Getting up to speed on inner sleeve, outer bonnet, isolator mounted rails and more...
February 18, 2014, by Philip Giddings
Portable protected and shock absorbing racks provide the most protection to equipment and should be used in all demanding applications, such as for rental and touring systems and for delicate or expensive equipment.
These racks come in three configurations: an inner sleeve and outer bonnet assembly; a shock mounted inner sleeve; and isolator mounted equipment rails.
The most popular with touring sound companies is the inner sleeve and outer bonnet assembly.
Inner Sleeve & Outer Bonnet
The inner sleeve is a self-contained rack with sides, a top, a bottom, and equipment and accessory mounting rails at the front and rear. Most are constructed of 3/4-inch (19-mm) plywood, although some designs consist of a tubular steel frame with side panels.
Typically the electronic equipment is mounted in the front, and the input, output, power, fan and other panels are mounted in the rear. All are recessed into the sleeve for added protection. The sleeve sits on a shallow dolly when in use. To transport the system, the bonnet is slipped over the sleeve and secured to the dolly, completely enclosing the sleeve.
The bonnet is held in place with recessed or surface mounted latches. The casters are mounted to plywood runners which then mount to the rack. This distributes the weight and captivates the carriage bolts between the plywood and the rack, making caster replacement easier.
The carrying handles should be located at 20 inches (508 mm) above the floor for easy lifting and should be on the sides; when the rack is tipped off the casters for shipping it will be resting on the front or rear, putting the electronics in a preferred position.
The inside of the bonnet and dolly are lined with foam and this provides the shock isolation to the sleeve. It is necessary to select a density of foam which will compress during impacts. The force of the impact on the foam depends on both the weight and the surface area the impact is applied to. Even a very soft foam will not compress if the force is spread over a large area.
For this reason it is best to line only the corners of the bonnet and dolly and to use a dense foam. The bonnet and dolly are generally constructed of 1/4-inch to 3/4-inch (6- to 19-mm) plywood which may have a thin bonded outer covering of fiberglass or of metal such as aluminum. The joints are reinforced with aluminum and the corners with metal caps.
Shock Mounted Inner Sleeve
A second type of shock absorbing rack, similar in principle to the bonnet type and built using the same techniques, is achieved by mounting a sleeve or metal frame inside a protecting outer rack, with removable front and rear doors.
The inner rack is suspended on rubber, spring, or foam isolation. Removing the doors exposes the inner rack.
This approach has advantages in use over the bonnet type although it is more difficult and expensive to build well. If foam is used in the outer rack and the sleeve slipped inside, the doors must be lined with a dense foam around the perimeter so that if the rack is transported face up or down and slips, the edge of the sleeve, not the equipment, rests on foam.
Rubber and spring isolators are difficult to install and maintain compared with foam, although when properly fitted they will not allow the inner sleeve to slide against the doors.
Isolator Mounted Equipment Rails
A third and less common type of shock absorbing rack suitable for light equipment, such as signal processing equipment, is constructed by mounting the front and rear equipment mounting rails to the side of a protective rack with rubber or spring isolators.
Once the isolator has been properly chosen based on the load it will carry, the inside dimensions of the rack can be determined so that the equipment rails will sit 17.75 inches (450.9 mm) apart. These racks are difficult to custom manufacture and are best purchased as a complete unit from one of the manufacturers specializing in this style of rack.
Shock Damage And Its Prediction
Shock damage to equipment is caused by acceleration forces developed during impact. Shock is measured in gs, one g being equal to the force of gravity. The shock which results during impact is determined by the speed prior to impact and the time taken to come to a stop or the shock rise time.
The shock rise time will depend on the elasticity of the striking body, the resilience of the impact surface, and the size of the contact area and its shape. The stopping time and the G forces are directly proportional.
For an object dropped from a given height, the G forces are predicted by the formula:
G = √b x 72/t
G = acceleration in gs
b = drop height in inches
t = shock rise time in milliseconds
A rigid load dropped 36 inches (0.91 m) to a concrete floor will experience over 200g while a 4-inch (101 mm) fall to sand will develop about 30g. Anything that can be done to mount sensitive equipment resiliently inside equipment cases is worthwhile.
In preparing rack layouts the designer may wish to consider the following guidelines:
1. Locate heavy items such as power supplies and amplifiers near the bottom for ease of installation and removal.
2. Locate items which receive a lot of operator attention, such as jackfields or signal processing, between the waist and eye level of the operator when he or she is in normal operating position.
3. Group together pieces of equipment which are related; for example, the paging and communications equipment in one rack, the switching and distribution equipment in another, and the effects and signal processing equipment in yet another, and so on.
4. Separate, when possible, signal equipment of different levels and types:
—Microphone level equipment
—RF (wireless microphone) equipment
—Line level equipment
—Control and power supply equipment
—Loudspeaker level equipment
5. Leave space around equipment which is known to run hot—at least 1 rack unit above and below. Consider using a baffled vent panel above these units.
6. Avoid putting anything with controls on the front panels at the very bottom of the rack where they can be hit (with feet or brooms, for example).
7. Some amplifiers create strong electromagnetic fields around themselves, and sensitive equipment can have hum induced into its circuits if located immediately above or below. A spacing of 2 rack units (2RU) is often sufficient.
8. Power amplifier racks that contain amplifiers which do not move air in or out of the rack, such as convection and side-to-side fan types, should have forced air cooling.
9. Racks which have many items mounted on the rear rails deserve special attention, as this can restrict access to the rear of the front-mounted equipment, making wiring and service difficult. More racks may be required to reduce the density.
In practice one finds that in trying to meet all the above goals, conflicts will arise; for example, should all the signal processing for all the loudspeaker systems be together, or should the individual processing be located near the amplifiers they are driving, where it would form a logical grouping? The creative designer must ponder the advantages of the different approaches and may decide that, because the wiring is so much simpler one way than the other, the answer is clear!
Philip Giddings, P.Eng., is the founder of design consultants Engineering Harmonics. With offices in Toronto, Vancouver, and Ottawa, the firm is a world leader in the design of performance sound, video, and communications systems for performing arts centers, sports facilities, public buildings, and houses of worship. He remains a senior consultant to the firm.
Editor’s Note: This article is excerpted from the much-lauded Audio System Designs and Installation by Philip Giddings. It’s now available in trade paperback (xxvi + 574 pp, index, ISBN 978-0-9920244-0-6) exclusively online from the publisher, Post Toronto Books, http://posttoronto.com/.