University of Edinburgh
 

Improving the Listening Environment for Deaf Children in Educational Settings

Presented on Tuesday 13 May 2010

Is Your Classroom Acoustically fit for Purpose?

Peter Grayson

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

Room Geometry

room geometry

Room Geometry - Distance

  • The further you are from the sound source, the more difficult it is to hear (speech recognition)
  • If a teacher is talking at about 70 dB(A) the front row students will hear the teacher @ 65 dB(A)
  • Middle row @ 59 dB(A) (already below class noise level!!)
  • Back row @ 53 dB(A) 15% -50% of consonants can be lost by distance. Children cannot understand lessons
  • Children don’t have the ability to fill in the gaps of missed speech until teenage years "auditory cognitive closure"

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.
  • It may not be obvious to the teacher that the students are having increased difficulty, because adults can understand speech in noise better than younger children.

  • 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 'data bank' from where they can retrieve it. Because they do not have those data banks, 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.
    All this means that children require a quieter environment and a louder signal than adults do in order to learn."

Signal-To-Noise Ratio (S/N)

  • Carol Flexer defines the S/N ratio as "the relationship between the primary or desired 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) 6 dB louder than the background noise (Noise). This is a Signal to Noise (S/N) ratio of + 6dB.
  • Whereas a child needs + 16 db S/N ratio and a hearing impaired child needs a +20 to +30 dB S/N ratio.
  • A typical classroom is likely to be between +5 and -7 dB.

  • Bradley and Sato (2004) used the WIPI speech discrimination test to find the optimum S/N ratio for 6, 8 and 11 year olds to achieve a 95% correct score. They evaluated a total of 878 students in 43 classrooms. Their findings were :  

results

Reverberation

  • Reverberation time (RT) 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.

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 favourably'
  • to make reasonable adjustments to ensures that disabled pupils are not at a
  • substantial disadvantage; and
  • to draw up plans to show how, overtime they will increase access
  • education for disabled pupils (school accessibility plans)

Recommendations by BATOD and ASHA

recommendations

Critical Distance

Critical Distance (Dc) is the distance from the talker at which the levels of the direct speech and the early components of reverberation are equal. Before the critical distance, the effective signal is the direct signal. Beyond the critical distance, the effective signal is dominated by reverberation and, at three times the critical distance;

Critical Distance

Critical Distance (Dc) is the distance from the talker at which the levels of the direct speech and the early components of reverberation are equal. Before the critical distance, the effective signal is the direct signal. Beyond the critical distance, the effective signal is dominated by reverberation and, at three times the critical distance; the direct speech makes negligible contribution.

critical distance

critical distance

Measuring for Speech Intelligibility

There are 3 ways of assessing a room for speech intelligibility

  1. Subjective
  2. Predictive
  3. Objective

1. Subjective

Subjective tests make use of various types of speech material. Frequently used speech elements for testing are phonemes, words, sentences

Tests we have used to assess speech intelligibility in classrooms are the AB word lists and the BKB sentence lists. The AB word list is on a CD from Southampton University and the BKB is on a DVD produced by our service.

2. Predictive

One of the first descriptions of a model to predict the effect of a transmission path on the intelligibility of speech was presented by French and Steinberg (1947) and later evaluated by Beranek (1947).

Articulation Index is a number that ranges between 0 and 1. In an environment with an AI of 1.0 speech will be completely understood.

The primary determinant for AI is the speech to noise ratio. A speech to noise ratio of 0 dB is equivalent approximately to an AI of 0.4. For acceptable speech intelligibility, a speech to noise ratio of approximately 9 dB or greater is required whereas a speech level of 3dB or more below the noise level (S/N = -3 dB) will produce inadequate communication.

3. Objective

One objective method to predict speech intelligibility is the 'Speech Transmission Index' (STI).

The STI is based on the application of a specific test signal.This system was developed by Houtgast and Steenken in 1971.

The STI measurement requires a special test signal from which the effective signal-to-noise ratio in each octave band at the receiving side is determined and used for the calculation of the STI.

Assessing for Speech Intelligibility

Nurseries

Nurseries are usually the most difficult places for

  • Carol Flexer refers to the fact that we hear with the brain, and the ears are only a means of getting the sound to the brain. She also states that children cannot listen like adults as their higher auditory brain centres are not fully developed until between the ages of 11 to 15.
  • Children cannot perform auditory cognitive closure like adults. (Auditory cognitive closure is the ability of the listener to 'fill in the gaps' of the conversation they are listening to.)

Noise Levels in Nurseries

Most of the interactions in nurseries tend to be group work activities and some can include movement.

Airey and MacKenzie recorded levels of 70 – 77 dB LAeq for 'typical group working conditions'.

Other research would suggest that the upper limit occurred when there was movement in the room

Reverberation Times in Nurseries

reverberation times

Noise Levels in Nurseries

noise leels

Case Studies

Case Study 1 Assessing a nursery

Tests performed

  • Reverberation Times
  • Background Noise Measures

The aim of the testing was to give advice as to how to improve the listening conditions in the room.

Plan of Room

This is a single storey building built as a church hall. The room is multifunctional as it is also used by other groups including scouts and guides

There were no external noise sources during the testing time.

plan of roomkey

Case Study 1 – Reverberation Times (RT)

Two visits were made to the nursery.

The first was to measure the noise levels and actual reverberation times. During this visit, measurements were also taken of the room so that the reverberation times could be calculated. The RT was calculated using the BRE spreadsheet from the NDCS Acoustics Toolkit. The calculated RT could then be used to show how it could be improved by adding acoustic treatment.

The second visit was to retest the room after some acoustic treatment had been completed.

The graph below compares the three sets of reverberation time measures

  • Actual RT - pre treatment
  • Calculated RT - when treatment had been completed
  • Actual RT - post treatment
graph

Case Study 2

  • Comparing subjective and objective measures of speech intelligibility.
  • Subjective Measure used - AB Word Lists
  • Objective Measure used - STIPA
  • Aim - to show the student and teacher the best seating position in the room

Case Study 2 - Plan of Room

This is a single storey building built away from any roads. There were no external noise sources during the testing time.

plan of roomkey

Case Study 2 - Reverberation Times

reverberation times

Case Study 2 - Comparing Subjective and Objective Scores

scores

Case Study 2 - Summary

Both tests produced similar outcomes and, in this case, the objective measures could have been used to show the optimum seating position for good speech intelligibility.

Case Study 3

Comparing speech intelligibility measures in 2 different rooms using an objective measure

  • Objective Measure used - STIPA
  • Aim - in both cases the hearing impaired students in these rooms were incapable of performing phonemically balanced speech tests so objective measures had to be used to assess speech intelligibility

Case Study 3 - Plan of Room

RC This is a single storey building built away from any roads. The only external noise source on the day of the test was from the class next door.

plan of roomkey

Case Study 3 Plan of Room

Y1 This room is on the ground floor of a 2 storey building. There was no noise from any traffic. External noise sources on the day of the test were from the class next door and some from the room above

plan of roomkey

Case Study 3 - Reverberation Times

RC

reverberation times

Y1

reverberation times

Case Study 3 - Noise Levels

RC

noise levels

Y1

noise levels

Case Study 3 - Objective Measures

RC The objective measures were obtained using STIPA and a Norsonic 118 sound level meter. The noise was generated using a classroom babble CD.

The output of the signal and the noise were both calibrated to 60 dB(A), 1 metre from the speakers.

noise levels

noise levels

RC The objective measures were obtained using STIPA and a Norsonic 118 sound level meter. The noise was generated using a classroom babble CD.

The output of the signal and the noise were both calibrated to 60 dB(A), 1 metre from the speakers.

noise levels

noise levels

Case Study 3 - Comparing Objective Measures

In Quiet

in quiet

In Noise

in noise

Case Study 3 - Summary

When the pupils were working the noise levels were similar but when the teacher's were teaching the teacher in RC was able to speak at a normal conversational level whereas the teacher in Y1 always had to have her voice raised.

The teacher in Y1 was unaware of the noise levels coming from the other classroom and the problems it was causing. She told me that when she had been observed by the head teacher, the head teacher was sat at the back of the room and said that she had had great difficulty understanding what was being said. There were no such difficulties in RC

The noise levels show how quickly the pupils in RC stopped talking and listened to the teacher but there is no such evidence in Y1.

Of the 2 rooms, RC is the better of the 2, acoustically.

I'm informed that doors are to be fitted to the Y1 room and this should ease the problem. Also it would be advantageous if an acoustic ceiling could be fitted in the room.

NB - if STIPA not available, a different pupil, who can perform phonemically balanced tests could be used.

Summary

Noise Survey - can be done simply with a simple sound level meter, your ears and some patience - see NDCS 'Acoustics Toolkit'

Reverberation Times - calculate the RT's of the room (using the BRE spreadsheet). This can be used to show how the RT's can be lowered using different materials.

Use the child to show the teaching staff how well the child performs in that particular listening situation

Thank You

p.grayson2@sheffield.gov.uk