Sealed tight but sounds right

Feb. 17, 2005
There are numerous ways of getting audible alerts out of sealed cabinets.

Dan O'Brien
Mallory Sonalert Products Inc.
Indianapolis, Ind.


Mallory Sonalert Products Inc.,
(317) 612-1000,


You've verified your equipment design meets all the relevant environmental standards. The enclosure is sealed tight against all outside conditions. As you tick off each specification, there's a nagging feeling you've forgotten something. And then you notice the requirement for an audible signal. "Oh, great," you think, "how do I put in an audible alert without ruining the integrity of the enclosure?"

There's no reason to panic. Audible-signal manufacturers deal with equipment integrity issues by a variety of means. Here are the most common ways.

How a solid-state audible signal works

The most common type of electronic audible signal uses a piezoelectric transducer. The transducer consists of a metal disc with an attached ceramic material that flexes in a specific direction when voltage is applied. Reversing polarity flexes the material in the opposite direction. Applying an alternating current to the transducer, such as a sine wave, generates an audible sound. The most common device works from 6 to 16 Vdc. An internal electronic circuit generates the ac signal for the transducer. The transducer mounts to the housing with silicone adhesive to seal the internal circuitry from the outside.

If the sound level requirement is moderate (60 to 80 dB @ 2 ft), mount the audible signal inside the equipment. Enclosing an audible signal typically dampens the sound level by 15 to 20 dB. By using an extra loud signal the sound intensity outside the cabinet may still meet the level requirement. Because every application is different, a designer should test the actual enclosure and signal to verify this method is viable.

If the alarm control circuitry resides on a circuit board then a board-mount signal makes most sense. Remotely mounted signals, such as those designed for panel mounting, can attach to an L-bracket inside the enclosure. Whether board mounted or L-bracketed, enclosure integrity remains intact.

What if an alert inside the cabinet isn't loud enough? If the equipment must meet a NEMA-4 , IP-65, or similar waterproof rating, create a watertight seal with a rubber gasket between the audible signal and the equipment panel. Get the gaskets from the audible signal manufacturer. Before manufacturers began offering rubber gaskets, equipment designers used silicone adhesive or glue. Over time those materials tend to dry out or harden. The seal fails and the enclosure loses integrity. And don't forget the difficulty in removing the device if the equipment is ever refurbished.

Put the rubber gasket between the inside of the panel and the audible signal to keep water from penetrating through the threads of the audible signal's nose cone.

Consider mounting the audible signal outside if a 1.25-in.-diameter mounting hole punched through the enclosure causes too much anxiety. The enclosure still needs holes punched through for power wires. But wires need much smaller holes which standard waterproof grommets or connectors can seal. Mount the audible signal outside to the equipment with a small box or an L-bracket.

What is a dB?

Sound intensity is measured in decibels or dB. The readings are a logarithmic ratio between two different power levels: a reference level and the measured level. The reference level is the threshold of hearing — the point where sound becomes so soft the human ear can no longer hear it. It corresponds to a sound pressure of 20 Pa or 0.02 mPa. Sound pressures below that value can not be heard.

On the opposite extreme, sound levels of 130 dB are more than 3 million times stronger than the threshold of hearing and can become physically painful to hear. Prolonged exposure to extremely loud sounds will physically damage the inner ear leading to hearing loss.

Sound becomes softer as you move away from the source because sound pressure drops with distance from the source. So always specify the distance for accurate comparisons when measuring sound intensity. Audible alerts are measured at a distance of 2 ft from the source.

Audible alert intensity is divided into three classes. Soft audible alerts are rated at 55 to 65 dB @ 2 ft, medium level alerts from 65 to 80 dB @ 2 ft, and loud alerts at 80 to 95+ dB @ 2 ft.

Specify a device with an output of at least 85 dB if the alert must have an intensity of 70 dB @ 2 ft and will be buried inside the cabinet.

If the audible signal will see rain or a water wash down, mount the nose of the device either pointing downward ( preferred) or horizontally so all water can easily drain out. Water accumulation in front of the piezoelectric transducer can kill the sound. And if the water should freeze, the expanding trapped ice will damage the device.

While the front of a piezoelectric signal is sealed against the environment, it can still take in water if a strong water stream is directed straight into the front opening. If the equipment is strongly washed down, the audible signal should be mounted away from the water stream. As last resort use an audible signal with a sound baffle. Before the equipment is "hosed" down close the baffle. Reopen it after hosedown. Sound baffle protection is not the best option because the operator must remember to open and close it — usually not a high priority.

With planning, audible signals can serve in equipment fortified against a harsh environment. Audible signal manufacturers have dealt with equipment integrity issues for many years. They can recommend strategies for any kind of environmental problem.

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