University of Edinburgh
 

Improving the Listening Environment for Deaf Children in Educational Settings

Presented on Tuesday 13 May 2010

Acoustics and Deaf Children; understanding the essential concepts

Richard Vaughan

Why are good acoustic conditions important?

United Slates Access Board, 2009
"Research indicates that levels of background noise and reverberation little noticed by
adults, who are mature and skillful listeners, adversely affect learning environments for
young children, who require optimal conditions for hearing and comprehension."

"Auditory Closure"

Arcodnicg to rsceearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteer are in the rghit pcale. The rset can be a toatl mses and you can sitll raed it wouthit poberlrm. Tihs is buseace the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

Why are good acoustic conditions important?
Good classroom acoustics are particularly important for:

  • deaf children, as hearing aids/cochlear implants amplify both wanted and unwanted sound.
  • children who have temporary hearing loss
  • children who have speech impairments or learning disabilities.
  • children whose home language is not the same as the teaching language

Back to some basics

Sound

  • Sound is made when objects vibrate and is an invisible vibration.
  • It travels in waves, spreading outwards from the source of the sound.
  • Sounds are different both in loudness (intensity) and pitch (frequency).

Sound level (Intensity)

Intensity or loudness is measured in decibels (dB)

  • Normal voice 60 dB at 1 m
  • Raised voice 70 dB at 1 m
  • Shouting 80 dB at 1 m

Frequency

Frequency (pitch) is measured in Hertz (Hz). All sounds are made up of different
frequencies.

A piano keyboard runs from low pitch on the left to high pitch on the right and an
audiogram is the same.

piano

The effect of distance

Signal drops by 6dB with every doubling of distance.

distance

Room acoustics - the distance effect

source 4ft 8ft 16ft 32ft
75 dBA 60 dBA 54 dBA 48 dBA 42 dBA

Speech Intelligibility

Among various methods for rating the understanding of speech is Speech Transmission Index (STI)

  • a special modulated noise test signal is played from a loudspeaker at the talker's location
  • microphone/receiver displays the STI value
  • the receiver can be moved around to surveythe whole listening area, identifying any
    difficult
STI
0-0.3 0.3-0.45 0.45-0.6 0.6-0.75 0.75-1.0
unintelligible poor fair good excellent

DfES Building Bulletin 93

Quote from the introduction:
"This represents a significant tightening of the regulation of acoustic design in schools, to reflect a general recognition, supported by research, that teaching and learning are acoustically demanding activities.

In particular, there is a consensus that low ambient noise levels are required, particularly in view of the requirements of the Special Educational Needs and Disability Act 2001 for integration of children with special needs in mainstream schools."

BB93 Standards: three main acoustic criteria

  • Internal ambient noise
  • Reverberation time
  • Sound insulation

Noise

  • Affects the understanding of speech
  • ... it overlaps some of the speech to make it inaudible or masked
    (Dorks, Morgan & Dubno 1982, Helfer 1991)

noise

BB93: Internal ambient noise in unoccupied space

The indoor ambient noise level includes noise contributions from:

  • external sources outside the school premises (including, but not limited to, noise from road, rail and air traffic, industrial and commercial premises)
  • building services (eg; ventilation systems)

The indoor ambient noise level excludes noise contributions from:

  • teaching activities within the school premises, including noise from staff, students and equipment within the building or in the playground. (Noise transmitted from adjacent spaces is addressed by the sound insulation requirements.)
  • equipment used in the space (eg; machine tools, computers, overhead projectors, etc). However, these noise sources should be considered in the design process.
  • rain noise. However, it is essential that this noise is considered in the design of lightweight roofs and roof lights.

Indoor ambient noise BB93 standards

Upper noise limits given for different spaces including:

  • General teaching spaces: 35dB LAeq,30min
  • Classrooms designed specifically for use by hearing impaired students (including speech therapyrooms): 30dBA LAeq,30min

Signal-to-noise ratio

  • For adults to make sense of a speaker in noise they need to have the speaker's voice (Signal) 6 dB louder than the background noise (Noise). This is a Signal to Noise (S/N) ratio of + 6dB.
  • However, a child needs + 16 db SIN ratio and a deaf child needs a +20 to +30 dB SIN ratio.

Recommended Signal to Noise ratio for a deaf child is +2OdB for frequency range
125Hz to 750Hz and +15dB for frequency range 750Hz to 4000Hz (BATOO)

"The more favourable the S/N ratio, the more intelligible the spoken message." "S/N ratio is the key to hearing intelligible speech."
Carol Flexer, Hearing Journal (August 2002)

Reverberation

  • the low frequency sounds in speech are mainly vowels
  • vowels are the most powerful phonemes in speech - they can 'mask' the high frequency sounds. This makes it difficult to hear the consonants
  • it is therefore important to reduce low frequency reverberation as much as possible for good speech intelligibility

frequency chart

Reverberation time

  • Reverberation time (RT60) is the time taken for the sound level to drop by 60 dB after the source is turned off.
  • A classroom with a long reverberation time will cause syllables to be prolonged so that they overlap and hence degrade speech intelligibility.
  • Long reverberation times occur in large rooms with hard wall and ceiling surfaces. Adding acoustic absorption and reducing the ceiling height will reduce the reverberation time and will improve speech intelligibility.

reverberation time

Reverberation time BB93 standards

Maximum reverberation times given for different spaces, including:

  • Primary school general teaching areas: <0.6s
  • Secondary school general teaching areas: <0.8s
  • Classrooms designed specifically for use by hearing impaired students (including speech therapy rooms): <0.4s

Sound insulation

Defined under 3 categories in BB93

  • Airborne sound insulation between spaces
  • Airborne sound insulation between cirulation spaces and other spaces used by students
  • Impact sound insulation of floors

Sound insulation BB93 Standards

Complex to measure/calculate!

  • Minimum weighted BB93 standardized level difference, DnT (Tmf,max),w
  • Minimum sound reduction index, Rw and minimum Nd,e,w-10lgN
  • Maximum weighted BB93 standardized impact sound pressure level L'nT (Tmf,max),w

BB93 Chapter 6: Acoustic design and equipment for pupils with special hearing requirements

Guidelines include:

  • "Sound insulation must be of a high standard, with the lowest background noise levels possible to ensure that a good signal to noise level is achieved."
  • "Short reverberation times are also critical in ensuring that sound does not build up when the class are working in groups."
  • "Care must also be taken to ensure that the level of low frequency noise is kept to a minimum. For many people with impaired hearing, low frequency noise can have a devastating impact on speech recognition, masking many important speech sounds... "

Includes information about FM systems, Soundfield systems and other technology.

"Classrooms are not the only places where hearing impaired children interact.

It is often overlooked in school design, but critical learning and interaction takes place
outside the classroom, and if hearing impaired children are to be fully included,
attention should be given to all areas of the school where the children might be
expected to interact with others."

Typical problems encountered in schools

  • High ceilings
  • Hard surfaces
  • Poor insulation between spaces

NDCS Acoustic Toolkit

Provides advice and guidance to schools and local authorities to help them:

  • create a better learning environment to improve the attainment of all children and particularly those who are deaf;
  • prepare their Accessibility Plans and Disability Equality Schemes as required by the disability and special needs legislation;
  • meet their - 'anticipatory' duties under the Disability Discrimination Act;
  • develop their asset management plan; involve pupils in the development of plans and
    schemes.

Structure

The first section is aimed at non specialists in schools:

  • an overview of factors affecting school listening environments
  • a brief introduction to acoustics
  • surveys which are simple to conduct which will help identify problem areas within schools

practical suggestions to improve listening environments.

The second section is mainly aimed at specialists (education audiologists, teachers of deaf children, acoustic consultants or acoustical engineers):

  • build on the information schools have collected
  • explore in more detail any major problems
  • identify possible solutions.

Thank you

richard@connevans.com