Electro-Voice Sentry 100-A Owners manual

®®

®

SentrySentry

Sentry

SentrySentry

®®

100A 100A

100A

100A 100A

Professional
Monitor System

••

Near-field monitor for post productionNear-field monitor for post production

•

Near-field monitor for post production

••

Near-field monitor for post productionNear-field monitor for post production
and live soundand live sound

and live sound

and live soundand live sound

••

Wide, uniform coverageWide, uniform coverage

•

Wide, uniform coverage

••

Wide, uniform coverageWide, uniform coverage

••

Extremely smooth frequency responseExtremely smooth frequency response

•

Extremely smooth frequency response

••

Extremely smooth frequency responseExtremely smooth frequency response

••

High power handlingHigh power handling

•

High power handling

••

High power handlingHigh power handling

Descriptions

The Electro-Voice Sentry
speaker system is the direct result of “hu­man engineering” with the broadcast/record­ing studio engineer in mind. This system has
high efficiency, extended low-frequency
range, and a uniform frequency response and
dispersion, in a compact, no-frills package.

The Sentry 100A employs a Super-Dome
tweeter capable of handling 25 watts of in­put power (most tweeters operate in the 5­watt range), while faithfully reproducing the
program material with response out to 18,000
Hz and uniform dispersion (120° at 5,000
Hz). Never again will accidental, high-fre­quency blasts from tape head contact (in re­wind/fast-forward mode) leave a curl of
smoke where a tweeter used to be. The low­frequency section is an 8 in. direct radiator
woofer installed in an optimally vented en­closure with fourth-order Butterworth tun-

1

ing.

The optimally vented design is respon­sible for the unusual combination of small
size, extended bass response and high effi­ciency. Such performance is simply not avail­able in other enclosures of similar size.

The Sentry 100A is housed in a utility cabi­net wrapped in a special, scratch-resistant,

®

100A monitor

TM

matte black vinyl. The cabinet size is inten­tionally designed for rack mounting. When
coupled with the SRB-7 rack-mount/wall­mount kit, the Sentry 100A can be integrated
into virtually any environment that demands
conservation of space, such as mobile record­ing studio facilities. The steel-reinforced
grille is covered with a custom gray cloth.
This provides maximum protection, acous­tic transparency and a pleasing aesthetic
quality.

Loudspeaker Response Due To The

Acoustical Environment

Several factors must be considered when de­termining the overall response of a speaker
system in any listening environment: physi­cal characteristics of the room itself, place­ment of speakers, and position of the listener.

As pointed out in several texts on room
acoustics, as the source-listener distance in­creased, the sound pressure level (SPL) de­creased in the direct field at a steady rate (in­verse square law: 6 dB drop for every dou­bling of distance) until a certain distance is
reached. This point is often called the criti­cal distance (D
approaches a constant value (the reverbera­tion field). The listening position in the sound

). Beyond this point, the SPL

C

field determines the amount of acoustic
power output of the speaker system needed
to produce a certain sound pressure level at
the engineer’s ears. Generally speaking, the
amount of power output needed from the
speaker/amplifier system decreases as the
room becomes smaller and/or more rever­berant (shorter critical distance). In most
cases, the audio engineer will be working
well within the direct field. If indeed this is
the case, the amplifier power requirement is
entirely dependent on the loss in SPL due to
inverse square law.

The low-frequency response of the overall
system can be adversely affected by poor
placement of the monitor speakers them­selves. The Sentry 100A was designed for

- to 1/2-space use. This requires that the

4

speaker system be mounted as close as pos­sible to floor/ceiling and/or wall surfaces.
Placement in loose cavities or resonant
mountings can also seriously degrade the
overall response.

___________________

1. A.N. Thiele, “Loudspeakers in Vented Boxes: Part 1,” J.
Audio Engineering Society, Vol. 19, No. 5, pp 386-387
(1971).

1

/

Sentry® 100A Professional Monitor System

SpeakerPlacement For Good Stereo

Imaging

Results of testing by Electro-Voice and oth­ers, such as Juhani Borenious

2

, suggest a rec­ommended listening angle y of 60° ±10° for
an optimum stereo image. Rather than mea­sure an imaginary angle, exact positioning
can be obtained by placing the speakers so
that the ratio of distances h/b = 0.9 ±0.2. The
stereo image that results is further reinforced
by the uniform dispersant characteristics of
the Sentry 100A, particularly in the higher
frequency range, where the majority of ste­reo information lies. Off-axis degradation of
frequency response cannot be corrected by
supplementary equalization. It is therefore
critically necessary to use a speaker system
with uniform directivity as well as smooth
on-axis response.

Sentry® 100A Professional Monitor System

Power Handling Capacity

Power handling specifications are usually
meaningless because they fail to indicate the
nature of the test signal and/or how this test
signal relates to actual use. The 30-watt
specification for the Sentry 100A is based
on filtered random noise (FM interstation
noise and tape hiss are common forms of
random noise), which is fed to the speaker
for an extended time (more than 15 hours).

Random noise testing is used because, like
real music and speech program material, it
contains many frequencies at once. Low fre­quencies, which cause large excursions of
the woofer suspension, are present; and mid
bass frequencies, which contribute mainly to
woofer voice-coil heating, are present as
well. Thus the woofer is simultaneously
tested for mechanical fatigue and voice-coil
overheating. Similarly, the tweeter is tested
for both mechanical and thermal failure at
appropriate power levels.

There is no generally accepted standard for
testing loudspeakers for power capacity. At
Electro-Voice, we expect each speaker and
system to survive 15 hours continuous ap­plication of rated power without failure of
any component or permanent change in per­formance.

One noise test standard, the West German
D.I.N. 45573, specifies the random noise

2

spectrum. This spectrum agrees approxi­mately with studies of voice and music spec­tra that appear in several textbooks on acous-

3,4

tics.

However, only a very small percent­age of the power applied to a Sentry 100A
would be in the range of the tweeter using
this spectrum.

A study done by John P. Overley of Electro-

5

Voice

resulted in the more realistic spec-

trum.

The curve shows the relative levels in oc­tave bands of average peak energy found in
many musical passages of a symphony or­chestra. “Based upon peaks as short as a frac­tion of a second in duration . . . it represents
the approximate distribution of energy vs.
frequency under highest signal conditions . .
. exactly those conditions which should de­termine the power handling requirements of
audio components.”

6

The musical passages
were taken from disc recordings that were
played back on “carefully equalized high
quality transcription equipment.”

7

The test signal actually used in developmen­tal testing of the Sentry 100A is shown in
Figure 9. It is an approximation to measured
spectra of the output of a lead guitar ampli­fier driven into heavy clipping, and repre­sents a worst-cast situation. The Sentry 100A
will survive 30 watts of this input for at least
15 hours.

The power-handling specification applies to
long-term application of power; for short du­ration peaks, the loudspeaker system is ca­pable of handling many times the rated
power. For example: for a few milliseconds,
the system will handle 10 dB peaks; if the
average input power level were 30 watts,
then it would handle peak power inputs on
the order of 300 watts.

Crossover Network

The integral crossover network is a 12-dB/
octave dual-section type, crossover occur­ring at 2,000 Hz. In addition, the Sentry 100A
has a continuously variable, shelf-type high­frequency control which allows adjustment

____________________

2. Juhani Boreniou, “On Loudspeaker Response in Sound

Control Rooms,” J. Audio Engineering Society, preprint
(1980).

3. H.F. Olson, PhD., “Acoustical Engineering,” D. Van

Nostrand Company, Inc., Princeton, New Jersey, p. 588
(1957).

4. L. Beranek, “Acoustics,” McGraw-Hill Book Company,

New York, p. 338 (1954).

for individual listening preferences, with
both boost and cut capability (+2 dB to –4
dB from nominally flat). The high-frequency
control is conveniently located on the front
panel for easy access.

SRB-7Rack Wall Mount Kit

One of the fundamental design requirements
for the Sentry 100A provided for the ability
to install the system in standard EIA 19 in.
racks. The hardware needed for this proce­dure is available as an optional kit. This same
kit provides for flush and angle wall-mount­ing as well. (Refer to the SRB-7 engineer­ing data sheet for complete details.)

A second nameplate, which can be applied
over the original label, has been included
for those situations where mounting the Sen­try 100A in an inverted position is preferred
(woofer nearer to ceiling).

warning:

Never mount the Sentry 100A by the back
panel or by only one of the other panels.
Failing to comply with this will cause the
panel to separate, resulting in the speaker
falling. For the safest method of mounting
the Sentry 100A, Electro-Voice strongly sug­gests using the Model SRB-7 rack/wall
mounting kit.

______________________

5. John P. Overley, “Energy Distribution in Music,” IRE
Transactions of Audio, Vol. AU-4, No. 5, Sept.-Oct.
(1956).

6. Ibid. p. 121.

7. Ibid. p. 121.