Building, Design for Sound and Acoustics, Part 1: Understanding the Basics of Sound

By John J. Lupo
Division Manager, North Florida, Dynamark Systems

sound-frequency-0516This is part one of a multi-part series of articles that will provide a solid understanding of the principles of sound and acoustics as it relates to design and construction of both public and private spaces. Parts 2-5 will cover decibels, STC ratings, soundproofing and room acoustics.

A basic understanding of sound is important for the proper planning and design of any room or public space. If you have experienced not being able to clearly understand a speaker at an event or gathering or maybe attending a classical concert only to be distracted by poor sound quality you understand what I will be referring to. Sound and acoustics can be a very complex subject to dig into. My intent is not to make you an expert but to give you knowledge that will be useful for the purpose of designing your theater or listening room.

Quick Point # 1

Air is considered an elastic medium. This means that air molecules will expand and contract.

Sound energy radiates from the source and travels through air in waves. A good example would be to throw a rock into a pond and watch the ripples of water expand from the contact point.

Sound Waves travel by causing a chain reaction of the air molecules. The increased air pressure created from the source expands the surrounding air molecules. The air molecules collide with neighboring air molecules transferring the energy much like the old 5-ball pendulum desk ornament.

Visualize a forest. As the wind blows across the tree tops the branches move in the direction of the wind returning to their original position as the wind passes, never moving from their rooted location.

All of this happens with incredible speed and is completely invisible to the human eye. This reaction is repeated until the energy of the sound wave fades. As the waves travel through air they will gradually loose energy / intensity.

An example would be;

2x the distance = 1/4 the intensity

4x the distance = 1/16 the intensity

Sound Waves travel through air at sea level @ 1130 feet per second. Sound will travel faster in water or metal, than through air. The denser a medium the faster it will travel through it.

So that will give you an idea of how sound travels, but how do we hear it, you may ask?

On the receiving side our ears are design like to collect sound waves. The waves are transformed by the inner ear to electrical signals, and then passed to the brain for processing. Our brains do all the tough work transforming the sound waves into the wonderful sounds we hear.

Quick Point # 2

There is no sound in outer space. Sorry to burst the bubble of Star Trek and Star Wars fans. All those explosions and battles would occur in complete silence. No medium = no noise

Sound Frequencies Explained

If you plan on working with sound in any capacity an understanding of frequencies is crucial to the audio quality within the defined space.

All sound produces waves at a measurable frequency. Sound waves are measured in frequencies (number of complete cycles per second).

The length of a sound wave varies depending on its frequency, ranging from 56′ for a 20 Hz tone to as little as 1-1/16″ (.0565″) for a 20,000 Hz tone.

Sound frequencies are expressed as Hertz (Hz), a measurement of many cycles per second the sound wave completes. Humans have the ability to hear frequencies from 16 Hz (cycles per second) to approximately 20,000 Hz. The range is commonly referred to as “20-20K”.

Sound Frequencies Explained

An understanding of frequencies is relevant to the audio quality of the room. Sound waves are measured in frequencies (number of complete cycles per second). The length of a sound wave varies depending on its frequency, ranging from 56′ for a 20 Hz tone to as little as

1-1/16″ (.0565″) for a 20,000 Hz tone.

We express frequencies as Hertz (Hz), a measurement of many cycles per second the sound wave completes. We have the ability to hear frequencies from 16 Hz (cycles per second) to approximately 20,000 Hz. It is commonly referred to it as “20-20K Hz “.

The Frequency Scale

Every sound produces a wave at a measurable frequency. Let’s use music as a point of reference.

Each note of the musical scale produces a sound wave at a precise measurable frequency. This is referred to as the fundamental frequency. The fundamental frequencies are divided into three main groups.

1) Bass frequencies 20 Hz-250 Hz

2) Mid Range frequencies 250 Hz-2,000 Hz

3) Treble frequencies 2,000 Hz-20,000 Hz

The upper mid range and treble frequencies are shorter in length and contain less sound energy. They will lose energy quickly making them easier to deal with in regards to room acoustics and soundproofing. The low bass frequencies (20-125K) are much longer in length and contain more sound energy. Low frequencies carry this energy for longer distances making these frequencies more relevant to room acoustics and soundproofing. Lower frequencies play a dominant role in both room acoustics and sound quality by creating standing waves (room modes). These waves distort or color the sound in the room producing a negative effect on audio quality. The relevant range of standing wave frequencies in room acoustics will range from 20 Hz through 300 Hz.In Part 2,Understanding Acoustics,I speak more in-depth regarding how these frequencies relate to room acoustics.

Quick Point # 3

1) Studies have shown that most of the usable sound information for human’s lies between 300 Hz and 4,000 Hz.

2) Our hearing is less sensitive to low frequencies. We require increased volume levels to hear these frequencies. As the frequency of the sound rises our sensitivity increases.

3) With the use of our stereo hearing (2 ears) we have the ability to accurately locate the source of the sound. By calculating the delay between the times the sound reaches each ear our brains compute the location of the source. We process localization better on the horizontal than the vertical plane.

4) We can filter frequencies and focus on what we desire to hear. We can concentrate on the violins over the rest of the orchestra if we choose or recognize our baby’s cry from other babies in the room.

Harmonics and Frequencies

The Harmonics of the fundamental frequency give each instrument or sound its own particular tonal quality. It’s the harmonics that makes a Saxophone sound like a Sax or a flute sound like a flute even when they are producing the same frequency tone. In this writing I will touch on harmonics but focus my attention mainly on the fundamental frequencies. A tone produces a fundamental frequency and multiples of that frequency referred to as harmonics.

Quick Point # 4

The sounds we hear are a combination of fundamental frequencies and their harmonics.

To use light as an example, when we walk outside we see sunlight as white or colorless sunlight. When viewed through a prism we see all the colors that combine to make the light we see. Sound works in much the same way. We don’t hear the individual frequencies but we hear the total sum of all the frequencies and harmonics blending together to create our own personal symphonies.

Amazing stuff, this ear and hearing thing.

I hope this gives you some useful information. Feel free to contact me with any questions or comments.

John J. Lupo
Division Manager, North Florida
Dynamark Systems

This article was reprinted with permission from John J. Lupo and originally appeared here.