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The Colors Of Sound

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[Publisher’s note: Dagogo is excited to present Steve Turnidge’s following new article to its readership.

Thank you, Steve!

This article is presented by Dagogo Senior Reviewer David Blumenstein.]

About the author:

Steve Turnidge is a noted mastering engineer at UltraViolet Studios with hundreds of albums and thousands of music tracks to his credit. With over 35 years experience in the Pro Audio electronics industry, he specializes in mixed digital and analog printed circuit board design.

Steve is also a published author having written both Desktop Mastering and Beyond Mastering for Hal Leonard Books, and taught Audio Recording at Shoreline Community College. He currently teaches mastering at local studios. He was a past chair of the Audio Engineering Society (AES) Pacific Northwest Section and Governor of the Recording Academy.

Website: http://www.arsdivina.com

 

 

Introduction

As a mastering engineer for over twenty years, I have had the opportunity to dig deep into the actual simplicity that is sound, and would love to share some of my findings with the Dagogo readers here.

I have been hearing that people are investigating using studio monitors for their main listening systems. This may be an excellent move for a listener that wants to learn more about the processes that go into the crafting of fine audio pieces, but it may not be the best for those who want to get the very best experience from their love of listening to music. Getting a bit deeper into what sound actually is physically and physiologically can help make decisions as to how to arrive at a system that provides a sustainable and enjoyable listening experience.

Much of this information may seem simplistic, but simple truths are at the base of all understanding.

 

The Ocean of Air

Let us back up and start at the beginning. First, we are at the bottom of an “ocean of air.” The weight of the atmosphere exerts a rather large pressure on us – roughly 14.7 pounds per square inch at sea level. Impressed upon this barometric pressure is sound pressure; the relatively rapid and usually slight variation of the air pressure.

This impression upon the atmosphere is created by our loudspeakers (as well as any sound producing item) – the speaker drivers cycle between pushing out (compressing) and pulling in (rarefying) the atmosphere. The speed with which this mechanical action occurs is called frequency – how often per second the speaker transits from its rest position out into the room to its farthest excursion (amplitude), back through the neutral state and pulling in, creating a lower pressure, then returning to the neutral position, which is the speaker at rest.

If this happens 20 times a second, we have 20 cycles per second or Hertz (Hz). If it happens 20,000 times a second, we have 20 kilohertz (kHz).

There is an interesting thing about digital audio workstations and waveform displays in general. You are actually seeing a representation of the movement of air and the driver, seeing the speaker driver from the side, going up and down over time.

Figure 1 – Waveform

In this image, the blue line down the middle of each channel represents -infinity dBFS – in other words, as quiet as you can get. In even other words, the blue line represents the barometric air pressure in the room.

When the green line rises above the blue line, it is a physical representation of the speaker driver moving out into the room, compressing the air. Then, passing and falling below the line represents the driver moving into the speaker, rarefying the air, and returning back to the rest position.

In this case, we have a stereo signal; two channels of audio – one for left and one for right. Since there is just a single trace on the waveform, and your speakers may have several drivers, the high frequencies are riding on the low frequencies – and the low frequencies are typically much higher amplitudes.

As this complex waveform is presented to the loudspeaker, it reaches crossover circuitry where the highs and lows (and often mid-ranges or more) are filtered and sent to their respective driver. These drivers come in distinct sizes – larger drivers for lower frequencies, and smaller drivers for high frequencies.

Figure 2 – Waveform Bass

Note in this figure that the low frequencies have very large excursions. This is not an arbitrary distinction, but physiologically determined. We, by design, have very little sensitivity to low level low frequencies. Consider the equal loudness Fletcher Munson curves:

Figure 3 – Fletcher Munson Curves

The Y axis is increasing sound pressure level (SPL), and the X axis is increasing frequencies. Note the dashed line – below this SPL we have no audible sensation. Each solid line of the graph is called a Phon, and these are set by the subjective level of each decade of SPL, finding the level at each frequency that appears to be the same level as at 1K for that decade. Note that the graph dips down in the higher frequencies, around 3.5KHz, where our hearing is naturally very sensitive. This is evolutionary – these are the frequencies of predators in the bushes and crying babies… and sibilance and feedback.

Heading left on the graph, we see we have very low sensitivity to low level low frequency sounds. It is a good thing that we don’t hear these well, because your heart beat and blood flow would drive you insane!

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