Are Regulators afraid to be useless? Is the equipment still valid? In the following post we will explain the reasons of this upgrade, what are the changes and the scope of them.
When performing in-situ measurements many technical limitations appear. Most important for Airborne Sound are:
Volume of the venue:
- Most of the houses have rooms with reduced volume.
- High Modal response in small volumes ( • Old ISO 140 parts 4 and 7 have informative annex with recommendation about how to perform the measurements that are sometimes impossible to meet due to space limitations.
Reliability of the measurement
- Old procedures have poor reproducibility.
- Measurements made in light structure Buildings at low frequencies are not accurate.
In order to address these limitations and improve the reliability of the measurements, few changes have been introduced in the measuring procedure:
- Introduction to manual scan.
- New low-frequency procedure for reduced volume venues (<25m3) at 50, 63 and 80Hz third octave bands.
The proposed method aims to allow dynamic measurements in the field. The immediate advantages are the possibility to:
- Reduce the equipment and cable needed for the measurement.
- Reduce de measuring time.
- Allow engineer to remain in the venue to control the background noise (ISO 140 norm part 4 and 7 do not allow to stay in the venue to the engineer).
However, there is a drawback that has to be taken into account. Walking can cause uneven exploring speed and additional noise.
There are two scanning routes:
- Standing position routes. Suitable for limited furniture spaces.
- Rotating position routes over a fixed position. Suitable for Sound pressure fields with a correlation coefficient know.
It is possible to calculate the effectiveness of any manual analysis with defined geometry by calculating an equivalent discrete number with uncorrelated positions for each route, (Neq)
New Low Frequency measurement procedure
In typical venues, the spatial variability of the SPL increases at low frequencies. The smaller the room is, the higher is the uncertainty:
Most of the living spaces (specifically bedrooms) in houses and hotels have small volume. In these spaces (<25m3, where there are less than 5 modes below 100 Hz) high modal response appear. The SPL might show differences of 17 to 28 dB, between the level measured in the center of the space and close to the limits (≈ 0,5 m form the edge).
18 m3 receiving room at 80 Hz 1/3 octave band
This situation presents problems when measurements are made in the central area. The soundmeter is placed in the nodal plane at mid height of the room, so the measurement can be over or under estimated. The norm requires minimum distances to the edge of more than 0.5m for 100 -5000 Hz.
ISO 140-4 information annex D for low frequencies suggested minimum distances of 1,0-1,2 m for frequencies below 100Hz.
The repeatability is improved by the use of additional microphone positions for SPL in the corners below 100Hz. Similar approach is made in the ISO 10052 norm when measuring the Building service equipment.
The aim is to use the SPL measurement of the center zone and the corners to estimate the average SPL for the whole space.
This can be achieved by using the empiric ponderation:
For the frequency bands with modes N<5 below 100 Hz, the quadratic mean error is close to 0 dB when low frequency method is used . The 95% of confidence intervals are similar to the ones of the central zone for mid frequencies stationary positions of microphone measured under the ISO 140-4 norm for 100 and 500 Hz.
In dBcover we keep track of the norm changes and upgrades of the norms and keep working to improve our Airborne sound insulation solutions (dBsonic), that allow to correct installations that do not comply with the regulation in a fast and efficient way. An example of this is the dBsonic HM-3D solution.