Loudspeakers: Pro Audio Cabinets, Information for the Budding Builder!

Generating low frequencies is often an issue for people. If you are using relatively small cabinets, getting enough sound pressure over a wide frequency range is often difficult without using stacks of efficient speakers to move a lot of air. Hoffmans Iron law states that you can only have two out of three attributes of a loudspeaker, loudness, low frequency and size. This means that in general if you want to go both low and loud, you need a large cabinet. Whilst there is some truth in this, it is because many existing manufacturers miss the mark by some margin, there is still plenty of room for optimisation within the laws of physics.

For a Pro Audio cabinet, in general, high output is desired but the bottom Octave of bass is not usually required as it is power inefficient, difficult to control in venues and high level subsonics can have some nasty effects. On humans, a feeling of nausea can come on and the very long wavelengths mean directivity is not easy to control, even with DSP and delays. A bass guitar low E is 43 Hz and a low B is 31 Hz if you are an extended range player (5 string plus basses). This also is around the bottom of a piano keyboard and for other typical instruments where amplification is required. So, there is really no need to have any fundamentals lower than this. In fact many professional setups, even in stadiums roll off the bass below 39 Hz with a reasonably steep filter. (sometimes 6 dB per Octave until 30 Hz and then 18 to 24 dB per octave below this). This helps the “iron law” as there is no need to go really deep so cabinets can be optimised for the range that they are needed for.

For the amateur “gigging” person, playing drums, keys or bass in a small setup (with no front of house amplification), it is often hard to get enough low end to be audible above the other players (guitars for instance often have very high efficiency speakers and huge midrange output but cannot push enough air to make effective bass and vocals are not really needed to be much below 80 Hz so also easy to make loud. The bass player therefore often brings large amount of speakers and big amplifiers so they can be “heard”. It is often also desirable to improve the punch from a kick drum. So for this typical small band setup for pubs etc, some speaker optimisation is desired.

For the bass player, sound output should extend down close to the bottom fundamental that you need, whilst the in room bass response is often boosted by the walls of the building, this is mostly due to the speakers being relatively close to walls and the omni directional nature of the loudspeaker cabinets. So, high frequencies are beamed forward but the rear firing radiation from a loudspeaker can be reflected by the venue walls and boost the average bass response. Unfortunately, this also often means due to distances between speaker and walls, some frequencies are boosted and some are cancelled leading to a lumpy bass response with some notes booming out and others being almost lost. Things get better in larger rooms, especially if the speakers are not stuffed in corners as the multiples of the room modes become much closer together. However, a large room filled with many people is going to require more output at these frequencies to provide the required sound pressure.

With traditional smaller cabinets you must have the following:

1. A large cone area (Sd) to be relatively efficient and move enough air.
   Multiple drivers can add significantly to cone area and two 8 Ohm drivers wired in parallel will give approximately 6 dB more output (3 dB from increased cone area assuming both are powered equally and around 2 dB more due to the amplifier giving more output into 4 Ohms. Typically 5 dB more is obtained as most amps don’t exactly double in to half the impedance but the reduction in thermal power compression of the drivers and coupling between cabinets if they are close will usually add another dB or so.
2. A large X max on the drivers so the volume of air moved can be large.
   This is the throw of the speaker cone combined with the Sd area gives the volume displaced, Vd. More air moved means more bass energy available. A premium driver with twice the X max of another cheaper driver of the same diameter could potentially mean using only a single driver instead of two, making boxes smaller, lighter and cheaper. For a sealed box, in general below the speaker box tuning resonance, the cone excursion gradually becomes greater with descending frequency. The advantage of this is that at very low frequency, the cone still have some control from the box volume and it can extend the frequency range due to a low roll off rate below that resonance but output in general in the bass is compromised as only one side of the loudspeaker is radiating energy in to your room.
3. Some form of porting such as a tube, slot, transmission line or passive radiator.
   (Horns too but not likely to be small!) All these methods use the rear wave of the speaker to enhance output over a certain frequency range and can add another 6 dB of output in the ports resonant range compared to an equivalent sealed box. This is the equivalent of adding a second driver so, if tuned well at the tuning frequency, it will perform as if you had two drivers instead of one. However, care should be taken on all these methods as below the lower cut off frequency of the “port”, the cone loading rapidly drops and excursion of the speaker could reach mechanical limits. The output below resonance also drops twice as fast per octave as a sealed box as the port then tends to cancel the output of the driver rather than enhancing it.
4. The porting has to be efficient!
   Small ports will have high air velocities inside which make chuffing noises and even with smooth edges, above 10 m/s air flows tend to be audible. Small ports can work well for hi fi but for pro audio, as the system is often used at higher levels, the narrow port can “choke” the system so it behaves more like a leaky sealed box so that 6 dB advantage starts to disappear again. Port area has to be appropriate for the Vd of the speaker and the tuning. In other words, the greater the volume of air displaced by a cone, the larger area the port needs to be. For pro audio, only a relatively large port will therefore maintain its performance at high levels but this can mean that a large port needs to be longer and box volumes also increase to maintain tuning making the box large again.
5. Tuning the port correctly is essential.
   If the port is tuned too high (say over 60 Hz), then at low frequencies such as a low B on a 5 string bass guitar (31 Hz), the cone excursion becomes excessive and increases distortion as little useful output will occur. If tuned too low, (say 30 Hz) the upper bass frequencies will tend to cause high cone movement. The port loads the cone to limit cone excursion around its tuning frequency only and this tends to work over around 1 Octave of range. Ensure the second octave will not cause X max to be approached by using Win ISD or a similar modelling program. Typical tunings for a single resonance port will be 40 Hz to 55 Hz range depending on application for pro audio. This is another reason that bandwidth is limited in pro audio and that large X max drivers are required.
6. Using a dual resonance port system will control the loudspeaker over a wider range of at least 2 Octaves.
   This is because the frequencies of the two resonances can be spread apart sufficiently so that when one resonance starts to unload the cone, the second takes over. This is very common in boxes such as 6^th order bandpass enclosures which have two port outputs usually with the second resonance around twice the first. However, these boxes have different chambers on opposite sides of the speaker cone so each output is 180 degrees out of phase with the other. These are only good for low frequencies so additional drivers would be required for the mid and high frequencies.
7. Some manufacturers use a double bass reflex tuning which can have two or three ports depending on design. Both these systems compromise the output as the two port system loses some energy within the cabinet rather than projecting it outwards and the three port system shares the rear pressure energy from the driver between the two chambers and ports so although there is good cone control over an extended frequency range, port output is smaller. These designs tend to be used more for hi fi than pro audio due to the output compromises.
8. Another way to do this is using the “Compact Transmission Line” loading technique designed by LFS. This is a truncated transmission line which has a modest volume immediately behind the speaker. This system uses a combination of “reflex loading” (or the Helmholtz resonance) of the external section of the port length against the larger internal volume of the speaker and larger part of the port tuned to one frequency whilst the “transmission line” length due to path length (or ¼ wave resonance) creates a second frequency. In CTL cabinets, the reflex tuning is deliberately set to a lower frequency than usual to extend the bass lower and control cone movement to around 30 to 40 Hz and the ¼ wave tuning is set to over twice that frequency so that over at least two and a half Octaves of bass, the cone is controlled and the output efficiency of the port output is still high at its terminus (rather than losing the energy inside the box). (CTL is a trademark of LFS and patent pending).
9. The CTL range is designed for maximum output over a wide frequency range and low distortion, even driven hard. Because cone control is improved over a wide range of low frequencies, the X max is less relied on than for conventional bass designs so it is possible to utilise a more efficient (higher sensitivity) bass drive unit and even units with higher Qts or above 0.4 to around 0.6 which would normally be used on sealed boxes only as the line damping prevents booming. Bass players can now use a single lightweight box and have enough sound pressure level to hang with even a loud drummer. (if you need more than this, you would DI to front of house. See article on front of house speakers)
10. The CTL range is generally 8 Ohms so you can double up cabinets if required for larger rooms and the load is no worse than 4 Ohms (most amplifiers do not like much less than this). They are high sensitivity at 96 dB/Watt to 102 dB/Watt without the need for horn loading and use cones in the 6 inch to 12 inch range. They use Speakon connectors with a pass through connection. Speakon connectors are safe and reliable and pretty much an industry standard. New designs will keep being added to the website as they become available so please come back regularly to see if there has been an update. Updates will be announced on our Linked in page and our Facebook page.
11. Finish is up to you. The designs allow you to build a system which is pre tuned and will work if you follow the instructions. What you put on the outside, carpet, vinyl, paint or laquer is entirely up to you. The build is relatively simple for the home woodworker or you can have a panel pre-cut by your wood supplier so you can simply glue and screw it together before finishing. All tips to prevent compromise are included in the build plan and there is a cutting plan sheet available and even some recommendations as to where to purchase the hardware. There will eventually be some online resources to assist building cabinets on the website.