Sound level meters are instruments capable of measuring the intensity of sound waves. Sound is a series of pressure waves that travel through a medium such as air or water. The stronger the pressure waves the louder the sound. These pressure waves are received via our ears and perceived by our brains. Sound ranges from the things we love to annoyances that can be harmful.
We are bombarded by sounds nearly every waking moment of every day. Though many of these sounds are things we want or need to hear, too often they are part of the din that surrounds us in the modern world—traffic, machinery, alarms and more.
It is a well-established fact that noise has several effects on health. Elevated sound levels can cause hypertension, hearing loss, heart disease, sleep disturbance, stress, and mental lapses. Some noises don’t even have to be at a particularly high level to cause these maladies. The effects noise has on health are serious enough that exposure to high noise levels is regulated by occupational health agencies (such as OSHA), the Environmental Protection Agency, state laws and local ordinances.
Sound level meters are able to quantify sound to ensure that the exposure to it is below the threshold of what is considered safe.
Measuring Sound
As noted above, sound is simply a series of pressure waves that travel at a frequency detectable to the human ear, generally about 20 to 20,000 Hz though it can vary considerably between individuals. Sound levels are measured in decibels. The decibel scale is logarithmic rather than linear meaning every 10 dB on the scale is equivalent to a 10-fold increase in sound intensity. Around 120 dB is the threshold of pain for humans. Even brief exposure at that level can cause permanent hearing damage.
How sound level meters work
Sound level meters generally consist of an omnidirectional condensing microphone, electronic circuits, and the meter display.
The microphone of the sound level meter contains a flexible membrane that moves slightly when the sound waves hit it. Those movements are converted into an AC electrical signal that is a precise electronic picture of the sound waves. Those AC signals are converted to DC by the a root-mean-square (RMS) detector circuit. The output of the RMS circuit is linear in voltage and is passed through a logarithmic circuit to give a linear readout in decibels.
Most sound level meters include a variety of filters, amplifiers and user adjustments to make measured results more clear and accurate. The meters generally measure over a range of about 30 to 130 dB.
Weighting
Humans do not hear sounds across all frequencies in the same manner. Our auditory system responds more to frequencies between 500 Hz and 8 kHz and is less sensitive to very low-pitch or high-pitch noises. Therefore, sound level measurements with a flat response will not accurately reflect how we perceive sound. For this reason, sound level meters use frequency weighting to ensure that the meter is measuring a similar range of sounds to what we actually hear.
Common frequency weightings include:
- A-weighting is the most common used in noise measurement. Like the human ear, A-weighting cuts off the lower and higher frequencies that most people cannot hear. A-weighting is often used for the protection of workers against noise-induced deafness and is the only weighting mandated by international standard (IEC 60651, IEC 60804, IEC 61672, ANSI S1.4). A-weighted measurements are expressed as dBA or dB(A) .
- C-weighting follows the frequency sensitivity of the human ear at very high noise levels. At levels 100 dB and above, the ear's response is flatter across most frequencies. C-weighting therefore includes much more of the low-frequency range of sounds than A-weighting. C-weighting is often used for peak measurements as well as in some entertainment noise measurement, where the transmission of bass noise can be a problem. C-weighted measurements are expressed as dBC or dB(C) .
- Z-weighting is a flat frequency weighting, implying zero weighting. Instead, it is a simple range of 10Hz to 20kHz ±1.5dB. Z-weighted measurements are expressed as dBZ or dB(Z) .
Dosimeters
Sound level meters generally only measure the current sound levels. In many commercial and industrial settings where long-term exposure to sound can be a health hazard, dosimeters are used. Dosimeters are specialized sound level meters that integrate measurements over time, providing an average noise exposure reading for a given period of time. They are important personal noise monitoring instruments that help institution comply with Health and Safety regulations such as the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard or EU Directive 2003/10/EC.
Classes
Standards organizations divide sound level meters into different classes depending upon their frequency range and accuracy. For example the American National Standards Institute (ANSI) classifies sound level meters as either type 0, 1 or 2 according to the precision of the instrument. Type 0 sound level meters are laboratory grade. Type 1 sound level meters have an accuracy of ±1 dBA and are used for precision measurements in the field. Type 2 meters have an accuracy of ±2 dBA and are used for general purpose measurements.
The IEC divides sound level meters into two classes.
Class 1 instruments have a wider frequency range and a tighter tolerance than a lower cost, Class 2 unit. For most applications other than law enforcement or research, there is little reason for using a Class 1 instrument. National standards permit the use of Class 2 instruments for regulatory compliance.
Calibration
Like most measuring devices, sound levels meters should be periodically calibrated to maximize accuracy. Some meters include a built-in calibration check that will quickly test a specific decibel reading. In lieu of an internal calibration check, acoustic calibrators can be used to verify accuracy of the instrument.
Things to consider when selecting a sound level meter:
- Is a specific range needed?
- What class instrument is required?
- Is weighting needed?
- Are any regulatory standards needed?
- Is data logging/recording preferred?
- Are IP ratings necessary for ingress protection?
- Do you want any advanced functions such as averaging or alarms?
If you have any questions regarding sound level meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
