subwoofer boxes explained
all info found here just brought it over
sealed enclosure design
The simplest of all loudspeaker designs, the sealed enclosure system consists of a driver mounted on one side of a sealed box. The sealed enclosure system is characterised by excellent transient response, good low frequency power handling, smaller box size and lower sensitivity to misaligned parameters when compared to other alignments. However, sealed enclosure systems tend to suffer from higher cutoff points and lower sensitivity than the other low frequency systems. There are two types of sealed enclosure systems: the infinite baffle (IB) system and the air suspension (AS) system. The IB system normally uses a large enclosure where the compliance (or "springiness") of the air within the enclosure is greater than the compliance of the driver suspension. The AS system normally uses a small enclosure where the compliance of the air within the enclosure is less than the compliance of the driver's suspension by a factor of 3 or more.
Sealed enclosure systems are probably the best starting point for the beginner DIYer because of the relative ease in achieving the desired frequency response. They are usually the subwoofer system of choice for audiophiles because of their excellent transient response (i.e. no boomy sound) characteristics when designed and built properly.
ported enclosure design
A ported enclosure system consists of a driver mounted on one side of a box that has an open tunnel or port which allows the passage of air in and out of the box. At low frequencies, the vent contributes substantially to the output of the system. The ported enclosure system is characterized by lower distortion and higher power handling in the system's operating range, and lower cutoff frequency than a sealed enclosure system using the same driver. Distortion rapidly increases below the cutoff frequency however as the driver becomes unloaded, and the transient response of a ported enclosure system is usually inferior to that of a sealed enclosure system using the same driver. However, the lower cutoff frequency and better power handling within the system's passband often makes ported systems the alignment of choice for many speaker builders.
Ported enclosure systems are much more sensitive to misaligned parameters than sealed enclosure systems, which makes their construction more difficult for the beginning DIYer. I advise that you don't attempt to build these systems, unless you're certain that the ]t/s parameters for the driver that you want to use are correct.
Almost any driver can be used in a ported enclosure system, however, only drivers which have a Qts value between 0.2 to 0.5 will generally give satisfactory results. If the driver has a Qts above 0.4, try using it in a sealed enclosure or single reflex bandpass system instead.
passive radiator enclosure design
Passive radiator systems are very similar in operation to ported systems. However, instead of a port, the passive radiator system uses a passive radiator (also known as a "drone cone") to extend the system's low frequency response. The response of a passive radiator system is similar to that of a ported system using the same driver. However, the cutoff (-3dB) frequency is slightly higher, and the cutoff slope is deeper, mostly due to the presence of a "notch" in the frequency response corresponding to the passive radiator's resonance frequency. However, this notch is normally located far outside of the passband of the system, and therefore usually of little audible significance. The larger the passive radiator, the lower the passive radiator's resonance frequency (for the same target Fb), and the further the notch is out of the passband.
To design a passive radiator alignment, start with a simple ported alignment using that driver that provides the desired box size and frequency response. Then, use the diameter of your chosen passive radiator as the "port diameter", and use this to calculate the required port length. Work out the volume occupied by this port and then use this to calculate the mass of air occupied by this port. The result is the required mass of the passive radiator. If it is too small, use a larger passive radiator and repeat the calculations.
Vas: 2 cu.ft.
Fs: 30 Hz
Diameter: 8 in.
Ported Alignment (QB3):
Vb = 0.70 cu.ft.
Fb = 39.4 Hz
Now, we need to select an appropriately-sized passive radiator. ALWAYS use a passive radiator that is larger in diameter than the active driver, as the displacement of the passive radiator usually has to be 1.5 to 2 times that of the driver. If it's not possible to use one large passive radiator, then you can use two or more smaller ones, and tune them by working out the effective diameter from the combined surface area of the radiators.
Note that the effective diameter of the radiator is approximately equivalent to the diameter of the passive radiator's face plus 1/3 of the surround. If unsure, use the quoted Sd for that radiator, then use the following equation to determine the effective radius:
R = (Sd/PI)^0.5
In this case, we choose to use a passive radiator that has an effective radius of 5 inches (roughly corresponding to a "12-inch" passive radiator).
"Port" Radius = 5 in.
Required Port Length = 186.1 in.
"Port" Volume = (PI*R^2)*h
= (3.14 *5^2)*186.1
= 14609 cu.in. = 8.45 cu.ft. = 0.2393 m^3
Mass = "Port" Volume * Density of Air
= 0.2393 * 1.21
= 0.289553 kg
= 290 g
The passive radiator therefore has to have a weight of 290g. To achieve this, start with a passive radiator with lower mass, then add weight to make up the difference. To measure the resonance frequency of the passive radiator, install it in a free-air baffle (e.g. the box it's going in, without the driver in place), then hold a driver, driven by a sine wave generator, as close as possible to the passive radiator, then vary the frequency. At the passive radiator's resonance frequency, you should see the greatest peak to peak excursion of the passive radiator.
Like their ported cousins, passive radiator systems are much more sensitive to misaligned parameters than sealed enclosure systems, which makes their construction more difficult for the beginning DIYer. I advise that you don't attempt to build these systems, unless you're certain that the t/s parameters for the driver that you want to use are correct. Almost any driver can be used in a passive enclosure system, however, only drivers which have a Qts value between 0.2 to 0.5 will generally give satisfactory results. If the driver has a Qts above 0.4, try using it in a sealed enclosure or single reflex bandpass system instead.
4th order enclosure design
The 4th order or sealed rear chamber bandpass system is basically a sealed enclosure system with the addition of an acoustic filter in front of the driver. The resulting system usually provides a lower cutoff frequency, the tradeoff being a larger enclosure. The enclosure can be reduced in size by using two drivers in an isobaric configuration. 4th order bandpass systems usually demonstrate better power handling characteristics than the other main systems considered here. Its transient response is second only to the sealed enclosure systems, making it a good choice for subwoofer applications.
As all of the output of the 4th order bandpass system is via the port, the largest port diameter possible for the enclosure should be used in order to minimize port noises. The ports should be flared whenever possible, for the same reasons.
The 4th order bandpass system rarely exhibits a perfect bandpass response - there is usually some out-of-band noise present in its output. A simple notch filter can be used to reduce this noise if it is audible. Alternatively, a low-pass filter can be used in series with the driver, but the in-band response of the system may also be affected if this approach is taken.
6th order enclosure design
The 6th order bandpass system is similar to the 4th order bandpass system , except in this case both the front and the rear volumes are tuned via vents. The power handling of the 6th order bandpass system ranges from excellent within its passband to poor for frequencies lower than its passband. The transient performance of 6th order bandpass systems is usually worse than the sealed, ported and 4th order bandpass systems, making it more suitable for sound reinforcement, multimedia and other less critical applications, rather than high-end audio. Like ported systems, the driver becomes unloaded at frequencies lower than the passband.
As all of the output of the 6th order bandpass system is via the two ports, the largest port diameter possible for each volume should be used in order to minimize port noises.
As with the 4th order system, the 6th order bandpass system rarely exhibits a perfect bandpass response - there is usually some out-of-band noise present in its output. A simple notch filter can be used to reduce this noise if it is audible. Alternatively, a low-pass filter may be used, but the in-band performance may be affected.
transmission line enclosure design
The transmission line system is a waveguide system in which the guide reverses the phase of the driver's rear output, thereby reinforcing the frequencies near the driver's Fs. Transmission lines tend to be larger than the other systems, due to the size and length of the line required by the design. The payoff is an extended low end response and a characteristic sound that's appealing to many. Usually, only drivers which have low Qts (0.25 - 0.4) , Qes (0.3 - 0.4) and Fs values are suitable for transmission line systems.
dipole enclosure design
Dipole subwoofers are quite different to the other subwoofer systems described on this site because of the way they treat the the driver's output. Your typical subwoofer driver produces sound from both the front and the rear of the cone, and the output from the rear is out of phase with the output from the front, which results in very reduced response levels, unless the rear wave is treated in some fashion. The other subwoofer systems described on this site all employ some means of dealing with the driver's rear radiation to improve overall low frequency response, the result being a "monopole" bass system that theoretically has the same response characteristics in all directions. However, for dipole bass systems, the rear radiation is left untreated, and instead the overall response of the system is adjusted by varying the size of the baffle and the "Q" of the system to achieve the best overall response characteristics.Driver Characteristics
The drivers used in dipole systems tend to be quite different to those in "monopole" bass systems. The driver's Qts tends to be particularly high (in some cases, as high as 2.0), the idea being to introduce a "bump" in the driver's frequency response around Fs that will compensate for the 6dB/oct rolloff in the response that will occur when the driver is mounted in an open baffle. Alternatively, a "normal" driver can be used in a dipole bass system, but a considerable amount of equalization may have to be used to make up for the loss in low frequency performance.
A dipole bass system has a "figure of eight" response pattern, which is entirely different to the "spherical" response pattern of your typical "monopole" subwoofer. The system's output is most powerful directly in front and behind the baffle, and decreases to zero at the sides, where the front and rear waveforms cancel each other. This response characteristic is said to be one of the major advantages of a dipole bass system, as the restricted dispersion results in fewer boundary reflections, which in turn is supposed to result in a smoother in-room response.
Dipole bass systems tend to be rather large, employing multiple drivers, primarily to make up for the output reduction due to the 6dB/oct baffle loss. This is not the type of system to use if you've got a small living room, and it's certainly not suitable for car audio!
The response of the system will be affected by a 6dB/oct drop in output below a particular frequency referred to as Fequal, that's directly dependent on the size of the baffle. At Fequal, the magnitude response (SPL) of the baffled driver will be the same as its infinite baffle response. Above Fequal, the response will rise to a 6dB peak at Fpeak (approximately equal to 3*Fequal), and at higher frequencies, the response will depend largely on the shape of the baffle. A completely circular baffle will produce the worse response characteristics, with deep nulls at multiples of Fpeak.
The following table demonstrates the relation between the baffle's effective diameter (i.e. the diameter of a circular baffle that has the same radius as the smallest dimension of the baffle), Fpeak, and Fequal:
Speed of sound, c=344 m/sDiameter (metres)Fp (Hz)Fe (Hz)0.438002670.576002001.153001001.43240801.56220731.7220067 From the table, it's plain to see that it's nearly impossible to push Fequal much lower than 80 Hz unless a fairly large baffle is used. The tradeoff here is efficiency; the smaller the baffle, the lower the final efficiency of the dipole system. OTOH, the larger the baffle, the higher the efficiency, but response at the upper end of the passband could get somewhat irregular as Fpeak is reduced.
Almost all dipole bass designs incorporate some means of boosting the response at low frequencies to compensate for the baffle loss. Typically one or more of the following methods are used:
- A high-Q driver is employed (the high Q results in a peak in the driver's free-air response at its resonance frequency).
- The Q of the system is increased by employing a series resistor (Qes is increased, which results in an increase in Qts).
- Active equalization is used to boost the low frequency response.
- Active or line-level filtering is used to cut the higher frequency levels to match the low frequency response.
Last edited by forceww; 02-19-2009 at 10:35 PM.