Mechanical Sectioning .vs. Silicon dots for resonance control

[bolserst]

Originally Posted by Capaciti

the main disadvantage of the mechanical sectioning using stripes isn't loosing radiating area, but shifting longitudinal resonance modes to critical frequencies ranges.

E.g. the final ESL using this methods. The intention is to increase low frequency sound pressure by adding several resonances in the desired frequency range. Lets say at 60 80 and 100 Hz. It works, but if you look to distortion, you will understand that those speakers sound compressed an coloured.

In addition those resonances have critical impact to spectral decay and even not limited to the resonce frequency but in higher ranges as well. In worst case spectral decay will be disturbed at 1000-3000 Hz and as a result voices do not sound as cristal clear as they should be expected from an ESL.

Yout testing panel show two rows of dots. This you should avoid, since this means that your panel show additional longitudinal resonance modes in comparison to a single row array. You need to verify which maximum membran width can be realized using one row, without the membran collapsing to the stators. Thats ugly trail and error processing.

Capaciti

When I add silicone dots to an ESL panel, the main purpose is to add stability and adjust the resonant frequency upward. I have not noticed any increase in resonance modes with two rows of dots, but I usually apply a tightly woven damping cloth to the back stator similar to what QUAD did in there ESL63. This dampens all of the diaphragm resonance modes except the fundamental which remains with a Q between 2 and 3.

Viewing pictures of the beautifully constructed Capaciti panels, I have often wondered why so many rows of silicone dots are used when Audiostatic uses only one row. Whatever the reasons, it obviously works well from what I have seen in posted response measurements.

Concerning sectioning of the diaphragm into strips of different widths. I have never understood the benefit for extending LF response. If anything, it seems like you are loosing radiating area for LF. With 3 strips as you mentioned with resonances at 60, 80, 100 Hz. The 100Hz section rolls off at 12dB / octave below 100Hz, so by 60 Hz it is adding nearly nothing to the LF output. You have effectively thrown away that area in terms of LF output. Perhaps I am thinking of it wrong, but I figured you want as much area as possible pushing out the LF sound. Maybe it works OK for response equalization if no resonance damping is used on the strips leaving them with hi-Q fundamental resonance. But, this compromises spectral decay as you mentioned. Personally, I much prefer the sound once some damping is added.

However, if the diaphragm is sectioned in to 3 equal widths with resonance of, say, 70Hz, all the diaphragm area would be moving together equally for the LF output. This should give similar results to a silicone dot damped diaphragm with the same total area and same resonant frequency, shouldn't it?

bolserst

[Capaciti]

Hi Bolsert,

if you damp the rear your will loose transparency of sound. I tried this this with different materials and the result is that you loose breathing of voices and that the sound seems to stick more to the panels.

My measurements prooved that by showing increased spectral decay in the midrange.

I do not know waht others think about the quads, but imo they do not sound very open, but sound like wrapped into something. And this something are the dust cover, the streamin resistance of the PCB stator and the rear damping.

But you are right, dampening makes resonance control much easier and in addition the rear damping makes anESL less critical to rear wall interaction.

Splitting in different sections as ment for the finals do not mean that you gain sound pressure in the lowest frequencies (e.g. 40 Hz) but in the upper low range (60-120HZ), without electrical equalization. If you listen to a Final ESL you will recognize that there is no deep bass.
Imo some Fullrange ESL do not suffer from the aspect of missing deep bass, but from a sound pressure dull in the range between 80 and 200 Hz, where the open baffle effect kills efficiency. There is no worse sound for an fullrange ESL havin a dull from 80-200 Hz and a extended pressure at 40-50 Hz. It sounds boomy at low and anemic at upper bass frequencies. This is why most people call ESL-Fullrangers a one note bass transducer.

As a result dont focus on best performance at 40 Hz, squeezing out the last decibels in that range, but take care that your frequency range is linear from 80 to 500 Hz, which is tricky or even impossible enough for a small ESL with limited area.

Capaciti

[Calvin]

Hi,

damping mats are quite effective in damping resonances as well as sucking life out of the ESL Their effect is frequency and amplitude dependent, which is not what one wants to achieve in first place. And besides it spoils the optics, which is what we all like most about ESLs...don´t we?

jauu
Calvin

[SY]

Visual appeal of ESLs was NOT the strong point of mine (rebuilt and modified Acoustats)! I tried them with and without damping mats. I'm not sure that I'd call the effect of the damping bad, just different. It definitely helped to tighten the mid bass and focus the images better, at least in my room. I had about 50% of the active area in thin felt, spaced about a centimeter from the diaphragm.

[bolserst]

Originally Posted by Capaciti

Hi Bolsert,

if you damp the rear your will loose transparency of sound. I tried this this with different materials and the result is that you loose breathing of voices and that the sound seems to stick more to the panels.

My measurements prooved that by showing increased spectral decay in the midrange.

I too have found that damping material can have a negative effect in the midrange, but improved transient response in the mid-bass. As usual with speaker design...there are trade-offs. The trick is to find a cloth that will provide enough acoustic resistance to damp the diaphragm resonance adequately without reflecting back the midrange sound and creating a resonant cavity between the damping material and the diaphragm. Very thin synthetic felt (1/32”) works quite well. The more common 1/16” felt also works reasonably well, but the deficiencies in the midrange start to show up. I have also found some tight weave polyester cloth that work…damping is not quite as good in the bass, but midrange effects are nearly unmeasurable. Still searching

Originally Posted by Capaciti

Splitting in different sections as ment for the finals do not mean that you gain sound pressure in the lowest frequencies (e.g. 40 Hz) but in the upper low range (60-120HZ), without electrical equalization. If you listen to a Final ESL you will recognize that there is no deep bass.
Imo some Fullrange ESL do not suffer from the aspect of missing deep bass, but from a sound pressure dull in the range between 80 and 200 Hz, where the open baffle effect kills efficiency. There is no worse sound for an fullrange ESL havin a dull from 80-200 Hz and a extended pressure at 40-50 Hz. It sounds boomy at low and anemic at upper bass frequencies. This is why most people call ESL-Fullrangers a one note bass transducer.

As a result dont focus on best performance at 40 Hz, squeezing out the last decibels in that range, but take care that your frequency range is linear from 80 to 500 Hz, which is tricky or even impossible enough for a small ESL with limited area.

Ah! This makes sense to me now. By splitting up the panel in to different width sections they are NOT trying to extend the LF response. Rather, they are trying to fill in the valley between where the dipole baffle effect starts rolling off the bass and the large resonance peak from a single diaphragm at 40-50Hz. But, they use up panel area doing this and are not left with enough for adequate deep bass output.

The Audiostatic ES100 is a perfect example of a one note bass transducer with anemic mid-bass. With the right kind of music, it could sound wonderful…very transparent midrange with amazing imaging capability. But with a lot of pop music or large orchestral type music it fell short sounding thin yet boomy at the same time.

I am not trying to create a full range ESL. Rather, I am trying to extend the LF of the ESL from the 200-300Hz used by the hybrids I’ve built down to 80Hz. Besides the obvious benefit of extending the ESL “magical” sonic qualities down another octave or two, my thoughts are that the crossover will be more seamless here since the wavelengths are much longer. However, as you mentioned, this area(between 80-200Hz) is where most ESLs perform with the poorest efficiency as the dipole baffle roll off is in full effect. My thought was to set the diaphragm resonance near the desired crossover point and damp to Q of 2 – 3. This way I will effectively gain 6dB efficiency to counteract one octave of the dipole roll off. Any remaining deficiency in response in this range would need to be made up by 1) increased area, 2) adding some small wings on the edges of the ESL to delay the dipole baffle roll off, or 3) increased signal drive in this frequency range(Acoustat uses separate LF transformers, Quad ESL57 adds windings for the LF, active EQ) 4) drop the efficiency of the midrange and highs to match the lows.

1) & 2) are easy enough...hoping I don't have to resort to complications like 3)
4) is also easy, but throwing away ESL efficiency never feels like the right solution

[MJ Dijkstra]

The efficiency of a damping cloth can be increased by glueing the cloth on the perforated stator (quad63) or in case of a wire stator, using a seperate perforated sheet with the cloth glued on (DIY netherlands). A fixed cloth won't move and is purely resistive. I found that a thin speaker grill cloth just 'wrapped around' the speaker worked but only very moderate compared to silicone dots in taming the fundamental resonance. I think this is because the wrapped on cloth will move along thus being less effective.
I do not know how silicone dots compare to the fixed cloth.

[bolserst]

Originally Posted by MJ Dijkstra

The efficiency of a damping cloth can be increased by glueing the cloth on the perforated stator (quad63) or in case of a wire stator, using a seperate perforated sheet with the cloth glued on (DIY netherlands). A fixed cloth won't move and is purely resistive. I found that a thin speaker grill cloth just 'wrapped around' the speaker worked but only very moderate compared to silicone dots in taming the fundamental resonance. I think this is because the wrapped on cloth will move along thus being less effective.
I do not know how silicone dots compare to the fixed cloth.

I agree, a fixed cloth does improve the damping. Also the closer you can get the cloth to the diaphragm the better the damping. If you attach the cloth on the inside of the back stator(like QUAD 63) you can use a cloth with a more open weave and still damp the resonance. The open weave then has less effect on the midrange. ALso the open weave does leave the ESL with a more transparent appearance which most people like.

Are you saying that you have found adding silicone dots to reduce the Q of the fundamental resonance? In my experiments, the Q and height of the response peak stayed pretty constant (+15db to +20dB) as I added more and more silicone dots. The only change was the frequency of the resonance increased as the number of dots increased.
Hmmmmm....perhaps I need to try some different brands of silicone. Maybe some have better damping properties than others.

[Capaciti]

Hi bolsert,

a peak up to 20db at resonance indicates a mismatch between membran width and tensioning of the mebran.

I guess you tension the membran mechanically. You should not. imo a heavily streched membran do not sound best. The more important is, that you need months or even years break in time to reach the point , where the tesion reduced itself to a constant and long lasting value. E.g your resonance of 60 Hz wuill decrease to possibly 40 Hz and Q decreases as well.

Better just to make thermal tensioning. glue the membran with lowest tension to the frame. The tension should bejust as high that major wrinkles are reduced.

Take a heat gun, best using one with a temperature indicator and set it to 250°C. Move it like a spray gun over the entire area. Keep a distance of about 5cm to the membran. Repeat it 5 times, let the membran cool down for 20 minutes and repeat the treatment 5 times again.

This wont end up in the best tension you might achieve figuring out a combination of mechanical and thermal treatment, but for the beginning it provides much better results than mechanical tensioning only.

I am sure that you end up with completely different results, which might surprise you

Capaciti

[Calvin]

Hi

My thought was to set the diaphragm resonance near the desired crossover point and damp to Q of 2 – 3.

This can be done quite successfully without damping but with usage of a dampened highpass of 2nd order. See this thread: Ping Calvin especially the pdfs Multidoc3 and 4.

jauu
Calvin

[harry_at]

Originally Posted by Capaciti

Take a heat gun, best using one with a temperature indicator and set it to 250°C.

Hi Capaciti,

I think you mean 150°C.

Regards,

Harry