question on tunning and hz

[6three6mafia6]

i got a question, ive seen in realm of excursion videos of xxx's and brahmas going through some test tones that are real low like around 20 hz 25 hz, how is it that these subs can play that when number one the sub only goes down to 28hz and im guessing there tuned higher than that?and what makes a sub not be able to play upper notes (not midbass).and what were to happen if someone were to make a box tuned to like 20hz , i know that tunning a box higher is for spl purpose so someone please clear this up.

[GlassWolf]

geesh.. where to start.
firstly, it's "tuning" and "they're"

that out of the way, 20Hz is the threshold of human hearing. subs can play down to about 9Hz in actuality, depending on the enclosure.
28Hz is usually the Fs of the sub, or the point at which the sub has the lowest impedance and least resistance to current. The free air resonant frequency.
This has nothing to do with how low the sub actually is capable of playing really.
in a sealed box, with cabin gain factored in below transfer point, a sub can play well below 20Hz without any roll off or decrease in volume.


Now with a ported box, they are usually tuned for SQ around 38 to 35Hz. for SPL, around 45-70Hz.
Now, below the tuned frequency, the sub will still function down to any oint set by an infrasonic 'brick wall' filter.
there will be roll off though, usualy fairly steep meaning much more power is required below the tuned freq to produce the same volume of sound at that frequency.
roll off is usually around 18 to 24dB per octave.
that means every time you halve the frequency below the tuned point for the ports, your natural volume will be 18 to 24dB quieter.
every 10dB is half the audible volume, so 20dB would be a quarter of the volume produced at the tuned frequency.

tuning the box close to the Fs of the sub generally allows for a very efficient box, which lets the sub produce more bass at the tuned frequency of the ports, with less power due to being near the sub's Fs, requiring less poer naturally anyway.

tuning a box to 20Hz would essentially make it most efficient at 20Hz, which is where your infrasonic filter should be set, meaning you're going to cut out anything produced at that point, ruining the entire point of the ported box anyway.
it's like building a fast car, then sticking it on a road with a near-zero speed limit.
self-defeating.

did I get everything covered?

[6three6mafia6]

kiki youre a genious, thats just what i needed to hear.

[GlassWolf]

glad I could help.
Sometimes I have trouble putting things into simple terms when it comes to more technical stuff.