Tell me about Resonant Frequency

[blue fury 3]

I guess I don't know much about this subject. I did a quick search on here and didn't find all the info I was looking for on the questions that I've had lately. To my understanding, it's just the natural frequency of the sub. I'd like to know a little more than that.

Recently I saw a topic where some 8's with ~38hz FS were for sale and someone described them to play really low on music, how nice they are for music, on and on. I didn't know if these subs were meant to play lower than 38 hz or if those people just called +38hz 'low'. Correct me if I'm wrong, but the FS is 1 frequency and not including the + and - frequencies (to some small extent) meaning that the sub can play above and below the FS. So if the box that these 8's are in is tuned much lower than 38hz with no high pass above it, wouldn't these subs be playing 38hz pretty frequently? Why doesn't this damage them?

I guess I just want to know what you can and can't do with the sub in terms of box tuning and playing music while knowing the FS. I'm not really satisfied with how I worded the paragraph above so maybe it can be better explained though back and forth conversation.

[///M5 2,833]

All objects have natural frequencies. This includes your whole body or any of your parts. For instance your stomach, ever wonder why you get sea sick? You excite the natural frequency of your stomach and your body doesn't like it.

Subs of course also do. The lower generically means that they will be able to play with authority lower, but in a box designed with some savvy you can get a sub to play well below its Fs. It just isn't as easy as it is with subs with a lower Fs. If you don't understand the whole gamut of T/S parameters though, I would encourage you to follow manufacturers suggested box sizes and not arbitrarily design your own.

[blue fury 3]

All objects have natural frequencies. This includes your whole body or any of your parts. For instance your stomach, ever wonder why you get sea sick? You excite the natural frequency of your stomach and your body doesn't like it.

Subs of course also do. The lower generically means that they will be able to play with authority lower, but in a box designed with some savvy you can get a sub to play well below its Fs. It just isn't as easy as it is with subs with a lower Fs. If you don't understand the whole gamut of T/S parameters though, I would encourage you to follow manufacturers suggested box sizes and not arbitrarily design your own.

The bolded line is what I'm asking about. I know the idea and definition of natural frequencies but I don't know how a sub behaves when playing frequencies very close to the FS. EXAMPLE: a sub with FS of 35hz in a box tuned to 28hz (random figures) playing music. There has to be 35hz notes played at some time or another, right? That said, I don't understand how a sub could be in this situation and not be damaged.

[Impious 1,476]

My first question is, why do you think a speaker would be damaged by playing at it's Fs?

The woofer doesn't self-destruct at Fs. Think of a weight at the end of a spring, with the spring being suspended vertically and the weight attached at the bottom of the spring. If you grab the weight, pull it down and then release it, the spring will rebound and continue oscillating up and down at a specific rate. In speaker terms, the mass of the soft parts would be the weight and the suspension would be the spring. At some specific frequency, the weight of the moving mass will be in balance with the compliance of the suspension and there will be a natural oscillation of the "weight on the spring" (the moving mass on the suspension). This is the Fs, and can be identified by a peak in the driver's impedance.

[blue fury 3]

My first question is, why do you think a speaker would be damaged by playing at it's Fs?

Well... I never specifically read it but I just took the natural frequency idea and used it with some info picked up from various places- basically I was given the idea that it it's frequency and with the continuous power, the soft parts would go out of control. I supported this thought with that bridge that failed a while ago. The wind caused it to sway and swing at its natural frequency until its supports couldn't resist the force from how far it was swinging. The bridge obviously collapsed so I thought that this is along the same lines- the FS after a short time would cause the mass on the soft parts to vibrate beyond what was safe and eventually lead to some sort of failure.

I apologize if I sound stupid, but it's never been explained to me.

The woofer doesn't self-destruct at Fs. Think of a weight at the end of a spring, with the spring being suspended vertically and the weight attached at the bottom of the spring. If you grab the weight, pull it down and then release it, the spring will rebound and continue oscillating up and down at a specific rate. In speaker terms, the mass of the soft parts would be the weight and the suspension would be the spring. At some specific frequency, the weight of the moving mass will be in balance with the compliance of the suspension and there will be a natural oscillation of the "weight on the spring" (the moving mass on the suspension). This is the Fs, and can be identified by a peak in the driver's impedance.

Impedance rise right? I have VERY little understanding of this so I'll try to look for good articles on this. Know any?

[///M5 2,833]

The bridge that recently failed in MN did not go into resonance. You are referring to the Tacoma Narrows which failed in the 40's. That failed not at its first resonance but a higher resonance excited by high air flow.

Nothing will go "out of control" unless you force it to, but you should use your subsonic to prevent overexcursion or really know what you are doing and understand your source, levels, and listen for stress.

Impedance rise is generally caused by a box response and not a driver in free air.

[blue fury 3]

The bridge that recently failed in MN did not go into resonance. You are referring to the Tacoma Narrows which failed in the 40's. That failed not at its first resonance but a higher resonance excited by high air flow.

Nothing will go "out of control" unless you force it to, but you should use your subsonic to prevent overexcursion or really know what you are doing and understand your source, levels, and listen for stress.

Impedance rise is generally caused by a box response and not a driver in free air.

The Tacoma Narrows. Thats the one I was thinking of. I could have sworn my professor said it was at its natural frequency, he's such a nice guy and was literally a rocket scientist with a Ph. D. in that field as well as Mechanical Engineering iirc. I'll have to ask him about that again.

The FS usually is below the playing frequency right? If you were to apply a decrease in level say, -9db (random figure, not a slope but just a decrease) on all the frequencies there and below to prevent over excursion, would the sub be able to play all the frequencies below? I hate to use 'what if' situations because of the unlikelihood that I'll actually do it.

More importantly, is there any other reason than over excursion not to have the sub play at or below its FS?

[killa101 0]

The bridge that recently failed in MN did not go into resonance. You are referring to the Tacoma Narrows which failed in the 40's. That failed not at its first resonance but a higher resonance excited by high air flow.

Nothing will go "out of control" unless you force it to, but you should use your subsonic to prevent overexcursion or really know what you are doing and understand your source, levels, and listen for stress.

Impedance rise is generally caused by a box response and not a driver in free air.

The Tacoma Narrows. Thats the one I was thinking of. I could have sworn my professor said it was at its natural frequency, he's such a nice guy and was literally a rocket scientist with a Ph. D. in that field as well as Mechanical Engineering iirc. I'll have to ask him about that again.

The FS usually is below the playing frequency right? If you were to apply a decrease in level say, -9db (random figure, not a slope but just a decrease) on all the frequencies there and below to prevent over excursion, would the sub be able to play all the frequencies below? I hate to use 'what if' situations because of the unlikelihood that I'll actually do it.

More importantly, is there any other reason than over excursion not to have the sub play at or below its FS?

You can play frequencies below the subs Fs without a problem. How deep your sub will play depends mostly on the enclosure being used. If you are using a ported enclosure then you will have a cut-off below the tuning frequency. So your sub will be limited in low-end frequencies below the tuning.

To answer your question about lowing the power to the sub...If you were to not exceed the sub's mechanical limits then you could potentially play all the low frequencies. However, you wouldn't really have any audible output. Your sub would basically be playing in free air.

You can get your sub to play low but doing so will often require a loss of something else. You will typically need a large enclosure or a very inefficient sub to get the low-end response.

The only problem you may have is if you were to tune a ported enclosure to the resonant frequency of the sub. In that case you will have very sloppy bass.

[blue fury 3]

Thanks for all the help. I think the last question I have is this: Take a sub meant for music with say, 50hz for the FS in an appropriate sized box tuned to say, 33hz. As long as you don't give it enough power to bottom out, would it sound just the same as the same setup with the other sub having an FS of say, 25hz? These figures are just based on normal situations and not times where the super lows are desired.

[Impious 1,476]

Well... I never specifically read it but I just took the natural frequency idea and used it with some info picked up from various places- basically I was given the idea that it it's frequency and with the continuous power, the soft parts would go out of control. I supported this thought with that bridge that failed a while ago. The wind caused it to sway and swing at its natural frequency until its supports couldn't resist the force from how far it was swinging. The bridge obviously collapsed so I thought that this is along the same lines- the FS after a short time would cause the mass on the soft parts to vibrate beyond what was safe and eventually lead to some sort of failure.

The bridge you are referring to (as pointed out by M5) failed not necessarily because it was excited at resonance, but because it was excited at resonance and driven beyond it's mechanical limitations. Just the same a subwoofer would be damaged if you drove it past it's mechanical limits.

But going back to our weight on a spring...what happens when the weight is pulled downward and released? Does the spring oscillate until it self destructs, or does it oscillate until it's energy is expended?

So, how do we determine the time it takes a subwoofer excited at resonance to dissipate it's energy and cease oscillation? By the Q factor. The Q of the driver describes the amount of damping provided at resonance, or for how long after the signal is removed the driver will continue to "ring". (The terms "overshoot" and "ringing" are commonly used to express the amount of damping a driver has.) Qms is mechanical damping (i.e. the suspension). Qes is the electrical damping (i.e. the motor). And Qts is the total Q of the driver (Qts = Qms * Qes/Qms + Qes). A high Q indicates a driver that is underdamped, meaning it will "ring" for a longer period of time or that it has more "overshoot". A low Q indicates a driver is more overdamped, meaning it will have less "ringing" or less "overshoot".

So the Qts tells us how a driver will behave at Fs based on the damping provided by the motor and the suspension. And, in case you've never noticed...in the vast majority of drivers on the market, the Qms is much higher than the Qes. Likewise, the total Q of the driver (Qts) is typically just slightly lower than the Qes and significantly lower than Qms. So what does this mean? It tells us that the motor provides much more damping and control over the motion of the cone than the suspension.

Impedance rise right? I have VERY little understanding of this so I'll try to look for good articles on this. Know any?

Well, the impedance does increase but I don't believe "rise" is typically the word used to express that idea as that's most associated with enclosure and heat effects, again as M5 pointed out.

[blue fury 3]

Thanks guys, I think that's all for now.

[jcarter1885 995]

Im on the same boat as the OP, I have been doing research on subwoofers (how they work, the different parts and what they mean). I got a handle on the terms and meanings of T/S Parameters but on how to apply it point of view or someone not so knowledgeable in this area.

I been looking at subwoofers trying to decide what to get, lately its been a toss up between a 15" Kicker CVX and 15" Sundown Z15 in a ported enclosure around 28-30Hz to pick up some impeccable lows.

I know the Kicker is 20.1Hz (Fs) and the Sundown is 28.585Hz(Fs), for example if I tune an enclosure at 28Hz (how will each driver be affected since the Kicker Fs is lower than the tuning and the Sundown is right in the area). If I tune the enclosure at 30Hz (how will each driver be affected). Any advantages and disadvantages for each woofer in that certain enclosure would be appreciated, and any other info about understanding subwoofers T/S Parameters in lamens terms would help also. Thank and Appreciate you guys.

[Impious 1,476]

Plug 'em into an enclosure program and play around.