Loudspeaker enclosures

Rohit Balkishan Dubla

A typical loudspeaker driver will sound very weak if it is used all by itself (especially for bass notes), without an enclosure. The reason is cancellation of pressure waves between the front and rear of the driver's cone. Therefore, an enclosure for a driver is designed in order to prevent rear waves from cancelling the front pressure waves. This has the advantage of allowing the driver to better reproduce bass as well as play louder than without an enclosure.

Types of enclosures

Enclosures can be typically categorised into the following basic types:

• • Open baffle

  • Closed or sealed

  • Vented or ported

  • Isobaric

Open baffle enclosure

This is nothing but a plank onto which a driver is mounted. This is the least efficient of all baffle types because it still leaves the rear and front of the driver unisolated, so there still is cancellation between the front and rear pressure waves. However the amount of cancellation is reduced in comparison to no baffle at all.

This type of baffle exhibits a figure-of-eight (dipole) radiation pattern and is commonly known as a dipole speaker. Since the baffle area can be very large in order to get good bass, sometimes the baffle is modified to look like an "H" in cross-section with the driver mounted at the centre. The dipole radiation pattern has distinct areas where the SPL is very less (the sides) as compared to the front and back and this is advantageous where it is desired to concentrate the sound to a certain listening area.

The low frequency response of the enclosure is directly related to the driver's low frequency response and rolls off at a rate of about 6 dB/oct below the -3 dB lower cut-off frequency in unison with that of the driver (basically a 1^st order high-pass filter characteristic). Only drivers with a large cone excursion and high power handling (and therefore, expensive) are suited to dipole arrangement. This is even more important since dipoles have to be electronically equalised (i.e., power input to the driver is increased as frequency goes down) to compensate for the 6 dB/oct roll-off if reasonable bass response is desired.

Closed or sealed enclosure

As the figure shows, this is the simplest enclosure that gives good to very good performance. This enclosure is also very tolerant of driver mismatches in that the sound degradation is rather graceful and not very audible. Depending on driver and enclosure we can have a variety of bass responses ranging from maximally flat to "punchy" or in between. This type of enclosure has the best (or tightest) bass with little or no boominess if properly designed. A variation of this enclosure known as "acoustic suspension" exists. This is basically a very small sealed box onto which a driver with very high compliance is mounted. This system works by using the enclosed air as the primary mode of suspension than the driver's own suspension. However only drivers with very high suspension compliance are suited to the acoustic suspension type of sealed box.

The low frequency response of this enclosure is a combination of the driver's own roll off and the enclosure's roll-off and as a result the response rolls off at a faster rate of about 12 dB/oct below the -3 dB lower cut-off frequency (basically a 2^nd order high-pass filter characteristic). However the roll-off may be less steep if the box is very large in volume. Sealed enclosures (especially acoustic suspension types) allow greater power handling for a given driver since the air enclosed forms a very good restoring force for the cone thereby ensuring greater electrical power before cone excursion limits are reached.

Vented or ported enclosure

Placing a hole on any one panel of a sealed enclosure converts it to a ported or vented enclosure. The vent or port has some mass of air in it which has a certain resonant frequency (Helmholtz resonance). When the driver's rear pressure wave approaches this frequency, the port augments the driver's output thereby improving its overall efficiency. The port can be tuned in order to give the enclosure desirable characteristics like extended bass - for an enclosure of the same volume and using the same driver, the vented enclosure will give bass response that goes lower in frequency as compared to the sealed enclosure. Port tuning can be achieved by changing either the port diameter (or area) and/or its length (by using a tube, as the figure shows). This makes the vented box the choice for most small speakers, and commercial offerings. However the extended bass response comes at a price - it is not as clear or tight as a sealed box would give. Also, a vented enclosure is very sensitive to the driver, i.e., if the driver is changed the box tuning (and therefore, the response) will no longer be valid.

The low frequency response of this enclosure is a combination of the driver's own roll off, the enclosure's roll-off and the port's roll-off and as a result the response rolls off at an even faster rate of about 24 dB/oct below the -3 dB lower cut-off frequency (basically a 4^th order high-pass filter characteristic). Vented enclosures ensure very little cone movement at the tuned frequency (and thus maximising driver efficiency), increasing a bit as the frequency reaches the lower cut-off point. However below the lower cut-off frequency the port starts to unload the cone - the driver behaves as if it is not in an enclosure and as such has only its own suspension to rely upon in order to control the cone's movement. Thus at frequencies well below the enclosure's cut-off, for the same power input there will be much larger cone excursion - this has the risk of physically exceeding the driver's excursion limits and damaging it. Therefore it is advisable to filter out frequencies below the enclosure's cut-off in order to prevent this.

Isobaric enclosure

The isobaric enclosure is not an enclosure in the true sense of the word. It refers to a method of loading a loudspeaker driver using an additional, similar driver. This type of enclosure or loading is typically used when the goal is to have a relatively small cabinet with bass output equivalent to that of the same driver in a box of double the volume. In short, an isobaric box has approximately half the volume for the same bass output as a compared to a "normal" box using the same driver. Addition of a second similar driver effectively halves the V_as of the main (or radiating) driver, which in turn means that it needs half the volume it normally would have for a given bass response.

The image above shows two ways in which drivers can be loaded isobarically - the first and more common way is to mount the drivers with cones facing each other and electrically wired out-of-phase, such that when one cone pushes, the other pulls (i.e., both cones move in the same direction). It is known as the cone-to-cone or "clam-shell" configuration. The second way is to mount drivers with cones facing the same direction and thus wired in phase (cone-to-magnet). This is more complex to build since it involves making an additional panel which along with the driver will be effectively embedded inside the box. This type is much harder to trouble-shoot should the inner driver get damaged as it requires opening up the box from the side to access the inner driver. A third way (not shown) to isobarically load the drivers is the magnet-to-magnet construction. It has the same construction as the cone-to-magnet, except that the inner driver will face away from the outer one and be wired out-of-phase (just as for the cone-to-cone configuration).

Since one driver has the job of loading the main (or radiating) driver (and does not play any role in producing sound) but still needs the same power as the main one, in effect this renders the isobaric configuration half as efficient as compared to a larger box with a single driver of the same make. Therefore, it requires twice as much power to achieve the same SPL as the single-driver in a larger box, which is a major reason why it is used only when space and not efficiency is a major concern. However this configuration also has double the power handling capacity as compared to a single driver enclosure.

To be noted is that the depictions in the image above show isobaric pairs in sealed boxes. Therefore, except for the halved V_as (and 3 dB less efficiency) the box calculations and performance are exactly the same as a sealed box using a single driver - equivalent to designing a box with the same T/S parameters as the single driver but with half the V_as. Needless to say, isobaric pairs can be used with vented enclosures also, and again except for the halved V_as and reduced efficiency everything else remains the same as that for a single driver. In case of the cone-to-magnet configuration above, the port would be on the volume enclosed by the box to the left of the inner driver. To have a port on the same panel as the outer driver would require it to be ducted, with the tube extending from the outer panel, through the inner panel into the volume behind the inner driver.