The significance of high Qms..?

Ro808

Member
2010-03-27 4:25 pm
The Markaudio drivers remind me of Jordan.

I've listened to this.
 

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Given that Mark Fenlon was a Ted Jordan apprentise for 5 years, doing most of the actual engineering. The first generation were broadly similar to the Jordans, and teh Jordan J6T did become the first Alpair 5.

With the second generations Mark set out on his own and the drivers have been improving since then. I rank the A11ms ahead of the Jordan Eikona2, i have both. The Jordans still have issue sin the top octaves, Mark has mosrtly massaged tha tproblem out of his drivers

dave
 
My view on Qms is that it is useful to look at non-linear behaviour of real-world drivers (not TS models!). Mechanical losses have a strong tendency for non-linearity. At low cone excursion, the suspension and other moving parts tend to "stick", where as at high excursion they "slip" more easily. I guess I am trying to say that there is a transition from static friction (stiction) to dynamic friction, which is not exactly breaking news. Since static friction is much higher than dynamic friction, mechanical losses depend strongly on the cone excursion. Small signals tend to get lost in stiction at low cone excursion ("loss of detail"). This is not the case at high cone excursion, where dynamic friction prevails. One way to reduce these mechanical non-linearities is to minimize mechanical losses in the driver. This is why I tend to prefer drivers with high Qms.

Hey Matthias, your fellow countryman Jürgen Strauss seems to support your supposition. In this video he gives some insights into the optimization of his two-way constant directivity monitor horn speaker, SE-MF-4. This is the part where he explains the value of high Qms/low Mms for sound reproduction, focusing on compression levels at low excursions:
Ju?rgen Strauss explains his loudspeaker design philosophy - YouTube
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
Interesting ideas and video. Thanks. I haven't thought much about Qms in isolation, but my first successful Open Baffle build used a woofer with a Qms of about 16, which is rather high. Qts 0.62 They did well for the low to medium levels played in my modest sized living room.
 

ScottG

Member
2003-02-04 12:23 am
US
The fact that Qms is by definition measured at resonance does not take away from the fact that the underlying cause - friction - is at work as soon as the cone moves. Regardless of frequency.

("Friction"/Mechanical loss) Which opposes the coil jumping out of the gap (..along with counter current).

It's that displacement and the resulting VC position in the gap relative to current that leads to higher non-linear distortion.

If you don't oppose the coil's movement (and assuming you have freq.s low enough to increase excursion), then you have that higher non-linear distortion problem at lower freq.s..

Again, I'm pointing-out the exception - low freq. higher excursion operation.

The notion that a high Qms is ALWAYS desirable is incorrect. Rather it depends on the driver and its use.
 
No, he didn’t. He only presumed that the friction in spider and surround has to be low for a good driver. That is about our discussion here too.
One thing is for sure: a speaker hás to sound different at low levels compared to high levels. The effect that mr. Strauss is referring to could very wel caused by our ear’s nonlinearities.
 
One thing is for sure: a speaker hás to sound different at low levels compared to high levels.

Serious question: assuming the speaker is operating within its limits (ie, no excessive distortion from being driven too hard), and putting aside psycho/physio-acoustic effects, why should a speaker sound different at low levels vs high levels?


I've made attempts to measure this "sticktion" that supposedly occurs at low levels.

What I did was this: place a very sensitive microphone (the sort that gets used for choral recordings) very close to the speaker cone. Then, using REW, run a frequency sweep.
Then, I decreased the signal level by 6dB and ran the sweep again.

I repeated this process, looking for any evidence that the speaker suspension (or whatever) was "sticking" - perhaps a change in the frequency response curve or similar.
For the penultimate sweep, I only heard a short range of frequencies in the kHz area, where we know the ear is most sensitive. For the final sweep, I heard nothing at all.

The microphone picked up everything perfectly well, though.

What I found was this: there was no change to the measured frequency response, down to levels below my threshold of audibility.

If sticktion was occurring, I would expect some changes. Perhaps the lower bass would still come through (larger excursions), but the upper bass (less excursion) would disappear or reduce in level more than just the 6dB changes I was making.
No changes were observed, so I can't support the sticktion theory.




The sensible explanation I can find for this stuff is related to two things:

- Acoustic signal-to-noise-ratio
- Our hearing mechanisms


For the case of acoustic SNR, let's say that music has 30dB of difference between the loudest peaks and the quietest details. It might be more than that, but it's a number we can play with and adjust later.

I'm sitting in my listening room right now, and I can hear the fridge in the kitchen. It's pretty quiet, perhaps as low as 30dBSPL at my ears.

The implication for music, though, is this: if I'm listening at 60dBSPL peaks (a comfortable, level, if a little on the quiet side), then the lower-level details are at 30dBSPL. That's the same level as the fridge, so it's likely that some of those details may be masked.

If I want to hear those low-level details clearly, then I must increase the level of the program material, or switch off the fridge. Since the fridge is keeping my drinks cool, there's only one solution.

When designing speakers, cone break-up of a mid or woofer can be a problem. In order to stop those peaks from becoming audible, the aim is to attenuate them. I've seen some designers aim for 20dB, others as high as 40dB. Some settle in the middle, at 30dB. We'll take that number.

So, in order for the low-level details of the music to completely mask the noise level of my fridge, they must be at 60dBSPL. The peaks, therefore, would be at 90dBSPL, which is a fairly high level: holding a conversation would be difficult.

Taking a more difficult goal of signal being 40dB over the noise, we must then subject our ears to 100dB peaks. Bear in mind that the goal here is to simply make sure we're hearing all the detail in the music.

If we want to hear what's 40dB down in the recording (instead of 30dB), we must increase the levels again, resulting in 110dB peaks.


We can clearly see, then, that ensuring proper acoustic SNR will improve the amount of detail we can hear, subject to certain factors.




The mechanisms behind human hearing have been studied by better scientists than I. There's plenty of reading around online for that side of things.
I'd also suggest that subtle (or not) tactile sensations can contribute to our overall enjoyment of music.


I'd also mention that a speaker's harmonic distortion generally rises with SPL, and they generally produce low-order distortion, which we might find pleasant to hear. That deviates from strict "high fidelity", though, towards more subjectivist landscapes.


Chris
 
All credit to Chris for putting the work to actually test this. I'm a bit disappointed it doesn't support what I thought would be the answer, but if that's what the results are, then that's the way it is.

I have been committed to low Rms drivers for a few years now. My system is built around that, bi-amped OB's with Supravox 285GMF, 2xFostex FX120's, and Aurum G3 ribbons. What I hear from this sort of approach is a room-filling sound at low playing levels, and more of a coherent 'flow' to the music. I just I wish I knew why this is ! ( ps. it's not something I suddenly discovered, I've been building speakers since 2002 )

I will say that the Purifi 6.5 drivers deliver a very consistent sound from low to high signal levels, I've just received a pair to build into units for a guy at work, they are not super-low Rms, but are some of the best I've heard for resolving fine details and delivering tone at low levels. They did this straight out of the box by the way. Shame I wasn't able to demonstrate this very well, last Saturday, Chris !
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
Couldn't this be swamped by other nonlinearities ?
Yes, I would think so. But an electrical recording straight from the driver terminals is about the cleanest way I can think of to record what the driver is doing. If it is "sticking" would that show up at the terminals? I don't know, but many things do.

One might compare a few low vs high Qms driver and look for things. What to look for? Harmonics? IMD or other things? The stiction would be a form of hysteresis, right?