An introduction to audio engineering. Here we skip over the basics so that we can delve deep into the issues that come up when designing, deploying, powering, tuning, and protecting a medium to large-sized loudspeaker system. We will talk about crossovers, coverage patterns, speaker arrays, delays, limiters, choosing the right amplifier power for the speakers, field repairs, and more. Original class recordings: https://www.youtube.com/watch?v=pHRYpP_cSqs https://www.youtube.com/watch?v=26lZdc9HhtI

1. SOUND 201
Large-scale Sound Reinforcement
Langton Labs
August 18-19th, 2014
Michael Broxton
Contact: broxton@gmail.com

2. WHAT IS INTHE CLASS?
• The physical behavior of sound & the sound field
• The human perception of sound
Today we will discuss the fundamental nature of sound.
Tomorrow we will discuss its reproduction using loudspeakers.
• Understanding loudspeaker specifications
• Moar volume: loudspeaker arrays
• Tuning the system: crossover, gain structure, delays, and EQ
• Tips for running live sound

3. THE SOUND WAVE
Sound is our perception of a mechanical wave of pressure or
displacement traveling through a medium such as air.
Broadly speaking, a sound wave has three properties that
carry information that determine its behavior.

4. THE SOUND WAVE
Sound is our perception of a mechanical wave of pressure or
displacement traveling through a medium such as air.
Broadly speaking, a sound wave has three properties that
carry information that determine its behavior.

5. THE SOUND WAVE
Image credit: Daniel Russel. Acoustics andVibration Animations
Although sound waves coming from multiple sources
sum together in a straight-forward manner…

6. THE SOUND WAVE
Image credit: Daniel Russel. Acoustics andVibration Animations
Although sound waves coming from multiple sources
sum together in a straight-forward manner…

7. THE SOUND WAVE
Image credit: Daniel Russel. Acoustics andVibration Animations
Although sound waves coming from multiple sources
sum together in a straight-forward manner…
…their summation can lead to complex patterns
of constructive and destructive interference.

8. THE SOUND FIELD
Of course, sound waves are not 1-dimensional.
They travel through space, and the evolve over time.
y
x
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop

9. THE SOUND FIELD
An ideal point source radiates sound
equally in all directions as a spherical wavefront.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop

10. THE SOUND FIELD
When wavefronts from two point sources (e.g. stereo
loudspeakers) interact, there is a pattern of interference.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop

11. THE SOUND FIELD
Null zones of destructive interference span areas of
constructive interference where sound amplitude is doubled*.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop
* - But perceived “loudness” is not doubled! We’ll cover that later…

12. THE SOUND FIELD
The spacing between the point sources effects the interference pattern.
As point sources get closer together,
the null zones get farther apart, and
vice versa.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop
When sources are really close together,
they form a dipole radiator that acts
like a directional point source.
Example: mid and high range driver
in the same loudspeaker.
Example: two adjacent mid-range
drivers in the same loudspeaker.

13. SPEAKER CROSSOVERS
This pattern of interference must be considered
when designing loudspeakers with multiple drivers.

14. THE SOUND FIELD
The frequency of the sound also changes interference pattern and
spacing between the null zones.
Low frequencies push the null zones
apart, but they also grow larger and
more noticeable to the listener.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop
High frequencies can push the null
zones so close together that they are
so small as to not be noticeable at all.
Example: stereo subwoofers Example: stereo tweeters

15. THE SOUND FIELD
For mid/high frequencies we begin to perceive a stereo effect
when the spacing between the sources is large enough, despite
the dense interference pattern.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop

16. THE SOUND FIELD
But sub-woofers are more tricky, though, because the pattern of
interference is human-scale. We also do not tend to perceive
a stereo image at low frequencies.
Click on the image to try this demo online using Johannes Singler’s WaveWorkshop

17. SUB-WOOFER ARRAYS
To address this problem, it is best to place subs near each other so that
they appear as a single, coherent source.
A rear-delay or end-fire array, used in
conjunction the the proper delays, can reduce
the amount of sound energy emanating from
the back and sides of the stage.
There are many
clever ways to array
sub-woofers to
achieve advantageous
cancellation effects.
Often these are used
to achieve a cardioid
pattern of sound
energy.

18. OTHER FACTORS
There are many other important factors that effect the sound field.
• Reflection
… off walls, the ground. Plays a particularly important role indoors.
• Refraction
… around objects, around people.
• Absorption
… helps to attenuate reflections, or as the sound passes through humid air.
• Scattering
… diffuses sound energy, spreading it out in a random manner.
These are topics that are important to consider in room acoustics.
Another class!!

19. FOR MORE FUN EXAMPLES
Try the Ripple Tank app (mac, iPad, or web)

20. LET’STALK ABOUT REAL SOUNDS
They are the (linear) sum of many individual pure waveforms each
with their own frequency and phase.
Add together
Final waveform
The real sounds we encounter in the world are complex.

21. DUAL DOMAINS
The Fourier transform is the mathematical tool we use to decompose
a time signal into its frequency components, and vice versa.
It is quite useful, but no matter
how complicated the time
waveform, the Fourier
transform only gives us
information about the
average power in each
frequency over all time.
However we simultaneously
perceive sound in terms of
both time and frequency.

22. A MIXED REPRESENTATION
Our ear does a neat trick: it decomposes the sound into a mixed
time-frequency representation.
Time
(linear)
Frequency
(logarithmic)
We can do this digitally (albeit imperfectly) using a tool called a
spectrogram or a windowed or short-time Fourier transform (STFT).
!
Using the spectrogram, we can see both spectral and temporal aspects
in the music in a way that is similar to how we hear it.

24. GETTO KNOW YOUR FREQUENCIES
“Subs” “Mids" “Tops”

25. PERCEPTION OF FREQUENCY: PITCH
The human range of hearing is from about 20 Hz to 20KHz
Frequency
(logarithmic)
We perceive pitch logarithmically in relation to frequency.
!
Each frequency doubling is perceived as an equal
(perceptually linear) increase in pitch: i.e. one “octave.”

26. PERCEPTION OF SOUND INTENSITY: LOUDNESS
Next let’s talk about another perceptual phenomenon: loudness.
Unit: dBu (voltage) or dBm (power)
Oscilloscope
Sound pressure
Measurement
Device
Phenomenon
Sound Pressure Meter
Electricity Human Perception
Voltage, current, or power Acoustic Energy “Loudness”
Unit: dB SPL
Your Ear
Unit: Phon

27. BRIEF DIGRESSION:THE DECIBEL
Our perception of loudness is also logarithmic.
What we perceive to be 2x as “loud” is actually 10x the acoustic energy intensity.
What we perceive to be 4x as “loud” is actually 100x the acoustic energy intensity.
What we perceive to be 8x as “loud” is actually 1000x the acoustic energy intensity.
etc.
The decibel is a logarithmic measure relative to some reference level.

28. =
= µ ( )
Again, we use this because it is
convenient, and matches our
perceptual experience.
For example:
dB sound pressure level
or
db SPL
BRIEF DIGRESSION:THE DECIBEL

29. BRIEF DIGRESSION:THE DECIBEL
Depending on where you are in the signal chain, you may find yourself
using a different one of these scales.
But they are all compatible, inasmuch as a +10dB in one scale leads to a
+10dB increase in the others! It is designed to be simple and intuitive.
Electrical Energy
=
= µ ( )
=
=
Unit: dBu
=
=
Voltage
e.g. Mixers
Unit: dBm
Power
e.g.Amplifiers
Acoustic Energy
Unit: db SPL
Pressure
Power
Unit: db SWL
Loudness
A phon is equal to the sound
pressure level (in db SPL) of an
equivalently “loud” 1-KHz tone.
= -
=
Beware… there are many decibel scales & reference levels!

30. CONTROLLINGVOLUME OR “GAIN”
Unit: dBu (voltage) or dBm (power)
Oscilloscope
Sound pressure
Measurement
Device
Phenomenon
Sound Pressure Meter
Electricity Human Perception
Voltage, current, or power Acoustic Energy “Loudness”
Unit: dB SPL
Your Ear
Unit: Phon

31. LEARNINGTOTHINK INTERMS OF DECIBELS
+3dB is:
2x the acoustic power
but only 1.23x as “loud”
+6dB is:
4x the acoustic power
but only 1.5x as “loud”
+10dB is:
10x the acoustic power
and 2x as “loud”
Some implications to think about:
• A 1-2 dB change in volume is barely
perceptible
• Doubling amplifier power does not
double loudness

32. SOUND INTENSITY AND ACOUSTIC ATTENUATION
Unit: dBu (voltage) or dBm (power)
Oscilloscope
Sound pressure
Measurement
Device
Phenomenon
Sound Pressure Meter
Electricity Human Perception
Voltage, current, or power Acoustic Energy “Loudness”
Unit: dB SPL
Your Ear
Unit: Phon

33. SOUND ATTENUATION OVER DISTANCE

34. SOUND ATTENUATION OVER DISTANCE

35. PERCEPTION OF SOUND INTENSITY:“LOUDNESS”
Unit: dBu (voltage) or dBm (power)
Oscilloscope
Sound pressure
Measurement
Device
Phenomenon
Sound Pressure Meter
Electricity Human Perception
Voltage, current, or power Acoustic Energy “Loudness”
Unit: dB SPL
Your Ear
Unit: Phon

36. EQUAL-LOUDNESS CONTOURS
The equal loudness curves are psychoacoustic
measurements of how humans perceive loudness.

37. THE RANGE OF HUMAN HEARING
They can help us to draw a boundary around the
perceptible range of sounds.

38. THE RANGE OF HUMAN HEARING
Or the sounds that represent speech, or music.

39. As the sound engineer, it is your
job to protect your audience
from hearing damage.
• Use limiters to prevent
transients from damaging hearing.
MEASURING NOISE EXPOSURE
Measures you should take:
• Measure the average noise
“exposure” over the scale of
minutes or hours.
Rough guidelines for dance music:
• 90-100 dB SPL (average) is a good, relatively safe
volume early or late in the night.
• 100-110 dB SPL (average) is a good sustained level
at the peak.
• Beyond this, you risk damaging ears and speakers.

40. PROTECTYOUR HEARING!
More info: https://www.etymotic.com/pdf/er_noise_exposure_whitepaper.pdf
Occupational Safety
and Health Administration (OSHA) &
National Institute for Occupational
Safety and Health (NIOSH)

41. PROTECTYOUR HEARING!
Passive Protection Active
$300$10
$150

42. PART II
• The physical behavior of sound & the sound field
• The human perception of sound
• Understanding loudspeaker specifications
Yesterday we discussed the fundamental nature of sound.
Today we will discuss its reproduction using loudspeakers.
• Moar volume: loudspeaker arrays
• Tuning the system: crossover, gain structure, delays, and EQ
• Tips for running live sound

43. THE IDEALSVS. REALITY
The ideal loudspeaker would:
!
… radiate sound like an ideal point or line source …
!
… play music with a flat frequency response over all audible frequencies …
!
… and get arbitrarily loud.
This is not physically possible!

44. THE IDEALSVS. REALITY
For starters, different frequencies of sound have different properties.
Low frequencies:
• Diffract more, reflect less
• Require a driver that can move
a lot of air!
• Are highly omnidirectional
High frequencies:
• Diffract less, reflect more
• Requires a driver that can
move very fast!
• Are highly directional

45. LOUDSPEAKER DIRECTIVITY
(at least down to the low frequencies)
A good loudspeaker has been
optimized to produce roughly
equal acoustic power over a
limited arc of angles.
Managing the pattern of sound
dispersion is called pattern
control and it is the key to
understanding how multiple
loudspeakers interact.

46. LOUDSPEAKER DIRECTIVITY
The first consequence of this is that you should put people’s ears where
the speaker is producing the best possible sound.

47. LOUDSPEAKER DIRECTIVITY
As an aside: this has implications for where you place
stereo loudspeakers, and the directions you point them.

48. ANATOMY OF A LOUDSPEAKER
Bass-reflex vent
Bass-reflex vents
Direct Radiating Woofer
Horn-loaded mid
Horn-loaded HF
compression driver

49. THE IMPORTANCE OFTHE ENCLOSURE

50. DIRECT RADIATING LOUDSPEAKERS
Direct Radiator Reflex Enclosure

51. HORN LOUDSPEAKERS

52. FOLDED HORN

53. OUR NEW SOUND SYSTEM
LA400
LA460
LA215
LF Subsystem: 1x 15-in, vented
HF Subsystem: 1x 2-in exit/3-in
voice coil compression driver on
Wave Guid Plate.
LF Subsystem: 1x 15-in, vented
HF Subsystem: 1x 1.4-in exit/
1.75-in voice coil compression
driver on constant directivity
horn.
MF Subsystem: 1x 8-in cone,
horn-loaded
LF Subsystem: 12-in woofer,
bent bass horn
LF Subsystem: 2x 18-in, vented
LF Subsystem: 2x 18-in, vented
LA128
LA128z
Coverage angle (+/- 6dB): 180°
Coverage angle (+/- 6dB): 360°
Coverage angle (+/- 6dB): 360°
Coverage angle (+/- 6dB):
90° Conical
Coverage angle: 60° x 45°

54. OUR NEW SOUND SYSTEM
LA400
Power handling: 500W @ 8 Ω
Freq response: 45-250 Hz
Sensitivity: 107 dB SPL/W @ 1m
LA215
Power handling: 600W @ 8 Ω
Freq response: 69 Hz - 18 KHz
Sensitivity: 97 dB SPL/W @ 1m
LA460
Power handling:
full range: 500W @ 8 Ω
bi-amp (LF/MF): 500W @ 8 Ω
bi-amp (HF): 150W @ 8 Ω
Freq response: 62 Hz - 20 KHz
Sensitivity:
full range: 97 dB SPL
bi-amp (LF/MF): 97 dB SPL
bi-amp (HF): 108 dB SPL
LA128
Power handling: 1600W @ 4Ω
Freq response: 31-200 Hz
Sensitivity: 98 dB SPL/W @ 1m
LA128z
Power handling: 2000W @ 4Ω
Freq response: 31-200 Hz
Sensitivity: 98 dB SPL/W @ 1m

55. LOUDSPEAKER ARRAYS
Line Source ArrayPoint Source Array
In order to create high sound pressure levels over a large
areas, you need to array many loudspeakers together.

56. LOUDSPEAKER ARRAYS
Image credits: McCarthey, Bob. Meyer Design Reference for Sound Reproduction. 1998

57. LOUDSPEAKER ARRAYS
Image credits: McCarthey, Bob. Meyer Design Reference for Sound Reproduction. 1998

58. ANOTHER FUN LOUDSPEAKER SYSTEM
20 x 12" Mid-bass drivers in 10
cabinets (2500 Watts / cabinet)
18 x 21" drivers w/ 6” voice coil
15mm excursion, & neodymium
magnets (4000W / 2 drivers)
12” and 2” horn-loaded
drivers,“cat’s eyes” horn
flare

59. SYSTEM PROCESSOR

60. SYSTEM PROCESSOR
1. Configure crossover frequencies
2.Add driver alignment delays, polarity, and EQ
3. Calibrate the gain structure & set the limiters
4. Rough balancing of frequency response
5. Careful system EQ

61. CONFIGURE CROSSOVER FREQUENCIES

62. SYSTEM PROCESSOR
1. Configure crossover frequencies
2. Add driver alignment delays, polarity, and EQ
3. Calibrate the gain structure & set the limiters
4. Rough balancing of frequency response
5. Careful system EQ

63. DRIVER ALIGNMENT DELAYS, POLARITY AND EQ
Bi-amplified loudspeaker Speaker system with
flown tops
Sub
Top
2m
5m
5.4m
Delay the subs by: 0.4m (1.2ms)

64. SYSTEM PROCESSOR
1. Configure crossover frequencies
2.Add driver alignment delays, polarity, and EQ
3. Calibrate the gain structure & set the limiters
4. Rough balancing of frequency response
5. Careful system EQ

65. CALIBRATE GAIN STRUCTURE & SET LIMITERS
As a general rule, use an amplifier
delivering 1.5x - 2x the speaker's
average ("RMS") power rating.
Note: amplifiers are a fixed-gain device. the knob of the front of
the amplifier attenuates the input, rather than “turning up” the output.

66. CALIBRATE GAIN STRUCTURE & SET LIMITERS
Setting gain structure involves two steps:
1. adjust levels so that all parts of the signal chain clip at the same time.
2. use limiters to prevent the amplifiers from clipping

67. SYSTEM PROCESSOR
1. Configure crossover frequencies
2.Add driver alignment delays, polarity, and EQ
3. Calibrate the gain structure & set the limiters
4. Rough balancing of frequency response
5. Careful system EQ

68. SYSTEM PROCESSOR
1. Configure crossover frequencies
2.Add driver alignment delays, polarity, and EQ
3. Calibrate the gain structure & set the limiters
4. Rough balancing of frequency response
5. Careful system EQ

69. POWERINGTHE SOUND SYSTEM
There are a few things to consider here:
• Power to the mixer, stage monitors, laptops, etc. should be a single
independent power circuit.
• Each amplifier will take somewhere between 5-15A on a normal 120V AC
line. AC circuits are typically 20-30A. Plan accordingly!
• Make sure your extension cables are rated for the power you are delivering.
• Avoid power strips. Plug amps directly into the line, using splitters if
necessary.

70. TROUBLESHOOTING
Blown fuse / tripped breaker
Unexpected resonance in a speaker
Clipping
A dead speaker or amp
If this happens: Do this:
Generator runs out of gas Hang your head in shame. :)
Reset breakers. Replace fuse.
Adjust crossover LPF
Turn down the mixer
Depends on the situation…
Late DJ Have laptop or DJ iPod at the ready
Mic feedback during live act Reduce gain, twiddle EQ

71. THANKS!
Michael Broxton
Contact: broxton@gmail.com