What are Exceedance Levels?

Exceedance levels represent the percent of the run time that was spent at or above the corresponding dB level.  Example: An L40 equal to 73 dB would mean that for 40% of the run time, the decibel level was equal to or higher than 73 dB.

What is LDN (Day/Night Sound Level)?

This measurement is a 24-hour average sound level where 10dB is added to all of the readings that occur between 10 PM and 7 AM.  This is primarily used in community noise regulations where there is a 10dB penalty for night time noise.  Typically LDN’s are measured using A weighting, a 3dB exchange rate and no threshold.

The Sound Level Meter won’t calibrate and it looks like the piece of foil under the grid on top of the microphone is ripped. What should I do?

NEVER remove the protective grid from the top of the microphone. The thin piece of material that looks like foil is actually the diaphragm. The diaphragm vibrates when exposed to sound waves. Those vibrations are how the meter measures the sound level.  If the diaphragm is damaged, then the microphone is damaged beyond repair. A replacement microphone must be purchased.

When is an Octave Band (OB) Filter used, and how should the Sound Level Meter be setup for use with the OB filter?

When a measurement is made with a SLM without the OB filter, the SLM is measuring sound levels over a range of frequencies and combining them into one overall (broad-band) sound level.  An OB filter is used to find the sound level just at one specific frequency.  When used with an OB filter, the SLM should be set to Fast Response (for quicker settlings after changing frequency selection on the OB filter), and Linear Weighting.  The Linear weighting setting will not attenuate, or lessen, the sound level at any frequency.

Should the meter be pointed directly at the sound source? Or should the meter be held at an angle to the source?

Quest SLM’s may be used with different types of microphones: free field or pressure. The type of microphone determines the angle at which the SLM should be held.  If using a free-field microphone, then the meter’s microphone should be pointed directly at the sound source.  If using a pressure type microphone, then the meter should be held at approximately 90 degrees to the sound source. This angle only really matters at higher frequency noises.  Refer to the user’s instruction manual for information on the type of microphone used on the meter.

What is the foam ball that came with the meter and how is it used?

Each SLM is shipped with a foam windscreen. The windscreen is placed over the microphone when taking measurements and helps prevent false high readings due to wind blowing across the microphone.  The windscreen will also help protect the microphone from dust and debris.

How does background noise affect measurements?

Background noise can cause considerable error in measurement when the background noise level is close to the level of the noise source of interest. When it is not possible to eliminate or reduce the background noise, a correction factor may be applied to the overall measurement to find the level of the noise of interest.  The user must first make a measurement of just the background noise, without the noise source of interest. Then make another measurement with the source of interest.  If the difference between the overall measurement and just the background noise is greater than 7dB, then the error due to the background noise is less than 1dB.

What is the difference between Type 1 and Type 2?

This is an accuracy specification.  There is an entire ANSI standard written around the difference between Type 1 and 2.  The accuracy of the measurements varies depending on the frequency of the sound being measured.  Basically Type 1 means approximately ± 1dB accuracy and Type 2 means approximately ± 2dB accuracy.  But again, this varies depending upon the frequency of the sound.

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