Ovih dana sam naisao na tekst koji odgovara na vecinu dilema zbog kojih sam otvarao ovu temu. Nadam se da nece smetati jezik i velicina posta...
(izvor:
Bass Guitar Magazine -
http://www.bassgearmag.com)
Sealed enclosures
Here’s how a sealed enclosure works. The back of the speaker is completely sealed off from the front. The air inside the enclosure acts as a spring, which helps control the movement of the cone. When the speaker moves out, the pressure inside is decreased. When the speaker moves in, the pressure inside is increased. A sealed cabinet is considered a punchier, more accurate sound.
Sealed cabinets are much easier to design and build than ported enclosures and are typically smaller in size. There is also much more room for error in design and construction because a small change to the internal volume doesn’t affect the lowest audible frequency significantly. You might often see the low range or note of a cabinet’s specifications defined as F3 or F10. Mid-band response is basically an average level of the speakers’ or cabinet’s overal l output within the usable frequency range. F3 is the frequency 3dB down from the mid-band and F10 is the frequency 10dB down from the mid-band. Sealed enclosures have better transient response, which means the system will respond more quickly to a sudden change. This is one reason why they sound punchier and more accurate. When you’re playing a run, it sounds more articulate with better note separation. Sealed enclosures also have good power handling capability and gradual frequency roll-off.
The disadvantages of sealed enclosures are lower efficiency and poor deep or extended bass output. The sealed design will never play as low as the resonant frequency of the speaker. It seems like the easy answer would be to design speakers for sealed enclosures with low resonant frequencies, right? That’s certainly one part of the puzzle, but a lower resonant frequency can negatively impact other design and performance goals. Speakers with lower resonant frequencies are typically low in output and narrow in frequency range. Another disadvantage is higher distortion. Maximum cone movement occurs at the resonant frequency of the enclosure (Fc). Basically, the speaker is working harder where the cabinet is most demanding. Dampening improves below Fc, so control of the cone and mechanical power handling of the speaker are good.
Ported Enclosures
Ported enclosures are also referred to as vented, or bass-reflex enclosures. This design requires a more scientific approach, and there is less room for error in design and construction.
Ported cabinets allow for an extended bass response. The result sonical ly is more “rumble” and deeper bass tone. A port (or vent) is used to tune the enclosure to a specific frequency (Fb). The surface area and length of the port are crucial to the tuning. The Fb of the enclosure does not change with speaker selection, but F3 does. The port uses the speaker’s rear output to enhance the speaker’s front output, which increases bass output (or SPL) above F3 (see Figure 3 again). This minimizes the movement of the speaker cone, so mechanical power handling at and above the tuning frequency is very good. The port is actually producing most of the output at the tuning frequency and the speaker’s excursion is minimal. Distortion is lower at this point due to less cone movement.
There are some disadvantages to ported enclosures. Transient response is poor compared to a sealed enclosure. The result is decreased accuracy. Also, there’s less control below the box tuning, which allows the cone to move more freely. This can result in damage to the speaker mechanically, a phenomenon known as over-excursion. A poorly designed ported enclosure can cause all sorts of problems. Tuning the enclosure too high can also be a problem. This can create a ringing at Fb and result in a one note wonder with inadequate frequency range (Figure 5). While mechanical power handling is typically a good advantage of a higher tuning, remember that the enclosure is not helping the speaker below Fb. If there is a sudden peak at a lower frequency there could be a potential for over-excursion problems. The speaker’s cone will literally jump out of the box. Low tuning can also generate problems. A large enclosure is required for a lower tuning. This can severely lower the speaker’s mechanical power handl ing. Loose, rumbl ing bass with no definition may be the result sonically. You sacrifice the effectiveness of the cabinet and the speaker. The F3 of a smaller cabinet that is tuned too low will be very high. Sure, transient response and punch will improve, but at this point, it’s more effective to use a sealed enclosure. A port without adequate surface area can create unwanted noise at higher volume even when the tuning and size of the enclosure is good for the speaker. If the air travels through the port with too much velocity, a noise often referred to as “chuffing” may occur. Port placement can be another issue. A lot of enclosures are rear-ported, which can be effective. Space is saved on the front, port noise is less of an issue, and it can create a more room fi l l ing sound. Room placement, however, is more of an issue. Poor room placement can reduce performance. Placing the cabinet next to a wall might restrict bass or it could make the bass sound too boomy. If you’re using the cabinet as a stage monitor, a lot of the sound wi l l be going in the wrong direction.
A parameter called EBP (calculated by dividing FS by Qes) is often used to determine if a speaker is best suited for a sealed or ported enclosure. An EBP close to 100 usually indicates a speaker is best suited for a ported enclosure. An EBP closer to 50 usually indicates a speaker is best suited for a sealed enclosure. Aspeaker with an EBP 50-100 might work well in both types of enclosures. This is purely a general rule of thumb. Many great designs violate this.
Of course...
Anytime you’re dealing with TONE, there’s some voodoo or black magic involved. I’ve seen many successful designs that didn’t look so good on paper. There is definitely some art and science involved in the voicing and EQ of the amplifier. I also believe that sometimes you just get lucky and find that perfect combination of speaker voicing, cabinet, amp, and instrument and it just doesn’t matter if it works out technically.
i da bude mali definitivno ported. Kompresija je super samo kod grdosija poput ampegovih 8x10 gde ima dovoljno vazduha unutra i gde ima dovoljnih velikih površina membrana da pokreće velike količine vazduha potrebne za niske frekvencije.