Sound Waves, Transducers and Analog Audio - Part II

In the previous post we discussed sound waves, transducers, alternating current and direct current. Then we looked at how a sound pressure wave is converted to an analog audio signal with a microphone. Here's a recap:

• A sound pressure wave is a vibration of air molecules
• A transducer converts one form of energy to another
• A microphone is a transducer that converts a sound pressure wave into an analog audio signal
• Analog audio refers to AC (alternating current) that is alternating (moving back and forth) at frequencies and amplitudes proportional to the sound wave in front of the mic

Got that? Let's move on

If you have a dynamic mic like a Shure SM58 put a finger across the three XLR pins on the back of it. Talk into the front. You won't feel a thing but trust me, there's a voltage there.

• A microphone outputs a very weak signal: sometimes as little as 2 millivolts (that's thousandths of a volt). This signal is not surprisingly called mic level
• The pro audio equipment that you're going to plug your mic into is designed to operate at an average of 1.23 volts. This average level for pro audio gear is referred to as line level

Just to give you some common reference points for voltage:

An AA battery cell has a potential of 1.5 volts. This can't physically hurt you.

The AC voltage from a household wall socket in the US averages about 117 volts. This can potentially hurt you so don't muck around with it.

Back to our analog audio signals:

• We know our microphone has a mic level output of about .002 volts 
• We need to boost the mic output up to around 1.23 volts line level before we can send it off for further processing or amplification

Let's do the math:

1.23/.002 = 615

To bring a microphone's output up to line level we're going to have to amplify it around 615 times. That's quite a lot! The circuit that's going to do this amplification for us is called a preamplifier.

If you've spent any time around audio engineers the subject of preamps or "pres" has probably come up. There are many different preamp designs by many different manufacturers. Heated debates often ensue regarding which preamp pairs best with which mic and in which application. I'm not going to get into those debates here but I will say that these circuits are very purposefully designed to sound different.

• Some preamps provide what's referred to as a "transparent" sound in that they don't change the overall tonal characteristics of a microphone
• Other preamps are popular among engineers for the exact opposite reason; they add their own tonal "color" to the signal

After the preamp boosts the mic signal to line level it's often sent off for further processing. This could involve adjustments to tonal qualities (equalization) or output levels. Note that at this stage our analog audio signals are still at line level.

After signal processing the line level output is sent to a power amplifier where it's boosted further. During this stage the signals are given enough power to physically push loudspeaker or headphone drivers back and forth.

Most of you are going to find this power amplification stage built into your active studio monitors. 

• An "active" device is capable of amplification
• A "passive" device is not (a passive speaker enclosure would require an external amplifier)

Like microphones, the drivers (speakers) in your monitors are also transducers. They convert analog audio signals to sound pressure waves.

• When the alternating current from the amplifier moves in one direction it pushes the driver out. When the current moves in the opposite direction it pulls the driver in
• The mechanical movements of the driver cause the air molecules around it to vibrate and create sound pressure waves in the air

To review let's take a look at a block diagram of the sound reproduction system we have described in these two articles:

• A microphone converts or transduces sound pressure waves to an analog audio (alternating current) signal of corresponding frequency and amplitude
• the weak output signal of a microphone is referred to as mic level
• A preamplifier boosts the mic output to a stronger level that pro audio gear requires to operate. 
• this is called line level 
• At this point signal processing such as EQ or adjustments to output levels can be applied
• After processing a power amplifier then boosts the line level signal to an amount that is capable of physically pushing a driver (speaker) back and forth 
• The mechanical motion of the driver transduces the electrical energy of the analog audio signal back to a sound pressure wave in the air

Got it?

If not let me know and we'll try to clear it up!

Next time we're going to talk a little bit about what those waveform graphics you keep seeing in all those textbooks really mean.

Karl Wenninger is an audio engineer, synthesist/sound designer, composer, guitarist and DIY audio electronics enthusiast. As an adjunct professor he has taught Pro Tools at The New School for Jazz and Contemporary Music, Computer Music at York College and Audio Post-Production for the Media Arts Program at NJCU. He was an program administrator and associate professor at the former Digital Media Arts program at Touro College in New York City for over a decade.