Etech Acoustics Exam

If the number of compressions and rarefactions of the sounds propagation in the air occurs at a rate of 20 cycles per second, what frequency (in hertz) is attributed to this sound propagation?

20 cycles per second = 20 Hz

1)     A concrete floor has a value of alpha=0.02 at 1000 Hz. What percentage of sound energy striking it will be reflected back?

Percentage of reflected sound energy = 1-   =98%

1)     The vertical section below shows a portion of a speech auditorium. Sketch on it the sound paths (direct and first reflected) from the speaker to the listener in row 5 which are the most useful in providing high intelligibility for this listener. Consider only the paths in the plane of this section, i.e., neglect lateral side-wall reflections.

1)     To avoid echo sound, what is the maximum time allowed for the first sound reflection to reach the listener?

55 ms = 0.055 s

The combined sound level of 35 dB and 95 dB is

95 dB

The combined sound of 65 dB and 70 dB is

a)  71 dB          Between 4 & 8 add 1 dB to highest value

For fibrous dissipative materials, absorption is normally higher at:

Higher Frequencies than Lower Frequencies

List the three major types of absorbers used in construction practices

a.  Fibrous Materials

b.  Panel Resonators

c.   Volume Resonators (Or Cavity)

DB addition

0-1 add 3

2-3 add 2

4-5 add 1

9 and over, use highest #

High reverberation time means

  Low Room Absorption

The noise level at 5 meters from the sound source is 50 dB. Assuming a free field, what would be the sound level (IL) at 10 meters from the source

44 dB   Sound level decreases by 6 dB for every doubling of distance

A reduction of 3 dB in noise level is

Barely Perceptible

If a change in intensity level were 10 dB, the subjective change in loudness would be

Four times as loud

Given an air conditioner produces sound intensity level of 40 dB at a distance of 5ft from the source, the intensity at four times this distance is

28 dB

If you add absorption material in a room, the sound level will be greater

  Near the source

The free field conditions can prevail

  A and B (OUTSIDE AND ENCLOUSED)

Give the minimum and maximum audible frequencies (show unit)

Minimum audible frequency 20 Hz and Maximum is 20,000 Hz

The STC values for Grade II are based on the outdoor noise level that is:

Average

Name the acoustical fields that can exist inside a room

A) Near Field

B) Reverberant Field

C) Free (Far) Field

The distance along the center line of our lecture hall from the front to the last row of seat is about 46 feet. How long does it take a sound, such as a hand clap, produced at the front to reach the last row? Express the answer in milliseconds.

T = 0.0407 sec = 40.7 ms Determined as follows T = D/C = 46/1130 = 40.7 ms

equation for reverberation time

TR = 0.05x (volume/sum of sound absorption)

equation for sound absorption

area x absorptance

Explain the purpose of a quadratic residue diffuser (QRD) and its application.

A QRD diffuser a series of harmonics shown in a panel with a variety of depths to diffuse reflected sound. They are used to control reflections in the listening environment, particularly from rear walls and first-reflection points. They are applied in almost any space, including at sunken highway edges.

Background noise levels should be the same for all activities

false

In acoustics, every doubling of mass thickness(doubling the width of a brick wall from 4” to 8”) will increase the transmission loss by:

6 dB

What is the difference between FSTC and STC?

STC of a wall system (enclosure) is a laboratory determined value. A lab test is a more controlled environment.

The on-site construction is not as perfect and, therefore, the term Field STC or FSTC represents the value of the wall as constructed. In general, Field STC is 5 dB lower than the STC for the same wall.

In summary: FSTC is the STC value obtained for a field tested partition between two spaces or enclosure STC is the value obtained from a laboratory

Increasing the width of sound absorptive (felt) materials is effective up to what maximum thickness:

6”

The type of mounting and assembly of absorption panels does not affect the coefficient of absorption

FALSE

Give the recommended STC between two classrooms:

a. Quiet STC: 42

b. Normal STC: 40

The slide in acoustics lecture shows the STC criteria for airborne sound insulation between dwelling unit based on Grade II – bedroom to bedroom. What does Grade II specify?

The Department of Housing and Urban Development issues acoustical design criteria based on 3 grades, each specifying a region: Grade I, II, III. Grade II is the most important category and is applicable in residential urban and suburban areas considered to have an average noise environment.

What are flanking paths?

Flanking paths are the means for sound to transfer from one space to another other than through the wall. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing or corridors.

From the slide in Acoustic lecture, give the recommended floor IIC between two bedrooms located above each other, and belonging to two different apartments:

For 2 stacked bedrooms belonging to different apartments, IIC = 52 (see table in slide lecture 3)

What are STC and IIC specified for?

STC is specified as sound reduction metric between 2 spaces separated by a partition or indoor-outdoor spaces separated by an enclosure. IIC apply to floor decks sound reduction

Mechanical devices make noise by vibration and these include HVAC equipment. Give three ways of quieting these machines:

- Enclose the equipment with a heavy drape

- Install vibration isolation under the equipment to decouple it from the floor

- Use flexible connection at duct take-off

Give the STC value for total privacy

STC to be between 46-50

Sound attenuation in ducts can be achieved with

Inner glass fiber board liners installed inside the duct

What do STC and IIC represent?

Sound Transmission Class (STC) is a dimensionless parameter that rates partition assembly to block sound transfer between two spaces.

Impact Insulation Class (IIC) is a dimensionless parameter that rates floor assembly to block impact vertical sound transfer between two spaces.