1145 - Wireless Sound Level Sensor

• Make sure the microphone end is placed so it points directly at the sound source. 
• When using a ‘Value Rises Above’ or Higher than’ condition to start logging it can be frustrating if classroom noise triggers a recording. It is often best to choose a higher trigger value than planned with a pre-trigger time set so the start of an event is captured. 
• This sensor is not waterproof, it has an operating range of 0 to 95% RH (non-condensing). Do not place the sensor in an environment in which high humidity levels are possible as this may result in damage or malfunction.
• If the sensor has been left in the cold, let it warm to near room temperature before waking it from sleep.

Sound level (dBA and dBC ranges)

The unit of sound intensity level or loudness is the decibel (dB). The sensor can measure the sound level in decibels using either the dBA or dBC range. The term dBA and dBC refer to the types of filters used to measure the sound (dB) - either an A filter or a C filter. 

The dBA range is the range most widely used as it indicates sound pressure level with ‘A’ weighting to give a response similar to the normal human ear in the range and intensity that it ‘hears’ sounds, so a noise which sounds louder will produce a larger dBA reading. 

The response of the human ear to levels of loudness will vary with the frequency of the sound wave it is detecting (frequency = sound vibrations produced per second). They are very sensitive in the frequency range of approximately 500 to 6,000 Hz (mid-range frequencies) and much less sensitive to higher and lower frequencies. The A weighing filter covers the full range of human hearing (high and low frequencies sensitivity is reduced).

The dBC range is similar to A but it can also measure sounds whose frequency is outside the range of normal human hearing particularly those sounds with very high (10,000 Hz +) or very low (<800 Hz) sound pressure/frequency levels. The ‘C’ weighting is useful for measuring high level noise such as engines and machinery.

The loudness of a sound depends on the amplitude of the sound waves. Increasing the ‘volume’ of a radio increases the amplitudes of the sound waves produced by the radios speaker, so the sound from the radio is louder. 

Changing the loudness does not alter the frequency of a sound.

Waveform ranges (mV)

The Waveform range has a maximum frequency response between 20 to 10,000 Hz (10 kHz), its sensitivity then reduces for sound with a frequency between 10 kHz to 16 kHz.  It is used for examining the frequency content of sound.

Sound waves are longitudinal. As sound passes through the air, the air particles oscillate about fixed points from left to right and the energy is also transferred from left to right. It’s these vibrating air molecules that cause the human eardrum to vibrate, which the brain interprets as sound. 

The amplitude of a wave is the maximum displacement of a particle from rest. The louder the sound, the larger the amplitude wave.

Wavelength is defined as the distance between two successive particles that are at exactly the same point in their paths and are moving in the same direction (same phase). It is given the symbol λ and is measured in metres.

The frequency of a wave is the number of complete cycles or oscillations of disturbance each second. The SI unit of frequency is Hertz (Hz). 

Pitch depends on the frequency of the sound waves. Making the pitch higher increases the frequency.Different musical instruments playing the same note can produce different waveforms. 

Conversion to Pascals

Sound pressure is the difference between the instantaneous pressure at a point in the presence of a sound wave and the ambient atmospheric pressure. The SI unit of sound pressure is the pascal (Pa). 

To find the value of the pressure wave that caused the sound (approx.), record data using the mV (or the amplitude) range. Find the maximum amplitude and divide this by 7.9433 to convert to pascals (Pa).