What Is Sound: A Guide to Acoustic Design

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What’s that annoying buzz or siren piercing through your headphones?

What about the pitter-patter on your window from the rainfall?

That’s the power of sound – the vibration and propagation of waves that brings auditory experiences to our ears.

Sound shapes our everyday moments through frequencies, amplitudes, and other characteristics.

Within the realm of architectural acoustics, managing sound takes on heightened importance; controlling noise and echoes makes indoor spaces more comfortable for human activity and speech.

This article will explore what defines sound, how it behaves, problematic reverberations, and strategies like sound absorption and diffusion to improve indoor acoustics.

Let’s dive deeper into the fundamentals underpinning sound, sound measurement, and practical acoustic design considerations that make for quality sonic environments tailored to human needs.

What is Sound and How is it Measured?

An infographic poster with diagrams showing sound as waves and how frequency, wavelength, amplitude, decibels (dB), and human hearing range are measured.

Sound is the vibration and propagation of waves through a medium like air, water or solids. Sound is characterized by properties such as frequency, amplitude, wavelength and decibels.

The frequency of a sound describes how many cycles or oscillations of the pressure waves pass a fixed point per second. Frequency is measured in Hertz (Hz).

What Is The Human Hearing Range?

The human ear can detect frequencies approximately in the range of 20 Hz to 20,000 Hz. Lower frequencies are perceived as lower pitched sounds, while higher frequencies sound higher pitched.

The wavelength of a sound wave is the distance between corresponding points on adjacent sound wave cycles.

What Is Wavelength and Pitch?

Wavelength correlates with pitch – shorter wavelengths are higher pitched sounds with tightly packed waves. Lower pitched sounds have longer wavelengths between wave cycles.

Amplitude refers to the maximum height or intensity of the sound wave. Larger amplitude means higher pressure variations and louder volume.

Amplitude is related to the sound energy transmitted through the medium.

What Is The Speed of Sound?

The speed of sound depends on the transmission medium’s density and elasticity. In dry air at sea level, the speed of sound is approximately 343 meters per second or 767 miles per hour.

In water, sound travels much quicker at over 4900 feet per second.

How Loud Is Each Decibel?

Decibels provide a logarithmic scale to measure sound pressure levels relative to a reference value.

Zero dB approximately corresponds to the lowest threshold of average human hearing sensitivity, tested at 1 kHz.

120-140 dB marks the threshold of pain, beyond which sound pressure levels can cause actual pain and injury to hearing. Examples of different decibel levels:

  • 0 dB: quietest audible sound
  • 30 dB: soft whisper
  • 40 dB: quiet library
  • 60 dB: normal conversation
  • 80 dB: busy traffic, alarm clock
  • 85 dB: lawnmower, blender, hairdryer
  • 90 dB: truck traffic, leaf blower
  • 100 dB: chainsaw, motorcycle, orchestra
  • 110 dB: rock concert speakers, hammer drill
  • 120 dB: jet plane at takeoff, sirens
  • 130 dB: amplified stadium concert, pneumatic drill

How Do Sound Waves Behave Differently Indoors vs Outdoors?

A 3D rendered room with green and blue particle sound waves reflecting off the walls, floor, and ceiling, illustrating the complex sound reflections and reverberation that occur indoors. The dark background makes the colorful sound wave particles stand out prominently.

Sound waves behave very differently indoors compared to outdoors.

When sound waves are produced outdoors, they are free to travel unimpeded in all directions outward from the source. There are minimal obstructions or reflecting surfaces.

Indoors, sound waves interact with the various obstacles within a room, including walls, ceilings, floors, windows, doors, furnishings, equipment and people.

Hard, reflective surfaces cause sound waves to bounce off rather than pass through. This is called sound reflection.

What Is Reverberation?

Sound reflection indoors results in the sound reverberating within the enclosed space.

The sound waves continue bouncing around the room off all the hard surfaces.This increases noise buildup and exaggerates resonances.

What Is Sound Diffusion?

Another phenomenon is sound diffusion. This occurs when sound scatters in many different directions after striking a rough or irregular surface. Diffusing surfaces help break up focused sound reflections.

What Is Sound Refraction?

Sound can also be refracted or bent as it passes from one medium into another with a different density.

For example, sound waves bend slightly when moving from air into a glass windowpane or vice versa. The amount of bending depends on the relative densities of the two materials.

What Is Sound Absorption?

Unlike hard surfaces that reflect sound, soft and porous materials like insulation or fabric readily absorb sound.

The acoustic energy is converted into minute vibrations within the material rather than being reflected. Good sound absorbency is vital for controlling noise and reverberation indoors.

Due to all these interactions, the behavior of sound waves indoors can be extremely complex.

However, two common problematic effects of sound reflecting indoors are flutter echo and room modes or standing waves.

What Is Flutter Echo?

Flutter echo occurs between two large parallel hard surfaces facing each other, like two blank walls. The sound bounces rapidly back and forth between the surfaces, creating an annoying fast repetition.

Slight delays between the direct and reflected sound can also cause resonance issues.

What Is Room Modes?

Room modes or standing waves result from the interference of direct and reflected sound waves indoors.

The reflections from walls or other room boundaries interfere with each other constructively and destructively.

This produces areas of focused high and low sound pressure. These resonant room modes can exacerbate noises.

How To Design Acoustics?

Careful acoustic design is required to minimize problematic sound behaviors indoors caused by excessive reverberation and resonance issues.

Materials selection and room geometry adjustments can help absorb, diffuse or otherwise mitigate sound reflections and improve indoor acoustics.

What Causes Problematic Reverberation and Echoes Indoors?

An illustration of a large unfurnished room with hard surfaces depicting excessive reverberation and echo issues through visual sound waves endlessly reflecting off the walls and ceiling, demonstrating the need for sound absorption.

Reverberation refers to the persistence or decay of sound in an enclosed space after the original sound source has stopped.

As sound waves reflect off the various surfaces, the reverberation time is extended. This is distinct from echoes, which are discrete, delayed repetitions of a sound.

Having Bad Reverberation Times

The optimal reverberation time depends on the room usage. For speech intelligibility, a shorter RT60 or reverberation time of 0.4 to 0.6 seconds is recommended.

For music performances, longer RT60 times around 1.0 to 2.0 seconds provide better acoustics. Excessive reverberation reduces speech clarity and causes noise buildup.

Hard Surfaces Cause Problematic Reverberation

Hard, reflective room surfaces are the main cause of problematic reverberation. The more sound reflection, the longer the reverberation lasts.

Insufficient sound absorption leads to poor speech intelligibility and an unpleasant listening experience.

Flutter echoes can also be very distracting and annoying. These occur between two large parallel reflective surfaces when sound bounces rapidly back and forth repeatedly.

The repetition rate or delay depends on the distance between the surfaces and the temperature. Flutter echoes are characterized by a distinct “fluttering” sound and can persist for multiple seconds.

Another problematic indoor echo is the slap echo. This is a single distinct reflection of a sound wave arriving at the listener with a short delay after the direct sound.

The discrete delayed echo can be distracting and make speech difficult to comprehend. Slap echoes are common in bare unfurnished rooms with hard surfaces.

What Are The Solutions for Reverberation and Echo Issues?

Several solutions can help address excessive reverberation and problematic echoes indoors:

  • Add sound-absorbing materials on walls, ceilings and floors to reduce reverberation time. Porous or fibrous materials work best.
  • Install acoustic wall panels and bass traps to absorb sound and limit flutter echoes. Face parallel walls with absorption.
  • Add furnishings, curtains and other soft materials to scatter and absorb sound.
  • Break up problematic room geometry and parallel surfaces. Angle walls or add irregularities.
  • Diffuse focused sound reflections using uneven or angled architectural features.
  • Adjust room proportions based on intended use and optimal reverberation time.

What are the Key Strategies for Controlling Noise?

Wide angle photo of an open concept living space with acoustic design elements like absorptive panels on the walls and ceiling, diffusing panels behind the couch, acoustic transparent curtains, and potted plants to help control noise and improve room acoustics.

There are four main strategies used in acoustic design to control noise and improve room acoustics:

  1. Sound Insulation
  2. Sound Absorption
  3. Sound Diffusion

1. Sound Insulation

Sound insulation involves using dense materials to prevent or attenuate the transmission of noise from one room or area to another.

Mass barriers placed between the sound source and the receiver reduce the passage of sound energy.

Examples include insulation in cavity walls, double glazing on windows, solid core doors, and baffles around equipment.

For blocking low frequencies, adequate mass and airtight construction are key, as low sounds can infiltrate tiny gaps. Appropriate sound blocking between rooms, floors, and outside prevents distractions.

2. Sound Absorption / Acoustic Treatment

The goal with sound absorption is to convert acoustic energy into thermal energy or heat, rather than reflecting it.

This is achieved using porous, fibrous materials or sound-absorbing panels to capture sound waves.

Good sound absorbency reduces reverberation time and noise buildup. Absorption is most effective at mid to high frequencies.

Materials like acoustic insulation, carpets, and ceiling tiles are great sound absorbers. Fabrics and other soft furnishings also help dampen noise levels through absorption.

Special acoustic wall panels with foam or perforated surfaces work well to control flutter echoes and resonance issues by absorbing focused sound reflections.

3. Sound Diffusion

Sound diffusers work by scattering focused sound reflections in many different directions. This prevents distinct echoes and flutter reverberation caused by sound bouncing repeatedly between two parallel surfaces.

Diffusers feature uneven surface profiles, edges, and geometry that break up and redirect concentrated sound energy.

This surface irregularity diffuses the sound reflections to cover a wider area. Some common diffusers are quadratic or polycylindrical shaped panels. Diffusion helps improve speech intelligibility.

Choosing optimal sound control strategies depends on the types of noise sources, and whether the goal is absorbing overall sound, blocking between rooms, covering pathways, or diffusing focused reflections.

Most acoustic designs employ a combination of absorption, blocking, diffusion and masking cover to address the various noise issues in a space.

Careful acoustic engineering creates comfortable sonic environments tailored to human activities and needs.


This chapter covers the fundamentals of sound and acoustic design in indoor spaces. It begins by explaining what sound is—the vibration and propagation of mechanical waves characterized by properties like frequency, wavelength, and amplitude.

Sound pressure level in decibels and the human audible frequency range are discussed. The chapter then contrasts how sound waves behave differently indoors versus outdoors.

Outdoors sound travels freely, while indoors it reflects off surfaces causing reverberation and resonance issues. Key indoor sound behaviors like flutter echo and room modes are described.

Next, the causes of problematic reverberation and echoes indoors are examined. Hard, reflective surfaces and inadequate absorption lead to excessive reverberation that reduces speech clarity.

Flutter and slap echoes are also explained. Solutions involve adding absorbent materials, diffusers, and breaking up room geometry.

The four main strategies of acoustic design—absorb, block, cover, diffuse—are then detailed. Absorption converts acoustic energy to heat using porous materials.

Blocking prevents noise transmission between spaces with barriers. Covering contains sounds with overhead structures.

Diffusing scatters sound reflections. In summary, this chapter provides a comprehensive overview of the fundamentals of sound and acoustic design considerations for improving indoor sound quality through strategies like absorption, diffusion, and noise blocking.


Dominic is the chief editor of the Burton Acoustix blog which writes about acoustics and soundproofing to help readers with their queries and questions they might have with regard to improving any sound or noise issues that they faced in their life.