موضوع: Live PA - Basic Sound Engineering Overview

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  1. #1

    Live PA - Basic Sound Engineering Overview

    Live PA - Basic Sound Engineering Overview

    Live Public Address (Live Live P.A.) systems
    come in many different shapes and sizes and can
    often confuse the newbie into not knowing even
    just the basics. This article is aimed at giving
    a basic overview of non-specific equipment
    configurations in an attempt to de-mystify some
    of the typical errors a newbie can make with
    Live P.A. systems.

    The function of a Live P.A.
    Two types of Live P.A.
    Reinforcement
    This is for speech or music which would sound
    good in a small room without artificial
    assistance.
    In the case of a classical guitar which is a
    very quiet instrument, this natural sound is
    only good for an audience of around 200 - 300
    depending on room size.
    For an audience of 500 - 600 it is possible to
    reinforce the sound so that everyone can hear
    clearly and to most people it will still sound
    natural.
    Amplification
    This is where the original sound is
    insignificant in comparison with the amount of
    sound coming from the Live P.A.
    The aims of public address:
    1.) To provide adequate volume ( not necessarily
    loud ).
    2.) To provide adequate clarity.
    A Thought on Acoustics
    A discussion on sound reinforcement is
    impossible without a mention of acoustics.
    The free field
    If the venue is an outdoor event then the
    engineer need not concern himself/herself a
    great deal with acoustics as this is the ideal
    situation.
    Sound in the open air travels away from the
    source and keeps going until it's energy is used
    up ( inverse square law ). There are no walls
    for the sound to bounce off and return to
    interfere with the next wavefront.
    Indoor Live P.A.
    Sound behaves in much the same way as any other
    wave, it bounces off walls (reflects), and bends
    around them (diffracts), and cannot pass
    directly through materials. Therefore speaker
    placement becomes important as does speaker
    coverage.
    Consider Figure 1.0.
    The sound waves being ommited from the cabs have
    a coverage of 120 degrees therefore it can be
    seen that there will be obvious 'blind spots' in
    the coverage.

    Figure 1.0 Shows a small venue and a typical
    coverage angle of a driver.
    It should be pointed out that the directionality
    of sound waves is somewhat frequency dependent
    and that the above diagram shows a potential
    problem for high frequencies.
    High frequencies have a smaller wavelength than
    low frequencies hence they are very directional
    ( λ = v/ƒ ), λ = wavelength, v = velocity, ƒ =
    frequency .
    Objects placed in the Live path of HF block
    them, whereas LF tend to bend (diffract) around
    them. Therefore a subject positioned behind the
    wall in Figure 1.0. will hear an attenuation in
    HF hence a dull sound.
    Golden rule number 1.
    Always ensure nothing is in the line of sight of
    a HF driver otherwise you may encounter loss of
    HF.
    Reflection and phase cancellation (comb
    filtering )
    Consider Figure 1.1.

    Figure 1.1 Shows the Live path of a sound wave,
    the venue is assumed to have reflective surfaces
    and after a period of time the wave can be seen
    to have travelled around the room bouncing off
    the surfaces and crossing over other sound
    waves.
    This situation can create what is know as 'comb
    filtering'. As you can imagine the sound takes
    time to travel around the room (340 metres/sec)
    and if the reflected wave (having being time
    delayed) coincides with another wave whose
    polarity is the inverse or a fraction of, then
    cancellation will occur.
    Conversely, if the combination of merging waves
    have the same polarity then addition will take
    place.
    The name comb filtering is adopted because
    looking at the frequency response of the
    product, the shape of the teeth on a comb can be
    seen. Showing areas of addition and subtraction.


    Figure 1.2 The peaks and troughs of comb
    filtering can be seen in this frequency response
    plot.
    Golden rule number two.
    Ensure the minimum amount of reflection by
    pointing the speakers in a suitable direction.
    The main thing is to keep comb filtering to a
    minimum this is sometimes easier said than done
    as most venues have reflective surfaces. There
    will always be pockets of 'bad sound' and
    pockets of 'good sound'.
    If you wonder round a venue and listen to the
    mix you will find these spots, it is your job as
    an engineer to keep the 'bad spots' to a minimum
    through speaker positioning, coverage and
    equalisation.
    In professional venues architectural
    acousticians get paid lots of money to design
    environments which produce 'good spots'
    throughout the venue by such methods as
    absorption paneling.
    Standing waves
    Standing waves are a result of sound being
    reflected back and forth between two parallel
    surfaces.
    As the first wave reflects it meets a newly
    arriving wave and the result can be that a
    stationary wave is produced which resonates at a
    frequency dependant on the transmitted waves and
    the distance between the Live parallel surfaces.

    The wavelength of the transmitted waves in
    relation to the distances between the Live
    parallel surfaces is important for consideration
    then.
    If this distance equals the wavelength or a
    ratio of it then a standing wave could be be
    made to oscillate.
    Example
    The wavelength of a 20 Hz wave is 17 metres, if
    this wave was transmitted between two Live
    parallel surfaces whose distance was 17 metres
    an oscillation could occur.
    Standing waves can be a problem in venues where
    the dimensions of the venue coincide with Live
    particular wavelengths.
    At LF, standing waves can 'creep up' on the
    engineer as they gather energy and appear to
    'feedback', which could of course occur if the
    standing wave was picked up by the microphones
    on stage and amplified.
    Careful use of room equalisation and speaker
    positioning can combat standing waves to a
    degree.
    What's wrong with a lot of Live P.A.'s
    Low efficiency speaker systems
    cure - ensure you have efficient speakers.
    Not enough amplifier power
    cure - ensure you have plenty of amp power
    Poor frequency response
    cure - ensure all components in the chain have a
    'flat' response
    Miss half your audience
    cure - ensure you have enough speakers that are
    angled to cover everyone
    Room reverberation swamps the sound
    cure - choose speakers with suitable directional
    and dispersion qualities, thus avoiding
    reflective surfaces
    Basic systems for two different sized rooms
    Example 1
    A small sized room having the dimensions of
    around 30 by 30 by 10 feet.


    Figure 1.3
    The system in block diagram form.

    .


  2. #2

    live PA part2

    Figure 1.4
    This would be a suitable setup giving adequate
    coverage.
    The power amp would be rated at around 150/200
    watts per channel and the speakers would be full
    range.
    Example two
    A medium sized room having the dimensions of
    around 50 by 40 by 15 feet.


    Figure 1.5

    Figure 1.6
    This system is known as a two-way system, and
    for this room a total power of around 1KHZ would
    be adequate. The audio spectrum is split in two
    at around 250 Hz. Thus two power amplifiers are
    nessessary. Percentage wise the Low end would
    have around 65%. Leaving a further 35% for the
    mid and top end.
    Large Live P.A.
    Large Live P.A. systems can often be anything
    from two-way to five way and can contain massive
    amounts of drive untis for each seperate
    bandwidth.
    A large Live P.A. system would have two
    engineers. One for the front of house one for
    the monitirs mix.
    Often delayed loudspeakers are needed.
    In a large open air concert say, a person
    standing 340 meters from the stage will not hear
    the emitted sound wave until one second has
    elapsed if he/she is standing 640 meters then
    two seconds will have elapsed before the sound
    can be heard.
    This is due to the speed with wich sound travels
    through the air. i.e. 340m/s.
    By the time the sound has travelled this
    distance it has suffered great losses.
    Therefore, further speakers will be needed for
    the audience to the rear of a concert.
    The sound from these drivers need to delayed in
    order for the sound emitted from the main
    drivers to be of the same phase.

    Figure 1.7 The basic configuration of delayed
    loudspeakers.
    The delayed signal could come from groups, from
    the main mix or aux's etc.
    Further points to note:
    A rock Live P.A. should be as intelligable as a
    West end musical.
    It should cover the audience evenly allthough
    this is not always possible.
    The system should be visibly in tune with the
    type of work and surroundings

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