Acoustics Practicals
Noise Rating Methods:
Sound level and sound pressure are commonly misconceived as indifferent but the human ear is more sensitive to some frequencies over others, and its frequency response also differs. This means that when deciding if a building is fit for a specific purpose, instead of taking a single dbA reading of sound loudness, its “noise floor” must be defined. This takes into account the frequency distribution and is especially important in recording studios but background noise can also be a problem in many commercial buildings, offices, hospitals and schools.
The NR curves were developed in 1971 by the International Organization for Standardization (ISO) to determine acceptable indoor sound levels by measuring sound pressure (in dB). A noise rating graph, shown below, shows acceptable sound pressure levels at different frequencies. For much lower frequencies, the pressure level can be a lot higher before it becomes a nuisance to the human ear.
{draw:frame} (http://www.engineeringtoolbox.com/nr-noise-rating-d_60.html)
Practical Experiment:
Using the Norsonic Sound Analyser 110 an investigation into the background noise levels of the studio live room was made. Measurements were taken at standard centre frequencies (1/1) and 1/3 octave band measurements, as well as a single dBA reading.
dBA value = minimum 16.5dB < 18.8dB > 22.9dB maximum
{text:soft-page-break} 1/1
1/3
{text:soft-page-break} Noise Rating / Criteria:
The noise rating levels or criteria defined for a specific location should not exceed a certain limit, the ISO determines that:
NR 25 – is the limit for broadcasting and recording studios, concert halls and churches (equivalent dB level = 35)
NR 30 – is the limit for level for private homes, hospitals, cinemas and conference rooms (equivalent dB level = 40)
NR 35 – is the limit for libraries, museums, court rooms, hotels and executive offices (equivalent dB level = 45)
NR 40 –...