The acoustic short circuit
If a loudspeaker driver is being used on its own, i.e. without cabinet and baffle, sound is radiated on both sides of the driver. If the cone moves in the direction of listener, the air in front of the driver is compressed, while it is stretched behind the driver. Considering low frequencies the air around the driver can compensate the sound pressure difference - and the low frequencies are not radiated to the ambient environment. This cancelling out of sound is called acoustic short circuit.
This short circuit may be prevented, by stopping the air using a short cut to get to the front of the driver. Therefore, the driver is mounted on a large panel called baffle. To reproduce a 50 Hz tone, the baffle needs to be 3 meters in diameter. Then the way around the baffle is so long that the pressure change at the front of the cone cannot be compensated by the pressure change at the back.
Unfortunately, a baffle of 3 x 3 meters is sometimes not practical. Other solutions had to be found.
The closed cabinet
The easiest solution to prevent that the air at the back of the loudspeaker travels to the front, is to be place the driver in a closed cabinet. The rear radiation remains in the enclosure and doesn't interfere with the front radiation.
But this method also has disadvantages:
- 1. the rear radiated is not used at all (this would be desirable for the low frequencies, since these often show radiation problems).
- 2. The air enclosed in the cabinet has an effect of a spring on the driver cone. In order to radiate low frequencies the driver needs to have a long excursion and that's exactly where the back-propelling spring tension causes the cabinet's resonance frequency to rise considerably (sound radiation of bass drivers below the resonance frequency may usually be neglected).
The closed cabinet also has advantages: an accurate pulse response and is easy to build
The bass reflex speaker
The bass reflex speaker also uses the backwards radiated sound for reproduction. This type was speaker was patented in 1932 by A.C. Thuras, however, many refinements were necessary until it was quite easy to calculate the required parameters. In 1973 Richard Small published some essays that helped - in conjunction with the input of many others - to succeed.
The bass reflex box is constructed and built up like a closed cabinet, however, an opening (or port) is extended into the interior of the cabinet by means of a tube. This bass reflex port should ideally be located close to the bass driver. The air pressure generated within the cabinet doesn't simply exit into the ambient environment, the air rather starts resonating inside the tube and acts as an independent sound source radiating with a phase shift.
If drivers, cabinet size and bass reflex tube and their dependencies are tuned properly, the low frequency sound generated inside the tube is still radiating while the sound radiating from the front of the driver is already diminishing. Therefore, the bass driver's excursion doesn't need to be as long as in closed cabinets.
The transmission line speaker
The transmission line speaker system is used and understood in many different ways. This system consists of a closed cabinet with a long, adjoined channel. This channel needs to have the length of the lowest reproduced wavelength divided by 4 (e.g. for 30 Hz: 11 metres wavelength / 4 = 2.8 metres).
The long channel acts also as a resonator, closed on the side where the bass driver is located and open on the opposite side. Therefore, the channel resonates with 1/4 or 3/4 or 5/4 of the wavelength. Since the higher frequencies ((3/4 and 5/4 wavelength) cause interference, the channel needs to be damped slightly with light material, somewhat more towards the bass driver, somewhat less towards the opening.
Transmission line speakers have the disadvantage that they are difficult to tune - DIY only makes sense having extensive measuring instruments available and with a lot of know-how.
Horn loaded speakers
A horn is an outstanding transmitter of sound, generated by a driver and passing it on to the ambient environment. Therefore, horns are usually used where high volume matters. In hi-fi applications, however, they cause a problem: The ideal horn should have a diameter of the longest wavelength divided by 2. At 30 Hz the horn should have a diameter of 5.5 metres.
This is not practical. Partial horns or folded horns are used to get by, sometimes even the walls of the listening room are used (e.g. Klipsch Horn).
In hi-fi applications horns play a secondary role, although, there will always be enthusiasts preferring this concept. At rock concerts where extreme volumes are often required, they are quite common.
Band pass cabinets
In principle the band pass cabinet consists of a closed system where a cabinet with a bass reflex port is attached to the front.
This concept is suitable for subwoofers, especially for passive radiators since no steep filter is necessary. This is a filter prohibiting the subwoofer of transmitting frequencies around 200 Hz that can be located by the human ear.
If you want to calculate the correct cabinet size yourself, please download lspCAD lite from our server or you may use our online tools (link top right).
Dipole and RiPole cabinets
Picture above: comparison of different dipolar speakers. They all have in common that they radiate forward and - with reversed phase - backwards (represented by green coloured axis).
In a listening room they collide, a state often called acoustic short circuit. In places where rear and front sound radiation are equally high, they cancel out one another. That's the case with the two first dipolar speakers vertical to the radiation axis; their radiation characteristics resemble a figure of eight while the others rather resemble a cardioid since the backwards radiating sound is more restricted.
In the picture to the right, the radiation axis is in light green and the radiation areas are marked in red. Above they are symmetrical (like a figure of eight), below - due to smaller opening on the left side of the dipole - they are generating a smaller area (minor area) and on the right a larger area (major area), encircling the minor area even further should the radiating differences become bigger.
More about dipolar speakers you'll find in Hobby HiFi 2/2005, or in our discussions with Axel Ridtahler, having patent rights for two types of dipolar speakers that are called RiPoles by us. Axel Ridtahler also wrote the PDF Overview..