For a bass guitar cab, weight would be a consideration, as well as durability. This would suggest plywood, and adding mass via damping might not be good for your spine.
my present bass cab have 40mm plywood panels, with some matrix bracing
I guess just a little exstra mass added to thin plywood wouldn't hurt
but with more weight on bracing to get the strength, yes
so, when is it 'lossy', and when is not 😕
I guess just a little exstra mass added to thin plywood wouldn't hurt
but with more weight on bracing to get the strength, yes
so, when is it 'lossy', and when is not 😕
Hi,
In this case forget it, as if it was worth considering for live bass in the
first place, which it isn't. Live all you you need is a "semblance" of the
recorded sound with a flexible rig that allows you to "approximate".
Midrange resonances of a live bass cabinet are pointless to reduce
if all the other cabinets involved have (and they do) big issues.
rgds, sreten.
forget what ? adding mass ? maybe
depends on my bracing, which I said is where I will put weight
I will consider excessive use of pine lists, which are strong and very light
plywood with pine lists, would this be considered 'lossy' ?
In both cases I was thinking about cabinet structural resonances rather than internal standing waves.
The cabinet structure is driven both by the chassis reaction force and the internal air pressure (and probably by extenal pressure to a small degree). At KEF, my recollection was that the compliant woofer mounts reduced cabinet wall vibration by about 15dB, this showing that the chassis drive was considerably higher than the acoustical drive.
Regards,
David
Cabinet walls moving isn't really the problem so much as the uneveness of their motion. At resonance the walls become transparent if no damping is present. Because the resonances are narrow band they ring with significant duration and become audible.
And don't think that concrete gets rid of all flex. Stiffer means the resonances move up the band, but I have yet to see a concrete cabinet that had its resonances so high that they were totally out of the audio band.
Tall buildings are made of concrete and steel but they are happy to sway several feet back and forth at resonance.
David
Hi David,
It is very possible for the concrete building to have an infrasonic Fs and not to tough to excite that with an earthquake. I'm not seeing the connection. I don't think it's reasonable to expect that a woofer(s) can do the same exciting in a thick walled concrete cabinet as it does to the flexy walled Harbeth. That's the only point I was making other than I don't see the advantage to a flexy wall as I don't equate the woofer box to a musical instrument.
I'll have to look at this later as my wife needs the computer now. 🙂
It is very possible for the concrete building to have an infrasonic Fs and not to tough to excite that with an earthquake. I'm not seeing the connection. I don't think it's reasonable to expect that a woofer(s) can do the same exciting in a thick walled concrete cabinet as it does to the flexy walled Harbeth. That's the only point I was making other than I don't see the advantage to a flexy wall as I don't equate the woofer box to a musical instrument.
I'll have to look at this later as my wife needs the computer now. 🙂
Here's the connection: at resonance mass reactance and stiffness reactance balance out and there is no resistance to motion unless there is damping. Concrete has little inherent damping so there will be resonances ("rings like a bell").
You've coined the term "flexy walls" which is intentionally misleading. For a cabinet to be a "musical instrument" implies strong resonances which is the farthest from the intention of the thin wall, high damping approach. Listen to the samples at the German site a few pages back and the only samples free from audible ringing were the ones labeled as having significant Bitumin layers.
The Harwood goal is to increase the ratio of damping to stiffness. Stiffness does not have to be low as long as the damping is high in proportion. For most builders a practical amount of damping may limit what wall thickness works best but we are really just trying to reduce the Q of the inevitable cabinet resonances. As with all systems, mechanical or electrical, Q is simply the ratio of reactance to resistance. Making a cabinet from concrete guarantees high Q, the opposite of what we want.
This is at least the third time, to my memory, that this particular debate has run its course. Those that oppose Harwood's thinking insist that if a cabinet is stiff enough (thick enough, heavy enough) it will be perfectly inert. It will not be. It will have multiple resonances and at each resonance it will be essentially transparent.
The example of the sky scraper is to point out that high stffness and high mass do not prevent resonance. The trend in earthquake proof buildings is to place large pendulum masses in the top floors. The pendulums swing thru a vat of oil to dissipate energy (the damping).
Damping is the key.
David
You've coined the term "flexy walls" which is intentionally misleading. For a cabinet to be a "musical instrument" implies strong resonances which is the farthest from the intention of the thin wall, high damping approach. Listen to the samples at the German site a few pages back and the only samples free from audible ringing were the ones labeled as having significant Bitumin layers.
The Harwood goal is to increase the ratio of damping to stiffness. Stiffness does not have to be low as long as the damping is high in proportion. For most builders a practical amount of damping may limit what wall thickness works best but we are really just trying to reduce the Q of the inevitable cabinet resonances. As with all systems, mechanical or electrical, Q is simply the ratio of reactance to resistance. Making a cabinet from concrete guarantees high Q, the opposite of what we want.
This is at least the third time, to my memory, that this particular debate has run its course. Those that oppose Harwood's thinking insist that if a cabinet is stiff enough (thick enough, heavy enough) it will be perfectly inert. It will not be. It will have multiple resonances and at each resonance it will be essentially transparent.
The example of the sky scraper is to point out that high stffness and high mass do not prevent resonance. The trend in earthquake proof buildings is to place large pendulum masses in the top floors. The pendulums swing thru a vat of oil to dissipate energy (the damping).
Damping is the key.
David
Totally agree with David. Persuit of stiffness over all else is the road to bell making. Sensei says 'Power without control is meaningless'. A high damping to stiffness approach is the the most efficient way.
Cute reference on building sway and damping.
Equation: Formula for Calculating a Skycraper's Sway | Wired Magazine | Wired.com
Reminds me of the Dual tonearms with tuned mass dampers.
David
Equation: Formula for Calculating a Skycraper's Sway | Wired Magazine | Wired.com
Reminds me of the Dual tonearms with tuned mass dampers.
David
Not necessarily a good idea 😉. May ruin your impression of other speakers.
A personal ancedote. I was in a back room of a US Harbeth distributor listening to some violin on one of the larger Harbeths (don't remember which). I went to the front of the store to get one of my CDs when two guys came in the store. First one says "Wow, there's someone playing violin on the back." Second one says "No speaker will ever sound as good as that live performance." That was ten years ago and I still remember that the Harbeth immediately made you quit analyzing the speaker and just enjoy music.
A personal ancedote. I was in a back room of a US Harbeth distributor listening to some violin on one of the larger Harbeths (don't remember which). I went to the front of the store to get one of my CDs when two guys came in the store. First one says "Wow, there's someone playing violin on the back." Second one says "No speaker will ever sound as good as that live performance." That was ten years ago and I still remember that the Harbeth immediately made you quit analyzing the speaker and just enjoy music.
Very interesting thread so far! Any comments on the practical applications above? I'm thinking on the mains a couple small cross braces for the side panels and lots of dynamat extreme that I have around, plus some fiberglass stuffing. Have acoust-x paint/coating for the inside of the subs.
That is exactly how I feel you have a speaker that is for "keeps".
Every speaker design has its merit. However, with the stiffer enclosures with a higher resonance, do you sometimes have to look at factoring that into the voicing of the crossover to try and control its coloration of the sound?
What I mean by this is say that your speaker cab resonates and produces a 1khz to 1.5kz peak. Can you factor this into the design of the crossover/ plan to EQ it out of the sound?
with the lossy design these peak resonances happen at a lower frequency where it really, in my eyes, is of less importance.
This just occurred to me, with lossy cabinet designs the main benefit is cleaner mids.
However, when it gets to the bass frequencies the only "lossy" design that I can think of is an infinite baffle and many from what I recall argue that it is more of a large sealed design than anything else.
that said, since with a lossy design is geared to lower the cabinet resonance away from the mid frequency range, would the inverse be true for a subwoofer?
In other words, if I have a sub that is designed to handle 20-80hz, should you then try and tune/ brace the box so that the resonance is say 200hz? This way that cabinet does not resonate in its targeted output range?
However, when it gets to the bass frequencies the only "lossy" design that I can think of is an infinite baffle and many from what I recall argue that it is more of a large sealed design than anything else.
that said, since with a lossy design is geared to lower the cabinet resonance away from the mid frequency range, would the inverse be true for a subwoofer?
In other words, if I have a sub that is designed to handle 20-80hz, should you then try and tune/ brace the box so that the resonance is say 200hz? This way that cabinet does not resonate in its targeted output range?
Another example of something quite rigid flexing .... the power of resonance:
breaking a wine glass using resonance - YouTube
Think it is sort of intuitive that if you put a finger softly to the glass it would probablt not break at all ..... damping ... 🙂
In theory, yes, but not so easy in practice. Cabinet resonances are difficult to see in frequency response curves unless you have very high resolution in a very anechoic environment. They are better resolved in delayed resonance curves.
As such EQ would have to bery narrow band and precise. Normal crossover design wouldn't do. I have found that you tend to naturally put a broad depression around the area of narrow band peaks, if you tune by ear. It isn't a great solution but it sometimes helps.
David
I think that is the one practical case. With a sub you can get the resonances well above 100 with a very stiff construction approach.
For 2 way and 3 way designs you can never make the cabinet stiff enough to get the resonances out of band.
David
ok, interesting.
I bring it up because it occured to me that it is the inverse or what you are doing with the higher frequencies for the in-room subs.
So tune the box for as high a resonance frequency as possible while tuning the speakers for as low a frequency as possible for, hopefully, some of the cleanest sound one can achieve....in theory.
you would most likely have some muddy sound in the 80-200hz region I am assuming simply by judging the graphs that I have seen around the web....the question is though, does that muddiness matter in that frequency band.
I'm about to make a new sub and I just got the panel sheets today. I made some knock-tests on different materials just of curiosity. I'll make more measurement as the project goes on.
Sorry the scales vary in graphs. Pay attention to dominant freq and ringing
The thread of my sub here http://www.diyaudio.com/forums/subwoofers/223960-bench-style-cabinet-subwoofer-2x15-closed.html
Method:
sheet of board held hanging in air with fingers
Microphone 10cm from the board on the opposite side of knock (knuckle)
Apparatus Zoom Q3, software 2.01 Gain L 24/96600Hz wav-format
Analysis Audacity 2.0.0 Windows 7 64-bit (program 32-bit)
Spruce plywood 21mm 42x104cm
MDF 25mm 40x85cm
I don't show all measurements but the size of the sheet makes a big difference.
A following post same method, different finished speaker cabinets measured.
Sorry the scales vary in graphs. Pay attention to dominant freq and ringing
The thread of my sub here http://www.diyaudio.com/forums/subwoofers/223960-bench-style-cabinet-subwoofer-2x15-closed.html
Method:
sheet of board held hanging in air with fingers
Microphone 10cm from the board on the opposite side of knock (knuckle)
Apparatus Zoom Q3, software 2.01 Gain L 24/96600Hz wav-format
Analysis Audacity 2.0.0 Windows 7 64-bit (program 32-bit)
Spruce plywood 21mm 42x104cm
MDF 25mm 40x85cm
I don't show all measurements but the size of the sheet makes a big difference.
A following post same method, different finished speaker cabinets measured.
Attachments
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