A-level Applied Science/Physics of Performance Effects/Sound

Before designing a sound system, you need to understand the nature of sound.

how sound travels

frequency and pitch

amplitude and volume

• Measure the speed of sound

Know how temperature, pressure, humidity, etc. affect the speed of sound

Measure reflection, refraction and diffraction

Measure resonance

Understand interference

Measure the absorption of sound at different types of surfaces

Measure the reflection of sound at different types of surfaces

consider how the sound is to be transferred from the stage to the auditorium.

ensure that the sound quality is reasonably consistent throughout the auditorium

• the type and quality of equipment used; • the number and type of microphones and loudspeakers used; • the positioning of loudspeakers and microphones; • the shape of the performance area; • the position of the audience; • the position of the performers; • the materials used in the performance area.

Introduction

Many people use one or two rooms in their living space as "theatrical" rooms where theater or music room activities commence. It is a common misconception that adding speakers to the room will enhance the quality of the room acoustics. There are other simple things that can be done to increase the room's acoustics to produce sound that is similar to "theatre" sound. This site will take you through some simple background knowledge on acoustics and then explain some solutions that will help improve sound quality in a room.

Room Sound Combinations

The sound you hear in a room is a combination of direct sound and indirect sound. Direct sound will come directly from your speakers while the other sound you hear is reflected off of various objects in the room.

The Direct sound is coming right out of the TV to the listener, as you can see with the heavy black arrow. All of the other sound is reflected off surfaces before they reach the listener.

Good and Bad Reflected Sound

Have you ever listened to speakers outside? You might have noticed that the sound is thin and dull. This occurs because when sound is reflected, it is fuller and louder than it would if it were in an open space. So when sound is reflected, it can add a fullness, or spaciousness. The bad part of reflected sound occurs when the reflections amplify some notes, while cancelling out others, making the sound distorted. It can also affect tonal quality and create an echo-like effect. There are three types of reflected sound, pure reflection, absorption, and diffusion. Each reflection type is important in creating a "theatre" type acoustic room.

Reflected Sound

Reflected sound waves, good and bad, effect the sound you hear, where it comes from, and the quality of the sound when it gets to you. The bad news when it comes to reflected sound is standing waves.more on standing waves These waves are created when sound is reflected back and forth between any two parallel surfaces in your room, ceiling and floor or wall to wall. Standing waves can distort noises 300 Hz and down. These noises include the lower mid frequency and bass ranges. Standing waves tend to collect near the walls and in corners of a room, these collecting standing waves are called room resonance modes.

Finding your room resonance modes

First, specify room dimensions (length, width, and height). Then follow this example:

Working with room resonance modes to increase sound quality

1. There are some room dimensions that produce the largest amount of standing waves.

a. Cube

b. Room with 2 out of the three dimensions equal

c. Rooms with dimensions that are multiples of each other

Absorbed

The sound that humans hear is actually a form of acoustic energy. Different materials absorb different amounts of this energy at different frequencies. When considering room acoustics, there should be a good mix of high frequency absorbing materials and low frequency absorbing materials. A table including information on how different common household absorb sound can be found here.

Diffused Sound

Using devices that diffuse sound is a fairly new way of increasing acoustic performance in a room. It is a means to create sound that appears to be "live". They can replace echo-like reflections without absorbing too much sound.

Some ways of determining where diffusive items should be placed were found on this website.

1.) If you have carpet or drapes already in your room, use diffusion to control side wall reflections.

2.) A bookcase filled with odd-sized books makes an effective diffusor.

3.) Use absorptive material on room surfaces between your listening position and your front speakers, and treat the back wall with diffusive material to re-distribute the reflections.

How to Find Overall Trouble Spots In a Room

Every surface in a room does not have to be treated in order to have good room acoustics. Here is a simple method of finding trouble spots in a room.

1.) Grab a friend to hold a mirror along the wall near a certain speaker at speaker height.

2.) The listener sits in a spot of normal viewing.

3.) The friend then moves slowly toward the listening position (stay along the wall).

4.) Mark each spot on the wall where the listener can see any of the room speakers in the mirror.

5.) Congratulations! These are the trouble spots in the room that need an absorptive material in place. Don't forget that diffusive material can also be placed in those positions.

Speed of sound in air

Impact of temperature
θ in °C	c in m/s	ρ in kg/m³	Z in N·s/m³
−10	325.4	1.341	436.5
−5	328.5	1.316	432.4
0	331.5	1.293	428.3
+5	334.5	1.269	424.5
+10	337.5	1.247	420.7
+15	340.5	1.225	417.0
+20	343.4	1.204	413.5
+25	346.3	1.184	410.0
+30	349.2	1.164	406.6

Mach number is the ratio of the object's speed to the speed of sound in air (medium).

Sound in solids

In solids, the velocity of sound depends on density of the material, not its temperature. Solid materials, such as steel, conduct sound much faster than air.

Experimental methods

In air a range of different methods exist for the measurement of sound.

Single-shot timing methods

The simplest concept is the measurement made using two microphones and a fast recording device such as a digital storage scope. This method uses the following idea.

If a sound source and two microphones are arranged in a straight line, with the sound source at one end, then the following can be measured:

1. The distance between the microphones (x) 2. The time delay between the signal reaching the different microphones (t)

Then v = x/t

An older method is to create a sound at one end of a field with an object that can be seen to move when it creates the sound. When the observer sees the sound-creating device act they start a stopwatch and when the obsever hears the sound they stop their stopwatch. Again using v = x/t you can calculate the speed of sound. A separation of at least 200 m between the two experimental parties is required for good results with this method.

Other methods

In these methods the time measurement has been replaced by a measurement of the inverse of time (frequency).

Kundt's tube is an example of an experiment which can be used to measure the speed of sound in a small volume, it has the advantage of being able to measure the speed of sound in any gas. This method uses a powder to make the nodes and antinodes visible to the human eye. This is an example of a compact experimental setup.

A tuning fork can be held near the mouth of a long pipe which is dipping into a barrel of water, in this system it is the case that the pipe can be brought to resonance if the length of the air column in the pipe is equal to ( {1+2n}/λ ) where n is an integer. As the antinodal point for the pipe at the open end is slightly outside the mouth of the pipe it is best to find two or more points of resonance and then measure half a wavelength between these.

Here it is the case that v = fλ

References

Acoustic Room Treatment Articles

Room Acoustics: Acoustic Treatments

Home Improvement: Acoustic Treatments

Crutchfield Advisor

Calculation: Speed of sound in air and the temperature

The speed of sound, the temperature, and ... not the air pressure

Properties Of The U.S. Standard Atmosphere 1976