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That air conditioner noise is annoying!

That air conditioner noise is annoying!
The sound of the HVAC System in your office or factory may be annoying depending on who you ask; the person who has to live with it, or the person who has to pay for it. To measure background noise levels objectively, we use Noise Curves.

International standards organizations (ISO, ANSI and ASA) recognize the need to objectify judgements on the amount of ambient noise in enclosed spaces, and provide us with definitions for various Noise Curves.

Regulations and building design documents often stipulate that the ambient noise in an unoccupied room should not be louder than a particular noise rating.

What are Noise Curves?

When measuring ambient noise one has to consider the full range of audible frequencies. Figure 1 is an example of a spectrum of ambient noise measured in an office. One might say that this spectrum could be rated with the single value 48 dB, this being the highest Sound Pressure Level (SPL) across the range (at 63 Hz). No single point on the entire spectrum is above 48 dB.

Figure 1: A Measured Ambient Noise Spectrum

To then give the same rating to all rooms that have a maximum ambient noise level of 48 dB (at any point in the ambient noise spectrum), is a little simplistic because we are concerned with the human annoyance factor; we have to consider that people find higher frequencies more annoying while we are more tolerant of lower frequencies. In Figure 2, curves are added that consider this weighting. These are the so-called Noise Curves. The particular curves in Figure 2 are Noise Criteria (NC-2008) curves.

Figure 2: Spectrum with Noise Curves

The lowest noise curve above the measured spectrum is NC-45. We therefore say that the room measured has an NC-45 rating.

NC-45 is above the recommended NC rating for offices, as can be seen in the tables below. The tables show recommended ratings for some Noise Curves. (Click here to read about other Noise Curves.)

Type of Room NC and RNC Curve RC Curve
Concert halls 15-18 -
Small auditoriums 25-30 -
Large auditoriums 20-25 -
Broadcast studios 15-25 -
Drama theatres 20-25 -
Private residences 25-40 25-30 (N)
Schools 25-35 25-40 (N)
Hotels 25-50 25-45 (N)
Offices 25-40 25-35 (N)
Type of Room NC and RNC Curve RC Curve
Conference rooms 25-35 25-35 (N)
Hospitals and clinics 25-45 25-40 (N)
Movie theatres 30-40 -
Churches 30-35 25-40 (N)
Courtrooms 30-35 25-35 (N)
Libraries 30-35 30-40 (N)
Restaurants 40-45 -
Light maintenance shops 45-55 -
Shops and garages 50-60 -


Notably, in Figure 2, the point on the spectrum closest to the NC-45 curve is at 8000 Hz. Therefore attenuating the cause of the high frequencies in the ambient noise by 5 dB would give the room a noise rating, of NC-40 and ultimately reduce the annoyance factor of that background noise to an acceptable level – job done!

Measuring Noise Curves with XL2 Analyzer
The XL2 Audio and Acoustic Analyzer, is a handheld acoustic measurement tool that allows you to measure Noise Curves at the press of a button, and immediately provides reports on the internal SD card.

The XL2 is used extensively for measuring Noise Curves in offices and factories, while the device is also trusted by acoustic consultants for measurements in concert halls and broadcast studios where low Noise Curve ratings are critical.

Tech Talk
The XL2, with the Spectral Limit Option installed, measures all supported Noise Curve types at the same time, allowing you to switch to your required report after the measurement. The result also indicates the frequency band, between 16 Hz and 8 kHz, that makes the most significant contribution to the noise rating, so that you know which frequency should be reduced to improve the rating.

For RNC curve types the XL2 reports any large fluctuations or surging at low frequencies, e.g. caused by fans.

Following the RNC specification further, the XL2 measures the octave-band sound pressure level every 100 ms. The minimum measurement time is 20 seconds.

For curve types NC-2008 and NC-1995, the speech interference level (SIL) result is calculated by averaging the octave-band sound pressure levels at 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz.

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