Assessing Noise and Reverberation

classroom

 

 

 

 

 

 

 

 

In the last few weeks we’ve discussed vocal fatigue’s prevalence among teachers and how it affects learning. As a result of this, we have looked at how simple improvements can be made to better in-class acoustics. Is it necessary to make improvements to your classroom?

 

In this article, the evaluation of noise and reverberation will be addressed – we hope that this is helpful to individuals who plan on using the acoustic health checks & calibration services in their understanding of noise and reverb measurement.

 

The National Union of Teachers recognises the following acoustic challenges facing classrooms in terms of noise transfer between rooms and the excessive reverberation in rooms:

 

  • the acoustic properties of classrooms built in the 19th and early 20th century are often inadequate to modern school setups
  • modern building methods are often reliant on lightweight materials, which do not provide adequate sound insulation
  • ‘open-plan’ classroom designs
  • multi-purpose areas, such as halls, which must be acoustically suitable for a range of different activities
  • specific subject classrooms in which noise can be a particular problem, such as music and design technology classrooms

 

Measuring sound

 

1. Decibels

Sound levels are measured by the decibel scale(decibel is abbreviated: dB). The softest audible sound (which is near total silence) is 0 dB.  A sound ten times louder than silence is 10 dB. A sound 100 times stronger  than near silence is 20 dB.  Again, a  sound 1,000 times more powerful than silence is 30 dB. When the intensity of sound is doubled, the decibel level does not double because the dB scale is logarithmic and not linear.

 

Some common sounds and their average decibel levels are displayed below.

 

Source

dB

Faintest audible sound

0

Whisper

20

Quiet home environment

30

Soft hi-fi

40

Speech

50 – 70

Café

80

Pneumatic drill

90

Accelerating motorcycle

100

Rock concert

120

Jet engine

140

 

2. Signal-to-Noise Ratio (S/N)

Signal-to-noise ratio is a simple comparison that is useful for estimating how understandable speech is in a room.  The sound level of the teacher’s voice in dB, minus the background noise level in the room in dB, equals the signal-to-noise ratio (S/N) in decibels.  S/N levels are typically lower in the back of classrooms furthest from the teacher (the signal strength of their voice reduces with distance) and also where the noise level in a classroom is at its maximum level, for instance near a classroom heating and cooling unit.

 

According to research done by the The National Union of Teachers, in classrooms with a signal-to-noise ratio of less than +10 dB, students struggle significantly to comprehend their teacher’s instructions.  The teacher’s voice needs to be between 10 and 15dB as a minimum above the background noise level.  For hearing impaired students, audiologists recommend a difference of 20 – 30dB between voice and background noise.

Teaching SNR. The power of the teacher's voice falls off with distance (black line) below the background noise level (pink area). More speech sounds are lost (blue discs)
Teaching SNR. The power of the teacher’s voice falls off with distance (black line) below background noise level (pink area). Vital speech sounds have a negative SNR even if the average SNR is positive (blue discs)

 

3. Reverberation and Sound Absorption

In an ideal world, classroom reverberation rates (RTs) should not exceed 0.4-0.8 seconds. The regulations set out in Building Bulletin 93 recommend a maximum reverberation time for classrooms between 0.6 and 0.8 seconds. The lower scale is recommended for primary schools and the larger secondary schools.

 

Long reverberation times may add to background noise and can negatively affect comprehension, but short reverberation times can limit the strong reflections needed to reach students at the back of the classroom. Thus it is important to find a balance.

 

reverb in schools

 

 

 

 

 

 

Of course many classrooms fall outside these limits. Luckily rooms can be treated for reverberation.  This includes putting tactics in place to absorb sound or to reduce the levels of sound. As you may imagine, the first option is more effective. To find out more about reducing reverberation levels, revisit our article on “Simple Steps to Better In-Class Acoustics”.

 

4. Flutter Echo

Flutter echo occurs when a sound bounces from its source between two flat, hard surfaces that stand parallel to each other, such as a floor and ceiling, or two walls.

 

Flutter echo is similar to reverberation in its treatment. Reducing the sound level, or covering the hard surface on which sound is bouncing will eliminate flutter echo.

 

5. Noise from Adjacent Rooms

If you are unsure whether a wall that divides two classrooms is a sufficient barrier to sound, you can apply the following test to clarify the matter: set up a television (or any sound generating electronic device) in front of your classroom and set its volume so that it can be easily heard in the back of the classroom.  Next, visit the neighbouring classroom. If the television’s sound is audible, the wall dividing the classroom is not a strong enough barrier to sound. If the television’s sound is faint, the wall adequately absorbs sound.

 

It is also important to consider the sound absorption of exterior walls, because activities outside the school may produce noise that leaks into the classroom. Very often schools have exterior walls made of brick or concrete, which is a good sound insulator, but the windows are inadequate in blocking outdoor noise.  Double glazing provides better noise reduction than single-paned glass (as well as providing better thermal insulation and decreased energy costs).

 

By looking at decibel levels, signal-to-noise ratio (S/N), reverberation and sound absorption, flutter echo and noise from adjacent rooms, you will be able to measure the levels of noise affecting your classroom. We hope that you’ve found this article helpful in making your classroom an excellent learning and working environment.

 

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