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The Birth of the Array: Part 4

The date is 1 June 1875… “Come here Mr Watson, I need your artisan ministry to alleviate the constrictive impediment of this redundant bottle top eradication apparatus!” As Alexander Graham Bell struggled with the faulty bear bottle opener, his beer roadie, Mr Watson couldn't hear him in the next room. The Scottish engineer gave up on the beer and went back to the traditional health giving properties of Scotch. He considered that, what he really needed was “a contrivance of electrified wires for the purpose of telegraphing speech”. The next day he invented the telephone, and Watson never heard the end of him.

Mr Bell demonstrating to Mr Watson his new Scotch-decanting device.
Mr Bell demonstrating to Mr Watson his new Scotch-decanting device.
The world's first and very wooden intercom
The world's first and very wooden intercom

Part of the new invention was an electro mechanical transducer that could convert the electrical energy of voltage and polarity oscillations of an electrical flow, into a mechanical motion capable of moving the air, in a manner that imitated the variations in the electrical flow. This device could also work in reverse in that vibrating air could induce a similar electrical current flow that could intern, be converted back into mechanical energy at the other end of the wire. Thus both the microphone and speaker were born and it was then only a matter of time before someone would be standing up yelling 1–2 testing.

This harks back to the history lesson at parts one and two of The Birth of the Array . Well let's now jump forward to the conventions of now. Just as a sports car has always had the internal combustion engine in the front and the trophy chick in the passenger seat, early speaker system designers discovered that you have to connect the ‘noise motor', to the environment efficiently if you want to get a viable energy transfer.

Think of it like this; like hot sports cars, it's about matching the engine to the transmission and suspension system. A big motor in the wrong body isn't going to perform. It's even more important if you want to get anything but noise from a loudspeaker. So let's take apart some speaker boxes.

The Speaker Box: Why a box?

full range speaker box This style of commonly available speaker is called a 2-way or full range speaker box. It has a horn-loaded tweeter for the high frequencies and a mid-low frontal radiating speaker for the mid-low frequencies. A ‘passive' crossover sends the appropriate frequencies to the correct speaker components. You may ask, “why a box, why not just in free air, and what are the holes in the front for?


a bass box Here we have a bass box - just one speaker for reproducing low frequencies. We could just have a small sealed box and it would produce low frequencies. So why put a big hole in the front. Let's look at the speaker first and how it works

Most loudspeakers are really an Electro-Magnetic motor

Cone & Suspension

A loudspeaker is a form of AC electric motor with a cone attached to transfer the mechanical motion of the motors output, to acoustical energy. It consists of a coil of wire set in a permanent magnetic field. When current flows through the wire, the wire becomes momentarily magnetised.

Wire to voice coil: The coils magnetic field is either attracted to or repelled from the magnet, depending on whether it is a positive or negatively charged field. Since the incoming signal is alternating continuously, the cone is vibrating hundreds or thousands of times a second.

So you see there is not much too how it works but the devil and money is in the detail of how to get it to sound good.

An oldie but goldie. This Altec 604 driver was an attempt at full-range point source sound. It has a top-end driver mounted on the back firing down a horn through the centre of the driver. The English company Tannoy built something like as well. They both sounded great and their well over 50 years old.

The dual concentric driver in all its cutaway glory. A fine piece of precision engineering that would have cost several weeks wages in 1949. There are Hi-fi enthusiasts today that still say there's nothing better for orchestral reference. Some others might say this design is old world enough to live in an Amish community.

speaker As the speaker cone moves back and forward to imparting vibrations to the air. It produces as much energy at the back of the cone as it does from the front. In a sealed box this would be wasted energy. In open free air, the speaker cone will vibrate but no low frequencies will be audible.

So, why put it in a box? And, what is the hole in the front of the box for?

It's another long story. The basics of speaker box design well and truly precede the line array, the speaker box or even the speaker. To answer these questions, we go way back to the early 19 th century before Mr Bell was born.

Introducing Hermann and the Helmholtz Resonator:

Hermann Ludwig Von Helmholtz (1821-1894) A German Physicist helped prove the law on the conservation of energy. He was also interested in music and sound, and built an electromagnetic tuning fork and other test devices. He wrote extensively on psychoacoustics. His papers on how sound generation, harmonic structure and sympathetic resonance are all still underpin most of the principles used by designers today.

In his maths class, it's “Machen Sie auf oder machen Sie aus!”

The blackboard behind demonstrates mathematically his theory that German Death Metal is not inspired by Satan.

Herman was not just a maths freak, he was very practical, and his maths were followed by experimental devices that formed the basis of new technological developments and new theories.

Hermann's electronic tuning fork helped the telephone get going and a stack of other inventions that are still around.

If you don't recognise that audio device, maybe the next one will be familiar?

The resonant frequency of a Helmholtz resonator depends on its volume, and a cylindrical resonator permits the volume of the resonator to be changed by sliding the tubes in and out. The notes (and hence the resonant frequencies) are engraved on the side of the apparatus. The little spheres are also tuning devices to demo the theory of resonating air to produce pitch.


Why the funky Christmas decorations?

Helmholtz is demonstrating the form of resonator that he developed for picking out particular frequencies from a complex sound. It was part of a greater thesis that went on to explain just about everything about sound and instrumental timbre. We won't go into now but it is sufficient to say his machine works as follows.

The resonators pictured were developed for picking out particular frequencies from a complex sound. The resonator consists of a body to contain a volume of air, a hole or neck in which a small mass of air can vibrate back and forth, and a slender exit that can be held in the ear canal (or, today, connected to a sound level meter). The enclosed volume of air acts as a spring connected to the column of the small mass of air, and vibrates at a frequency dependent on the volume of the air in the larger enclosed area.

Some small whistles are Helmholtz resonators. The air in the body of a guitar acts almost like a Helmholtz resonator. An ocarina is a slightly more complicated example. Loudspeaker enclosures often use the Helmholtz resonance of the enclosure to boost the low frequency response. Here we analyse the process of what is happening:

The vibration here is due to the 'springiness' of air: when you compress it, its pressure increases and it tends to expand back to its original volume. Consider a 'lump' of air at the neck of the bottle (shaded in the middle diagrams). The air jet can force this lump of air a little way down the neck, thereby compressing the air inside. That pressure now drives the 'lump' of air out, but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring (diagram at right). The jet of air from your lips is capable of deflecting alternately into the bottle and outside, and that provides the power to keep the oscillation going.

So let's apply this to a speaker box to get more ‘Doof'

Remember that a loudspeaker is vibrating back and forward pushing and pulling on the air, which is an elastic mass resisting movement. It can be compressed but when it is surrounded by lower pressure, it wants to expand back to the same pressure.

speaker If the speaker resonates in free air, the energy from the front and back cancel themselves out. Without a baffle to separate the positive and negative air movements, no bass sound wave energy is audible. The purpose of the tuned box is to delay the energy long enough to have the bass frequencies exit the port in time (polarity) with the forward facing speaker. This gives us a lift in the bass. It also has other technical advantages and allows a designer to extend or restrict the range and frequency response of the speaker.

Above could be called a phase inverting acoustic capacitor. The size of the port and the volume of the box determine its tuning. If this is matched correctly to the speaker, more ‘doof' will be your reward.

Here is a test you can do your self. Run some full-range music into a speaker box, unscrew the bass driver and hold it in the box, then listen to the loudspeaker loading in and out of box. You will hear a big difference.

Try it with a horn driver on and off a flare. This time the driver will get a lot quieter when not attached to the flare. The difference in each case is how the driver (motor) is ‘grabbing' the air. This is called loading the speaker. In free air it is ‘unloaded'.

The energy generated from behind the speaker driver is ‘delayed' physically for a short time and then ‘springs' from the port in time (phase) with the motion of the front of the cone. This is the basic principle of the bass reflex or Helmholtz resonator as applied to a speaker box. The simulated box pictured also could be altered to have other feature that will maximise the low frequency output.

Let's add a horn somewhere

The energy from the rear of the cone could be emerging from a tuned expanding pipe, otherwise known as a horn. By a calculation including horn length, flare rate and mouth area, a low frequency cut off can be determined that will give considerable amplification at the designed frequency. The cut off frequency for the horn would be ¼ wavelength of the lowest frequency. In other words, if the horn length was 7' long, the lowest frequency would be a 28' wave. That's about 41Hz. (Low E on a bass).

There is a fair bit of woodwork here but you will more than triple your amplifier's effective power by the extra bass output.

rear horn loaded box This is a rear horn loaded box and it has a very long path length. It also opens at the back and is designed to sit with the rear opening in a corner of the room, using the corner walls as an extension of the horn. It would have had massive efficiency and with only a few watts in, it would have had loads of output at very low frequencies.

How does a flared pipe (horn), differ from a straight pipe or tuned chamber?

If a tuned ported box is like an acoustic capacitor in that it stores energy and releases it in a burst, the horn is like a transformer that can convert the very high acoustic pressure at the speaker end of the horn to a large area at the open end. The air at the open end acts as an invisible diaphragm and ‘hangs on' to a large air mass to move it very efficiently.

Horn Loaded Boxes

Horn Loaded Boxes

If the first horns were real hollowed out animal horns or modified crab shells, horn loading must be pretty old technology. A horn in modern sound reinforcement had two purposes;

Directivity . A horn provides more sound pressure level (SPL) at a given listening area by increasing the directivity of the sound towards the listener. There is more sound energy directed at the listening area, and less sound outside of that area. By analogy, think of focusing a beam of light (from a flashlight or torch). A widely focused beam spreads the light around, reducing the intensity at any one point. However, a narrowly focused beam provides much more light intensity at the centre, and much less in the surrounding area.

A practical self-demo:

You can get more low frequencies from your bass boxes if you put them in the corners.

Consider a point source speaker driver hanging high up in the air. Sound will be radiated off in all directions. Now if we sit the driver on the ground, it has only half the area to radiate into and acoustic power will be increased by two at any position. If we place the driver at the corner of a floor and a wall, we now radiate into quarter-space, and SPL is increased by four times. Likewise, a driver in a corner will be constrained to one-eighth of the free space area, and SPL is eight times louder. Without any other effect considered, the corner ‘loading' has given us an increase in apparent power by just sending most of it in one direction. (Not withstanding the possible losses we have already done to death).

Impedance transformation leading to efficiency increase.

That title sounds pretty much like ‘Boffo' talk; keep going, it gets explained.

The second common property of a horn is acoustic impedance transformation. Impedance refers to the resistance when it is presented to alternating current. Since the driver is working into a narrow opening, it is presented with high pressure as it forces the wave though a small opening. Hence it is called a compression driver.

high-frequency compression driver In this high-frequency compression driver, the diaphragm is a metal dome facing a ‘ phase plug '. The phase plug is there to create a number of time correcting path-lengths for the sound wave to exit the driver and enter the horn entrance all travelling the same distance from their point of creation. In other words, it prevents distortion.

This couples a compression driver to the air. When we constrict the area/volume of air into which the driver radiates, we increase the acoustic impedance of the air bring it closer to that of the driver. (We stiffen the air up so it is more like the diaphragm in resonant behaviour).

Now, we provide a gradual increase in area until we get to the horn mouth, which is much larger than the throat. The mouth couples the horn throat to the air in the room. The air at the mouth of the horn acts like a big invisible diaphragm.

Some compression drivers have very limited travel and power handling, and require this acoustic transformation in order to work at their designed efficiency. However, some modern compression drivers are much more tolerant of power handling. These can often be used in horns that do not just focus on providing impedance transformation, such as Constant Directivity horns .

A constant directivity horn has as its main virtue, even frequency response across the face of the horn with minimal hot spots. Horn speakers have of course some disadvantages also, as the length of the horn ideally should be 1/4 of the wavelength of the lowest frequency that should be transmitted; this means that horn speakers tend to be big, very big if the low frequency response should be good. Real horn speakers are usually some compromise from the theoretical dimensions and therefore some horns sound really good and some are really bad.

A horn-loaded system for the horn hi-fi sicko! This is a four-way system; check the ‘organ pipes' in the centre. Remember low frequencies are long wavelengths. A horn-loaded system for the horn hi-fi sicko! This is a four-way system; check the ‘organ pipes' in the centre. Remember low frequencies are long wavelengths.

The drawback of horn-loaded boxes is that the horn loading (i.e., area of maximum efficiency), only works at limited frequency band pass. This is just as a tuned organ pipe only works at one note. Horn-loaded speaker systems are generally very ‘lumpy' as far as their frequency response is concerned and require a lot of equalisation to correct. This brings it's own complications.

So what does it all mean?

It all equals a point source sound system. Everything from the little Bose box in the lounge room to the big horn show comes back to a few well-established principles. Up to year 2000, nothing much had changed. We were all still using Dr. Herman's formulas with a bit of Harry Olsen thrown in. Dr. Heil had cracked a mention and the bands were getting smaller and louder.

Next issue we will see if we can put up a big old-fashioned PA and get good results

Discover More:
The Birth of the Array: Part One
The Birth of the Array: Part Two
The Birth of the Array: Part Three

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