Ported, Sealed or Transmission Line

Again I sum it up..."Ported speakers make excellent bird houses".

Tobias writes:

Sealed box designs need more amplifier power, on the whole, because the cones have to work against the very stiff spring of the air trapped inside the box.

Although stiffness is provided by the air in the sealed box, the spider on a driver designed for acoustic suspension is less stiff, and compensates.

When Vilchur designed the acoustic suspension speaker he modified the driver spider to provide less spring restoring force on cone excursion because he knew the air in his sealed box would provide some restoring force.

The main reason the acoustic suspension speaker needs more amplifier power is the energy of the back wave is totally absorbed by the stuffing (converted to heat). In a reflex design, the out-of-phase back wave energy hits the port and is inverted 180 degrees, and is now in phase with the sound coming off the front of the driver. This extends the bass response, although the eventual rolloff when it does occur is much steeper (typically 12dB per octave as opposed to 6 dB per octave).

There are many good speakers using the bass reflex system.

Regards,

Thanks for the correction, Metralla.

I knew about the spider design ( and the more resilient cone surround ) but I hadn't absorbed the back wave concept. Needed more stuffing, possibly.

Let's also not forget that while all Acoustic Suspension speakers (AS) are sealed boxes, not all sealed boxes are AS. When the compliance of the driver exceeds that of the air in the box (i.e. when the box gets larger and larger), this alignment is referred to as an Infinite Baffle (IB). And, the issues which have been raised in this thread regarding the AS speakers are ameliorated. Correct me if I am wrong, but Sean's projects tend to go in this direction - big woofers in big boxes that make big bass.

Again, while a TL is superior to a sealed box, it's really hard to go wrong with a sealed speaker.

Eldartford, there are plenty of excellent sounding ported speakers on the market. Dogmatic comments in audio lead to a lot of instances where the person ends up in the wrong.

Infinite Baffle sounds like a cable manufacturer's technical White Paper. :o)

It's also what you get when you set a speaker in a wall, or in a floor like some subwoofers of the fifties. ( Think of the low end from an 18-inch EV hung just under your feet! )

The term means that the driver's back wave is " baffled " as it tries to come round and cancel out the front wave, which is in opposite phase. It's a British term and in England the baffle of a speaker is its front panel. When a speaker is mounted in a wall, the back wave has, for practical purposes, an infinite distance to travel before it can cancel anything out.

Of course all dynamic loudspeaker designs try to either eliminate or control the cancellation effect, which is most serious in the bass. That's why we put drivers in boxes in the first place.