University of Edinburgh
 

Audiology Update

Presented on Thursday, 25 November 2010

Assessing and Maximising Audition in the classroom

Brian Shannan, Tony Murphy, Richard Vaughan

Learning in a Sea of Noise: Classroom Acoustics and Access to Verbal communication

Speech Intelligibility

"We would never teach reading in a classroom without lights. Why then do we teach in 'acoustical darkness'. Speaking to a class, especially of younger children, in a room with poor acoustics, is akin to turning out the light."
John Erdreich PhD

Factors Affecting speech intelligibility

factors affecting

And: the individual child.

Populations Affected by Noise: Reduced speech perception

  • Young (<13-15 years old)
  • Sensori-neiural hearing loss
  • Conductive hearing losses
  • Articulation disorders
  • Language disorders
  • Non-native language speakers
  • Auditory processing dosorders
  • Minimal and unilateral hearing losses

Acoustic closure

Carol Flexer, in an article for Hearing Journal (August 2002) stated "People can fill in the blanks of missed information only if they have that information already stored in their brain's 'databank' from where they can retrieve it. Because they do not have those databanks, children need a sharper auditory signal than adults do. Thus, while a classroom might sound fine to an adult, it may be woefully inadequate for typical children who are neurologically undeveloped have not had decades of language and life experience."

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

Auditory Cognitive Closure

Arocdnicg 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.

Children cannot listen like adults!

  • Children are not as able to compensate for the acoustic smearing effects of reverberation.
  • They need a better signal to noise ratio to perceive speech clearly.
  • Noise and reverberation work together to crease an adverse listening and leraning environment.

Classroom Acoustics effects: summary for normal hearing

  • The newer or more difficult the task, the greater the chance for noise interference
  • More than a simple masking effect
  • Slower reading acquisition - phonemic perception and reading comprehension
  • Learned helplessness - lack of perseverance for difficult tasks
  • Attention and distractibility effects
  • Deaf children are affected more by noise and reverberation than children with normal hearing.

Room Geometry

Sound is louder the nearer you are to the source of the sound.

Inverse square law

  • Sound intensity at source reduces in proportion to the square of the distance, irrespective of directivity.
  • Sound spreads out and gets weaker as energy is spread more thinly.
  • At the sound source there is contsant power.
  • It is the area of sound propagation that is increasing and therefore the intensity of sound at distance decreases.
  • Therefore, it is difficult to hear at distance!

loss of sound with distance

Background Noise

Noise competition

  • Unwanted or unintended sounds that interfere with the signal
  • Classroom noise level at night 30-35 dB, 4045 dB when heating is on, 57 dB when teacher and children are present
  • A major effect is classroom organisaion and control
  • Major changes in SPL over the course of a day with peaks on bus, at dinner time in school corridors

Noise control

School noise:

  • Sources of noise within classroom, within school, within locality of school
  • Background-steady state, eg; fan from OHP
  • Intruding - sudden onset, dropped file, school bell
  • Internally generated - students' and teachers' talking, scraping chairs, shuffling feet
  • Problem areas: corridors, school halls, dining room, high noise generation with high social content

Noise transmission

  • Direct - walls set into vibration
  • Via acoustic leaks - via plug sockets, doors
  • Via adjacent rooms
  • Noise sources: heating/air concitioning (doct-borne noise)
  • Noise within classroom - teacher control has been identified as the most important aspect of noise control by deaf children.

background noise

The problems with noise

  • Background noise refers to any undesired auditory stimuli that interferes with what a child wants or needs to hear and understand (Crandell et al 1995).
  • Excessive noise is a much more serious and widespread problem than poor room acoustics.
  • Noise creates noise.
  • The poorer the acoustics, noisier the environment, noisier the children.
  • It may not be obvious to the teacher that the students are having increased difficulty because adults can understand speech in a noise better than younger children.
  • All this means that children require a quieter environment and a louder signal than adults do in order to learn.

The Signal to Noise Matrix

Inputs Enhancers Outputs
Target speech Proximity
Reverberation: Early reflections
Signal
Activity noise Proximity
Reverberation: Early and late reflections
Noise

Services/
apparatus noise

Proximity
Reverberation: Early and late reflections
Noise
External noise Proximity
Low attenuation within room boundaries
Reverberation: Early and late reflections
Noise

Reverberation is controlled by the acoustic treatment of room boundaries.

Impact of external noise is controlled by the acounstic insulation within room boundaries and the structural design of the building to control conduction of sound within the 'skeleton' of the building.

Signal to Noise Ratio

  • Carol Flexer defines the S/N ratio as "The relationship between the primary or dsired auditory signal to all the other unwanted background sounds". She also states "The more favourable the S/N ratio, the more intelligible the spoken message". and "S/N ratio is the key to hearing intelligible speech".
  • For adults to make sense of a speaker in noise they need to have the speaker's voice (signal) d dB louder than the background noise (noise). This is a signal to noise (S/N) ratio of +6 dB.
  • Whereas a child needs +16 dB S/N ratio and a deaf child needs a +20 to +30 dB S/N ratio.
  • A typical classroom is likely to be tween +5 and -7 dB.

The spectral distribution of useful information in the original speech signal

Most importan range is from around 700 to around 3,000 Hz - contains 65% of the useful information

Extending down to 350 Hz and up to 6,000 Hz adds another 30% (total = 95%

Useful information covers a range of 30 dB

Signal-to-noise ratio Speech audibility Phoneme recognition
-15 dB 0% 0%
0 dB 50% 85%
+15 dB 100% 98%

Increased difficulty tuning into the small differences between words - with a hearing loss OR when any child is listening in noisy classroom conditions.

Visual Analogy of SNR

visual analogy

 

Reverberation

Reverberation time (RT60) refers to the amount of time it takes for a sound to decay 60 dB from its initial offset.

For example if a 100 dB SPL signal is delivered into a room and takes 2 seconds to be reduced to 40 dB SPL, the RT of that environment would be 2 seconds.

Reverberation is not all bad - early relflections can increase loudness with out reducing clarity.

The late components of reverberation

  • They arrive too late to be integrated with the direct signal or the early components (more than 1/10 of a second)
  • If their level is still high enough, they interfere with the current sound by both physical and perceptual masking.

textual analogy

Reverberation times for schools:

  • Primary class <0.6s
  • Sexondary class <0.8s
  • Hearing Impaired Base <0.4s
  • Is this appropriate?

Some Guidelines

Building Bulletin 93 (BB93) "Acoustic Design of Schools" was introduced in 2003. This document recognises that there is a need for a 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.

The Disability Discrimination Act requires schools

  • Not to treat disabled pupils "less favourable'
  • to make reasonable adjustments to ensure that disabled pupils are not at a substantial disadvantage; and
  • to draw up plans to show how, over time, they will increase access to education for disabled pupils (school accessibility plans).

Recommendations by BATOD and ASHA

Acoustic Parameters British Association of Teachers of the Deaf American Speech Language Hearing Asssociation
Unoccupied noise level 35 dB(A) 30 - 35 dB(A)
Reverberation time (unoccupied) 0.4s across frequency range 125 Hz to 750 Hz 0.4s
Signal to noise levels +20 dB across frequency range 125 Hz to 750 Hz > +15 dB
  +15 dB across frequency range 750 Hz to 4 kHz  

In groups

  • Identify three areas within the school that are poor acoustically
  • Identify three areas in the school that provide a positive listening environment
  • Try to identify some simple measures to reduce noise
  • How can you encourage the development of listening skills - identify at least two activities.

Summary

  • What we want to hear is called the SIGNAL
  • What conflicts with the signal is called NOISE
  • Deaf children require the signal to be more intense than the noise
  • If the signal is heard at an intensity of 70 dB SPL and the conflicting noise is heard at an intensity of 60 dB SPL, then the signal to noise ratio is + 10 dB (that is the difference between the two values).
  • The '+' indicates that the signal was the most intense
  • A '-' would indicate that the noise was the most intense, eg; if the signal to noise ratio was -10 dB and noise was 60 dB SPL, then the signal must have been 50 dB SPL.

Summary - reflection

  • When a sound hits an object, some of the energy is absorbed and some is reflected (or bounced) off the object.
  • If the object is solid like a brick wall virtually all the sound is reflected.
  • If the object is soft (eg; thick carpets or heavy curtains) more of the sound is absorbed.
  • This reflected sound can interfere with the cound coming direct from the source.
  • In classrooms, may different types of surfaces are present - all reflect sounds by different ammounts.
  • Sounds are reflected of walls, floors, furniture etc
  • The reflected waves can then hit other surfaces and be re-reflected.
  • Thise multiple reflections mean that the sound coming from a source (eg; a teacher) may take a long time to decay and may interfere with the sound coming directly from the speaker.

References

  • Bess, F H, Dodd-Murphy, J and Parker, R A (1998) Children with minimal sensorineural hearing loss. far and Hearing, 19 339-354.
  • Cunningham, J, Nicol, T, Zecker SG, Bradlow, A and Kraus, N (2001) Neurobiologic responses to speech in noise in children with learning problems: Deficits and strategies for imprrovement. Clinical Neurophysiology 112, 758-767.
  • Elliott, LL (1979) Performance of children aged 9 to 17 years on a test of speech intelligibility in noise using sentence material with controlled word predictability. Journal of the Acoustical Society of America, 66, 651453.
  • Johnson, CE (2000) Children's phoneme identification in reverberation and noise. Journal of Speech, language and Hearing Research 43, 144-157.
  • Nilsson, M, Soli, SD and Sullivan J (1994) Development of the Hearing In Noise Test for the measurement of speech reception thresholds in quiet and in noise. Journal of the Acoustical Society of America 95, 1085-1099.
  • Smith E, Lemke, J, Taylor, M Kirchner, JL and Hoffman H (1998) Frequency of voice problems among teachers and other occupations. Journal of Voice 12, 480-488.
  • Soli, SD and Sullivan, JA (1997) Factors affecting children's speech communication in classrooms. Journal of the Acoustical Socity of America 101, 3070.