A sound level meter records a physical sound level and displays it as a measured value.
A sound level meter consists of a microphone capsule that converts the sound level into an electrical voltage. This signal is weighted the frequency range to simulate human hearing. The A and C curves are widely used. The signal is then averaged in the time domain. These are usually the "slow + fast" settings, which make the display slower, that we can also follow the sound level with our eyes. High-quality sound level meters have energy-equivalent averaging and can also calculate the LEQ.
Measured parameters
The most important measurement parameters are precisely defined in the IEC61672.
These are in particular:
- LAEQ
- LCEQ
- LAFMAX
- LCPEAK
All values are measured in dB. This often leads to confusion. However, the values differ considerably.
We have written a separate article about the different quantities that can be used to measure sound.
Cheap sound level meters (Amazon/Ebay) can at best display the LAFMAX. Therefore, these devices are completely unsuitable for standard-compliant measurements.
Dynamic range
Humans ear work in the range from 0dB (hearing threshold) to about 130dB (pain threshold).
High-quality sound level meters with a standard ½” capsule have an inherent noise of approx. 18dB(A) and measure up to well over 140dB.
It is important to distinguish between the terms measuring/dynamic range and inherent (self) noise.
The acoustic noise of a sound level meter is the displayed value, i.e. complete silence when there is no acoustic input signal.
In order to measure a useful signal with sufficient precision, its level must be at least 6-10 dB above the background noise. The background noise is made up of the inherent noise of the measuring system and all other interference components (e.g. traffic noise). The measuring range indicates the level range in which measurements can be taken with sufficient accuracy. For marketing reasons, the inherent noise is often "offered" as the lower measuring range.
Frequency range
The audible audio range covers 20-20kHz. Below 20Hz is the infrasound range. Above 20kHz is the ultrasonic range
Accuracy
The standard IEC EN 61672-1:2014-07 Electroacoustics - Sound level meters - Part 1: Requirements (IEC 61672-1:2013); German version EN 61672-1:2013 sets out extensive requirements for sound level meters. The standard defines devices according to class 1 and class 2. The differences are most clearly visible in the tolerance for different frequencies.
octave/third octave filter
For further investigations, the audio range is often divided into several frequency bands. The individual bands do not have a constant bandwidth, but are logarithmic. This relationship also forms the basis of music theory. In an octave band, the upper limit frequency is twice as high as the lower limit frequency. If you divide an octave band into 3 bands, you get the third octave bands.
The center frequencies are standardized in IEC61260.
FFT
The FFT (Fast Fourier Transform) is used for narrowband analyses. The FFT is a good way to determine individual tones/frequencies. The frequency resolution can be increased as desired, but the temporal resolution decreases to the same extent.
Room acoustics reverberation time
Many high-quality handheld sound level meters can also determine room acoustic parameters such as the reverberation time (RT60). The older method with switched off noise is usually used here. The powerful chirp method , with which large rooms can be measured even with the lowest levels, can be found in PC-based systems such as Akulap .
Psycho-acoustics
Psycho-acoustics is a relatively new field. Here, our hearing ability is simulated using very complex algorithms. Psycho-acoustic measurements do not focus on the sound level with an accuracy of fractions of dB. Rather, the parameters of loudness, tonality and sharpness are used to reflect our hearing impression. Loudness in sone, for example, is more widely used in the assessment of fans, air conditioning systems, etc.
