Enclosure resonances, not a big deal?

[planet10]

Originally posted by xpert
I just see that planet_10 has the status of a moderator. Best I quit.

Moderator doesn't matter. I'm just a member unless i flash my cop badge (except that i have to be on better behavior)

dave

[xpert]

Originally posted by planet10


Moderator doesn't matter. I'm just a member unless i flash my cop badge (except that i have to be on better behavior)

dave

I dropped you an e-mail.

[pjpoes]

Ok a few question/comments:

1) Does anyone know if a standard output level is used when this type of cabinet measurement is taken. I know that a voltage is often given, 2.83 volts for instance, which is fine if you want to compare, realistically, a speakers voltage sensitivity. However, when taking measurements of a cabinets vibrations, the greater the driver output, I would think, the greater the vibrations transferred through the cabinet. In seems in this case that for accurate comparisons, you need to set a fixed decibel output, not voltage, as a very low sensitivity speaker might only have 75-80db's of output with 2.83 volts, while another might be doing 98.

A good comparison is that my TC sounds subwoofer, which has 36mm of excursion, is fairly inefficient, and it's voltage sensitivity is in the low 80's. Looking at 200hz, the TC woofer (with no filter) is around 82db's, where as the B&C in the Abbey is closer to 95db's or so. It's why the subwoofer was tested with such a higher drive voltage, I was trying to equalize the output of the two.

2)If the resonance found on the box is in a range above or below where the speaker is primarily use, as well as being well down in level, relative to the speakers main output, how might it impact the sound.

Like the subwoofer. If the strongest resonances detected were 200hz, and the subwoofer is crossed at 80hz, what's it matter. There is no doubt the panel vibrations are excited in relation to the speaker output, and if the woofer is rolling off at 24db's per octave, we are talking about it being down what, over 20db's at 200hz once a filter is applied.

3) How would adding dampening defeat the impact of bracing? I understand where you are coming from, the bracing increases the rigidity, which increases the resonance. The dampening increases the mass which lowers the resonance. Thats what you are trying to say? However, the dampening converts a portion of the energy to heat, removing it from the picture.

[Pano]

Good questions.

[planet10]

Originally posted by pjpoes
3) How would adding dampening defeat the impact of bracing? I understand where you are coming from, the bracing increases the rigidity, which increases the resonance. The dampening increases the mass which lowers the resonance. Thats what you are trying to say?

That is it.

Consider this. If a resonance is not excited it is as if it doesn't exist. Further, energy available to excite the resonance is inversely proportional to the square of the frequency

Bracing increases the resonance of a panel. The higher the better, particularily in a sub. Get it high enuff and it will never get excited. By increasing the mass with the damping you are bringing the resonance back down where it is more likely to get excited.

I also have a strong hunch that the damping decreases the Q of the resonance, meaning that there is greater liklihood of sufficient energy to excite it (more area under the curve). And, as the paper linked above points out, you are less likely to hear a higher Q resonance.

dave

[gedlee]

Matt

Your story is nothing! Once while my computer was doing its bi-weekly backup in the middle of the night, the hard drive crashed and crashed the system during the backup. The backup was ruined - unreadable (not closed properly), and I lost several months of data and more AND the backup!! Even regular backups are not safe if you don't backup the backups. Which I actually did, but only every other month or so. Now I backup the backup regularly.

To be valid one would have to compare the radiated sound level from the cabinet vibrations to the sound radiated at that same voltage from the drivers. It could well be that high efficiency would be a benefit here since for a given voltage the SPL from the driver is greater. This would certainly be true for structural excitations, but not really for airborn ones. But, as I said before, I believe that the structural ones are the more serious and high effieciency makes the ratio of driver output to cabinet output greater for a given enclosure. In essence reducing the cabinet vibrations audibility without chnging the cabinet at all. Interesting!


David

I think that you last paragraph is written wrong:

"I also have a strong hunch that the damping increases the Q of the resonance, meaning that there is greater liklihood of sufficient energy to excite it (more area under the curve). And, as the paper linked above points out, you are less likely to hear a higher Q resonance."

Damping will DECREASE the Q and this might tend to make it more audible although that depends a lot on the details.

I find damping in an enclosure to be effective without adding much moving mass. Take CLD, for example, there isn't much moving mass addition, the inner plate doesn't really couple to the outer one, but the damping is very effective. If you just add dead damping weight to the inside then yes, this is not nearly as effective for the reasons that you state (if stated correctly).

[planet10]

Originally posted by gedlee
backup

One of my computer clients has it set up like this. OS and aps are on an internal mirrored RAID. Data is on another internal mirrored RAID. Data is backed up daily to an external RAID and the 2nd drive of that swapped regularily (once or twice a week). As well, copies of the the last 3 incremental backups are written to the RAID with the OS on it. Regular backup of projects are done to DVD.

I think that you last paragraph is written wrong:

"I also have a strong hunch that the damping increases the Q of the resonance, meaning that there is greater liklihood of sufficient energy to excite it (more area under the curve). And, as the paper linked above points out, you are less likely to hear a higher Q resonance."

Damping will DECREASE the Q and this might tend to make it more audible although that depends a lot on the details.

yes that is what i meant to say... i'll go back & fix it. Thanx.

I agree on the CLD. I just never saw the point of things like bitumenum damping pads.

dave

[pjpoes]

I know this has gone off topic, but as to my horrid computer story. I'm a 27 year old PhD graduate student. It's never been my experience that anything I was working on would justify such measures of backing up. This is all new to me, and I've learned some really hard lessons all too many times just in the last two years. It used to be that I never worked on any single project or paper long enough to worry about losing it, and even then, a simple disc or flash drive was perfectly fine. Now things are quite different. Years of work can go down the tubes, it's a huge deal. I lost a flash drive that had the only proper copy of my original masters proposal a while back, had to start from scratch. Thankfully it wasn't a very good draft, and rewriting it helped it improve it greatly. I now have my thesis and data backed up on the schools network, and I may start working on a copied version straight from the network from now on, so nothing like this can happen again.

[xpert]

Originally posted by pjpoes
Ok a few question/comments:

1) Does anyone know if a standard output level is used when this type of cabinet measurement is taken.

The question is meaningsless in case the cabinet "resonance(s)" aka "transparency" are identified within the sound, not as vibration. The challenge is the seperation of cone sound and panel sound. To sort out the bad from the good, isn't it?

Originally posted by pjpoes
2)If the resonance found on the box is in a range above or below where the speaker is primarily use, as well as being well down in level, relative to the speakers main output, how might it impact the sound.

Nil, obviously.

Originally posted by pjpoes
3) How would adding dampening defeat the impact of bracing?

Why should it?

The calculus of Distributed Mode Loudspeakers (DML) aka NXT panels has been pushed a lot recently. From that the radiation of a vibrating panel is by far better understood now. It can be seen at a glance that the resulting soundfield has a complicated relation to vibration and vibration depends in a complicated manner on the forces applied to the panel. Beyond the first one or two resonance modes chaos rules literally.

An other striking outcome of DML development is that resonances are not "bad" for themselves. The impact of a resonance is: It alters (a) amplitude response and (b) group delay, phase. It is NOT non linear distortion. Resonance is a linear process that can be filtered out - of course without loosing "detail" (someone argued against in this thread ...). If the audibility is in question the deviation from an ideal transducer has to be measured in the LINEAR domain => amplitude & phase vs. freqency as being literally the same as "impulse response". That done group delay and amplitude response graph could be searched for "resonances". If the amplitude response isn't worse than +/- 0.5 wideband, means over one/two++ octaves or +/-3dB smallband then no issues are expected. The same with group delay figures below 1ms or so.

What has to be understood that all is in amplitude & phase vs. freqency. There are no secrets left to be revield in a further analysis of resonances on panels. The ear has no detection mechanism beyond listening to the soundfield. Otherway it would be superstition. If all the bad things about resonance won't show up in amplitude & phase vs. freqency it won't be heard either. It's pointless to debate such. At least if any data is missing ...

I know that the "feel" for an enclosure is a strong motivation to overdo the build in DIY very often. In doing that the DIYer does what he is enABLed to. To alter "inner details" of driver construction, soundfield generation are most often beyond the capabilities. That's o/k. But it doesn't mean that overdoing of cabinet building is a must. I don't support this.

'Hope that answers the query for my data on the topic. Crossbracing of 15mm plywood stiffens against the two first modes of panel movement, for dampening I use some kind of dense felt made from wood fibres, very cheap. The latter helps mostly against cavity resonances. Seconded by deliberately stuffing even of reflex boxes. I never had problems with a cabinet build as such. What makes a difference is changeing the 6" boomers against 12" midrangers.

http://www.exdreamaudio.de/?Akustik:Geh%E4use

ciao

(ps* I want to delete m account, how?)

[Geoff H]

Back to the original question

"One of the recent trends in loudspeaker design has been to incorporate dipole or open baffle alignments into the design. This has the obvious benefit of removing resonances within the body of air behind the diaphragm, but can this same result be achieved through another technique?"

None that I am aware of.

The air trapped behind the cone can cause non linear distortion due to the air requiring an increasing pressure as it compresses. I think it is a square law characteristic. Pushing down on a tire pump will reveal this. Drivers designed for true acoustic suspension have methods to counteract the problem.

One thing about non linear distortion is that it creates harmonics of the exciting frequency. Thus, if the output of a transducer (driver, baffle and enclosure) has such overtones it has suffered some nonlinearity.

A vibrating panel is unlikely to resonate to produce a sine wave. It will have harmonics from each mode of vibration.

But as Dave pointed out in reply 85, they are only a problem if excited.

My present system is possibly one of the best examples to prove the point. 3mm steel plate open baffles. The mass is high enough not to be excited. I still have a little "feelable" vibration to contend with.

I nearly gave up on the idea after cutting the steel. It rang like a bell when tapped with a hammer. It got worse when I cut the holes. A few angle braces improved the situation.