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Acoustics 101 for Architecture & Interiors: Part 5

acoustics-101-5

06.20.2018

ACOUSTICS 101 SERIESPart 1 Part 2 / Part 3 / Part 4 / Part 5 / Part 6 / Part 7


A primer series on basic theory and the latest technologies for interior design + building sciences.

In this episode we’ll de-mystify acoustic ratings, re-visit the basics of modern-day acoustics and clarify language and terms from traditional to cutting edge.

A brief overview of today’s Topics:

  • Overview | Acoustics is Fun!
  • Relevance of Acoustics in Architecture and Interiors?  | Acoustic Signature & Acoustic Isolation
  • Acoustic Language & Terminology  |  Universal, Traditional + Quantum
  • Sound & Vibration
  • Acoustic Ratings De-Mystified
  • Summary

Overview | Acoustics is Fun!

With the recent popularity in Acoustics and Neocon upon us, it’s a apropos to re-visit why we care about acoustics.  Acoustics is an amazing thing.  It makes us want to dance at concerts, brings life to the sound of films, inspires us at the Symphony, makes us cry at the Opera, and lets us rock out to loud music yet still hearing every tiny detail with great clarity.  Acoustics is the science and art of sound, which brings us so many inspiring things, including my personal favorites:  music and language.

Before the advent of written word, all of history was passed down with language and spoken word in stories and with folklore.  While it is true that we are a visually dominated species, sound and hearing are a very close second.  In this day and age, advances in technology and the maturation of both fields make anything truly possible.  Acoustics is the brush and architecture is the canvas.   

The Relevance of Acoustics in Architecture + Interiors  |  Acoustic Signature & Acoustic Isolation

Architects + Interior Designers create environments for specific purposes, invoking a sense of being and existence, imbued by design.  Historically, designers have always been conscious of how sound creates a mood and feel in a space.  They knew how an excellent acoustic environment can inspire people to great heights, while a poor one can ruin a business or cause permanent damage to one’s hearing.

Every environment is affected by sound.  Where there is air, there is sound.  A quiet living room or soft tea house is calming and peaceful, while loud school gymnasiums and busy airport terminals are agitating and tense.  Libraries and Yoga studios encourage study and meditation while theatres and concert halls invigorate and inspire with music.  Or not.  This is the purpose of acoustics; to give designers the tools with which bring their designs to fruition with efficacy.

The field of acoustics can be most easily and accurately imagined as two symbiotic parts of a whole:  Acoustic Signature + Acoustic Isolation.  The first is a subjective quality that is difficult to quantify, yet extremely important.  The second is very scientific, easy to quantify and also very important.  Together, they makeup the lion’s share of all things acoustic.

Acoustic Signature

is the unique quality of how something sounds;  how we like or dislike a sound.  “I like her voice”, or “what a beautiful song”, are purely subjective, yet extremely important statements.   The subjective nature of sound can dramatically affect us emotionally and consequently, physiologically.  Everyone knows the joy of hearing a favorite song or the sound of a loved one’s voice.  The uniquely identifiable aspect of a sound is like a fingerprint or signature;  hence the term Acoustic Signature.   Acoustical Design is the craft of creating a specific acoustic signature for a room.  Sometimes referred to as studio design, it can apply to any space where a specific acoustical experience is desired.

In the late 1800’s, a young Harvard physics student spent years making subjective observations and conclusions about sound.  He listened to the sound of various halls on campus in an effort to learn enough to hopefully improve the sound of the infamously poor-sounding Fogg Lecture hall.  This subjective set of observations and conclusions became what we now know as Architectural Acoustics.  The young man’s name was Wallace Clement Sabine;  the founder of modern day acoustics, creator of the term ‘reverberation’ and eponym of the term ‘sabin’, the standard unit of measure for acoustical absorption even today.

Currently, there are no units of measure, standardized test procedures or measurement techniques with which to quantify acoustic signature;  only observation and subjective opinion.  However, new standards and terminology like ‘dimensionality’, ‘micro-dynamics’ and ‘life-like imaging’, may soon bring more definition and scientific clarity to the creative design side of acoustics.

Acoustic Isolation

is being able to keep sounds from moving between spaces or through walls, ceiling and floors.  Historically, it has been the most complex, laborious and expensive part of acoustics because of the challenges of how easily low frequencies transfer through walls.  Traditional acoustics uses mass and density to create isolation barriers that naturally result in heavy, large and expensive structures which are often ineffective especially at low frequencies (ex: trucks, airplanes).  Quantum Acoustic solutions for isolation are in the preliminary stages with estimated performance as high or better than traditional solutions with increased low frequency performance at a fraction of the size with a smaller footprint, saving many square feet while increasing the overall volume of the room.  For example, a 3.75” thick quantum iso-wall can be layered on an existing wall with no air gap vs. an 18” thick traditional, free standing, staggered stud wall plus 2” of air gap is like comparing apples to oranges.  Like many cutting edge technologies, it performs better at a fraction of the size with less materials and labor.  Now that quantum acoustics has solved the age-old challenges of acoustic signature design, one can expect to see new designs, applications and products based in quantum acoustics for acoustic isolation uses.

Acoustic Terminology  |  Universal Terms

Timbre  |  The Character of Sound

Per Merriam Webster, timbre is: “the quality given to a sound by its overtones: such as

a : the resonance by which the ear recognizes and identifies a voiced speech sound

b : the quality of tone distinctive of a particular singing voice or musical instrument”

Timbre in a broader sense is extremely relevant when applied to all sound, not just to the voice or musical instruments.  It includes more than just frequency response or amplitude, it takes into account character;  the identifiable traits of sound that give cues to the human ear which create emotional responses such as happiness, fear, excitement or trepidation.  While some elements of timbre are well defined, many are still to be defined or replaced by new more appropriate language.  Yet when we hear a certain timbre, we can always clearly identify the sounds and the emotions they engender.  This is one of the exciting forefronts of acoustical research:  how to further define Timbre and create new language to describe the character of sound.

Intelligibility  | The Clarity of Spoken Word

The impact of acoustics on comprehension and intelligibility of spoken language is dramatic.  Too much reverberation and intelligibility drops quickly.  Very little reverberation and intelligibility increases quickly.  It’s like trying to have a conversation at a basketball game or at a large party with 300 people in a ballroom where no one can understand the person standing next to them.

Sweet Spot  |  The Best Sounding Spot

Sweet Spot is an acoustic term borrowed from the sports arena.  It is the spot in a racquet, bat or club where contact with the ball is most effective.  In acoustics it means the spot or place in a room where things sound the best.  Traditionally, the sweet spot in any room is roughly one square foot or the size of one’s head.  This means only one person at a time can hear the best sound in the room.  Quantum technology creates wall-to-wall sweet spots the size of an entire room so everyone in the room can hear exactly the same thing, anywhere.

Acoustic Terminology  |  Quantum Acoustics

Quantization

is the new method to control sound energy using quantum physics; the science behind all electronics and cosmology.  The same technology which gave us computers, the internet, electricity and the light bulb are now used to control sound.  Using meta-materials and quantum physics applied to air molecules, Quantum Acoustics® controls Air; the medium on which sound travels.  It removes the source of acoustical issues before they begin, by quantizing any air molecule before it contacts an interior surface.  Air is forced to stop being a medium for the sound energy by forcing air molecules to behave as individual particles and not in groups or waves.  Consequently, sound ceases to exist because there is no medium to travel on.  It’s akin to knocking down the radio tower, causing the carrier waves to stop, which in turns prevents the audible radio signal to be heard.  Simply put, wherever applied, it removes the ability of the material surface beneath it to affect sound, like an acoustical cloak.  Quantum acoustics focuses on the organic qualities of sound and removing acoustical issues before they can exist rather than after-the-fact remediation. 

Acoustic Resolution

Acoustic Resolution is a measure of acoustical performance based on how many Non-Parallel Surfaces exist in each square foot of a surface in reference to the back plane of the device.  This is denoted with the symbol NPS/ft.and was created to accurately quantify and define identifiable qualities of sound.  With the invention of Quantum Acoustics, it became obvious there was a dramatic increase in performance, necessitating a proper, accurate unit of measure able to grow with future technological advances.  Like most of science and nature, resolution is directly related to performance: higher resolution values result in higher performance.  Similarly, there is no upper limit to acoustic resolution, it is only limited by materials science and technological innovation.  In this way It differs from the sabin and NRC which have upper limits.  Higher numbers of NPS/ft2 means better sound:  better imaging, greater clarity, better definition, increased intelligibility and larger sweet spots.

  • Traditional acoustic resolution is roughly 0.01 NPS/ft2
  • Quantum acoustic resolution ranges from 0.56 to over 9200 NPS/ft2

Acoustic Terminology  |  Traditional Acoustics

Using a variety of acoustical architectural design methods and panels, traditional acoustics controls sound energy frequency by frequency (like notes on a piano) and decibel by decibel (amplitude/volume).  Traditional acoustics seeks to control sound energy by identifying and then controlling problematic frequencies and amplitudes, attempting to remediate acoustical issues that exist in nearly every interior space.

Absorption

is perhaps the oldest traditional method of sound control.  Even before the term ‘absorption’ was invented, it was used in acoustic testing at Boston University by Wallace Clement Sabine.  Which is why the original unit of acoustical absorption is known as the Sabin.  Its’ purpose is to control sound by diminishing the amplitude or volume by trapping the sound energy in porous materials, which slow the sound down like a fast car trying to drive through a deep water puddle.  The car slows down as it hits the puddle and the energy transfers into the puddle.  Unpleasant or uneven frequencies are remediated using this traditional technique.  Fiberglass batting such as Owens Corning 703 has been the most popular choice for many decades, with newer materials like mineral wool made from inorganic woven stone fibers under brand names like Thermafiber or Roxul are becoming more widely accepted in recent history. 

Diffusion

is a control method in which sound energy is changed from a high concentration to a low concentration in a certain volume airspace.  Diffusion panels or ‘diffussors’ are typically made of wood, and look like complex block arrangements.  They are typically used in conjunction with  absorption and other traditional acoustic materials for best overall effect.  They are designed to take packets of sound energy and spread or scatter them more evenly.  As with many traditional acoustic panels, they are often more decorative than effective and when they are effective, they are only useful in limited frequencies or portions of the human range of hearing.        

Reverberation or Echo

is prolonged sound in a space which is audible to both the human ear and test equipment.  It is the sound which continues long after the original sound source has stopped;  echo.  A good example of a long reverberation would be the sound inside St. Peter’s Cathedral, or a large cave, subway station or tiled bathroom.  Sound here “echoes” or “reverberates” around and around the space, prolonging the sound from a few seconds to over a minute, creating what is known in audio production as a “wet” sound.  A “dry” sound is one with little echo/reverb usually leading to clarity and definition.  This characteristic of sound is well known in concert halls, opera houses, post production, and recording studios.

Sound

is energy riding on air molecules.  Technically, it’s the rarefaction and compression of air.  Imagine a slinky all stretched out, then all compressed.  Now repeat with variations.  That motion with variation is called sound:  energy moving on and though air.  Sound is analogous to how waves in the ocean are energy riding on water molecules.  Energy can also ride on other liquids or gases, but because we spent most of our time immersed in air and our ears are designed for best performance in air, we’ll just focus on air as the primary medium for now.

Sound rides on air.  Air being the medium upon which sound travels, is like a carrier wave to a radio signal.  As the air comes into contact with all areas of an interior space, the character of the air and every material the air comes into contact with affects the timbre of the sound.  Architecturally, every single material in a room has an audible and often dramatic affect on sound.  Consequently, materials science is an integral part of acoustic design.  How each material sounds and/or affects the air on which sound rides is critical when choosing architectural materials.

Vibration

is energy moving through solid materials like earth, stone, tectonic plates or building materials such as glass, steel, wood and drywall.  Sometimes vibration cannot be heard by the human ear.  A very low frequency vibration can be felt, but often not heard.  Intense vibration often results in the shaking or rattling of walls or ceilings causing audible sound via what is known as ‘diaphragmatic movement’.  In other words, vibration is often accompanied by a resulting sound energy.

In architecture, vibration is most commonly seen as HVAC noise travelling through structural members and becoming audible through diaphragmatic movement of walls.  Another prime example is road noise travelling via the ground plane, vibrating walls and creating airborne noise within interior spaces.  The obvious solution to controlling sound and vibration is to either control the source of the noise first, or control the materials and geometries in the room with a good acoustic design.

Sound & Vibration  |  Energy Moving through liquids and solids

Sound

is energy riding on air molecules.  Technically, it’s the rarefaction and compression of air.  Imagine a slinky all stretched out, then all compressed.  Now repeat with variations.  That motion with variation is called sound:  energy moving on and though air.  Sound is analogous to how waves in the ocean are energy riding on water molecules.  Energy can also ride on other liquids or gases, but because we spent most of our time immersed in air and our ears are designed for best performance in air, we’ll just focus on air as the primary medium for now.

Sound rides on air.  Air being the medium upon which sound travels, is like the carrier wave to a radio broadcast.  The sound energy rides on top of a wave of air.  The character of the air and every material the air comes into contact with affects the timbre and quality of the sound.  Architecturally, every single material in a room has an audible and often dramatic affect on sound.  Which is why material science is such an important part of acoustical design. 

 

Vibration

is energy moving through solid materials like earth, stone, tectonic plates or building materials like glass, steel, wood and drywall.  Sometimes vibration cannot be heard by the human ear.  Very low frequencies actually fall outside the audible range.  However, A very low frequency vibration can be so strong that it results in diaphragmatic movement of building structures, causing loud, cacophonous sound.  In architecture it is most commonly heard as HVAC noise travelling through structural members and becoming audible through diaphragmatic movement of walls.  Another example is road noise travelling via the ground plane, again vibrating walls and creating airborne noise within interior spaces. 

See Hanson's work at DHDI  |  http://deltahdesign.com

As acoustics is the science of sound + vibration, the timbre of the sound or vibration energy is affected dramatically by the behaviour and character of the medium or material on which it travels.  Simply put, the characteristics of the air or materials the energy touches or travels through affects the timbre and quality of the sound/vibration.  Consequently, material science is an elemental part of acoustical design.

Acoustic Ratings  |  Demystified

Decibel (dB)

Formal Definition

A logarithmic ratio unit of measure which is one-tenth of one bel, named after Alexander Graham Bell in the early years of telephony.  It is typically referenced to a voltage reference for use with audio or acoustic meters.  (0 VU)  There are weightings or filters which adapt the db for different uses, such as human hearing.  These weightings are designated by a letter after the symbol.  For example, the weighting which adjusts for the human ear’s fletcher munson curve is called the “A” weighting and is designated by the following symbol:  dB A.  There are 5 in total:  A, B, C, D and Z.  The Z weighting is actually no filter, just pure raw information and is widely used by acousticians and in testing.

Informal Understanding 

dB measures the volume of sound.  The louder the sound, the higher the number.  120 db is very loud, like a rock concert or jet airplane overhead.  10 db is very quiet like the desert at night or an anechoic test chamber.  It’s non-linear nature (logarithmic) means than 20 db is not twice as loud as 10 db.  To add to this confusion not all db’s  are the same:  db A is equalized to mimic human hearing whereas dB Z is un-weighted, raw measurement.  Basically, higher dB equals louder sound and because it’s logarithmic, twice the dB can be much louder than twice the number. 

Acoustic Resolution  |  NPS/ft2

Formal Definition

Acoustic Resolution is a measure of acoustical performance based on the number of Non-Parallel Surfaces in each square foot of a device.  (in relation to it’s backplane)  This is denoted with the symbol NPS/ft.and was created as a unit of measure to accurately quantify acoustic resolution. It was necessitated by a lack of existing measurement and to adapt to a dramatic increase in performance between Quantum Acoustics™ devices and Traditional Acoustic panels.

Acoustical resolution is directly related to acoustical performance: higher resolution values result in higher acoustical performance.  Like physics and nature, there is no upper limit to acoustic resolution, it is only limited by the technology of the time and materials science.  In this way It differs from the sabine and NRC which have upper limits.

Informal Understanding 

Acoustic Resolution is a real world unit of measure for acoustical performance.  More (higher number) NPS/ft2 means better sound:  better imaging, greater clarity, better definition, increased intelligibility and larger sweet spots.   Up is better, down is worse.  Think Heaven and Hell.

  • Traditional acoustic resolution is roughly 0.01 NPS/ft2
  • Quantum acoustic resolution ranges from 0.56 to over 9200 NPS/ft2

 

Absorption Coefficient (Sabine)

Formal Definition

One “Sabin” is one unit of sound absorption, named after Wallace Clement Sabine who founded the field of architectural acoustics while at Harvard University.  1 sabin is set as 100% sound absorption.  This applies to both imperial and metric units of measure.  1 metric sabin is a material with 100% sound absorption in one square meter.  1 imperial sabin is a material with 100% sound absorption in one square foot.  The most often used example of 100% sound absorption is an open window in a wall. 

Informal Understanding

If you want to absorb all sound, get a material rated for 1 sabin, which is the maximum rating.  Minimum is 0 sabins, with ratings being in 0.1 increments;  0.1, 0.2, 0.3 up to 0.9 and 1.0.

  • Max absorption = 1.0 sabin
  • Min absorption = 0.1 sabin
  • No absorption = 0.0 sabin

In this case, higher numbers mean softer while lower numbers mean louder.  (the opposite of db’s) However, this is can be tricky as sometimes it means 100% at x, y or z frequencies;  not all frequencies.  Sometimes manufacturers will claim 100% sound absorption or 1.0 sabins but in the fine print they’ll say it’s only at certain, limited frequencies much less than the full range we can hear.  While it’s legally correct, it’s not in the spirit of the unit of measure and less than helpful to designers.

Sweet Spot

Formal Definition

The spot on a racquet, club or bat where it makes the most effective contact with the ball.

Informal Understanding 

The best sounding spot in a room.  Borrowed from the world of sports, “sweet spot” is a colloquialism referring to the best sounding spot in a recording studio, theatre or performance space acoustically.  “Sweet Spot” is the location in a room where the critical elements of acoustics are in peak alignment:  Imaging, Definition, Clarity, Intelligibility and frequency response.  Traditionally, it is a small area approximately 1 cubic foot or less located at the mix position of a recording studio. In opera houses it is the front few rows of the first balcony.  In a theatre or performing space, it is typically on the orchestra level, on the center line and roughly 1/3 to 1/2 way back from the proscenium arch.  New acousical technology creates entire rooms where the sound is perceptively identical in every seat: from wall to wall. 

OITC  |  Outdoor-Indoor Transmission Class

Formal Definition

OITC is a measure of sound isolation between outdoor and indoor spaces with more emphasis given toward the lower band of audible frequencies.  Like most isolation standards it tests in a limited range of frequency:  80 hz to 4000 hz.   (Note:  human hearing range is from 20 hz to 20,000 hz.)

Informal Understanding 

OITC is like STC or NIC but for exterior to interior purposes.  In short it’s used to measure sound getting into living spaces or offices where the exterior is loud.

NRC | Noise Reduction Coefficient

Formal Definition

NRC or Noise Reduction Coefficient is an average of all the materials and the system holding it and their ability to absorb sound energy.  The American Society for Testing and Materials (ASTM) has standardized testing procedures to measure NRC including frequencies from 100 hz to 5000 hz.  (Note:  human hearing range is from 20 hz to 20,000 hz.)  It is most commonly used for ceiling tiles and other wall or ceiling mounted acoustical absorbers.

Informal Understanding 

NRC is basically a rating for sound absorption, much like the sabin only with a very limited range.  Basic Math says it measures less than 25% of the human hearing range.  NRC = 100hz to 5000hz.  Human Hearing = 20 hz to 20,000 hz.   It’s like being promised a dollar and getting a quarter.  Most truck noise, airplane noise and bass notes in today’s music are below the test range of NRC.  Most birds, sirens, crying children or popular music are above the test range of NRC.

STC | Sound Transmission Class

Formal Definition

STC is a unit of measure of how much airborne sound energy is attenuated across a boundary wall, ceiling, floor, door or partition.  It is mostly used for interior spaces but there are test procedures for exterior uses as well.  While measure in db, a sound transmission class rating is expressed in STC.  Example if a wall isolates 42 dB between the two adjoining spaces, it is said to be rated for 42 STC, not 42 dB.  Standardized testing is performed on frequencies from 125 hz to 4000 hz.  (Note:  human hearing range is from 20 hz to 20,000 hz.)

Informal Understanding

STC tells you how much a wall isolates sound from this side to that side of the wall.  Like NRC it covers less than 25% of the human hearing range.  If a wall is rated for 22 STC it means that 22 db of sound is prevented from getting through that wall.  It’s a widely used, moderately effective unit of measure due to it’s limited frequency response. 

NIC  |  Noise Insulation Class

Formal Definition

NIC is a unit of measure of sound isolation between spaces like STC.  Also like NIC is measure in decibels (db) but represented in NIC.  Example; the wall between the office and the conference room has a rating of 42 NIC.  Unlike STC, NIC uses both on site testing of the space after construction in addition to including more variables in the extensive pre-build calculations.  NIC tests test from 125 hz to 4000 hz.  (Note:  human hearing range is from 20 hz to 20,000 hz.)

Informal Understanding 

NIC is like a field test STC with more stringent requirements.  It is the next generation of testing;  more detail, more testing and more effective.  Like STC it measures less than 25% of the human hearing range.

IIC  |  Impact Insulation Class

Formal Definition

IIC is a unit of measure of isolation of impact sounds between floors and spaces in buildings.  The most common example of an impact sound is footsteps or footfalls.  It measures sound between 100 hz and 3150 hz using a standardized ASTM test and a tapping machine to simulate actual footfalls.  Like STC and NIC, a higher number of IIC means more isolation and therefore higher performance.

Informal Understanding 

IIC measures the sound of footsteps between floors in a building.  As structures get easier to build with minimal structure, the sound of people walking has become a significant sound isolation issue in multi-story buildings.  This is an important aspect and one which everyone has to deal with.

Acoustics in Summary

Acoustics is a fascinating thing that brings us a cornucopia of sound which enriches our lives.  The most obvious ways are via music and spoken language.  It effects us even more than light and photography, as our brains listen to and process sound even in our sleep while meanwhile, our eyes are closed.  

  • Acoustic Signature + Acoustic Isolation have a wide range of impact on our lives both positive and negative.  Thanks to advances in technology, all these can be controlled using conscious, intentional design. 
  • Sound is energy riding on air, vibration is energy riding on solid materials.  Consequently, the characteristics of materials affect sound and vibration dramatically.  Materials Science is an essential element of acoustical design. 
  • Traditional acoustics typically uses thick absorption and diffusors, while Quantum Acoustics® uses thin, lightweight devices made with meta-materials designed with nature based mathematics.
  • Architecture + Interior Design have the ability to create incredibly rich and pleasant soundscapes.  Much of acoustic design is common sense and when it gets complex, there are acousticians available to assist in the design and implementation of both traditional and new acoustical technologies.
  • Like any field of study, there is a great deal of acoustic-centric language, ratings and terms and specific language which all help to clarify, educate, quantify and specify materials for construction and interior design.
  • Acoustics is fun!  Whether it be listening to music, having a great conversation, bird songs or wind in the trees, sound enriches our lives in many ways we usually take for granted.  This is how deeply acoustic effects us subliminally:  that we love it and take it for granted but would grieve it’s loss with great sorrow should we not have the ability to hear the sounds of our world.

In our next episode of Acoustics for Architecture + Interiors, we’ll discuss more relevant topics in the exciting universe of acoustics:  same bat-time, same bat-channel.


ACOUSTICS 101 SERIESPart 1 Part 2 / Part 3 / Part 4 / Part 5 / Part 6 / Part 7