Sound, Distance, & Mallory’s Web Tool

Have you ever wondered what a sound is exactly?  A sound that hits the eardrum (and makes it vibrate) consists of small air pressure waves that have a frequency between 20 Hertz (Hz) and 20 kilo-Hertz (kHz).  This is why the wind can drown out sound- the wind’s large air pressure can overwhelm the eardrum which is sensitive to smaller air pressure waves.  The larger the air pressue wave, the louder the sound is, and sound level is measured in decibels (dB) which is an arbitrary scale ranging from 0 (threshold of hearing) to 130 (threshold of pain).

When specifying the sound level of a buzzer or alarm, a distance number is required because sound level decreases over distance.  People understand this concept intrinsically because if someone is right next to you, they can hear a whisper, but if they are across the room, you need to raise your voice to be heard.  A rule of thumb which can be used when converting a sound level from one distance to another is that the sound level drops six decibels (dB) every time the distance doubles.  So, if an alarm is specified as 100 dB @ 1 ft, then at 2 ft, the sound level would be 94 dB, at four feet, 88 dB, and so on.  This concept is important because manufacturers specify their audible alarms at various distances, so it can be difficult to compare them or to understand how loud an alarm might be in an application.  To make the sound level to distance calculations easier, Mallory has developed a sound level conversion web tool to compare audible sounds specified at different common distances.

Two common technologies used to generate alarm sounds in an industrial environment are piezoelectric and electromagnetic.  A piezoelectric buzzer generates sound when a voltage signal is applied at the appropriate frequency to a piezoelectric disc which causes the disc to flex up and down generating the air pressure waves needed by the ear to hear a sound.

An electromagnetic buzzer generates sound when a voltage signal is applied at the appropriate frequency to an electro-magnet which has a metal disc suspended above it.  As the electro-magnet turns off and on, the disc flexes up and down generating the air pressure waves.

Both technologies have their strengths and weaknesses.  Piezoelectric technology excels at producing loud sound levels with very minimal electrical current levels, and electromagnetic technology excels at producing loud sound levels in small packages at lower voltage levels.  Electromagnetic technology is more often used for small SMT or PC Pin type buzzers, and piezoelectric technology has various sized models which generate the very loud sound levels needed in noisy industrial environments.

Both technologies produce the highest levels in a very narrow sound frequency window which translates to producing a loud sound in a small package, but these devices would not be good for producing music which requires a wide sound frequency window.  Speakers, on the other hand, do have a wide sound frequency window making them good for complex sounds, but they are heavy, large in size for the sound produced, relatively expensive, and require high current levels.  For these reasons, speakers are not commonly used in alarm applications.  One example of a speaker being used for an alarm sound is the back-up alarm used on large vehicles such as construction equipment or garbage trucks.

Sound level can be measured using a microphone and a sound meter.  However, unless the sound is being measured inside an anechoic chamber like Mallory uses, the sound level reading will be affected by echoes, reflections, and dead spots generated by the various equipment, people, tables, and walls around the sound measurement set-up.  So, be cautious when comparing the sound levels of competitor’s buzzers to Mallory’s audibles because you will probably find that the Mallory alarm is louder than the competitor’s even though the competitor’s buzzer has a higher sound level listed on the print.

Mallory, an ISO 9001:2015 company, has been manufacturing audible and visual alarms in the USA since 1968.  With 12 active patents, Mallory is the technology leader in audible and visual alarm devices.