Decibel Sound Level Scale
The decibel sound level scale is an arbitrary scale that ranges from 0 dB (threshold of hearing) to 130 dB (threshold of pain). The chart below shows where some common sounds fall on this dB scale. Audible alarms are available that have sound levels as soft as 55 dB at 2 feet and as loud as 110 dB at 2 feet.
Fundamental Frequency & Harmonics
Below is a frequency scan of a piezoelectric audible alarm that has a resonant frequency of 2,800 Hz. As you can see, there is a strong frequency peak at 2.8 kHz and several smaller frequency peaks that follow called harmonic frequencies. The table below the chart shows that the size of the harmonic frequencies are significantly smaller than the fundamental frequency for this particular alarm unit. Because this alarm has a large fundamental frequency and much smaller harmonic frequencies, the sound quality of this part will be very good. When this alarm is activated, the listener will hear one clear frequency (also called sound pitch) from the alarm. Other electronic alarm technologies such as electro-magnetic or electro-mechanical type alarms often have much larger harmonic frequency components resulting in less clear tone.
Controlling Sound Level- Mechanical Method
There are two ways to control sound level in an electronic audible alarm. One is mechanical and the other is electrical. The mechanical method involves changing the size of the front hole opening of the audible alarm. The open area in the front of the audible alarm including the front hole opening is an acoustically tuned cavity. By partially covering the front hole opening, you are changing the cavity tuning making it less efficient. The more the front hole opening is covered, the more the sound level will decrease.
An example of how to do this is shown above. The picture above shows our manual sound baffle accessory with part number: SCVC being assembled onto the front of a panel mount alarm housing. The SCVC accessory consists of a screw and two pieces of plastic. After the sound baffle is attached to the front of the alarm, the sound level can be changed by manually turning the top piece of plastic. The sound level of the alarm will attenuate anywhere from 10 to 15 dB’s making the alarm sound about half as loud as before.
The main disadvantage of this manual method of controlling the sound level is that it is controlled by the operator. In some situations, it is dangerous to let the operator have the ability to decrease the sound level of the alarm. The most obvious potential problem is that the operator could turn the sound level down too much so that the alarm will not easily be heard the next time it is activated.