Classroom Acoustics & Design for Voice Audibility

• Spaces such as theaters, concert halls, temples, and classrooms are designed for voice audibility, which is essential.
• The main goal is acoustic comfort for proper speech intelligibility, avoiding listener discomfort.
• Low background noise and reverberation levels, along with the elimination of echoes and sound focusing, are required to achieve this.

Classrooms

• The ability to hear and understand in a classroom is critical for learning.
• Poor acoustics can result from insufficient insulation or conditioning, high background noise levels, or unfavorable architectural conditions, all of which have a negative impact on academic performance and attention.
• Classrooms must be protected from intrusive noises.
• Sounds should be amplified and distributed evenly.
• The spectral content of the message must not be altered.
• Inadequate reflections should be avoided.

Historical Evolution of Teaching Classrooms

• Schools emerged in the nineteenth century as purpose-built buildings, initially modified from other architectural forms without regard for educational needs.
• Modern architecture, influenced by the philosophy of "form follows function," began to focus on functional and hygiene aspects, facing challenges due to disorderly urban growth and poor environmental conditions.
• The German rational and organized approach led to consideration of gender separation, student numbers per area, air volume per student, hygiene, and illumination.
• School architecture evolved with urban developments and the political stability of each country.

Elements of Communication

• A communication chain is identified in the transmission of sound messages, consisting of a sender (teacher), a receiver (students), and a transmission channel which encompasses the physical characteristics of the classroom as well as sound propagation pathways.

Sender

• The sender is the sound source that encodes and transmits the message.
• The spoken message varies in intensity and frequency; consonants are essential for intelligibility due to their high-frequency energy, whereas vowels predominate in low frequencies and are more intense and sustained.
• Human voice has directionality, radiating more power in certain directions, influencing message comprehension based on the listener's position.

Transmission Channel

• The transmission channel in a classroom consists of the physical space and sound propagation pathways, sound energy reaches the listener directly and indirectly (reflected).
• The energy of direct sound is determined by the distance from the source, decreasing by 6 dB for each doubling of distance.
• Reflected sound overlaps, resulting in a constant reverberant field, with the critical distance being where the level of the direct and reverberant fields are equal.
• Early reflections arrive before 100 ms and improve intelligibility if not too intense; late reflections can cause echo and impair understanding.

Receptor

• The receiver, made up of listeners, assesses the acoustic quality of the space.
• Perception is determined by the type of message being conveyed.
• Semantics for the oral message and aesthetics for the musical message.
• Intelligibility is essential in the perception of oral messages, necessitating adequate intensity to overcome background noise and a good balance of clarity and loudness.
• It is critical in rooms dedicated to speech that the spectrum of the received sound be as similar as possible to the spectrum of the sound transmitted, emphasizing the significance of direct sound and early reflections.

Geometric Acoustics

• Analyzes sound propagation in an enclosure, the first reflections having more energy than the reverberant tail.
• These reflections are point-specific and depend on the room's geometry, following the law of reflection.
• While not entirely precise, this method is useful in understanding the space's acoustic properties.
• The surface must be large and smooth in comparison to the wavelength for a reflection to be specular.
• Diffraction and diffusion occur if the surface is small or uneven.
• The reverberant tail, which is made up of reflections of order greater than 3, is studied using statistical acoustics due to the high density of reflections at any point in the enclosure.

Forms

• The shape of the enclosure is critical to acoustic behavior.
• Forms in spaces intended for speech must allow for short sound paths, allowing for the message being spoken to be transmitted.

Flat Surfaces

• Non-parallel distributes sound energy more evenly, trapezoidal shapes are used to avoid floating echoes in large rooms.
• The front wall strengthens the speaker's voice, while the back wall using absorbent materials is used to prevent late reflections that can cause echo.
• The inclination of the roof and floor in large rooms aids in directing initial reflections to the most distant regions.

Curved surfaces

• Such surfaces can produce irregularities in the sound field by concentrating energy; examples include spheres, ellipses, and parabolas, which focus reflections on specific points.
• Convex scatters sound, preventing energy focusing issues.

Materials and Acoustic Conditioning Mechanisms

• Acoustic correction adjusts the sound quality of a space to its intended use, improving the quality of listening in places such as cinemas and classrooms, and lowering the sound level in noisy environments such as workshops and dining rooms.
• This is accomplished by selecting acceptable materials and mechanisms based on their absorption coefficient and the volume of the room, and the ideal reverberation time.

Types of materials and mechanisms

• Absorbents reduce reverberation; examples include fiberglass, mineral wool, and resin or polyurethane foams, which absorb sound energy and dissipate it as heat.
• Resonators reduce reverberation at specific frequencies, including membrane and cavity resonators (Helmholtz), which absorb resonant frequencies, optimizing acoustic response.
• Reflectors increase early reflections towards the audience and are smooth, rigid, and non-porous, with a low absorption coefficient, directing the sound energy in a concentrated manner.
• Diffusers distribute sound evenly and are used mainly in concert halls to enhance spatiality and acoustic quality while eliminating anomalies such as echoes and colorations.

Absorbents Materials

• These improve sound absorption and thus the acoustic quality of a room.
• They’re porous and fibrous, absorbing energy by allowing sound waves to penetrate and dissipate as heat due to internal friction.

Resonators

• Absorb specific frequencies, such as membrane and Helmholtz resonators.
• Membrane resonators vibrate when sound waves strike them, whereas cavity resonators use a spring-mass system to create an absorption peak at the resonant frequency.

Reflectors

• Designed to produce beneficial reflections, are smooth and rigid, reflecting sound energy in specific directions, increasing intelligibility and loudness, particularly in rooms intended for speech.

Diffusers

• Like surfaces with ornaments, polycylindrical and Schroeder diffusers reflect sound in multiple directions, improving acoustic quality and eliminating anomalies.
• Function best at certain frequencies, depending on their dimensions and Schroeder's number theory.