23:
1. Compare tbe absorption of 100m2 of plastcred wall with
that af 100m2 of carpetcd Boor at
(8) 125 Hz;
(b) 2(XX) Hz.
1.. An auditoriwn bas dimeosioos 40 m x 20 m and a ceiling height of 15 ID. The front and back walls &recovered
with plywood paneling; the side walls and ceiling &re
plaster.1be Boor is wood.1bere are 1100 wooden seats.
Estimate the reverberation time (SOOHz) when
(8) The hall is empty;
(b) Ha1f the seats are filled;
(c) All the seats are occupied.
3. Estimate the time delay ti of the first reftectcd sound
for a person seated near the center of the auditorium described in Exercise 2. Does the first reftection arrive from
tbe side or from overbead?
4. If the ceiling in this auditorium were covered with acoustical tile, by how much would the reverberation time be
decreased?
5. Specify reasonable values for the reverberatioD times.
100, 200, 500, and l(xx) Hz for a 2(XX)-m3CODCeIt
I-.
to be used primarily for orchestral music.
6. If two hard parallel wa11sare spaced30 m apart, ca1~
the repetition rate of the ftutter echo that migbt ~
What efforts might be made to prevent its occurrence?
7, Repeat ahistoric experiment done by JosephHenry ~
than 130 years ago. Clap your bands periodically as>"move away from a large, ftat wall. Determine how r.
away you have to be in order to distinguish the ~
from the original sound. Divide twice this distance by ~
speed of sound to obtain the "limit of pticeptibility," .
Henry called it [see J. Acoust. Soc. Am. 61 250 (1977)~
Compare your result to that given in SectiOD23.3.
8. Find the reverberatioo time at 8(XX) Hz for a very 1itt
room having a volume of l(xx) m3 when the temper8bJlt
is 20°C and the relative humidity is 30%. Assume d8
absorption by the walls is negligibly small. Would Y<MI:
answer be different if V = 100 m3 instead?
9. Show that in a free fieid, a graph of L p versus log r is I
straight line having a slope of -20 (see Fig. 23.1).
25
1. Calculate the frequencies of the fiBt three resonancesof
a room with dimensions S m x 10m x 2.S m. Do they
have any significaoce acoustically?
%. Suppose tbat two loudspeakeB. each 300 from the median plane. carry the same program material. but the
loudspeaker ODtbe left bas twice the signal strengtb of
the one on the righL Describe the location of the image. (Calculate sin 8[. and look in a set of tables or use a
packet calculator to determine the an~e 8[ corresponding to your value of sin 8[; sin 300 = 2.)
S. The sketch below representstbe moro described in Exercise 1. Indicate tbe locatian af tbe maximum sound pressure level when tbe moro is excited in each af its first
three resonances.
3. Draw a diagram of a room with dimensions in the ratio 4 : 3 and indicate the favorite listening areas for two
different placements of a pair of SpeakeBin a stereo system.
4. From the chart given in Fig. 24.2. detemline the distance
from a loudspeaker with Q
S in a room with A
500
at wbich direct and reverberant sound are equal in level.
(Hint: Lp - Lw should be 3 dB above the reverberant
=
level.)
=
6. Supposethe time/amplitudetrading ratio is found to be
100JLs/dB.If ODespeakeris 0.2 m farther away than a
second.how muchgreatcrmustits soundpowerlevel be
in order that the soundimage will appearto be on the
medianDIane?