criteria for Low Frequency SOUND and Infrasound

The United States government does not have specific criteria for low frequency noise. This applies to sound from wind turbines as well as other sources. We performed an extensive world-wide literature search of over 100 scientific papers, technical reports and summary reports on low frequency sound and infrasound - hearing, effects, measurement, and criteria. The search for potential criteria focused on relevant, objective standards published by technical societies such as the American National Standards Institute (ANSI) or the International Standards Organization (ISO). This research turned up criteria in the following areas:

1. Threshold of hearing (audibility),

2. Disturbance,

3. Perceptible vibration, rattle, and annoyance.

These criteria were evaluated both indoors and outdoors, and are discussed briefly below.

Threshold of hearing

The ear is the primary organ for sensing infrasound. Hearing becomes gradually less sensitive for decreasing frequencies. But, humans with a normal hearing organ can perceive infrasound at least down to a few hertz if the sound level is sufficiently high. The threshold of hearing is standardized for frequencies down to 20 Hz in ISO 226:2003. Based on extensive research and data, Moeller and Pedersen propose normal hearing thresholds for frequencies below 20 Hz down to 4 Hz.

Disturbance

A report prepared by the University of Salford for the UK Department for Environment, Food, and Rural Affairs (DEFRA) on low frequency noise proposed one-third octave band sound pressure level criteria and procedures for assessing low frequency noise. The guidelines are based on complaints of disturbance from low frequency sounds and are intended to be used by Environmental Health Officers. The DEFRA criteria are based on measurements in an unoccupied room.

Perceptible Vibration, Rattle and Annoyance

ANSI S12.9 /Part 4 identifies two thresholds: annoyance is minimal when the 16, 31.5 and 63 Hz octave band sound pressure levels are each less than 65 decibels (dB) and there are no rapid fluctuations of the low frequency sounds. The second threshold is for increased annoyance which begins when rattles occur. This also involves sound pressure levels in the 16, 31.5, and 63-Hz octave bands.

ANSI/ASA S12.2-2008 discusses criteria for evaluating indoor noise, and has two separate provisions for evaluating low frequency noise: (1) the potential to cause perceptible vibration and rattles in lightweight wall and ceiling construction, and (2) meeting low frequency portions of room criteria curves.

Field Program

Two types of utility-scale wind turbines were measured for this study. These two turbine types are among the most commonly used in the developers fleet and the wind industry in general: General Electric (GE) 1.5sle (1.5 MW), and Siemens SWT-2.3-93 (2.3 MW). Sound levels for these wind turbine generators vary as a function of wind speed.

Real-world data were collected from Siemens and GE wind turbines operating in Texas to compare to the low frequency noise criteria discussed previously. These data sets consisted of outdoor measurements at various reference distances, and concurrent indoor/outdoor measurements at four residences within the wind farm. Reference distances were 1,000 feet and 1,500 feet from the nearest wind turbine in all cases the measured sound levels reflect the operation of several other nearby turbines. These distances are meaningful since wind turbines are rarely sited closer than 1,000 feet from a residence.

Sound level data, wind speed data, and wind turbine operational data were measured during the field program. A wide variety of wind and noise conditions were measured during the one week program. In order to minimize contributions to noise from the wind itself, the most useful data had relatively low winds near the ground and strong winds near the hub height of the turbine.

results and comparison to criteria

Due to space constraints, only the results from the Siemens SWT-2.3-93 wind turbine will be discussed and shown. However, the results and conclusions for the GE 1.5sle wind turbine are very similar.

Threshold of hearing

Figure 1 plots the outdoors one-third octave band sound levels from the Siemens wind turbines for two different time periods under high output conditions. Also plotted on this graph are the ISO 226 audibility thresholds. The results show that infrasound (below 20 Hz) is inaudible to even the most sensitive people 1,000 feet from these wind turbines (more than 20 dB below the median thresholds of hearing). Low frequency sound above 40 Hz may be audible depending on background sound levels. It is worth noting that measurements with the wind turbines turned off under high wind conditions showed that low frequency sound from the wind itself is also above the threshold of audibility. Results from the indoor testing showed similar results with the windows open or closed.

Disturbance

The measurement results show that all outdoor equivalent DEFRA disturbance criteria were met. In addition, indoor measurements were made in two residences and the results show that all indoor DEFRA disturbance criteria are met.

Perceptible Vibration, Rattle and Annoyance

Outdoors -- the measurement results show that all equivalent outdoor ANSI/ASA S12.2 perceptible vibration criteria are met. The 31.5 and 63 Hz sound levels are below the level of 65 dB identified for minimal annoyance in ANSI S12.9 Part 4, and the 16 Hz sound level is within 1.5 dB of this level, which is an insignificant increase since the low frequency levels were not rapidly fluctuating. The low frequency sound levels are below the ANSI S12.9 Part 4 thresholds for the beginning of rattles.

Indoors -- the measurement results inside two residences show the ANSI/ASA S12.2 low frequency criteria were easily met for both windows open and closed scenarios. The ANSI/ASA S12.2 low frequency criteria for bedrooms, classrooms and hospitals were met, and the criteria for moderately perceptible vibrations in light-weight walls and ceilings were also met.

SUMMARY

Wind farms with Siemens SWT-2.3-93 and GE 1.5sle wind turbines at maximum noise at a distance more than 1,000 feet from a residence do not pose a low frequency noise or infrasound problem. At this distance the wind farms:

1. meet ANSI/ASA S12.2 indoor levels for low frequency sound for bedrooms, classrooms and hospitals;

2. meet ANSI/ASA S12.2 indoor levels for moderately perceptible vibrations in light-weight walls and ceilings;

3. meet ANSI S12.9 Part 4 thresholds for annoyance and beginning of rattles;

4. meet UK DEFRA disturbance based guidelines;

5. have no audible infrasound to the most sensitive listeners; and

6. might have slightly audible low frequency noise at frequencies at 50 Hz and above depending on other sources of low frequency noises in homes, such as refrigerators or external traffic or airplanes. However, audible noise in these low frequencies already exists from other sources.

The results of the program showed that two of the most common utility-scale wind turbines on the market do not pose a low frequency noise or infrasound problem at a distance of 305 meters (1,000 feet) or more. These conclusions are consistent with research done by others on different wind turbine manufacturers.