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.
Driving Electromagnetic Transducers
Electromagnetic transducers utilize electromagnets that enable these devices to produce low sound frequencies in small package sizes. The penalty is that these devices require more current to work, have limited voltage ratings, and the electromagnetic coil can create voltage spikes; which, makes the circuit designer’s job more challenging.
The following two circuits are self-contained in that the circuit produces the sine wave type signal required by the buzzer. The circuit challenge is to set the value of the capacitors, resistors, and/or inductors to produce the rated buzzer oscillating frequency.
The following three circuits provide a means to drive the electromagnetic transducer buzzer, but they require an input signal to turn the drive circuitry off and on at the appropriate rated buzzer frequency. Care must be taken not to over-voltage the buzzer.
The input signal to the previous three drive circuits can come from a microcontroller output or from a logic circuit similar to the following:
The capacitor and resistors values need to be set to provide a signal at the appropriate rated buzzer frequency. For example, if the electromagnetic buzzer is rated at 2.3 KHz, then the output signal’s frequency from the microcontroller or from the above circuit needs to be at 2.3 Khz.
The inductors needed for some of the above circuits may need to handle large current values and may need to have large inductance ratings. The inductance ratings needed can range from 1 milli-Henry (1000 uH) to 100 milli-Henry (100000 uH).
One source for these types of inductors is:
Bourns- JW Miller Inductors (www.bourns.com)
Product Series: JW Miller 5900 Series High Current Chokes
Tube and Tape-n-Reel Counts- All P/N’s
|Part Number||Tube Count||Reel Count|
Note: All other standard part number supplied by Mallory are bulked packed.