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RE: Ported or Infinite Baffle

Posted by tomservo (M) on July 23, 2023 at 06:02:53

In Reply to: Ported or Infinite Baffle posted by Freo-1 on July 22, 2023 at 18:40:02:

A woofer produces two acoustic signals, they are equal but opposite in polarity and without a cabinet, for bass, the two signals largely cancel each other out. The sealed box's job is to contain one side of the radiation while the Vented box's job is when "at port resonance" is a phase inverter and the radiation from the port is then additive to the woofers front side radiation instead of the opposite.

The way it works out is for a given box volume, that a vented alignment can be as much as +6dB more efficient and produces more low frequency sound for a given cone excursion AND SO for a given driver's linear excursion, the vented loaded driver can produce less distortion at a given SPL.
The reason is that one wants the knee at the LF corner to have the right shape, not a big peak and exaggerated bass, not an over damped curve with diminished bass. IF the driver's motor isn't strong enough, the LF corner is a peak, if it's too strong, it rolls off too early.
AND the vented alignment needs the stronger motor to work best and a higher sensitivity comes with the stronger motor.

But wait there is more (as they say). Assuming you listen to speakers in a room (an enclosed space, even a car) As the frequency falls, you have a series of room modes set by the rooms dimensions BUT once below the lowest mode, the room becomes a pressure containment vessel.
IF one had a concrete air tight room, one finds that a flat response woofer, produces a rising response as the F goes down.

In a perfect room case, the slope is +12dB per octave and so (as is often the case for car audio, windows up) the normal sealed box roll off of -12dB /oct is compensated by the +12dB/oct room gain slope and the result is for a sealed box woofer with a knee around 50Hz, will (in a car) gives flat or near flat response down to several octaves lower than the actual corner f. This room gain slope has been called the only free lunch in Audio.

A vented box has a roll off below the LF knee that is twice as fast, -24dB/oct and so in a car still rolls off at -12dB/oct even with the +12dB/oct room gain slope. As a result, the sealed box system can "sound like" it has more deep bass and below the corner F and it can or it does.

In a normal living room, the knee for the room gain is usually more like in the upper 20's rather than 50's or 60's like in a car and sealed boxes become large when the corner F is in the 20's and the lower efficiency makes them rather uncommon.

Transmission lines were an approach to dealing with the back radiation by delaying it enough so that the sum is equal or greater than one sided radiation. These were more popular before the Thiel&Small parameters were established. I developed a variation on that which was a combination of a horn and transmission line called Tapped Horn that had some of the gain from horn loading and could deliver up to around +9dB greater sensitivity and output over the same driver as direct radiator. Like bass horns, those are still pretty large (to the living room perspective) although they are smaller than a conventional bass horn for a given low F corner.
Hope that helps
Tom Danley
Danley Sound Labs

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