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Speech interference from aircraft noise is a major cause of annoyance for communities. Examples include:
- The disruption of routine activities, such as:
- radio or television listening
- telephone use
- family conversation
- The disruption of speech communication in
- industrial settings
In a classroom, communication must be clear and without interruption. The steady background sound level must be low enough for the teacher to be clearly heard. The intermittent outdoor noise events also need to be kept to a minimum. So, for classroom situations, it is important to evaluate:
- the steady background level,
- the level of voice communication,
- and the single-event level from aircraft over-flights that might interfere with speech.
Several related research studies have been conducted over the last 30 years. Several guidance documents have also been published. These have resulted in various federal noise level criteria for speech interference.
U.S. Federal Criteria for Interior Noise
Speech comprehension can be considered in two ways:
- Word Intelligibility - the percent of words transmitted and received. This might be important for students in the lower grades who are learning the English language. It might also be particularly important for English as a Second Language (ESL) students.
- Sentence Intelligibility - the percent of sentences transmitted and understood. This might be important for high-school students and adults who are familiar with the language. They don't necessarily have to understand each word in order to understand sentences.
In the "Levels Document", the EPA set a goal of an indoor 24-hour average level LEQ of 45 dB. At this level, 100% sentence intelligibility is expected for an average adult with normal hearing and language fluency.
Classroom Criteria for Steady Noise
For listeners with normal hearing and language fluency, complete sentence intelligibility is possible when the signal-to-noise ratio is approximately 15-18 dB. At least a 15 dB signal-to-noise ratio is recommended* in classrooms. This is to make sure that children with hearing impairments and language disabilities are able to understand high speech intelligibility. Also, if the average adult voice registers a minimum of Lmax 50 dB in the rear of the classroom, the continuous background noise level indoors must not be higher than an LEQ of 35 dB. (This is assumed to apply during school hours.)
*Recommended by American National Standards Institute (ANSI) and the American Speech-Language-Hearing Association.
For a speaker-to-listener distance of about 1 meter, the World Health Organization (WHO) reported that:
- Speech in relaxed conversations is 100% intelligible in background noise levels of about 35 dB
- Speech can be fairly well understood in the presence of background levels of 45 dB
The WHO also recommends a guideline LEQ of 35 dB for continuous background levels in classrooms during school hours. To determine eligibility for noise insulation funding, the FAA uses related guidelines. These guidelines state that the design objective for a classroom environment is an LEQ of 45 dB. For a more detailed discussion see:Classroom Acoustics
Classroom Criteria for Irregular Noise
The noise measured at an airport or airbase is not continuous. It consists of individual events where the sound level exceeds the background level. These events occur for a limited time period as the aircraft flies over. So, the classroom criteria described above is not applicable to aircraft noise exposure.
Speech interference from aircraft noise is basically determined by the size and frequency of the individual events. So, using a time-averaged metric alone (such as LEQ) is not necessarily appropriate. Single-event criteria are needed to account for those irregular noisy events.
In 1984, researchers recommended using Speech Interference Level (SIL) for classroom irregular noise criteria. This metric is based on the maximum sound levels in the frequency range that directly affects speech communication. This range is approximately 500 Hz to 2,000 Hz. An SIL of 45 dB was identified as the desirable goal. This would provide 90% sentence intelligibility during aircraft over-flights. Although early classroom level criteria were defined in terms of SIL, Lmax has since become more popular.
Both SIL and Lmax consider the maximum sound levels from intermittent noise events. They can also be related to existing background levels. An SIL of 45 dB is about equal to an A-weighted Lmax of 50 dB for aircraft noise.
In 1998, researchers made another conclusion. If an aircraft's noise reached the speech level of Lmax 50 dB, 90% of the words would be understood by students in a classroom. An indoor Lmax of 50 dB was then adopted as the maximum single-event level allowed in classrooms. This limit was set based on students with normal hearing and no special needs. At-risk students may be affected at lower sound levels.
SEL has been recommended by some as a better choice for estimating speech interference from aircraft overflights indoors. A maximum SEL of 64 dB is suggested. A 26 dB noise reduction is assumed when you move indoors from outdoors. So, a 64 dB SEL indoors is about equal to 90 dB SEL outdoors. Aircraft events with outdoor SEL values greater than 90 dB would disrupt indoor speech communication. The research indicates that speakers using a casual vocal effort can achieve 95% intelligibility when indoor SEL values did not exceed 60 dB. This translates to an approximately 50 dB Lmax.
ANSI 12.3 states that the criteria for allowable background noise level can be relaxed for irregular noise events. This is because speech is impaired only for the short time when the aircraft noise is close to its maximum. Consequently, when the background noise level of the noisiest hour is dominated by aircraft noise, the indoor criteria can be increased. The Leq of 35 dB for continuous background noise can be increased by 5 dB to an Leq of 40 dB. However, the noise level cannot exceed 40 dB for more than 10% of the noisiest hour. This is for a room that's less than 20,000 cubic feet.
The WHO does not recommend a specific indoor Lmax standard for single-event noise. Yet, it does place a guideline value at Leq of 35 dB for overall background noise in the classroom. Other reports published by the WHO also suggest a 50 dB Lmax.
The United Kingdom Department for Education and Skills (UKDFES) established a maximum 30-minute time-averaged metric for background levels of 30-35 dB. It also established a 55 dB maximum for intermittent noises.
In summary, the related scientific literature and international guidelines state that indoor background noise levels should be limited to an LEQ of 35 to 40 dB. Single events should have an Lmax limit of 50 dB.