A sound level meter (SLM) is a device used to make frequency-weighted sound pressure level measurements displayed in dB-SPL.
0.0 dB-SPL is the threshold of hearing, and is equal to 20uPa (microPascals). This correlates to what one would aurally perceive when in a deep cave or in a large anechoic chamber.
Packaged as a single-function handheld test device, SLMs are intended to be held at arm’s length during measurements (to reduce the effects of the body on the measurements) or secured to a tripod stand for more stability.
All SLMs feature an omnidirectional measurement quality condenser microphone, a mic preamp, frequency weighting networks, an RMS detector circuit, averaging circuits, the meter display, AC and DC outputs used to feed other measurement devices or for recording (see Figure 1, below).
Most SLMs have the same set of user adjustments, including SPL range selection, A and C weighting filters, slow and fast detector response, and minimum or maximum SPL.
The SPL range switch provides a balance between minimizing the preamp noise level and measuring a wide range of sound pressure levels.
Most of the commonly available SLMs measure from about 30 to 130 dB-SPL and do this in 3-4 ranges.
Figure 1: Functional drawing of a basic SLM (click to enlarge)
The more advanced and expensive SLMs feature removable microphones, 1-octave and/or 1/3-octave filter sets, additional weighting filters including B, D and Flat or Linear (no filter), additional detector response options (Impulse and Peak), averaging (over time) and data logging or storage (either on-board, as computer files or both).
Almost all SLMs are designed and specified to perform to one of four internationally standardized levels of accuracy:
Table 1: Permitted tolerances as defined by the IEC 60651 and ANSI S1.4-1983.
Note that these tolerances are at 1 kHz, the standard calibration frequency for SPL measurement. In order to ensure the flatness of the SLM there are additional tolerances specified for various frequency bands and microphone classes as well.
Class-0 SLMs are employed primarily to calibrate other SLMs and may be used for very high precision noise measurement in controlled spaces and/or for academic research.
Class-1 and Class-2 SLMs are most widely used by acousticians, sound system professionals, industrial designers/ manufacturers and researchers in academia and government. Measurements made with these levels of accuracy are generally acceptable as evidence in the resolution of legal disputes.
Class-3 SLMs are restricted to noise survey meters and dosimeters.
Most general purpose SLMs are provided with a 1/2-inch free field microphone, permanently attached to the SLM body.
Higher quality SLMs with removable microphones may be outfitted with capsule sizes ranging from 1/8-in to 1-in.
The smaller microphone capsules (1/8-inch and 1/4-inch) have three primary advantages:
—They are capable of higher SPL measurements;
—They remain omnidirectional up to higher frequencies;
—And they present less disturbance of the air due to their reduced physical size (the mic itself has less impact on the measurement).
Larger 1-inch capsules exhibit less self-noise and are therefore best suited to measurement in very quiet spaces, of very low-level noise sources, and of low-frequency energy.
One can also choose between three different classes of measurement microphones: Random Incidence, Free Field, and Pressure.
In reality, these three classes of microphone are all fundamentally “pressure response” microphones and are designed to sense pressure - not pressure gradients or particle velocity.