3. Sound Level Requirements:
Normal peak levels of 85 dB SPL are common in the
theater. A minimum headroom level above this figure
would be 10 dB; that is, the system should be capable of
handling peaks up to 95 d B - S P L Wherever possible,
JBL recommends that a headroom figure of 15 dB be
designed into the system.
The following factors have been taken into account
in determining system and amplifier choice for various
house sizes:
a. Room volume
b. Room boundary
c. Reverberation time
d. System sensitivity and power rating
e. System directivity index
4. Recommendations for Various Size Theaters:
A. For theaters seating up to 200 patrons; volume
approximately 1700 cubic meters (60,000 cubic feet):
*Preferred
system:
Model 4673 (direct radiator LF)
Power needed for 100 dB: 46 watts
Recommended power amplifier:
JBL 6230 in
bridged mode (300 watts into 8 ohms)
Characteristics: Broad-band sensitivity 97 dB, one
watt at one meter; 500-Hz crossover into Bi-Radial horn;
integral power response correction; smooth response
down to 45 Hz; ideal quality for both dialog and music;
shallow profile.
* Alternate system:
Model 4671 (direct radiator LF)
B. For theaters seating up to 350 patrons: volume
approximately 2800 cubic meters (100,000 cubic feet):
*Preferred system:
Model 4670B
Power needed for 100 d B : 30 watts
Recommended power amplifier:
Single section of
JBL 6230 (75 watts into 4 ohms, the impedance of the
low-frequency section of the 4670B system.)
Characteristics: Sensitivity, 100 dB, one watt at one
meter; 500-Hz crossover for most natural dialog and
music quality; integral power response correction; LF
response smooth down to 45 Hz; shallow profile
'Alternate vented horn systems:
Models 4672A and
4674A
Power needed for 100 dB: 30 watts
Characteristics: Mid-band sensitivity, 103 dB, one
watt at one meter; LF response limited due to horn load-
ing; to be specified where available amplifier power is
limited (typically, 50 watts or less)
C. For theaters seating up to 500 patrons: volume
approximately 4500 cubic meters (160,000 cubic feet):
'Preferred system:
Model 4670B
Power needed for 100 dB: 50 watts
Recommended power amplifier:
single section of
JBL 6260 (300 watts can be delivered into 4 ohms, the
impedance of the low-frequency section of the 4670B
system)
Characteristics: Sensitivity, 100 dB, one watt at one
meter; 500-Hz crossover for most natural dialog and
music quality; integral power response correction;
smooth response down to 45 Hz; shallow profile
Power needed for 100 dB: 46 watts
Recommended power amplifier: JBL 6230 in
bridged mode (300 watts into 8 ohms)
Characteristics: Broad-band sensitivity, 97 dB, one
watt at one meter; 800-Hz crossover with integral power
response correction; smooth response down to 45 Hz;
slightly less realistic on dialog than recommended sys-
tem; shallow profile
'Alternate vented horn system:
Model 4674A
Power needed for 100 dB: 50 watts
Characteristics: Mid-band sensitivity, 103 dB, one
watt at one meter; LF response limited due to horn load-
ing; should be specified only where available amplifier
power is limited (100 watts or less)
* Alternate vented horn systems:
Models 4672A and 4674A
Power needed for 100 dB: 23 watts
Characteristics: Mid-band sensitivity, 103 dB, one
watt at one meter; LF response limited due to horn load-
ing; to be specified where available amplifier power is
limited (typically, 50 watts or less)
The systems discussed above are shown in Figure
1 A, and typical free-field response curves are shown at
B and C.
2
