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Acoustical Layering
by Travis Ludwig © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any
commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com
Basically, a recipe is a list of ingredients and directions. We found a RECIPE that works for sound! But, it takes more
than a good recipe to make a great meal or dessert. For the best results, you need the best ingredients and the best directions
possible. You also have to understand the expectations of those who are going to taste the results of your effort. For example,
a chef may like extra spicy food. But, when preparing food for others, s/he must take into consideration the guests s/he will
be serving. The chef may need to vary the recipe slightly.
The Ingredients:
If a recipe calls for cream, it's not essential to use Alpine milk from hand-milked Bavarian cows. But, if you try to
use skim milk instead of cream, you may jeopardize the final results. The same holds true for sound reinforcement. A microphone
designed for hand-held vocals used as a lectern microphone simply isn't the ingredient for the recipe - it won't create the
desired results.
Here is a list of ingredients for your sound system:
* Well-Designed, Properly Installed Main Speaker System
* Quality Microphones for Speech
* Quality Microphones for Music Vocals
* High Quality Microphones and Direct Boxes for Musical Instruments
* Low-Noise Main Mixer with Sufficient Inputs and Outputs,Gain and Headroom
* Quality Equalizer(s) of the Right Type and Configuration
* Well-Built, Well-Maintained Cables
* Quality Monitor Speakers
* Quality Playback Cassette Decks and CD Players
* And, the Icing for the Cake (your mixing skills)
The Directions For A Great Mix:
Let's start with the basics (eggs, flour, milk. . .):
It is vital to understand the acoustical elements of the sound mix and their effect on each other. A multilayer cake represents
the concept that each ingredient builds on the other and they all work together to create a desired outcome. It should be
pointed out that you may not use all these layers. However, the principals remain the same.
...ambient noise in the room establishes the base layer of sound.
The bottom layer (ambient noise) is already established for you. The amount of ambient noise in the room establishes the
base layer of sound. In other words, the air system, conversations, babies crying, people moving, etc. creates noise the sound
system must overcome. The ambient noise will also change levels. For example, an empty room is much quieter than one filled
with people.
The second layer is acoustical instruments. It is important to begin first with the main instrument(s) like the piano,
then add the drums, and finally add any other acoustical instruments. For example, if the two main instruments are guitar
and piano, begin with the pianist playing a selection. Have the guitarist join after the first verse. If you have trouble
hearing the guitar, reposition him/her. If the guitar is still not loud enough, then a microphone will need to be added.
Please note, we advise natural acoustical solutions before adding sound reinforcement.
When using drums during a special event, test them next. Have the piano begin playing. Then begin the drummer after a
minute or two. Listen first to determine if you can still hear the piano (hint: the higher octaves of the piano are usually
easier to hear over other instruments). If the piano starts to get buried, increase the microphone level on the piano gradually.
Again, try natural solutions before adding microphones or raising volumes. Finally, test any other acoustical instruments
including acoustical guitar, woodwinds and brass.
The third level consists of electronic instruments such as keyboards, electric guitars, bass guitars, acoustic instruments
with electronic pickups, electronic drums, etc. The piano is usually the primary acoustical instrument, and you have already
set the piano level. Now add any electronic keyboards to the mix. At this point, you can have the drummer and other acoustic
musicians take five. Use the same procedure as before. Begin with piano and add one keyboard at a time. Then continue with
any other electronic instruments. When you have finished, take a break. Try to leave the room and listen to silence for five
minutes. Then go back and listen to the whole instrumental mix.
Last, but certainly not least, are the vocals. Beginning with the background vocals, add each vocalist one at a time as
you did the instruments. Finally, add the primary vocalists. Obviously, it is vital that the primary vocalist(s) are heard
and understood above all the other instruments and background vocals.
...you...may need to vary this recipe slightly.
A Personalized Touch:
As in our example of the chef varying the recipe slightly; you, as the sound operator, may need to vary this recipe slightly.
You know what your audience likes to hear. Also, you know the demands of your specific event. So, if you would like the background
vocalist to be as pronounced as the primary vocalist, add your personalized touch!
A Few Tips From The Chef:
1. Always listen for what is too loud as well as what is too soft.
2. If a musician or vocalist says s/he needs more of themselves in the monitors, first think of turning other instruments
or vocals down ...just a bit. Ask them what is too loud.
3. Always make level changes to the monitor mix or channel gain trim control when the musician or vocalist is not active.
4. Make small, gradual changes.
5. Occasionally turn down the master levels for the main system and listen to the monitor system to see how loud it
is. The monitor system may be causing the main system to sound poor. Also, walk to the first few seats to see if you hear
more sound from the monitors than the main system.
A Final Word:
Communicate with the musicians. Let them know this recipe may take several attempts before creating a cake that deserves
icing. It will take extra time, effort and patience on everybody's part. But the results will be worth it!
Tips on Dealing with Feedback Problems
By Dr. Dale A. Robbins
Feedback is technically known as audio oscillation. In simple terms, it is nothing more than sound coming out of the speakers,
going back into the microphones and reamplified in rapid oscillating cycles, creating a shrill whine or squeal. The object
of a system is to achieve satisfactory, high fidelity amplification of sound before feedback occurs. Under some circumstances,
limited by acoustics and poor equipment, this can be a real challenge.
There is no complete cure for feedback. If the volume of any mic is boosted loud enough, it will eventually cause feedback.
However, these are ways that it can be reduced and controlled:
Methods to Reduce feedback:
1. Turn the volume down.
2. Decrease gain on equalizer (EQ) or tone controls.
3. Reduce monitor system volume or its EQ gain.
4. Move microphones farther away from sound path of speakers.
5. Use a wide band graphic equalizer and/or parametric equalizer to reduce gain on susceptible feedback frequencies.
(Note: When using EQ to filter out feedback, a sacrifice in desired tone quality may result. But if used carefully, an acceptable
compromise of less tone for more feedback control can usually be achieved.)
6. Use higher quality, low impedance microphones and equipment.
7. Select tighter patterned, cardioid microphones when possible.
8. Decrease gain on reverb controls.
9. Improve room acoustics to absorb more reflected sounds.
10. Have an audio engineer to perform an acoustic analysis between your system and auditorium using a pink noise
generator and oscilloscope. By this method, he can identify feedback prone frequencies and help you tune your graphic equalizer
or suggest acoustic alterations to your auditorium.
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INTRODUCTION TO CHURCH SOUND SYSTEMS
(courtesy Shure Microphones)
Sound systems for houses of worship encompass a wide range of applications, from simple
speech reinforcement in a fellowship hall to multi-channel live music performance in a large sanctuary with recording and
broadcast capabilities. Though these systems may vary in size and complexity, they are all governed by the same physical principles
and they share certain types of equipment. Common components of these systems include microphones, mixers, amplifiers, loudspeakers,
and many types of electronic signal processing devices such as equalizers, compressors, and audio time delays. A complete
sound system involves some or all of these items. Proper selection and application of this equipment requires knowledge of
both the overall system goals and the individual component characteristics.
Since microphones act as the interface
between the sound source and the sound system, it is necessary to include some discussion of these two areas, and sound in
general, in order to understand how the microphone interacts with them. In addition, some related devices such as wireless
microphones, automatic mixers and signal processors will be discussed. The objective of this guide is to provide
the reader with sufficient information to successfully select and apply microphones and related equipment for a variety of
religious facility sound situations.
Houses Guide of Worship and Sound
Because good sound quality is the goal
of any house of worship sound system, it is helpful to be familiar with some general aspects of sound: how it is produced,
transmitted, and received. In addition, it is also useful to describe or classify sound according to its acoustic behavior.
Finally, the characteristics of good sound should be understood.
Sound is produced by vibrating objects. These include
musical instruments, loudspeakers, and, of course,human vocal cords. The mechanical vibrations of these objects move the air
which is immediately adjacent to them, alternately pushing and pulling the air from its resting state. Each back-and-forth
vibration produces a corresponding pressure increase (compression) and pressure decrease (rarefaction) in the air. A complete
pressure change, or cycle, occurs when the air pressure goes from rest, to maximum, to minimum, and back to rest again. These
cyclic pressure changes travel outward from the vibrating object, forming a pattern called a sound wave.
A sound wave
is a series of pressure changes (cycles) moving through the air A simple sound wave can be described by its frequency and
by its amplitude. The frequency of a sound wave is the rate at which the pressure changes occur. It is measured in Hertz (Hz),
where 1 Hz is equal to 1 cycle-persecond. The range of frequencies audible to the human ear extends from a low of about 20
Hz to a high of about 20,000 Hz.
In practice, a sound source such as a voice usually produces many frequencies simultaneously.
In any such complex sound, the lowest frequency is called the fundamental and is responsible for the pitch of thesound. The
higher frequencies are called harmonics and are responsible for the timbre or tone of the sound.
Harmonics allow us
to distinguish one source from nother, such as a piano from a guitar, even when they are playing the same fundamental note.
In the following chart, the solid section of each line indicates the range of fundamental frequencies, and the shaded section
represents the range of the highest harmonics or overtones of the instrument.
The amplitude of a sound wave refers
to the magnitude (strength) of the pressure changes and determines the loudness of the sound. Amplitude is measured in decibels
(dB) of sound pressure level (SPL) and ranges from 0 dB SPL (the threshold of hearing), to above 120 dB SPL (the threshold
of pain). The level of conversational speech is about 70dB SPL. A change of 1 dB is about the smallest SPL difference that
the human ear can detect, while 3 dB is a generally noticeable step, and an increase of 10 dB is perceived as a doubling of
loudness. (See Appendix One: The Decibel.)
Another characteristic of a sound wave related to frequency is wavelength.
The wavelength of a sound wave is the physical distance from the start of one cycle to the start of the next cycle,
as the wave moves through the air. Since each cycle is the same, the distance from any point in one cycle to the same point
in the next cycle is also one wavelength: for example, the distance from one maximum pressure point to the next maximum pressure
point.
Wavelength is related to frequency by the speed of sound. The speed of sound is the velocity at which a sound
wave travels. The speed of sound is constant and is equal to about 1130 feet-per-second in air. It does not change with frequency
or wavelength, but it is related to them in the following way: the frequency of a sound, multiplied by its wavelength always
equals the speed of sound. Thus, the higher the frequency of sound, the shorter the wavelength, and the lower the frequency,
the longer the wavelength. The wavelength of sound is responsible for many acoustic effects.
After it is produced,
sound is transmitted through a medium. Air is the typical medium, but sound can also be transmitted through solid or liquid
materials. Generally, a sound wave will move in a straight line unless it is absorbed or reflected by physical surfaces or
objects in its path. However, the transmission of the sound wave will be affected only if the size of the surface or object
is large compared to the wavelength of the sound. If the surface is small (compared to the wavelength) the sound will proceed
as if the object were not there. High frequencies (short wavelengths) can be reflected or absorbed by small surfaces, but
low frequencies (long wavelengths) can be reflected or absorbed only by very large surfaces or objects. For this reason it
is easier to control high frequencies by acoustic means, while low frequency control requires massive (and expensive) techniques.
Once a sound has been produced and transmitted, it is received by the ear and, of course, by microphones. In the ear,
the arriving pressure changes push and pull on the eardrum. The resulting motion of the eardrum is converted (by the inner
ear) to nerve signals that are ultimately perceived as sound. In a microphone, the pressure changes act on a diaphragm. The
resulting diaphragm motion is converted (by one of several mechanisms) into electrical signals which are sent to the sound
system. For both receivers, the sound picked up is a combination of all pressure changes occurring just at the surface of
the eardrum or diaphragm.
Sound can be classified by its acoustic behavior; for example, direct sound vs. indirect
sound. Direct sound travels from the sound source to the listener in a straight line (the shortest path). Indirect sound is
reflected by one or more surfaces before reaching the listener (a longer path). Since sound travels at a constant speed, it
takes a longer time for the indirect sound to arrive, and it is said to be delayed relative to the direct sound. There are
several kinds of indirect sound, depending on the acoustic space (room acoustics). Echo occurs when an indirect sound is delayed
long enough (by a distant reflecting surface) to be heard by the listener as a distinct repetition of the direct sound. If
indirect sound is reflected many times from different surfaces it becomes diffuse or non-directional. This is called reverberation,
and it is responsible for our auditory perception of the size of a room. Reverberant sound is a major component of ambient
sound, which may include other non-directional sounds, such as wind noise or building vibrations. A certain amount of reverberant
sound is desirable to add a sense of space to the sound, but an excess tends to make the sound muddy and unintelligible.
One
additional form of indirect sound is known as a standing wave. This may occur when the wavelength of a sound is the same distance
as some major dimension of a room, such as the distance between two opposite walls. If both surfaces are acoustically reflective,
the frequency corresponding to that wavelength will be amplified, by addition of the incoming and outgoing waves, resulting
in a strong, stationary wave pattern between the two surfaces. This happens primarily with low frequencies, which have long
wavelengths and are not easily absorbed.
A very important property of direct sound is that it becomes weaker as it
travels away from the sound source, at a rate governed by the inverse-square law. For example, when the distance increases
by a factor of two (doubles), the sound level decreases by a factor of four (the square of two). This results in a drop of
6 dB in sound pressure level (SPL), a substantial decrease. Likewise, when the distance to the direct sound source is divided
by two (cut in half), the sound level increases by 6 dB. In contrast, ambient sound, such as reverberation, has a relatively
constant level. Therefore, at a given distance from a sound source, a listener (or a microphone) will pick up a certain proportion
of direct sound vs. ambient sound. As the distance increases, the direct sound level decreases while the ambient sound level
stays the same. A properly designed sound system should increase the amount of direct sound reaching the listener without
increasing the ambient sound significantly.
If a handout is available online (e.g., a newspaper article) I might include the appropriate link to the information students
need on this page.
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Website design by
©2005 KC72576 Technologies.
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