Loudspeaker Basics 101 – Part 2

In Loudspeaker Basics 101 – Part 2 we review why speaker enclosures are generally required, the different designs that are available and various cabinet issues.

Introduction

There are basically two types of speaker (cabinet) radiation:

• Dipole

Dipolar sound radiation occurs when the sound pressure waves from the front and rear of the driver are free to mix in the listening environment.

• Monopole

Monopole sound radiation occurs when the drivers rear sound pressure waves are prevented from being radiated at all. (see ported enclosures)

When a typical mid-range or bass driver is placed in free space, there is very little difference in the distance from the rear and front of the speaker cone to your ear. However, the pressure wave that radiates from the drivers rear is the opposite to that radiating from its front. When the acoustical path distance difference to your ear reaches one quarter of the radiated wavelength, the two pressure waves will begin to noticeable cancel and the resulting SPL will drop by 3dB. In a perfect world when the wavelength difference is half a wavelength (phase inverted) there would be no SPL heard from the driver as the front and rear SPL’s are equal and opposite. In real life this condition can never occur due to various losses, reflections and numerous paths the sound takes on its way to your ear.

If the driver is mounted on a baffle the acoustic path length from the drivers rear is increased. This type of mounting is still referred to as a dipole due to its characteristic figure of 8 SPL radiation pattern.

The larger the baffle the longer the path length and therefore the lower the frequency at which there is a 3dB loss. Correspondingly there will be certain wavelengths where the rear SPL arrives at the ear in phase with the front SPL and adds to the SPL (An effect exploited by ported cabinets).

Baffle sizes required to get good bass response from this type of arrangement are not generally practical for residential use. Although some enthusiasts have sealed their baffles into chimneys to duct the rear wave out of the room/house in order to extend the drivers bass response.

NOTE: This issue does not affect high frequency and most mid-range drivers as:

1. Their rears are enclosed or sealed by design preventing the radiation of the drivers rear pressure wave.
2. The rear path length is large with respect to the radiated wavelength.
3. There are many room reflections that negate the cancelation effect.

We therefore have three fundemental approaches to extending the bass response of a driver:

1. Seal the drivers rear so that the rear pressure waves do not radiate into the room.
2. Select a path length from the drivers rear such that when it arrives at the drivers front plane it is additive.
3. Provide some form of resonant environment so that the rear SPL is additive to the drivers front SPL when it emerges.

These three techniques give rise to a large range of cabinet designs based upon a range of ‘variations on each approach’.

Types Of Monopole Enclosure

There are four basic types of speaker cabinet designs that provide just front or monopole radiation:

1. Sealed or Infinite Baffle
2. Reflex or Tuned Port
3. Transmission Line
4. Horn (Folded)

Each of these designs has a whole range of ‘variations on the theme’, making for dozens of types of speaker cabinet design. Each one striving to provide clean, tight and extended bass performance, generally from the smallest volume possible.

Remember that these are monopole radiators even if the port is to the rear of the cabinet. So you do not want the pressure changes inside the cabinet to be radiated from its surface by it moving or vibrating.

The physical design, shape and mounting of the cabinet is also important as it affects the way the speaker pressure wavefront radiates into the room (see later comments).

Lets briefly look at each basic designs operation, characteristics and technical issues.

The Sealed or Infinite Baffle.

This is the simplest of speaker cabinets. It is also the most inefficient as all the speakers rear energy is lost. Sealing the enclosure provides the driver with additional air resistance due to the compliance (stiffness) of the enclosed air. This has several effects:

1. It reduces the lower frequency output SPL.
2. It requires higher powers as the cones movement is more stiff due to the enclosed air mass.
3. It provides an extended and gentle bass rolloff of 12dB/octave after its resonate frequency.
4. It allows the speaker to handle much more power if designed to do so.
5. It provides better and more controlled damping for the cones movement.
6. Internal box SPL can be re-radiatted through the cone without correct damping/absorption and good design.

Ideally the enclosure should provide what is known as an adiabatic response – what heat energy goes in comes out. In order to achieve this the enclosure must be totally airtight and rigid enough to stop all panel vibrations. The energy that goes into compressing the air volume raises its temperature then as the cone moves forward and the pressure drops the stored heat energy is given up and the pressure drops. If this is perfectly adiabetic there is no energy loss from the airs compression and rarefaction nor from panel vibrations that also generate heat, and the cones movement will therefore remain linear. This is often helped with the addition of absorptive material.

Generally speaking this design provides a very well controlled bass performance at the expense of much higher powers, but at the same time creates very tight and well defined bass with little boominess. In order to get high SPL’s at very low frequencies, like sub-woofers, it is not uncommon to have amplifiers that produce peak powers of over 3000watts, even for home use.

Modern high power DSP controlled Class D amplifiers, speaker voice coil designs and extended excursions allow for exceptional extended bass responses and high SPL’s from relatively small enclosures. With the DSP controlled amplifiers providing a very well controlled performance and the ability to minimize damage to the speaker from being overdriven.

Reflex Or Tuned Port

A ported, vented or reflex speaker as they often referred to,  is very often used to increase and extend the LF output.

A bass reflex speaker is just like a sealed speaker, but in addition, has a an open tunnel, or port, which allows air to circulate freely in and out of the box. This vent helps increase the efficiency of the woofer, with substantial contribution to the low frequencies.

The ported enclosure is basically a Helmholtz resonator, the frequency of which is determined by the area of the port, the length of the tube and the volume of the enclosure. At the enclosures resonance the drivers rear sound pressure wave emerges from the port being phase inverted providing 3dB gain (x2) by being additive to the drivers front sound pressure wave. After this frequency there is no phase inversion and the emerging rear sound pressure cancels the drivers front sound pressure causing a rapid 24dB/octave rolloff.   Above this resonant frequency the mass of air inside the port is too great to respond to the speaker movement, and it acts ‘like’ a sealed enclosure. Below this resonance frequency the port acts like a large hole in the enclosure providing little rear pressure for the diaphragm and so power handling is poor. This requires that the power amplifier provides good high pass filtering to prevent damage and overload at these lower frequencies.

Advantages :

• +3 db efficiency compared to the equivalent volume sealed enclosure.
• Lower cutoff frequency. Normally after reaching the resonance frequency of the driver, the response will start to go down hill, but that is when the port starts to put in some work and extends the frequency response.
• Lower distortion at resonance frequency. At this point, the port does most of the work. The speaker barely moves at the resonant frequency of the box. This means less distortion and more power handling.

Disadvantages:

• After resonance frequency is achieved, the response is degraded with a steep roll-off of 24 db / octave.
• The vent, if not designed correctly or at high sound levels, can be noisy.
• Transient response is not as good as sealed cabinets.
• Progressively poor power handling below the speakers resonant frequency.
• Coloration is possible at the lower frequencies.

As with all speakers it is essential that the cabinet be well braced and very rigid to prevent  loss of energy and sound coloration from cabinet flexing. Absorption is also added to both damp the rear waves and aid in the reduction of the rear sound being re-radiated through the cone.

Transmission Line

The transmission line enclosure is a tuned length of usually folded space that increases the rear waves path length in order to extend bass performance and increase LF output. These folded spaces are designed to have a resonant frequency, below which there is a 12dB per octave roll off of SPL.

In order for the rear wave to be phase inverted so that it adds to the front wave the transmission line length is equal to one quarter the wavelength of the selected resonant frequency. The SPL phase relationship between the bass drivers cone and vent is in phase in the pass band until the frequency approaches the quarter wavelength at which point the vent is producing most of the output.  The line operates in conjunction with the driver over several octaves. As a result the cone excursion is reduced, providing higher SPL’s and lower distortion levels, compared with reflex and infinite baffle designs.

A transmission line speaker employs essentially, two distinct forms of cone loading. The upper bass is completely absorbed by the line allowing a clean and neutral response. The lower bass is extended and distortion is lowered by the line’s control over the drive unit’s excursion.

Low frequencies, which remain in phase, emerge from the vent which essentially acts as a second driver. The advantage of this approach is that the air pressure loading the main driver is maintained which controls the driver over a wide frequency range and reduces distortion.

The introduction of the absorption materials into the transmission line reduces the velocity of sound through it,  with heavily damped lines reducing the velocity of sound by as much as 50%, although 35% is typical in medium damped lines. This has the effect of dropping its resonate frequency and extending the speakers bass response. There are three basic damping approaches:

1. Filling the entire tube treats the line as a damper, aiming at completely eliminating the rear wave.
2. Filling half the cross section throughout its full length treat the line as an infinite baffle, basically damping high frequencies and wall to wall resonances.
3. Filling the tube from the driver to half the tubes length creates a quarter wave resonator, leaving the fundamental tone with its velocity maxima at the open end of the tube intact, while damping the higher harmonics.

Advantages:

• One of the exclusive benefits of this design is its ability to produce very low frequencies even at low monitoring levels – line speakers can accurately produce full range deep bass that usually requires a subwoofer.
• The resulting cone excursion is reduced, providing higher SPL’s at lower distortion levels, compared with reflex and infinite baffle designs.
• This design produces higher SPL and lower bass extension than ported or sealed boxes of similar size.

Disadvantages:

• The main disadvantage of the design is that it is much more labor-intensive to create and tune a high quality transmission line with consistent performance compared to building other enclosure designs.

Transmission lines tend to sound more natural because there is little build up of pressure behind the bass cone, with the rear radiating air passing through an internal labyrinth to reinforce the bottom end of the frequency band. This also means no rear sound is re-radiated through the bass driver. The other advantage is that the air mass in the transmission line loads the bass driver and lowers its resonant frequency. This allows for the extended low end response and keeps the bass driver well damped, requiring less excursion than sealed or ported speakers to produce the same output.

Horns

Acoustic horns, or waveguides, are used to match the high pressure and small radiating area of a cone or compression driver diaphgram to the much larger area of the external atmosphere, which is a low pressure environment. This is achieved by its expanding flare rate, the horn transforms the high pressure, high particle velocity vibrations at the small throat of the horn into low pressure, low particle velocity vibrations distributed over the larger area of the mouth, thus more efficiently coupling the wavefront to the air.

The horns flare rate provides this transition to match the designers required acoustical performance criteria. There are many classic curves, such as exponential, hyperbolic, conical, each with its own acoustical properties. Some flare rates favor LF output, others overall efficiency, and still others uniformity of frequency response and/or low distortion.

Horns may be straight as in the previous case of the DK30 horn or folded as shown above and below. The length of the horn is directly related to the lowest frequency that it can support so most low frequency horns are folded due to their length. Unfortunately this folding tends to restrict the range of operation to approximately two octaves due to the bends reflecting energy backwards and causing cancellation.

These horns not only provide increased sound energy transfer efficiencies of up to +10dB but provide the designer with the ability to tailor both the vertical and horizontal sound dispersion polar patterns for middle and high frequencies.

Mid and high frequency horns are not so large as to make them impractical for home and studio use. Bass horns however are quite large and in practice need to be folded to be of practical use. Some however have the space and money to do it ‘properley’!

Image Credit: Makezine.com

Just How Large Can You Build Your Sub Horns?

These stereo sub horns were designed and built by Roberto Dell Curti of Royal Device in Italy. The horns have enormous sensitivity providing 110dB/1 watt/1 meter from 10Hz.

Types Of Dipole Speaker

A dipole speaker is one that radiates sound press energy from both its front and rear as shown above. Its major disadvantage is that the lowest frequency that the speaker can effectively radiate is when the path distance difference from the front and rear waves reaches one quarter of the radiated wavelength. At this frequency the SPL will drop by 3dB and continue to rapidly rolloff at 24dB/octave. These designs are often enhanced by the addition of monopole speakers to handle the lower octaves.

Two common membrane types currently support this design:

1. Electrostatic panels
2. Ribbon panels

Both designs can provide exceptionally uncolored sound down to bass frequencies. The current very large designs can provide SPL’s that are high enough to support home theater performance. While these designs can produce a ‘second to none’ performance for classical music and some ‘pop’ music, all designs require additional moving coil monopole designs in order to create the required visceral effects to support the 0.1 LFE channel. The designs are well established by the manufactures listed below each of which adds there own variations in order to optimize the speakers overall response:

1. Quad Electronics
2. Magnepan
3. Martin Logan
4. Sound-Lab
5. Audiostatic
6. Alsyvox
7. Wisdom Audio
8. Eminent Technology

If you have the space and budget these designs are just about as good as it gets.

Surround Speakers

Surround speakers are often a custom design compared to the typical monopole speaker. They are generally required to create a diffuse sound field so that they cannot be located. However, small monopole speakers are frequently used for this purpose being located well above the ear so that there is little sound directed at the listener. Custom designed surround speakers are generally one of three types:

1. Monopole or Direct Radiating – a conventional small speaker, like the Genelec 80XX series below.
2. Bipole
3. Dipole

Polk Audio F/X-500i Switchable Bipole/Dipole Surround Speakers

Direct Radiating Surrounds

Direct-radiating surrounds are like your typical front speakers in that they use drivers that fire forward. Audio purists like myself tend to use these but they are placed well above ear level and in my case utilize additional 1D diffusers. In most cases all of the surround information is aimed at the seating position, and reflections are a lesser part of the setup.

Bipole Surrounds

This design uses typically two arrays of drivers which face in opposite directions, and are in phase with one another. They aim the surround information into and behind the seating area, but not directly at the listener, which avoids hot spotting. This effect occurs at higher frequencies as they tend to act like a beam drawing the listeners attention to the speakers location which can become distracting.

Dipole Surrounds

Dipole speakers, are similar to bipole speakers, except one speaker array, usually the rear one, is 180 degrees out of phase with the front array.  What this does is create an on-axis sound null, along which the listener sits, with a resulting direct sound level drop of up to 12dB. This causes this listener to hear mostly off-axis reflected sound. Dipole surrounds tend to produce a less-focused, diffuse and enveloping surround field. Some surrounds like the Polk F/X 500i shown above may be switched between bipole and dipole modes.

The Speaker Cabinet & Other issues

Construction

It’s all about rigidity. The cabinet must not flex or radiate any sound. Flexing absorbs energy that should be given back to the enclosures air mass and cabinet radiation makes the speaker no longer a monopole radiator. Both these effects impact the frequency response and can ‘color’ the sound.

This means that the cabinet panels must be very rigid and well damped to meet that requirement. Cabinets are often severely internally braced, especially bass and sub-woofer enclosures and made of high density materials like MDF that must not ‘ring’. Cabinets will often be filled with sound absorbing materials and may even be lined with bitumen pads in order to dampen any vibrations.

Image Credit: Part Express

Cabinet Edges

Sharp speaker cabinet edges can give rise to secondary sound radiation points due to diffraction, and create sound interference patterns that degrade the stereo imaging and frequency response. Mounting speakers flush with the rooms boundary and rounding cabinet corners will help negate this effect. Even the square edges of speaker grill frames can be shown to produce this effect.

Edge Diffraction Effects

It is not uncommon to round the speaker cabinet edges to reduce or prevent sound diffraction and interference effects. This technique is very clearly shown in the design of the Genelec, 8020, 8030 and 8040 series.

Cabinet Mounting

No matter how well made the cabinet it will vibrate or even move. If the cabinet is not isolated from the floor or wall those vibrations/movements will be transmitted to the supporting structure. As sound travels much faster in solids (x10 or more) than it does in air an effect called ‘early early sound’ can be created. This effect is shown below.

Flanking or Early Sound

The effect can result in the listener hearing the sound radiated from the rooms structure before the actual air born sound arrives at the ear; smearing the sound wavefront and stereo image. So ideally all speakers especially subs should be isolated from their supporting structures. See here for more information on sub isolation.

Driver Spacing.

Ideally we want the frequencies from each driver to arrive at our ears at the same time. This is particularly true when crossing over from one driver to another. Poor timing due to different acoustic centers and electrical filter delays can result in poor phase and frequency response that can impact stereo positioning and depth.

Most speaker designs are based upon none concentric drivers using separate drivers spaced across the front of the speaker cabinet face. Tannoy invented the first dual concentric speaker in 1947, launching the infamous Monitor Gold in 1967.

Dual Concentric Cross-Section

The following diagram shows the issue of different physical path lengths for different driver spacing and acoustical centers. This can be rectified by careful crossover design and/or physically setting the drivers into the cabinet in order to time align their acoustic centers.

This issue is at its most important at the crossover point when both drivers are radiating the SPL equally. SPL output from drivers should be uniformly additive in order to get a flat frequency and phase response. Again crossover design playing a big part in achieving a flat frequency transition between drivers.

Group Delay

There is an effect called Group Delay that (should only) become apparent at low frequencies. It is very important and can have a significant effect on aligning the performance of multiple subs and satellite speakers. So what is group delay? It is the difference in time that it takes a particular frequency to emerge from a speaker and is most apparent at bass frequencies. All frequencies that goes into a sub woofer do not come out with an equal delay. Different speaker designs, particularly subs, can have radically different group delay characteristics. So the SPL from different subs, as an example, will not be lineraly additive.

The Overall Group Delay for My 4 SVS subs.

We see from the above graph that 20Hz is delayed 65mS and 30 Hz is delayed 22mS. Provided these differences in delay are below certain criteria (no more than the time for 1.5cycles and ideally less than 1cycle at frequencies below 30Hz) they do not impact the perceived sound.

Just remember, DO NOT mix sub makes, models, designs, frequency responses or power handling. It can make optimizing and equalizing your bass frequencies very difficult. More about this in part 3.

Next

In part 3 we review some of the possible room placements for both satellite and in particular sub-woofer(s).

Click here for Part 1 that discusses speaker drivers and crossovers.

Click here for my posts on Acoustics 101.