"Fast" woofers.

[Adam_M]

Dan Wiggins, formerly of Adire Audio among other companies wrote a nice paper on the concept of "fast woofers" It's a good read!
https://www.adireaudio.com/wp-content/uploads/2018/12/Adire-Audio-Woofer-Speed-by-Dan-Wiggins.pdf

 

[PDS]

Maybe I'm not understanding this correctly, but, with everything else being equal, won't a speaker with a lower Q sound faster than one with a higher one?

Adam, thanks for the link!

 

[NTK]

Q should be optimized for the specific application. System will be sluggish if Q is too low (or ζ is too large). System will ring (resonate) if Q is too large (or ζ is too low).

Below is from Wikipedia showing the step response of a second order system with different Q. The rise time of the system determines the upper frequency limit of the system. You can see that a low Q (high ζ) gives a slower rise time. If the upper frequency limit is well above the system's operating frequency range, any reasonable value Q will be fine and will give no discernable differences.

 

[PDS]

Thanks, NTK! I'll give that a close look soon.

 

[MRC01]

When I think of a "one-note" subwoofer, I imagine one having a strong resonant frequency due to factors like its mass, suspension compliance and impedance. How would this appear in measurements? Seems it would have a response peak and long spectral decay / ringing at that resonant frequency. Is that right? Anything else?

 

[morpheusX]

Data-bass.com Myths article is a mandatory read

 

[PDS]

Good stuff, MorpheusX. Thanks!

Here's a quote from the article: "Traditionally it used to be thought that inductance did not matter in the bass range and that the effects of it were primarily above the sub range and could be ignored. As bass drivers have gotten more and more powerful with higher power handling and greater Xmax they have also increased the inductance effects of their coil / motor systems " and "Inductance affects all aspects of the driver and cabinet systems behavior. "

So when people experience bass that is perceived as slower or less tuneful in the same position, and in the same room, and assuming that the driver/system is competently designed, it could be because:
1) one sub plays lower than the other, which very well could excite more room modes.
2) There could be resonances in the system, such as cabinet vibration, which leads to time-smearing.
3) The Q of the system might be different.
4) The inductance of the voice coil might be different.

Do larger woofers tend to have higher inductance coils than smaller ones? If so, that by itself could lead to a difference in perceived sound quality.

 

[617]

When I think of a "one-note" subwoofer, I imagine one having a strong resonant frequency due to factors like its mass, suspension compliance and impedance. How would this appear in measurements? Seems it would have a response peak and long spectral decay / ringing at that resonant frequency. Is that right? Anything else?

anechoic frequency response will show this and any resonances, but the reality is that the frequency response of the subwoofer is completely changed and dominated by the room it is put in.

 

[ppataki]

The Thiele-Small parameters of the RSS315-HF-4 are such that it has a rather high F3 (–3 dB cut-off frequency) when placed in a sealed enclosure, of around 50 Hz. That's why its step response function decays so much faster than that of the SB-1000, which is much more extended in the low-frequency range.

That is only partly true

This is the FR of the SVS vs Dayton

The Dayton indeed has a lower Q

Let's see what happens if I compensate the Dayton with a low shelf filter
Now this is the FR of the SVS vs the compensated Dayton:

So the step response of the Dayton shall decay slower now, right?

It surely does:

But let's see again the SVS

38ms vs 60ms to reach down to 20% decay

I think this clearly shows that the Dayton is faster even when applying a low shelf on it

 

[LTig]

I agree that Damping Factor may be a misleading, perhaps worthless metric but wouldn't you agree that amplifier/speaker interaction issues with high output impedance amplifiers are real?
The dummy load used by JA when testing amplifiers seems to indicate that the combined amplifier/speaker response can be greatly affected depending on the output impedance of the amplifier.

Yes, the influence of a high output impedance of an amplfier on a varying input impedance of a speaker is real and therefore high output impedances should be avoided, but the change in the frequency response is not due to damping. Both impedances just build a simple voltage divider.

 

[NTK]

That is only partly true

This is the FR of the SVS vs Dayton

View attachment 124415

The Dayton indeed has a lower Q

Let's see what happens if I compensate the Dayton with a low shelf filter
Now this is the FR of the SVS vs the compensated Dayton:

View attachment 124416

So the step response of the Dayton shall decay slower now, right?

It surely does:
View attachment 124417

But let's see again the SVS

View attachment 124418

38ms vs 60ms to reach down to 20% decay

I think this clearly shows that the Dayton is faster even when applying a low shelf on it

You misinterpreted the step response. An ideal step response is a step with no decay. The existence of the decay is because the subs' FR aren't flat to DC. The faster the decay, the earlier the bass roll-off. Which means the Dayton has a poorer bass extension compared to the SVS,

 

[ppataki]

Actually the Dayton has a better bass extension if you check the frequency response curve
Lower decay time means that it comes to a rest faster hence producing more precise bass

 

[617]

You misinterpreted the step response. An ideal step response is a step with no decay. The existence of the decay is because the subs' FR aren't flat to DC. The faster the decay, the earlier the bass roll-off. Which means the Dayton has a poorer bass extension compared to the SVS,

Right, step, square wave response, CSD and group delay are all illustrated by the FR.

 

[NTK]

Actually the Dayton has a better bass extension if you check the frequency response curve
Lower decay time means that it comes to a rest faster hence producing more precise bass

Here are the frequency response and step response of typical LTI system. Both systems 1 (Fc 10) and system 2 (Fc 20) have 2nd order (i.e. -12 dB/oct) roll-offs. System 2 has poorer bass extension, and settles to zero faster than system 1.

Now, system 2 is changed to 1st order bass roll-off, but with the same corner frequency as before.

Remember. This is the step response. The ideal response is NOT settle to zero.

 

[Kip]

"Fast" subwoofers are not a thing, bass wavelengths are long and therefore take time to propagate, 12.5ms at 80Hz, 25ms at 40Hz, etc. All subwoofers are slow by nature. Ported vs sealed vs infinite baffles are not changing the speed of the driver, but rather how it interacts with the room.

The primary performance factors of a subwoofer are smooth frequency response and low distortion. If you can achieve those 2 things, it will sound good. Infinite baffle subs are a good solution for someone with power to spare and EQ. Group delay and resonances cannot be entirely discounted, but those are easy things to get right simply by designing the correct enclosure.

I assume you have seen the ASR review of your Magnepan LSR? You probably want to crossover at 80 or 100Hz, distortion rises quickly in the bass region.

I don't see how any driver cone is exempt from the physics that determine how fast they can go silent after playing a tone.

 

[Willem]

The "speed" of a subwoofer is not a driver property but a functon of the room integration: have room modes been suppressed by eg dsp room eq?

 

[egellings]

View attachment 124225View attachment 124227View attachment 124255

I'd cross him/her n at about 80Hz and the bottom end should be well taken care of.

 

[NTK]

I don't see how any driver cone is exempt from the physics that determine how fast they can go silent after playing a tone.

If you start or stop a sine wave, the transition means your waveform will have spectral contents extending to frequencies higher than the fundamental. The more abrupt the transition, the stronger the higher frequency contents.

For example, here is the spectral analysis of the above single period sine wave burst (normalized to the sampling frequency used in this simulation). In a multiway speaker, the high frequency (i.e. the "fast") components are handled by the mid-range driver and tweeter, and the woofer only have to reproduce the "slow" low frequency components.

Here is the signal separated into its low and high frequency components. (The filters used here are linear phase FIR filters to preserve the phase relationships.) The low frequency part is reproduced by the woofer, and the high frequency part is reproduced by the mid-range/tweeter. Together they sum back to the original (source) signal. You can see that the woofer doesn't need to be "fast". The "fast" parts of the signal with the abrupt transitions are taken over by the high frequency drivers which are "faster".

This is the spectrum plots of the original signal and the low pass filtered and high pass filtered components to show the spectral compositions of the low pass filtered and high pass filtered signals.

Therefore, the woofer still doesn't need to be "fast", and no law of physics is violated.

 

[ebslo]

I don't see how any driver cone is exempt from the physics that determine how fast they can go silent after playing a tone.

Welcome

If you read the rest of the thread (it's not particularly long) and ask specific questions on points that need clarification, you will come to see how no violation of physics is required.

That said, I'll try a simple appeal to intuition. Consider a subwoofer playing a 100Hz tone. The driver decelerates, stops, and accelerates in the other direction 200 times every second, or once every 5 milliseconds. Any subwoofer can do this just fine, and play at frequencies much higher than 100Hz. We often model a driver as a piston, but it's very much not like the piston in a car as there is no drive shaft or flywheel that would store angular momentum. The motor directly decelerates and accelerates the cone every cycle.

ps. In an interesting twist, it turns out the motor's max power, or its ability to accelerate the cone, has nothing to do with what frequency a driver can play, only the SPL it can generate.

 

[617]

I think a lot of confusion comes from not understanding that the kick drum isn't reproduced by the woofer alone. If you low pass a drum kit you will be left with a soft flub flub sound with no 'jump' or immediacy. Those qualities are given by the mid and tweeter.