The Science and Engineering of Sound

The Science and Engineering of Sound


Hi, John Hess from FilmmakerIQ.com and today
we’re going to dive into the science and engineering of sound and microphones. As vision is our brain’s response to electromagnetic
radiation in the form of light waves- sound is our brain’s response to atmospheric pressure
changes. Well, we’re not exactly a weather barometer – but the concept is the same. Let’s imagine a tightly wound string – that
of a guitar. When the string is plucked, the string vibrates, pushing the air molecules
around it. The push is called compression and it creates a peak of high pressure. Now
as the string retracts, we have rarefaction – a trough of low pressure. This repetition of compression and rarefaction
creates what’s called a longitudinal wave or a compression wave – that’s a wave where
the molecules are moving in the same direction as the wave itself. It’s worth noting that
the molecules pretty much stay in one place, it’s the energy that is moving through the
air at the speed of sound. If we plot the changes in air pressure created
by the sound wave against time – we get a waveform. Through waveforms we can visualize
some key aspects about sound. The first is amplitude. Amplitude is the strength
of the wave itself – The greater the distance from the centerline, the more intense the
wave. A common method of describing amplitude is something called RMS – root-mean-square
which is a mathematically handy way of determining the average amplitude of a wave. Then there’s Frequency – the rate of how
many cycles of peaks and troughs are in a given period of time. This is most commonly
measured in Hertz – or cycles per second. one hertz is one cycle per second – as we
go up in the number of hertz we perceive a rising in pitch. 440 hertz is the standard
that musicians use to tune their instruments which corresponds to the A on second space
treble clef. Since a sound wave cycle can begin at any
point in the waveform, we have to consider phase – when we record sound, at what point
do we pick up the sound wave? This isn’t an issue when dealing with a single wave,
but when you combine waves, when using stereo microphones and speakers, waves that are in
opposite phase will add together cancel each other out which not what we want. As we move into more complex waveforms we
start dealing with harmonic content and this is what makes music and voice so wonderful.
Let’s go back to our string example – let’s say this string vibrates at 440 hz – an A.
But it’s also vibrating at higher frequencies – The most basic are whole number multiples
of the fundamental frequency, such as 2 which gives us 880 hz this is a whole octave A above
the fundamental frequency, Its also vibrating at the third multiple for 1320 hz which is
an octave and fifth and E and so on up the multiples of the fundamental frequencies – some
of these resonate louder than others and there are some other oddball frequencies thrown
in there too and all this is happening every time that string gets plucked. These extra
frequencies are called harmonics. Though these harmonics aren’t as loud as the fundamental
frequency, they shape and color the sound making it possible to distinguish between
two different instruments playing the same pitch say a french horn and a trumpet. The final aspect of the waveform we need to
examine is the envelope. The envelope describes the shape of a sound over time which starts
with the attack – the time it takes for a sound to build up to full volume. The decay
– how quickly a sound levels off to a sustain after the intial attack, the Sustain – the
ongoing sound and finally the release, how quickly the sound decays after the note is
released. Now that we have a basic grasp of what sound
is and how to describe it in terms frequency, phase, harmonic content and envelope, lets
go back and look at amplitude or loudness – how we measure the strength of sound. The
human ear has quite an amazing ability to detect sound waves – capable of an energy
range of approximated 10^13 to 1. With such an enormous range, we have to use a logarithmic
scale to define loudness when working with audio. Fortunately our ears respond to sound
in a logarithmic fashion. Basically a logarithm of a number is the exponent
to which another fixed value, the base, must be raised to produce that number. So say we want the log of 10 using base 10.
10 to the first power is 10 – so the log is 1. The log of 100 is two because 10 to the
second power is one hundred. The log of 1000 is three – and so on. Logs make working with
exponential scales easier which is why they’re so useful in this discussion but don’t get
too hung up on the math. The unit used to describe sound loudness is
the decibel. Originally implemented by Bell Laboratories to describe signal loss in telephone
lines, the decibel – which was formally introduced in 1928, describes the relationship between
two different signals as 20 Log (signal strength using root mean square divided by the reference
signal strength). Now notice, the decibel isn’t a measurable
unit like a centimeter, it’s a comparison and you always need a reference. For determining
loudness of sound pressure, we use the quietest sound that can be detected by average human
hearing – this number has been internationally agreed to as 20 x 10-6 pascals or 20 micropascals. Now we can compare the sound pressure of noisy
things to this bare minimum and we can derive a decibel vaues from a soft whisper around
30 dB SPL to freight train at 100 ft from 70dB SPL up to a Jet Takeoff from 200 feet
registering at 120 dB SPL. Notice the distance is always notated as sound waves dissipate
according the inverse square law. Safety regulations have risen around these
decibel ratings – 85 dB and below you will be fine for 8 hours. Cut that time in half
each 3 dB you go up. But sound pressure level is only one kind
of decibel. For filmmakers, we are more likely to encounter decibels in regards to power
of the audio signal when carried through and audio system. Here the equation is slightly
different. A dBm=10 log P/Pref – where P is the measured wattage and the reference
is 1 milliwatt. Its really easy to get lost in the science
and engineering mathematics but the dBm is the most common expression used in audio equipment
and audio workstations. There’s a few concepts that are relatively easy to grasp: Turning up something by 3dB will double the
signal strength – but because our ears are so logarithmically sensitive, doubling the
strength doesn’t mean double the loudness. Consequentially Turning down 3dB halves the
signal strength which is also not as significant as you might think. And because we are using log, turning something
up by 10 dB means we are actually increasing the signal 10 times. 20 dB means we are increasing
the signal 100 times and 30 dB means the signal is boosted 1000 times. Does the math matter to the working professional?
Probably not. As you work in audio you will start to intuitively know what a boost of
3 db gain and 6 db gain sounds like. Just know that there’s a tremendous amount science
and engineering behind all
of this. Okay, so how do capture sound waves and turn
them into electrical current which we can record? There are a couple different ways of capturing
sound and we’ll cover three of the most common ways for recording.. The simplest is
the dynamic microphone. In a dynamic microphone a thin diaphragm is connected to a coil of
wire called a voice coil which is precisely suspended over a powerful magnet. As the sound
waves strike the diaphragm it cause it to vibrate, moving the voice coil through the
magnetic field generated by the magnet which generates a small bit of electricity which
is sent down the output leads. This is the audio signal – an electronic representation
of the actual sound wave. The advantages to simple and robust design
of the dynamic microphone is they can handle loud sources without much distortion. Unfortunately
this makes them weak when trying to capture soft distant sources because the diaphragm
needs lot of sound energy to move. A variation on the dynamic microphone is the
ribbon microphone. Instead of using a coil, ribbon microphones use a small strand of very
thin 2 microns thick aluminum ribbon. This ribbon is more responsive to high frequencies
with the drawback that the ribbon is fragile and prone to tearing. Ribbon microphones are
almost exclusively used in the studio, not for location audio. The final type of microphone commonly used
in film is the condenser mic. Condensers use two charged plates, one fixed and one which
can move acting like a diaphragm. Unlike dynamic mics, there’s no coil. Instead of using
the electromagnetic principle, condensers use the electrostatic principle. The two charged
electric plates create what’s called a capacitor. As sound waves strike the electrically charged
diaphragm, it moves in relation to the fixed plate changing its capacitance and generating
a very small electric charge which is amplified inside the microphone. The advantage of condenser mics is their response.
Because you’re not moving a coil, condensers can be more responsive in the high frequencies.
Because you don’t have any magnets, condenser microphones can be made very small. Because condensers work with electrically
charged plates, that means they require some sort of outside power. Some microphones have
the option of an onboard battery while all condensors can utilitzed something called
Phantom Power +48v from the audio recording or mixing board. There’s not one microphone that is perfectly
suited for all applications. We’ve already covered three kinds of sound conversion technologies,
lets take a brief overview of the kinds of microphones that are used in film and video
production and of course we’re just going to scratch the surface on what’s available. When choosing a microphone, we have to consider
the directional response which often times represented by something called a polar pattern.
There are a few common types of polar patterns The omnidirectional pattern means the mic
is responsive to sound from all directions, you don’t have to be “on axis” to be
picked up. These mics are useful for picking up sound in a general area but the draw back
is they will pick up all the unwanted sound in the area. An example of this omni directional
mic is the RODE Reporter mic which is designed from news interview applications – you don’t
have to point it directly at the source, just get it close to speaker. Directional mics are more picky. The most
basic is the Cardioid pattern. Notice how it picks up what’s in front but not behind.
An example is the RODE M1 mic, a dynamic microphone that is suited for live performance as it
picks up the sound on axis but won’t pick up what’s behind it, like crowd noise or
feedback from a speaker. The RODE NT55 is an example of a cardioid condenser which has
the option of switching from cardioid to omni-directional. The NT55 and other large diaphragm condenser
mics like the beautiful sound RODE NT-1 are well suited for voice over and other studio
recording applications. More directional are the Hypercardioid and
Supercardioid polar pickup patterns -the RODE NTG-2 shotgun microphone is an example of
a supercardioid mic which picks up the front and sides and rejects 150 degrees to the rear.
This condenser microphone used in conjunction with a boom pole, is great for recording location
audio while trying to filter out some of the unwanted ambient sound. For working in even
noisier situations you may look to something like the super long RODE NTG8 microphone which
is great for noisy outdoor use. Some condenser and ribbon microphones will
feature a figure 8 pickup pattern or bi directional pickup pattern. Mics like the RODE NT2000
can be switched into a bidirectional pattern which is useful for certain musical applications. There are two other kinds of microphones that
have a place in the film and video production world that we haven’t talked. The most common
is the lavalier or lapel mic. Mics like the RODE Lavalier and PinMic are small condenser
microphones with an omnidirectional pickup pattern that are designed to be placed on
the talent to capture sound. They work on proximity and when combined with a wireless
transmitter and receiver can offer the most freedom when recording audio. But because
of their size and the placement on the body of the talent, lavs won’t have quite the
same richness of sound as a shotgun or studio condenser mic. Another useful type of mic is called the boundary
mic. Boundary mics are omnidirectional condenser mics that are positioned flush with a surface
that capture sound as it rolls off the flat surface. Boundary mics find a lot of use in
stage production. Between hand held mics, shotgun mics, studio
condensers, lavaliers and boundary mics these will be the microphones you will need for
the vast majority of situations you will encounter when making films and videos. Remember sound is a crucial part of the filmmaking
process and in some cases, good clean sound is actually more important that a pretty picture.
We’ve just scratched the surface of the science of sound and microphone technology
but now it’s up to you to research and experiment with recording sound. Go out there and make
something great, I’m John Hess and I’ll see you at FilmmakerIQ.com