Frequency Response?

It's an interesting question, and while I'm not an EE, here's my understandiing of how it works (for a dynamic/cone woofer):

The woofer is a system composed of 4 main parts; a cone, an electric motor attached to that cone, a box housing that cone, and a suspension connecting that cone to that box. The motor applies force that causes the cone to move and create sound. (I'll avoid the "push air" vs "pressurize" question because I don't think it's relevant to answering the question from the OP). The other 3 components in the system all constrain the movement of the cone to one degree or another.

Assume for a second that the motor system is large enough to move the cone an infinite distance if the motor has enough "fuel" (LF signal) from the amplifier. In this case, your question goes to the behavior of the cone. If the cone is sufficiently large and/or capable of moving infinitely far, it will (theoretically) have infinite low frequency capability. But it doesn't - the combination of cone/suspension/box always limits the max movement of the cone.

So, the size of the cone and it's maximum travel limit the entire bass system's low frequency capability. Once the cone reaches its maximum travel, further "push" from the motor will fail to move it further. As more LF signal is converted to force by the motor and the cone approaches its maximum excursion, the cone starts to resist and dynamics are compressed. Once maximum excursion is reached, excess signal is largely dissipated as heat.

Conceptually - at the extreme - one of 2 things eventually happens:

Overdriving the motor by sending it too much LF "fuel" will melt the voice coil element in the motor. If the motor is sufficiently robust that the voice coil survives the heat build-up, the cone would eventually deform/crush itself.

It's also possible that the LF capability will be limited by an undersized amp (not enough LF fuel).

That's my understanding, anyway (hope it's accurate, but I'm interested in hearing if some of the tech types here have a better answer.)

Marty

Yes, they will go deeper. Sometimes I think the advertised frequency response of some speakers is just creative license. Saw an advertised rating of 40 Hz - 3dB for a sealed monitor with a 6.5" woofer. Don't think so. 60 or 70 maybe, but that doesn't sell speakers. Doesn't mean that speaker has poor bass, just a gradual roll-off and I would expect good transient response (tight bass). Some driver/box combinations have a steep roll-off and impedance curves going into the lowest frequencies.

Yes, the woofer does try to play lower frequencies, unless there is a crossover cutting off those frequencies lower than a certain point, say 30Hz. Usually the bass driver is left to attempt to play those lower frequencies, but is less precise as the frequency drops.

If you look at the graph of a woofer's response you will see at the left side it will fall off, as though a steep hill to the left. This is the visual of the speaker's worsening in performance of the lower frequencies. It may perform at +/-3dB at 40Hz, but at 30Hz may by down to -6dB or so. The more veritcal the line as the curve turns down the worse the performance in terms of capacity to play the lower frequencies.

Do not expect a speaker with specs like 40Hz +/- 3dB to do well at 25 or 20Hz. It will not. You will not get the lowest, deepest bass in abundance from such a speaker, no matter what anyone tells you in selling it. If you want lower bass, bass you can feel, etc. you simply have to find a speaker with the proper drivers/specs. At this time, while I can still move around big speakers, I choose not to use a speaker with specs like 40Hz for my reference. There are many speakers with just as good sound quality and far better lower end, say 16-20Hz performance, though you will usually pay a lot more for it. It is relatively easy to find competent speakers with bass specs such as 25-28Hz +/-3dB, and that would give you a lot of low end. Again, if someone is showing you a smaller tower speaker, or a bookshelf speaker with smaller drivers and a rating of 40-45Hz without any tight specification like "+/- 3dB" avoid it if you want powerful bass; it will never give it to you. It will perform more poorly if the number is higher, such as +/- 5dB, or even worse +/- 10dB. A very tight measurement is +/-2dB.

Having said all that, this doesn't tell you what the tonal character of the speaker is, which is why you do not typically want to buy a speaker unheard. The type of cabinet used, and whether it is ported or a sealed enclosure design of a box speaker, and the crossover network also vastly influence the sound the bass makes. So, you will want to hear a speaker you are considering purchasing if possible.

If you go the small speaker route with less bass you can always do a sub.

But, yes, any bass driver attempts to go lower along with the frequencies of the media unless cut off by a crossover. It would also attempt to go higher as well, but the crossovers limit it. Regarding a driver reaching "infinity", it will distort badly as it is pushed harder and attempts to play frequencies increasingly outside of it's optimum range of operation.

Thanks for all the great answers. I have another question. Since the driver continues to try to reproduce these lower frequences, does it do any harm to the driver sonically or physically?

Frequency respone specifications are often so generalized that they have very limited value.

Some mags like stereophile will do more comprehensive actual measurements of fr and others. Those are more useful in terms of measuring actual performance.

For the average joe, a reference test cd or record to generate test tones at various frequencies and your ears as the measuring device can be insightful. Or a spl level meter device can be used.

Of course music is a much more complex signal to reproduce than any test tone, but a suite of variable test tones can gjve at least some degree of quantifiable measurement of how you system and your ears perform.