I believe most accelerometers use a constant current supply(piezo at least) perhaps 5mA, to modulate an output voltage. no pre amp required, unless youre measuring VLFs. If you use a electromechanical velomitor instead, you only need a scope and the sensitivity of the device. Personally, id go for a velomitor.
Artificial tones are useful in exploring the lay of the land, but real music -- the actual simulus of import -- tells you whether you have been successful since it is the only stimulus when you are listening to music, the only energy source available to turn a potential resonance into an actual resonance. If under those conditions you get no resonance you have achieved the goal, even if it rings like a bell when pumping in an appropriate artifical signal.
dave
dave
The boy genius knows exactly what to do i have no doubts. Sometimes takes a bit of effort to motivate. The latest aXp has a project that takes it further. A buffer and maybe some gain is likely needed between the accelerometer with its CCS and the ADC, i consider that a preamp.
dave
Exactly. All this comes down to how to minimize unwanted ressonances from the paralell walls inside rectangular boxes that are done wrong in the first place. Any kind of stiffing or damping might work as a camuflage for the main problem, it won`t solve it.
You`re in to something but there`s a third way to do this too; make asymetric non-reflective cabinets w.o. absorbers. It can be done😉
An important side of this music reproducing-thing is to realize that any kind of loss, that be acoustic energy loss in absorbers/walls or electric energy loss in filters a.s.o. robs energy(=dynamics) from the music-material. This lost enegy can`t be re-gained or compensated by adding big amps, it`s lost.
Back in the old days when watts were few and made by tubes this used to be a main issue, but the most of this knowledge seems to be forgotten or outdated after the transistor arrived. But physics haven`t changed.
Ah, yes, the intelligent loudspeaker that resonates on sine waves but somehow behaves when driven with music. Somehow with all the noise-like components (all percusion instruments, felts on piano strings, breathiness on flutes, buzz on reeds, fuzz on guitar, pure noise on snares and castanets and cymbals), not to mention the tone variation of ensemble playing or standard vibrato, your loudspeakers manage to have resonances that the music never quite sees?
Have you patented this?
David S.
if the 'artoficial' sinewave tells the measuring equipment that you have serious resonances, that means the design is flawed. Music, real or not, will be reproduced with added distortions. Maybe some like the distrortions, but accurate reproduction of recorded material it is not. So if measured correctly, trust the measurements!
Quote lowtherdream: 'I always think of speaker cabinet as musical instruments..'
Quote PaleRider: 'You`re in to something..'
cmon, guys! a loudspeaker is not a musical instrument. a loudspeaker is a precision device, its only purpose is to reproduce what is fed into it.. the voltage, the amperage, whatever. It really is not a guitar. nor a drum.
PaleRider, if you really think speakers should have sound of their own, then why do you bulid speakers that seem to have 5 inch thick walls (MDF I assume) judging by your avatar pic. Let the speakers resonate away, like a violin! or a guitar
Quote PaleRider: 'You`re in to something..'
cmon, guys! a loudspeaker is not a musical instrument. a loudspeaker is a precision device, its only purpose is to reproduce what is fed into it.. the voltage, the amperage, whatever. It really is not a guitar. nor a drum.
PaleRider, if you really think speakers should have sound of their own, then why do you bulid speakers that seem to have 5 inch thick walls (MDF I assume) judging by your avatar pic. Let the speakers resonate away, like a violin! or a guitar
speaker dave, I beg forgiveness, I mixed that up with something else so I am quite confident no such theoretical basis is to be found. I spent most of the "free" time before lunch searching the net until I had to realize I had been severily wrong.
However, I came across some useful information, I hope Bracing
and then I went to the Linkwitz Lab homepages and re-read the "Issues in loudspeaker design, part N (mounting drivers to baffles, I think it was called).
Linkwitz had some measurements and comments that if correct, I think they will have me right in the middle of what speaker dave and Planet10 are arguing.
The varying energy content in different frequencies in audio waves I thought was the case, I had mixed up with the kinetic energy produced by the driver firmly attached to the baffle. He had measured an 8" driver, and at
20Hz, the kinetic energy, given in mWs was 58,36
640Hz and it was down to 0.06.
However, when a certain kinetic energy is likely to start excite a particular speaker box wall, for a particular driver, I could not figure out. I would guess there are too many combinations available to come up with a generalization, and that only measurements, or experience will tell.
Linkwitz was also mentioning box Q in conjunction with stiffening. He said stiffening raises Q, and he seems to be in the low Q corner, as he wrote that CLD for example, would reduce Q again.
I think someone already has been into this earlier in the thread, and I've been told this by someone in the past, if there is something in the above that I have briefly related to, I wonder if box resonance minimization in one box is feasable?
As mentioned, earlier I've been told that a stiff and light box is the way to go for woofer boxes, because it should be possible to push the resonances well behind 1000Hz, and then the available energy in the frequencies above this value, will have a very small likelyhood of being able to excite the walls. So in this scenario, I totally agree with Planet10 on a theoretical basis.
On the other hand, for higher frequencies, I wonder if it would not be easier to go the way of speaker dave, because a very heavy and well damped box is not as likely to be excited by higher frequencies. In this scenario, I would presume bracing would be kind of counter productive as it would raise the panel resonance threshold?
This is my present view on the subject. What do the rest of you think?
If the above would be correct, as a theoretical idea, if one did a two box tower that is quite popular these days, and made the top box (mid and tweeter registers) very, very heavy as the above would dictate, how do you think that would affect the light and stiff woofer box?
For returning somewhat to the original thread subject, for a higher frequency box, I think that those who don't mind MDF, it could be a suitable choice as the box is supposed to be heavy. I would consider sand loading the box instead, using something else.
Anyone worked with Corian? I've read that you can use a router, but what about making non-square boxes, that is, bending and the like?
What about other plastics/compounds for a heavy and damp box? If you would be able to construct a mould, wouldn't it be possible to, in a proper way wieght load the mould, if the chosen compound isn't heavy enough?
Now my imagination starts flowing, so I stop here. (And it is weekend time😉)
Thanx for both of those.
It is easy to argue that in this situation most of the energy feed into the box is of this nature.
In a multiway, this makes a lot of sense, the vast majority of the boxes i design or work with are 1-way.
dave
Where do you get that idea from? Can`t you see my comment?
To make myself a bit more clear; I`m trying to say that the problem here is not so much the choosen materials but the way speakercabinets are done.
Rectangular boxes w. paralell walls & lots of ressonances are not the way to go, think different.
The big challenge in speakerbuilding is to recreate music with dynamics still intact, and to do so anything that ressonates or absorbs energy (from the music-signal) must be banned.
The solution here is to build cabinets w.o. any paralell walls inside, asymertic or circular, whatever.
Speaking for myself, and not intending to hijack the thread... I'd like to see the original question slightly modified to "what are the characteristics of a better material for enclosure." That approach is being touched on here and there already. My reasoning is twofold. First, it removes the variations of economies, eg Canadian maple is a different commodity in Toronto than in San Antonio (hey, I'll swap you plenty of mesquite🙂). Secondly, it is conducive with allowing the DIYers to experiment with whatever various materials they choose to fit those characteristics.
Speaker Dave, let me start by saying that I appreciate you sharing your knowledge on these boards. Also, I realize that you have many more years of experience at building speakers than I do, and that you are respected on these boards, and I am a nobody.
However, for some reason you keep repeating the above as if Dave (p10) said that the resonance frequencies aren't played duing music. Are you actually not intellegent enough to understand his argument, whether you agree or not. As far as I can tell, his argument has NOTHING to do with music not including resonant frequencies, rather that the music does not excite them due to their low amounts of energy in the higher frequencies. He is talking about probability. Which is also why he would like testing to include music. Because in music, the probability is low. (His argument not mine). Agree with him or not, please understand that, and allow the conversation to have purpose and meaning, please.
Regards.
If the music includes those frequencies that means that p10 is plain wrong. Why? Because more of wideband noise energy is needed to excite resonance with higher Q. But less of sinusoidal energy of exact frequency is needed to make it RING LONG AND AUDIBLE.
The music can include the frequencies, but unless they repeat often enuff (ie like a sin wave does every cycle), insufficient energy is pumped into the resonance before it is drained off by the inherent damping in the box material. Music is transient and changing in nature.
Also consider (thanx buggsson for the poke to my brain) that the majority energy source for causing the potential for a resonance to be turned into a resonance is structure bourne (so has to travel longitudinally thru the cabinet material) and not air space energy (particularily if you can push up the frequencies of potential for a resonance) where air space damping has already drained much of the air space energy, and you avoid coniciding them with the frequencies of any standing waves.
dave
It wasn't clear to me initially whether his arguement was to do with the particular frequency range of music vs. the location of resonances (high or low in the band) or otherwise, which is why I repeatedly asked for explanation. It is perfectly fine to say that you prefer low crossover points (say 400) and then push your cabinet resonances above that point. Those are valid design choices, although they don't point toward a a solution for those of us designing more conventional 2 ways (and the Harwood approach does).
But the repeated claim was that some clever design approach allowed Dave to "get no resonance" on music "even if it rings like a bell when pumping in an appropriate artifical signal." I can only interpret that as describing a system that has one response to sine wave stimulus (an artificial signal) and a separate response to musical stimulus. As an electrical engineer that sometimes stayed awake during "signals and systems" class, I know that this is nonsense. If resonances are there then music will excite them just as well as sinewaves will. Take a look at Meyer Sound and Renkus Hines that both sell high resolution FFT systems that measure PA system response during concerts solely with music as the measuring stimulus. If there are resonances at any frequency then the music will stimulate it, as their measurements show.
There also seems to be a lot of confusion about "energy" and the "probability of resonances being stimulated". The most typical cabinet measurements will use a constant voltage sine sweep and either an accelerometer or nearby microphone for pickup. With signal energy being proportional to voltage squared or pressure squared, then flat voltage input would also be flat energy. The cabinet measurements of Harwood are typical of what I've seen, showing a few strongest resonances in the 200 to 700 Hz range but with fairly flat output above that range to the 2kHz region. There is no indication that cabinet response falls dramatically above 400 Hz as suggested. Now this is for flat voltage input and we can argue that the typical cabinet output would follow the flat voltage output multiplied by the typical spectrum of music. That would accentuate the middle frequencies and slope down towards high frequencies, but still wouldn't cause a nose dive above 400. Still, the Harwood criterion of which resonances were audible is based on speech and music input rather than sine waves, so the spectral balanc of music is already in the mix.
The notion that higher frequency resonances are narrower and therefore have miniscule probability of being excited is simply a canard. The sine wave curves show them at their true representative height. A peak at 100 and a peak at 1000, having the same Q will have the same height (and the same probability of being excited). Not that it matters but since Q's are fairly constant across the band, the higher frequency peaks tend to be wider in absolute bandwidth.
I accept people having different opinions about what constitutes ideal cabinet construction, but when technical sounding arguements constantly stray into inaccuracy, I feel the need to comment.
David S.
Or. more logically, it means that the stimulus has different characteristics.
Higher Q.
Higher frequency is related to this:
Svante post claiming 1/f^2 for airspace energy impingin gon cabinet
dave
The above deals with energy from the driver getting to the cabinet walls thru the air space. (i'm working on citations, they exist, i had a BIG smile on my face when i first confirmed Svante's post)
It is trivial to show that at least 2/3 of the energy transmitted into the cabinet is from the reactive mechanical motion of the driver chassis directly acting on the enclousure (in a closed or infinite baffle with the driver rigidly coupled to the baffle).
Energy is the potential to do work -- we need to do work to excite a resonance.
Work = force acting thru a distance (in our case distance is the driver excursion)
W = F x d
For every halving of frequency we need to increase excursion by a factor of 4. ie d ~ 1/freq^2
In a dynamic speaker a flat FR requires constant acceleration. ie acceleration is independent of frequency
force = mass x acceleration. Ignoring any decoupling of the outer cone from the inner cone the mass of the cone is independent of frequency.
So F = ma is a constant
Substituting into our first equation:
W = F x (a/freq^2) where a, F are constant, so W ~ 1/freq^2
ie energy (that is transmitted directly into a cabinet via the driver chassis) available to excite a resonance is proportional to 1/freq^2.
dave
(thanks buggsson, it was your comment that poked me to realize how easy that part was)
The more i think about it, i can't see any reason why the same analysis wouldn't apply to the airspace coupling... except that instead of direct coupling, one would have the "squishy" air as a coupling medium.
dave
You'll confuse yourself if you relate work to falling excursion. Excursion falls because we operate direct radiators with constant force into a constant mass (as you say). Because radiation resistance rises 12dB per Octave we can have flat output over the appropriate region with falling excursion. Rather than energy (which brings in time duration) it is better to think in terms of power radiated, which will be approximately flat with a reasonable speaker.
Either way, cabinet output doesn't fall 12dB/Octave with any measurements that I have seen. I just pulled out Martin Collums' High Performance Loudspeakers and he shows several plots that Barlow took. Like Harwood they show cabinet output with the same bandwidth as the woofer and generally flat output at high frequencies but, of course, strongly higher output at midrange panel resonance frequencies. An especially relevant plot shows output of a cabinet constructed of three different thicknesses of plywood. As the thickness goes up the primary cabinet resonance goes up in proportion, but the height of the resonance stays exactly the same.
I agree that pressure in the box falls 12dB per Octave but the predominant force to the cabinet is mechanical drive from the chassis (flat force with frequency, as you show). That is why we had such luck reducing cabinet resonance effects at KEF with compliant decoupled drivers.
This was an interesting article on cabinet dampng and bracing: Testing the Cabinet, Measurements, Conclusions — Reviews and News from Audioholics
Harbeth (UK) advocates "thin wall" cabinets. BBC-style thin-wall cabinets. Why so special?
David S.
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