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Loudspeakers

1          Preliminary

When you talk about loudspeakers, you probably think of a box with one or more speaker drivers mounted on it. These speaker drivers are usually mount on the front surface of the loudspeaker, and this particular surface is commonly known as the baffle.

No matter its operating principle, the speaker driver consists of a palindromic AC motor, which consumes power fed from an amplifier’s output. In particular, this power has the form of AC current of varying voltage and frequency over time.

As the AC current swings its polarity over time, the driver’s rotor moves back and forth its diaphragm, moving some air and causing successive condensings and thinnings (air pressure changes, because air has some mass, thus inertia), which are perceived by the human ear as sound.

As the amplifier’s output higher voltage pushes the driver to consume more current, the latter moves its diaphragm to a larger distance from its resting position, causing higher pressure level, playing louder, and vice versa. 

Due to the Laws of Physics, technical limitations and cost, most loudspeakers comprise two or more signal paths, each path whilst ends to one or more specific drivers. As the loudspeaker is fed with the audio signal, the signal flows through a crossover circuit and splits to two or more ways, ending to the corresponding driver. A 2-way loudspeaker’s crossover circuit splits the signal to two paths, feeding the woofer with the low – mid frequencies and the tweeter with the high ones, while in a 3-way loudspeaker the signal is split to one more path for the mid frequencies, feeding themidrange.

The frequencies where the signal is being split are known as the crossover points. In a 3-way design, the low to mid crossover point is usually located somewhere between 300 and 1000Hz, and the mid to high point can be found at some 2.5 – 3KHz, according to the physical properties the loudspeaker’s specific drivers. Some loudspeakers feature one more driver –a super tweeter- for the very high frequencies, say 10KHz and above.

Also, one more special loudspeaker is the subwoofer, which applies to low frequencies only, some 15 - 120Hz. The loudspeakers of this kind feature a built-in amplifier, the appropriate crossover circuitry and input and output physical connectors, to be placed among other equipment in an audio or cinema installation.

Though multi-way loudspeakers are the mainstream, there are also a few single way designs, comprising one driver only, thus they are commonly known as full range, as this driver is utilized to play all the frequencies. However, because of the Laws of Physics, such a loudspeaker shows a rather poor performance both at low and high frequencies. 

  

2          Enclosure design

In search of a loudspeaker, you probably can find products of various enclosure configurations of the woofer’s cabinet, according to application, specific requirements, cost and taste. Sound reproduction at low frequencies is a very demanding task, and the enclosure configuration comes in to utilize the backwards sound emission of the woofer. We can discuss the most common enclosure configurations as follows.  

2.1         Sealed

This approach is also known as infinite baffle, or closed cabinet. As its name implies, a sealed enclosure consists of a completely sealed box, so that the back emission of the woofer is isolated from the environment. Compared to other designs, a sealed box loudspeaker tends to give a perception of less bass, and this phenomenon can be also documented with its measurements. However, a sealed loudspeaker is capable of better sound quality, as the bass frequencies are reproduced with significantly more clarity.

2.2        Vented

Vented loudspeakers are also known a bass reflex. Such a loudspeaker is built on a cabinet featuring a tube inside it, which ends to a vent, thus the back emission of the woofer is exploited to give sound. According to special design needs and limitations, loudspeakers with more than one vent can be found around the market.

This particular tube operates as one more woofer, which sounds at a very narrow frequency range. Vented loudspeakers are capable to produce deeper bass, at a cost of less sound quality, compared to their sealed counterparts. As rich and deep bass is a very attractive thing to most people, some manufacturers equip their products with tube of a small diameter, which is capable of wider frequency response, and, as the tube diameter is getting smaller and smaller, the speed of the air is getting higher inside it, showing the “choof – choof” effect. Nevertheless, you can find plenty of loudspeakers with good vent performance, especially of large physical size.

2.3       Horn loaded

This kind of loudspeaker is usually intended to apply to post address installations, as it’s likely a design of large physical size. Horn loaded enclosure speakers are equipped with large woofers of 15 or 18 inches diameter.

Image 1.Most common speaker enclosures in side cross sectional view, sealed (on the left), vented (in the middle), horn loaded (on the right).

2.4     Other implementations

Among configurations mentioned above, loudspeakers of other design approach are available in the market. For example, you can find panel speakers, featuring a firmly stretched membrane, which operates as a sound producing diaphragm, moving back and forth in a magnetic field made with physical magnets (magnetostatic loudspeaker), or a permanently charged membrane which is vibrating in an electromagnetic field (electrostatic loudspeakers). This membrane is made of special, high tensile strength polymer sheet, usually Mylar, coated with conductive material.

Panel speakers are likely large in height and width whilst they have a very small depth and they emit sound in a bipolar fashion, made usually for domestic use.

Also, you can find a few designs made with common electrodynamics drivers, but with no enclosure. Such a speaker features only a baffle with all its drivers mount on it. These loudspeakers sound in a bipolar fashion as well.  

3          Active, passive and multi-amped loudspeakers

As mentioned above, any loudspeaker is an electrical load, consuming power from an amplifier’s output. According to how a loudspeaker is connected to the amplifier, it is either a passive or an active one – and this is a very important criterion.

3.1                    The passive design approach

As this configuration was the mainstream for decades, they are still the most common loudspeakers. A passive loudspeaker comprises a crossover circuit, which consists of some inductive and capacitive passive components (coils and capacitors respectively), and usually some resistors as well.

Each way of this circuit is meant to behave as an inductive or capacitive load, thus it partially suppresses the frequencies, forwarding the rest to the corresponding driver(s).

This design approach has the advantage of simple and easy design, relatively low cost, and it’s easy to manufacture. On the other hand, passive loudspeakers require an external amplifier, but this is not the case.

Image 2. Artistic depiction and block diagram of a passive loudspeaker, connected to an amplifier. The crossover circuit handles the signal at power level (0,5 – 100VRMS and above).

 

The true downsides of speakers of this kind are that they are prone to poor sound performance and power loss because of the passive crossover’s nature. As this crossover handles the signal at power level (at a voltage of 1-2 Volts and in excess of 60 or 70 Volts and large current, perhaps more than 10 Amperes), it suffers of thermal compression, resulting to inaccurate performance after it is playing loud for just a short period of time. Also, the passive crossover, with the speaker cable, consumes itself some 10% of the amplifier’s power, especially if long cable runs are used (in excess of 2m / 7f).

3.2     The active design approach

Compared to a passive one, an active loudspeaker is more complex, but this could be its only disadvantage. Active loudspeakers comprise an active crossover circuit, that is, an active circuit with resistors, capacitors and transistors, which functions as a zero gain preamplifier and equalizer. Also, active loudspeakers have their own built in power amplifiers, one per way.

So, an active loudspeaker requires a preamplifier to feed it with line level signal, thus it has the appropriate signal input, made with an XLR connector (usually balanced connection), or a common RCA socket for unbalanced connection. Also, as this loudspeaker has active components inside it, it must be plugged to a mains socket.

Image 3. Block diagram of an active loudspeaker, connected to a preamplifier. The crossover circuit handles the signal at line level (0,1 – 2VRMS).

As the active crossover handles the audio signal at line level (at a fraction of 1 Volt and some milliamperes only), it is capable of very accurate frequency separation, whilst each amplifier has to do a significantly lighter job, as it’s driving a single speaker driver, so they are capable of clearer sound performance with far less distortion of any kind. Also, active loudspeakers are more efficient and their connection to a system has to be considered much simpler.

 

Main characteristics

 

3.3         Frequency response

The frequency response is probably the most common –whilst very important- specification of any loudspeaker. If you take a look at the specs sheet of a loudspeaker, you should read this specification in a fashion like “40 – 20000Hz”. If the manufacturer is more serious and honest, this specification should look like “45 – 18000Hz ±3dB”. This tells you that if the loudspeaker is fed with audio signal of such a voltage, that it can force the speaker to consume power of 1 Watt, it will play sound within that frequency range at a level fluctuation of no more than 3 decibels louder or lower than its “average” level.

This specification is occasionally depicted as a curve in a two-axis graph, where the x axis measures frequency and y axis the sound level. Probably, the flatter the curve, the more precise the loudspeaker’s frequency response. Large raises or dips are commonly known as coloring of sound. 

3.4                    Power handling

First, please, think of an electric boiler. Such an apparatus is an electric load, which consumes constant power over time from the power grid, as it’s fed with AC current of practically constant voltage and frequency. A loudspeaker is an electric load as well, but when in operation, things go far complicated. If you turn the volume knob up, you increase voltage, forcing the loudspeakers to consume more current, thus more power, thus they play at a higher sound level, louder, and vice versa. So, loudspeakers are electric loads of varying power, but there’s an upper limit of course.

Simply speaking, power handling of a loudspeaker is a term that tells you the maximum power it can consume without thermal or mechanical damages.

Power handling is commonly specified with watts RMS, which is rather a false and misleading term. Probably, this specification states the maximum average continuous power of the loudspeaker. Average continuous power means that the signal consists of a single frequency and constant voltage, but of course this is far from reality. As the music has low and high passes over time, the signal voltage fluctuates, rendering the speaker to consume variable audio power over time. The loudspeaker is by nature capable of consuming power higher than continuous for a short period of time, and this is where the program power comes in. So, if the loudspeaker is specified with something called program power, this power is always higher than the average continuous power.

 

3.5                    Sensitivity and SPL

Sensitivity commonly applies to passive loudspeakers. The sensitivity of a loudspeaker tells you how loud it can play when consuming average continuous audio power of 1 W. Sensitivity is also called efficiency. So, the sensitivity is a very important characteristic, as it’s essential to determine how loud your speakers can play at a given listening distance.

When dealing with passive loudspeakers, the most straightforward and proper approach is to choose your loudspeakers first, and then choose an appropriate amplifier, according to your loudspeakers' specifications, and -perhaps- taste.

Power itself is neither enough to tell you how loud your loudspeakers can play, nor how good sound quality they are capable of. Nonetheless, sound level and sound quality are the most important things you should care about your loudspeakers. As of sound level, there are four specific things you should know, to determine how much amplification power you need, to fully drive them:

 

·       Sensitivity,

·       maximum power handling,

·       listening distance, (how far your ears are located from each one of the loudspeakers),

·       impedance curve (for passive loudspeakers)

 

Sensitivity expresses how loud the loudspeaker can play at a distance of 1m away from its tweeter, when forced to consume power of 1W. So, sensitivity is expressed like "xxdB/W/m" or something like this.

For example, if a loudspeaker is specified with a sensitivity of 90dB, when you put your ears 1m away from its tweeter (with your ears on tweeter axis), and force it to consume 1W of power, you will experience a sound at a level of 90dB SPL. Being that way, when you double the listening distance, the sound level is decreased by 6dB. So, this loudspeaker will play at 84dB at 2m, 78dB at 4m, and so on. In addition, when doubling the number of loudspeakers, the sound level is increased by 3dB. So, if you set a pair of loudspeakers of this example, being each one of your ears 1m far away from the corresponding loudspeaker, you will experience a sound of 93dB, or 87dB at 2m, or 81dB at 4m, or 75dB at 8m, and so on.

If sensitivity specifies an active loudspeaker, it tells what signal voltage should be applied to its input in order to fully drive the loudspeaker at its maximum sound level. Maximum sound level is the maximum level the loudspeaker can play without damage. For example, if an active loudspeaker is specified with sensitivity of 1.5VRMS and maximum SPL level of 115dB, this means that it must be driven with signal voltage of 1.5VRMS to give an output SPL of 115dB at a distance of 1 meter on tweeter’s axis. So, two loudspeakers in a 2-channel setup can achieve a maximum SPL of 118dB (+3dB) at 1 meter, or 112dB at 2meters (-6dB), or 106dB at 4 meters (-12dB) and so on.

Finally, in order to determine the maximum SPL a given passive speaker is capable of, you can do it according the following formula, using a pocket scientific calculator.

VMAX_SPL = Vref + 10log(Psp)

…where Vref is loudspeaker’s sensitivity, Psp  its power rating and VMAX_SPLthe max SPL it can play.

Let’s see an example. Suppose a passive loudspeaker rated 300W of average continuous power and an 88dB/W/m sensitivity. If you do the math, you will get a result of 113dB, which is the highest SPL of this loudspeaker, thus you can achieve 116dB with a pair of them, at 1 meter, driving them with 300W. Moving a step ahead, if the listening distance is 4 meters, these loudspeakers will play at 104dB (116-6-6=104).

Working in reverse, if your amplifier’s nominal power output is lower than loudspeaker’s power rating, you can substitute the Psp variable with your amp’s power output, thus you will determine how loud these speakers can play at amplifier’s max output.

 

3.6                    Nominal impedance

As an active loudspeaker is a device which comprises an active electronic circuit, its signal input behaves as the input of a regular preamplifier, having a fairly constant resistance of some 10 or 20 KOhms (maybe more). 

Impedance is a specification applying to passive loudspeakers only. As all loudspeakers are made of electric motors and fed with current of varying frequency, a passive one shows a resistance which varies over frequency, thus it behaves as an inductive and/or capacitive load. So, nominal impedance is expressed in Ohms, because it’s practically impossible to state a varying resistance with numbers. Most passive loudspeakers are specified as members of two main impedance categories, 4 or 8 Ohms. If a loudspeaker is rated 8Ohms, its resistance should not fall under some 6Ohms at any frequency, but this should not be taken for sure.

If the resistance of a loudspeaker is getting lower at a given voltage input, it consumes more current, thus the speaker becomes demanding of a more powerful (and expensive) amplifier. So, many speaker manufacturers are prone to rate their products as 8 Ohms, in order to persuade their customers that their products can be driven with faire ease. Of course, there are many 4Ohms rated loudspeakers in the market, most of which tend to fall to 3 and maybe less Ohms at some low frequency, usually because they comprise two 8Ohm woofers connected in parallel. Generally, most speaker manufacturers are loose when specifying the impedance of their products.

If a speaker manufacturer is more serious and honest, its product is specified with more detail than just an ambiguous 4 or 8Ohms statement. So, for example, such a product is specified as 8Ohms, minimum impedance 4,5Ohms at 180Hz with max phase deviation ±38 degrees. This specification tells many more things, as states that the speaker is rated 8Ohms, but it demands significantly more power at low frequencies, while its phase deviation is not higher than 38 degrees throughout its frequency range. It is not an important thing, if a speaker’s impedance falls to a low value at a mid or high frequency, because there’s not much power at those frequencies.

Moreover, an impedance measurement of a loudspeaker is far more meaningful than a couple of numbers in the specifications sheet. Let’s take a look at the following graph.

 

Image 4. Impedance and phase magnitude measurement of an 8 Ohms rated loudspeaker. (See text).

 

 

Image 4 illustrates the impedance and phase deviation measurement of an 8 ohms loudspeaker. This particular graph has two y axes, of which the one on the left measures ohms and the right one the phase shift in degrees. As you can see (solid curve), impedance falls to its minimum value (some 3 ohms) at two frequency points, somewhere around 70 and 150Hz, and again at 600Hz. While the latter point is located at a quite high frequency, where there's no that much energy in the audio signal, the impedance fall at 70-100Hz is significant. Frequencies around 70-150 have quite more energy. So, if an amplifier is set to drive this loudspeaker to its limit, playing something which contains very high energy at low frequencies, this should be a powerful amplifier, capable of delivering 500W at 4 ohms, as this is the power rating of the loudspeaker.

On the other hand, dashed curve depicts phase shift, which fluctuates from some +50 degrees at 1200Hz to -50 degrees at 50Hz. Therefore, this loudspeaker should be considered a difficult and very demanding load. Generally, it’s good to remember that a fairly easy – to - drive passive loudspeaker’s phase shift should not exceed some ±30 degrees.

 

4          Distortion

In audio applications, distortion is any alteration of the original signal. The most common distortion is harmonic distortion, which consists of overtones, whole number multiples of the original signal’s frequencies.

Though harmonic distortion is the most common specification in the electronic audio equipment, it’s unlikely in loudspeakers’ specs sheet, especially those of the consumer market, because it’s much higher than that of electronics. As all loudspeakers transduce electric to mechanical energy, they comprise mechanical parts which are prone to induce high distortion, much higher than that of the electronics (cd players, amplifiers, consoles, or signal processing equipment such compressors and limiters).

So, loudspeakers induce harmonic distortion of some 3% or 5% at average SPL, whilst they could be more distorting at higher levels in excess of 105dB at listening position.

 

5          Choosing, placement and proper use

When asking yourself what kind of loudspeaker you should purchase, you must consider some basic factors which can result to a decision easy to make. Those factors include budget, what kind of equipment you might already have and of course the nature of use.

For example, if you already have an integrated amplifier and are in search of a pair of loudspeakers for domestic use, you should probably look for a pair of passive loudspeakers. In another case, if you already have a signal source only, perhaps a CD player or a DAC and your computer, then you should consider of active loudspeakers as an attractive option, but you must have in mind the following. Active loudspeakers are not the mainstream in the hi-fi market (already), so they are more difficult to sell at a later time, in case you want to replace them with something new.

Another very important aspect when choosing loudspeakers is how you intend to place them in your listening room. Of course you should place them in such a position, so their baffle has a fair distance of 1 or more meters from the rear and sidewalls, but, no matter how big is your room, the distance between you and your loudspeakers is a fundamental thing, because bass is an expensive story in audio. As your listening position is going far and far from your loudspeakers, they should be of larger physical size, comprising bigger and bigger woofer(s). So, as a rule of thumb, it is good to remember that a rather small loudspeaker with just one 6.5” woofer driver will do at a distance of some 1.5 to 2 meters only. At a distance of 3 meters you will need at least two 6.5” drivers, while at 4 meters it’s better to have loudspeakers with two 8” woofers at least. Larger distance probably demands one or two 10” or 12” woofers, and so on.

Of course, a loudspeaker with two 8” or 10” woofers is quite big, and its drivers will spread at a distance of some 100 – 120cm vertically, and therefore such a loudspeaker is unsuitable for close listening, because it can’t give a consistent imaging of sound. When you attempt to listen to such a loudspeaker from a fairly close distance, it’s sure you will perceive tremble coming from top, whilst the bass will be coming from “down there”.  

Also, your speakers should be placed in such a position, so they are spread enough horizontally, to give you an adequate sound imaging. As a rule of thumb, it’s a good practice to place them in an equilateral triangle fashion, where the triangle’s peaks are located to your head and the speakers or slightly closer each other.

Because of the nature of high frequencies and the physical properties of the loudspeakers as well, high frequency sounds do not have much dispersion both horizontally and vertically, so it is important your ears are located at the height of the tweeters. If you listen to your speakers off tweeter axis, you will experience an altered frequency response with less tremble, mainly according to tweeter’s make andbaffle and crossover design.

 

6          Monitor loudspeakers

As this term implies, monitor loudspeakers are special loudspeakers, designed to meet professional needs. According to purpose, there are three categories of monitor loudspeakers, near field monitors, mastering monitors and video production monitors.

Near field monitors are likely used in PA systems and small, semi-professional recording or home studios, mostly placed at a close distance in front of the mixing engineer. Due to small listening distance, near field monitors are of small physical size, usually 2-way designs, comprising a tweeter and a 5.25” or 6.5” mid-woofer. Most of near field monitors are active, so they can be connected directly to the appropriate output of the mixing console.

Mastering monitors are bigger, usually 3-way designs, and they comprise at least one 8” or 10” woofer. Mastering monitors are used in mastering studios, to give the mastering engineer the most precise sound imaging and the highest sound quality. They are meant to be placed at a larger distance of some 2-3 meters. Because of the nature of their purpose, mastering loudspeakers should comprise drivers of the highest available quality, and they are active as well, so they are capable of as flat as possible frequency response and the lowest possible distortion. Because of their active crossover design and high quality drivers, mastering loudspeakers are capable of high performance for quite long periods of time, without suffering of thermal compression. So, because of their make, mastering monitors are never that cheap.

Both near field and mastering monitor loudspeakers have a balanced signal input, whilst they usually have such level controls on the back panel, so the engineer can adjust the bass, mid and tremble level separately, according to particular room conditions, the kind of music being mastered and taste.

As for the radio/video production monitors, they are made in a quite different way. Loudspeakers of this kind are not made to achieve the highest possible performance, but to simulate the sound performance of the spectator’s / listener’s equipment, which is likely a TV set or a car audio system, so that the engineer can have a close idea of how the content being edited will sound. These loudspeakers are usually passive and single way, with a full range driver.