Who, What, Where, When and Why of the Quick Sound Field

About 13 years ago New England Digital contacted us about a digital sampling booth. Traditional dry rooms did not record enough real sound, the open mic in a studio was too roomy of a sound. The problem arose not due to the quality of traditional studio acoustics but because of the heavy level of electronic processing. Synclavier type, multistep digital signal processes the original sound over and over, this magnifies even the smallest room coloration at the mic. Poor source material means poor results.

We had been experimenting by that time for three years with various gobo techniques that were producing QSF type acoustics. A number of studios had free standing columns of TubeTraps set out on a horseshoe pattern. Others with economy in mind, suspended on wires from the ceiling Tubes in the same pattern. We didn't have much hope for this "coma" class acoustics but at least it sounded great. NED wanted a booth controlled not gobo controlled acoustic space for the mic and instrument.

The goal is to capture the "total" sound of the instrument in an otherwise dry room. In the early days of recording, the method used to get this sound employed some 30 mics distributed at all odd angles and distances inside an anechoic chamber. The sum of the inputs, including their natural time delays due to various distances, were directly mixed. The result was the composit "total" sound of the instrument, a blend of on and off axis, near and farfield sounds. This cumbersome technique is only available in the rarest of circumstances and is essentially now lost to obscurity. The reason it works however is still valid. The listening human brain correlates into one total sound all similar sounds that follow the direct signal within 20 ms or so (the Haas effect).

We established a signal at the mic equivalent to this classic multi mic setup by creating multiple reflections in an otherwise highly absorptive room, a perfect Tube Trap application. NED loved it and still uses it. Their exhibit travels all over the world and the QSF sampling room remains clearly visible behind the control room window.

Shortly after the first NED exhibit aired, we started to receive orders for QSF sampling rooms. Within a month after most rooms were installed the studio would call up to order a smaller sized QSF type vocal booth for their complex. The story went like this. Vocal talent in the studio discovered the "new sound" of the sampling booth and wouldn't leave. They insisted on a TV monitor and an audio patch through into the jingle control room. They were using up a high ticket room on a low ticket application.

Well, it took no marketing genius to realize that the vocal booth to sampling room ratio is at least 1000 to 1. The market for the QSF technique would be the QSF vocal booth.

One of the most memorable inquiries was from Pete Townshend. He called also shortly after the NED project, about his nonparallel walled but terrible sounding sampling room. We discussed the ETC properties and diffusion theory of the QSF. He could hear it in his mind and bought one on the spot. We tailored it to fit his existing room, the Boat House at EEL PIE Studios, London. Upon installation, he wrote two separate times extolling the properties of the QSF mic space. He explained the user benefits so well, I couldn't resist asking if we could quote him. After weeks of silence (Pete does not do endorsements)... yes, he'd endorse the QSF acoustic space so we should make arrangements to do a photo shoot. It's been 12 years now and we still have his support.

There are two ways to get a QSF acoustic space. The QSF gobo method is a free standing Tube Trap technique. The QSF booth is a wall mounted version. The QSF gobo keeps the Tube Trap reflectors oriented towards the mic. Five or six Traps are placed about 18" apart in a horseshoe pattern with the mic in the middle and the talent at the open end.

Within the QSF gobo, the mic is in a shadow zone, protected from the sound of the room. Incoming wavefronts are absorbed by the full band width side of the Traps that faces outward. The multiple reflecting facet of the QSF space is developed by the inward facing reflectors. More early reflections at the mic means a stronger "direct" signal. Less room ambience at the mic means lower noise floor. The QSF gobo produces enhanced signal to noise ratios at the mic. You can now open up the mic to get more dynamic range.

Because of the multiple reflections in a QSF acoustic space, a single reflection, typically off a script stand or window is buried and not noticed. The comb filter coloration problems due to one or two early reflections is now masked by the dense group of early reflections in the QSF acoustic. The QSF booth has a distinct reflection rate around 1000 per second yet the RT-60 is 0.08 sec, 80 ms. That is why it's a dry signal with fantastic presence.

Mic techniques are relaxed in the QSF acoustic space. Close-mic/wind-screen routines are unnecessary. The talent can back off the mic, even change their head angle or position and the QSF signal at the mic remains stable. The implication for this that leaps into any engineer's mind is exactly true. Set up is very repeatable in a QSF space. Retake sounds just like the first take. The critical mic and talent positioning does not occur inside the QSF acoustic space. Many people mention that QSF acoustics should make a great Foley room.

There are two products that strongly complement setting up of the QSF space. The first is the StudioTrap, a 9" diameter Trap mounted on a music stand base. It is very versatile. You can lower the Trap for a sit down work or raise it for stand up vocals. It rotates easily to dial in the reflectivity. The StudioTrap is stored and handled much like a mic stand.

The second development is a spin off from the commercial side of the ASC product line. We had developed a fold-up, high strength cardboard type of product, actually a 1/3 Round Trap. Other models for this low cost, lightweight product are planned. The important feature, it is light enough to be glued up with mastic, the same type used for acoustic tile and Sonex installations. This "glue-up" Trap comes in stock lengths of 3 feet and quickly converts any small room into a QSF vocal booth. Well, enough QSF for now.