1 Live Sound Reinforcement The nature of sound. 2 Live Sound Reinforcement In this presentation we will discuss the nature of sound traveling through

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Live Sound Reinforcement

The nature of sound

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In this presentation we will discuss the nature of sound traveling through air and how it is perceived by the human ear.

We will fist analyze sound wave characteristics and apply this understanding to the audio spectrum and how this relationship pertains to sound equipment.

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Sound is produced by a rapid variation in the average density or pressure of air molecules above and below the current atmospheric pressure.

We perceive sound as these pressure fluctuations that cause our eardrums to vibrate.

When discussing sound, these usually minute changes in atmospheric pressure are referred to as sound pressure and the fluctuations in pressure as sound waves

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When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction, (also known as expansion).

The back and forth oscillation of pressure produces a sound waves.

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Sound waves are mechanical waves that is an oscillation of pressure transmitted through a solid, liquid, or gas (air), composed of frequencies within the range of hearing.

COMPARISON OF SOUND WAVES (LEFT) TO RIPPLES IN A POND (RIGHT)

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The action of a speaker cone creates compression when the cone pushes out and rarefaction when it pulls back.

This back and forth action of a speaker that pushes the air molecules can be represented in the form of a sinusoidal wave (sine wave). C = Compression

R = Rarefaction

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The speed of sound varies depending on the substance it travels through.

Sound travels 15 times faster in steel or iron than it does in air, as an example if two people are about 150’ apart along a railroad track and 1 person taps the rail with a rock the sound will be heard twice, once as the vibrations pass through the railings and again a split second later as they pass through the air.

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The speed of sound in air is 344 meters per second or 1130 feet per second.

Sounds will travel through air at this speed if they’re high frequency sounds or low frequency sounds.

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Breaking the sound barrier traveling at Mach 1 or 768 miles per hour. Which means Mach 2 is 1536 MPH. At these speeds what would MPG be………………?

The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft.

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We can further define sound waves to include amplitude which is the strength or intensity of a sound wave that includes both the height and depth of the wave form, think volume or sound pressure level (SPL)

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The length of a wave form can remain constant as we amplify it, in this instance we’re increasing the volume.

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Another component of a sound wave is it’s frequency.

Frequency is the rate at which the wave form repeats it self measured in cycles per second known as hertz

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In electrical terms frequency is the rate a which electrical current alternates, in our case the sound waves are represent as electrical energy.

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Another factor to consider is the wavelength, the wavelength is the distance a wave of a given frequency will travel through air in one cycle.

A general rule for wavelength is the higher the frequency the shorter the wavelength.

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Here we have the same wave manipulated to provide different results, such as amplification and frequency.

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This brings us to the range at which the human ear is capable of hearing in.

This range is called the audio spectrum or the audio frequency range and it spans frequencies of 20Hz to 20,000Hz (20KHz).

This range spans approximately ten octaves, or doubling of frequency.

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The octave represents a proportion (2:1) and its these between frequencies that the hearing process recognizes rather than the actual number values of individual frequencies.

As an example the middle range of the audio spectrum is 640Hz which is not the middle of the audio spectrum between 20Hz and 20KHz, it is half the number of octaves between 20Hz to 20KHz.

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The audio spectrum can also be evenly divided into decades which is a ratio of 10:1 also called the order of

magnitude.

20Hz to 200Hz 200Hz to 2000Hz 2000Hz to 20,000Hz

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When we bring them altogether we have a chart that includes the whole audio spectrum and it’s divisions.

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The audio spectrum is shown with the lower half and upper half from 20Hz to 640Hz and 640Hz to 20KHz.

The decades as an example can represent a sub-woofer (low), a mid-range and a tweeter (high), or bass, mid and treble like you have on car stereos (bass & treble).

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When we look at the decades they represent notes on a key board the EQs are centered on the octaves and notice between the 5th and 6th octave is the mid range of the audio spectrum.

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When you’re handling the different types of equipment in a sound system you’ll notice the references to the audio spectrum that we just discussed.

Another example is the cross over, separating the highs and the lows.

What range would be considered high for a cross over and what range is considered low?

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The upper half of the spectrum between 640Hz to 20KHz would be considered the highs and the lower half of the spectrum between 20Hz to 640Hz would be considered the lows.

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You were introduced to the term sound pressure level (SPL) earlier in this presentation, the SPL is how your ear drum responds to sound pressure.

Using the “A” weighted scale we can chart these sound pressure levels as a numerical value using the unit of measure dBA.

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Here is an SPL chart that provides a comparison of common sounds and they’re sound pressure value.

The threshold of pain starts at 120 SPL, (rock concerts are close to this level).

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Here are some more comparison of typical everyday noises and they’re SPL value.

In fire alarm one of the requirements for smoke detectors is that they sound

at 75dBA at pillow height, NFPA 72 article 7.4.4.1

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NFPA 72 also provides an ambient sound level chart so that installers can insure that the notification device will sound 15dBA above ambient noise levels, annex A table A.7.4.2.

There is a lot of emphasis on live sound reinforcement as it applies to fire alarm systems.

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NFPA 72 also provides a permissible noise exposure chart that has been established by OSHA not provided in your live sound book.

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It should be noted hear that long exposures to high levels of sound pressure will result in degenerative hearing loss over a period of time.

This hearing loss in other words doesn’t repair itself but instead worsens over time, we may be faced with an epidemic of young people becoming hard of hearing at much earlier ages than previous generations.

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What could make a large segment of the younger population susceptible to early age hearing loss?

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1 Live Sound Reinforcement The nature of sound. 2 Live Sound Reinforcement In this presentation we will discuss the nature of sound traveling through