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Part 2
Professional speakers designed for permanent installations come
in many flavors. Two-way, three-way, component systems and systems
supported with subwoofer/bass boxes. Two systems are the most
common. Two-way with a sub is a newer common preference and recent
new 3-way speaker designs are also getting good marks in a church
setting.
Speaker Coverage
For live sound all full range speakers have a certain dispersion
(coverage) pattern. This pattern is expressed as a horizontal
and vertical angle. A common dispersion angle of a speaker is
90° horizontal and 60° vertical. What this means is that
if you’re standing right in front of the speaker (on axis
with it) and then walk 45° to the left or right (keeping the
same distance from the speaker), when you reach that point, the
sound pressure level will be 6dB lower than from where you started.
The dispersion angles give the area the speaker covers, ±3dB.
This area isn’t a square or rectangular, it’s more
of an oval shape. There are, of course, other dispersion angles.
It’s common to see combinations such as 90°x45°,
90°x60°, 60°x60°, 60°x45°, 45°x45°
and even wide dispersions like 120°x60°. Determining what
dispersion pattern to use or if you need more than one speaker
each with a different pattern requires a good understanding of
audio system design and acoustics.
Something you need to know about dispersion characteristics is
that they are frequency dependant. Remember, sound waves have a
physical size. To control them, the “controller” must
be physically large compared to the wavelength of the sound wave
itself. The horn and baffle board on a speaker is what does this
controlling. If the horn is small, it can only control very high
frequencies. If it’s very large, it can control lower frequencies.
Smaller speakers will offer control down to 1200Hz and larger speakers
can go as low as 500 hertz. This isn’t always the case, and
you must read the specification sheet properly and know how to interpret
the polar plots to determine exactly what’s going on. Some
specification sheets will claim a speaker has a dispersion pattern
of 60° horizontal, when in fact it doesn’t get that narrow
until you’re up to 1,500Hz, well above the fundamental range
of speech! The desire is to have the right dispersion to cover the
seating area, but not the walls or ceiling.
Example of a polar plot for the horizontal coverage of a speaker.
If you’re looking at a 2-way or 3-way speaker, find out
what the crossover point is between drivers. If, in a 2-way speaker
the crossover point is at 1200Hz, that tells you there’s
very little dispersion control in the speech range. This means
there’s a great chance you’ll have problems with feedback
in the system – especially if you are not using or able
to use the sweetspot of the room.
Remember the problem with the mono left-right speaker system
in regards to the frequency response? When two sound waves meet
in the air, they will either add up and be twice as loud, or they’ll
cancel. What exactly happens is determined by the distance between
the speakers and how far the measurement point is between the
two (and whether it’s equidistant from the two or if it’s
closer to one side or the other). This effect is called comb filtering
and is quite detrimental to intelligibility since some seats may
have certain parts of speech boosted while other seats don’t
hear other parts at all. Even with a cluster system, this is a
concern when more than one speaker is used to cover the entire
room. There will be some degree of comb filtering wherever the
coverage patterns of two or more speakers overlap each other.
Because of this, it’s very important to understand exactly
what the dispersion patterns are doing with the speakers you’re
using in the room you’re putting them in. Simple tricks
like putting the overlap region in the isles can work well, but
it’s not always that simple. More often than not, compromises
must be made. Knowing what compromises are acceptable and which
are not takes experience of a seasoned church audio professional.
Make it Loud!
How loud is the speaker? Better stated, how sensitive is the speaker
to an electrical signal? This is often called the speaker’s
sensitivity and it’s measured by playing a test signal with
1-watt of power into the speaker and measuring how loud the speaker
is 1 meter away. A speaker with a sensitivity rating of 99dB (1W,
1M) is louder and more efficient than one rated only 95dB (1W,
1M). You would need two of the speakers rated at 95dB (1W, 1M)
to equal what the other single speaker can do. In addition, the
more sensitive speaker can use a less powerful amplifier and achieve
the same sound pressure level. This goes hand-in-hand with the
power handling of the speaker and the maximum output level. Will
the speaker be able to play loud enough without distortion for
both music and speech? If not, you need to either look at a speaker
that can, or add enough speakers to do the job. Doubling the number
of speakers only adds 3dB to the system. It takes an increase
of at least 6dB to double the sound pressure level, and, as humans,
we need about a 10dB change to think it’s twice as loud
or half as loud. So, if you’re looking at a speaker that
can play 95dB (1W, 1M), but you actually need 101dB (1W, 1M),
you’ll need 4 of these speakers. It may be a lot cheaper
and more aesthetically pleasing to simply find a speaker that
has a higher sensitivity rating so you only need one speaker.
Sure, the single, more sensitive speaker will cost more than one
of the less-sensitive speakers, but it’ll be a whole lot
cheaper than buying four of the other speakers and four amplifiers!
The book “Why Are Church Sound Systems & Church Acoustics
So Confusing” written by Joseph De Buglio contains many
tips and guidelines for choosing the right number of speakers
and amplifier power for the system. The book includes a chart
which suggests combinations based on room shape and denomination
(Traditional, Evangelical, and Pentecostal/Charismatic).
Many people are under the false impression that it takes a bigger
system for music than it does for clean undistorted speech. If
your minister raises and lowers their voice a lot, that could
be a range of 15 to 25dB. Music often is performed within a 10dB
range or 10 times the power. If a person speaking has a range
of 25dB – which many ministers do, the power demand can
be 128 times. This difference in power is tremendous; will the
speaker be able to handle this wide range of power? (Will the
amplifier be able to provide this amount of power without distortion?)
A system designed for music only or primarily won’t stand
up to the needs for clear, undistorted speech when the time comes.
Distortion
At one contemporary church, members complained it was much too
loud when the SPL (sound pressure level) reached 95dB. Down the
street was a Methodist church with a pipe organ and seating for
600 people. During a typical service, one could measure levels
of over 110dB when the organ was played and the congregation sang.
No one complains it’s too loud. What you need to understand
is that clean, undistorted sound is much more pleasing to our
ears than distorted sound. In fact, a speaker playing with only
50 watts of power and 10% distortion will always be perceived
to be louder (and often more annoying) than a speaker playing
with 200 watts of power and no or very little distortion.
Using speakers not large enough for your requirements or amplifiers
which can’t supply the needed power without distortion means
you’re not just annoying people, but turning them away unnecessarily.
E xample of normal and clipped (distorted) signals (From mm
Productions)
Remember, the acoustic output of any acoustic instrument always
results in a pure waveform that’s not distorted. Distortion
comes from the electronics we use (and in some cases, extreme
poor acoustics), not from the instruments themselves. If our desire
is to reproduce the sound of instruments and speech in a natural
way, we must avoid distortion at all costs.
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