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Home Theater Blog
Pulling back the curtain on all things audio… including home theater, wireless

The Physics of Sound

How To Make A Sound Wave

Whenever an object is plucked, tapped, rubbed, smacked or otherwise touched in any way, that object will vibrate. Newton summed it up in his third law, “For every action, there is an equal and opposite reaction.” This action/reaction creates a vibration where air molecules push and pull each other in a sort of molecular cha-cha. Before you know it, neighboring molecules start joining in on the fun. The result is a rippling effect as the vibration travels through air in the form of an energy wave. The spiral shape of your outer ear just happens to be flawlessly designed to collect and direct that energy wave into your ear canals. The anatomical intricacies of what happens there is a semester in med school. Suffice it to say, your ears, along with the help of your brain translate that wave into sound.

Sound Waves Can Be Fun

Hard to fathom that the music of Beethoven and the theme song to Barney are made up of the same basic physics of sound. Nevertheless, when you boil it all down, every sound wave has three distinguishing qualities:

    1. Speed:At sea level sound waves travel about 700 miles per hour. That’s 1100 feet per second or approximately one foot every millisecond.

    1. Frequency: This is the measurement of how long it takes for each wave crest to pass by. Since the speed of the wave itself doesn’t change, the frequency measurement depends on how close together the wave crests are spaced. This space is known as the “period”. At high frequencies, the wave crests are spaced about 1” apart and pass by 10,000 times a second (10KHz). Bass wavelengths, on the other hand, are about 10’ long and pass by 100 times a second.
    2. Loudness: The more the air molecules are mushed together and pulled apart, plus the closer you are to the source of the sound, the more the arriving wave will move your eardrum and the louder the sound will be. This loudness is normally measured in decibels (dB). These two waves will sound the same except the high-energy wave will be louder.

Phase: When two waves meet in stride they reinforce one another. But when they meet out of stride they can actually cancel each other out. This is an important phenomenon in speaker design because there are many opportunities for this canceling effect to happen.

Diffraction:This happens when the end of a wave rides along a boundary, say the front panel of a speaker box, then encounters the end of that panel. When this occurs it creates a sonic bump.

Dispersion: Sometimes waves beam much like light coming from a flashlight. Other times they spread-out in all directions like when a pebble is tossed in a pond.  Speakers do both. The type of dispersion depends on a single factor: if the wavelength is shorter than the wave-maker it will beam; if it’s longer it will spread out.

What Happens To Your Ear

Every sound has its own signature. After it arrives at your ear it gets fingerprinted in a way. A few milliseconds later a family of other sounds that bear the same signature arrive in the form of reflections. Your ear will associate these reflections with the first arrival that created the signature so that the cacophony of other sounds can be ignored. Then, by calculating the direction of these delayed arrivals, how long they were delayed and the way their signature has been imprinted you are able to tell a lot about what kind of environment you are in.

In order to pull off this auditory miracle your ear has to determine the directions of the original sound and its reflections. This is no simple task. You simultaneously take into consideration at least three different kinds of information to calculate the direction.

  • First, one ear hears the sound as louder simply because your head creates a sound shadow for the ear furthest away.
  • Secondly, the part of your ear that sticks out from your head modifies the sound in ways to tell you if it’s coming from the front, back, above or below.
  • And lastly, your brain calculates the phase of the wave by determining if the wave arrived first at your left ear versus your right ear.

All in all, you unconsciously apply a very sophisticated formula:

And you thought you were bad at math.

Sssh, Did You Hear That?

You can hear a sound at 1 dB or 150 dB. The price you pay to be able pull off this auditory wonder is that you’re fairly insensitive to changes in sound energy levels.  For example, a speaker receiving 100 watts of energy will sound only four times as loud as when its receiving 1 watt. What does this mean to you? It means that you don’t need to worry yourself with amplifier power as much as you might have thought. 70 watts, 100 watts, what’s the difference? Only about 1½ dB, actually.

Another neat loudness related trick your ear performs is it becomes increasingly sensitive to bass when things are loud and to midrange when things are quiet. The loudness button on your receiver is designed to compensate for this by boosting the bass at lower listening levels.

How the Ear Inspires Speaker Design

Understanding the physics of sound is essential to good speaker design. Speaker designers are obsessed with the challenge of solving and answering the following issues and questions in their attempt to recreate nature:

Tonal balance: The treble, bass and midrange parts should be in the right proportion to how they occur in nature.

A Flat Response Graph: A flat graph tells the designer that the speaker is reproducing the sound with the right balance (at least for the position of the measuring microphone). This accuracy is considered the most important aspect in the entire audio system. This doesn’t mean the speaker will sound good per se, only that the ration of frequencies it produces are proportionate.

Time-Delay: A speaker designer will want to know how much time-delay there needs to be for an arriving sound to be identified as a reflection rather than part of the original sound’s signature. The jury is actually still out on this one, but if the delayed arrival is soon enough, say from reflections off the grill frame or off speakers that aren’t mounted flush, it’s heard as part of the signature or, said in layman terms, a distortion in tonal balance. Interestingly enough, your brain mostly ignores reflected signals when assessing tonal balance.

Bass Reflections: Our ears cannot locate bass sounds unless a designer correctly associates a bass note’s overtones and locates them in space. This anomaly allows for speakers that specialize in reproducing low bass waves (subwoofers) to be placed away from the main audible speakers in a speaker system. The result is a kind of fake out. Your ear will be convinced that the speakers they are hearing are also producing the low base waves. All home theater surround sound systems take advantage of this phenomenon.

 

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Tags: Audio Technology, Guru Tips and Tricks, Home Theater

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