sound Field Room
Success with the Acoustic Island gobo in the larger studio spaces led to an extension of the principles into the smaller, dedicated sound rooms, such as a vocal booth, drum room, broadcast voice-over rooms and the like, including the sampling room.
2.1 The Basics
The acoustic devices utilized are the half and quarter round versions of the full round sound traps used in the gobo system. These segmented traps each have a reflector covering the central 1/3 to 1/2 of the surface of the trap. They are easily mounted in any position: horizontal, vertical or upside down. Their stiffness is due to a built-up-beam integral to the mechanically damped backboard structure.
The curved reflector in each trap serves to scatter midrange and high frequency sound. The lower frequencies are absorbed by the entire trap’s surface. The lows are scattered not directly by the trap but by the process of diffraction as they rebound off the thin reflective wall strips between each trap. Dispersion of sound here is a two stage process.
Two types of walls have been built. A bare gyp boardroom can be outfitted with a set of traps on 18” centers. Another approach, initially used, is a freestanding isolation booth. It uses lead-backed traps with “tongue and groove” Plexiglas strips in between. That combination produces an STC (Sound Transmission Coefficient) of 32dB yet provides 30% visual contact with the outside.