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Commercial Sound 70-100V Line

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Realistic Overview

The Looniest nonsense to evolve from the cold war besides nuclear weapons was mind control.   With slick sales techniques, fortunes were made tricking big department stores into believing that the public could be programmed by sound into becoming 'mindless consuming robots', without realising they already were.   The remanence of this today is the vacuous elevator music in shopping centres and commercial buildings.

Voices from above may have a religious connotation but -
Low fidelity, often distorted sound from excessive reverberant ceilings is unnatural and claustrophobic.
In our natural environment the sky is silent.

Quiet non-reverberant ceilings give a feeling of openness. It is natural to listen horizontally.
Speakers are put into ceilings because it is simple, often without thought of there being other ways.

  • Sound from above imposes.
  • Sound horizontally communicates.
  • Sound from below is intimate.

Music reproduced with 'fidelity' symbolises intimacy and honesty.
Commercial sound can be approached creativity by selecting combinations of different approaches.

People of limited acoustical knowledge (often architects) have traditionally designed commercial sound installations.   Most systems function at a basic level complying with fire regulations for evacuation announcements.   Few installations function efficiently, and the majority have complications that never get resolved.

There are many excellent books on commercial sound installation; one of the most respected is
'Sound System Engineering' by Don and Carolyn Davis.

Installation Basics

Quality cable and connections are essential, including clear and detailed installation documents.   Most small ceiling speakers are generally of poor fidelity, and there are exceptions. Cost is not a guide when profits are to be made.

Be suspicious of any one claiming to be 'expert' on how sound should sound.   Wisdom is to audition speakers before installation. One does not buy a car on specifications or the salesman's opinion alone; one takes it for a test drive.

The technical requirements are often for many small speakers to be spread over large areas.   Cable length can be miles / kilometres. To minimise cable loss, the amplifier output is increased to a higher voltage through a step-up line transformer.   Each speaker has a step-down line transformer.

The Line system actually operates at approx 30V RMS but can peak at 100V.   Some line transformers have a limited bandwidth that restricts fidelity. High power large line transformers can have more inductance, and therefore a better bass response. A skilled electronic technician can check samples.

Specifications provided with power amplifiers and speakers with line transformers, are referenced to load-power (Watts) only.   This information is not sufficient for accurate academic calculations. A skilled electronic technician will require one hour to make necessary measurements, to provide the missing information, which is essential to make accurate calculations for the installation.

  1. Decide on the total number of speakers to be used and the average power to each speaker.
  2. Decide the best voltage for the lines to be driven at.
  3. Decide on power and number of amplifiers to drive the lines.
  4. Do accurate technical measurements on the amplifiers, line transformers and speakers.
  5. Be practiced with all calculations and cross check.
  6. Find peace with the Gremlins.

Testing procedures and calculations for installation design ( a - j )
Amplifiers. Line Transformers. Speakers.

(a) Line Transformer Inductance:
With no output load. Insert a 100 Ohm resistor between the amplifier electronic output and the transformer primary.   Start at 400Hz with the amplifier output at full level (rail to rail).   Sweep the frequency down while monitoring the transformer secondary output with an oscilloscope, and a multi-meter across the 100 Ohm resistor. The transformer should have sufficient inductance to function down to 40Hz.

If there is buckling in the output waveform and increased current through the resistor, the transformer has reached saturation.   Do the same test with a speaker transformer.   If either transformer has the potential to saturate at the lowest bass frequency, under normal working condition, this will cause excessive current flow similar to a short circuit. The system will be unreliable.

(b) Measure the voltage ratio
(turns ratio) of the amplifier output transformer, all taps unloaded. The voltage ratio (turns ratio) on some transformers is between, (1:2) and (1:4).

(c) Measure the amplifier 'rail to rail'.
If the amplifier has a rail supply of +30V -30V (60V total) the output will be approx 20V RMS.   If the transformer has a turn's ratio of (1:4) the secondary will be 80V RMS.

Note It is essential to know the actual load impedance (Z) of the line transformer.   This information is rarely quoted in specifications that come with the amplifier. Deducing this from the power-load specifications supplied with amplifier and speakers is educated guess work, and rarely accurate. The amplifier line transformer must be actually measured.

(d) Measure Power
of the amplifier under load. Determine whether the amplifier electronic output is designed for 4 or 8 Ohm. The output transformer impedance ratio of is the square of the turn's ratio.   If the amplifier is designed to give 100Watt (20V RMS into 4 Ohm) and transformer turns ratio is 1:2, the secondary line voltage will be 40VRMS, into a load of 10 Ohm.   If the turn's ratio is 1:4, the secondary line voltage will be 80V RMS, into a load of 64 Ohm.

Note Too many speakers on the line will overload the amplifier. An infinite number of speakers are a short circuit (0 Ohm).   A short circuit at low level can easily destroy an amplifier. The total number of speakers must represent a load no lower than what the amplifier is designed for, regardless of power.   The load directly affects the running temperature of an amplifier, and therefore its reliability.

Recorded music is compressed within a limited dynamic range.   Taking 6dB dynamic headroom into calculation, the amplifier can be driven at a maximum average no greater than 1/4 full power.

(e) For this sample
100Watt amplifier to deliver 30V RMS of music on the line, with 6dB dynamic headroom, the line should be connect to the (1:4 turn's ratio) tap of the output transformer. This will allow the total number of speakers connected, to represent a load no lower than 64 Ohm.   If the total number of speakers to be connected represents a load less than 64 Ohm, then the choices are -

  1. Use the 1:2 turns ratio, which allows for a load of 16 Ohm,
    but the line level will have to be reduced to 20V RMS to allow 6dB headroom.
  2. Add more 100Watt amplifiers.
  3. Use a more powerful amplifier and repeat the calculations.

(f) Audition the speakers.
Assume the speakers have no association with the specifications supplied, regardless of model number or brand is printed on the box.   It is often not possible to know in what factory the speakers were made or how many agents they passed through.

(g) The speaker
line step-down transformer that has to be measured for -

  1. Sufficient inductance to pass 40 Hz.
    An inductance meter measurement is a guide, but it will have to be actually checked at full line voltage unloaded.
  2. Measure the speaker transformer turns ratio, all taps.

(h) Suppose 1Watt
is required to each speaker for best listening enjoyment. If each speaker is 8 Ohm, then 1Watt is 2.8V RMS. (2.8V divided into the 30V line voltage, is approx 10).   Therefore select the 10:1 tap on the speaker transformer.

(i) Calculate the reflected impedance
on the line, from each 8 Ohm speaker on the 10:1 tap.   The impedance is calculated from the square of the turn's ratio. (10x10=100) x 8 Ohm = 800 Ohm. If the total number of speakers connected to the line is 12, and each speaker is connected to its 10:1 transformer tap, then the total impedance to the line is (800 Ohm divided by 12) = 66 Ohm approx.

The 100Watt amplifier is driving the line through its transformer from the 1:4 tap.   The lowest load it can drive is 64 Ohm; therefore no more speakers can be added.

(J) From these calculations
this 100Watt amplifier is operating at 12Watt average.   This allows 6dB of headroom for the music transients to peak at approx 50Watt.

Print out the procedure from ( a - j ).   Create a spreadsheet and give yourself time to go over and vary the calculations until you are comfortable with them.

Background Column Theory

The majority of column systems consist of a vertical row of small speakers. Often seen in churches, shopping centres, travel terminals, gymnasiums etc.   Their intended application is for announcements and background music.   The advantage of a column is its simplicity and being visually unobtrusive. The fidelity of a column can be no better than that of the individual speakers, regardless of marketing claims.

Column dispersion
A single speaker has a varying conical dispersion.   As more speakers are added vertically, sound from each speaker, is squeezed by the ones above and below. This results in increased horizontal dispersion.

In reality the horizontal dispersion is wavelength limited and inconsistent.   High frequencies (wavelengths less than distance between speakers) result in intense vertical lobes.   These lobes cause phase cancellations and loss of intelligibility, the high frequency energy is decreased. One solution is to cross over the high frequencies to a single tweeter.

Some small expensive columns have a complex passive crossover network that reduces energy to the outside speakers as the frequency increases, so only the centre speaker remains working at the highest frequency.   At lower frequencies (wavelengths longer than column length) horizontal dispersion is no longer effective. An average column will have a limited and inconsistent horizontal dispersion between 2-3 octaves.

--- To be continued Graphics to be added ----

End of Topic 1
Created: 10-Dec-2008