As I mentioned in a previous post, I recently built an impedance rig to allow me to make impedance measurements using REW. In that post I showed that the frequencies of the peaks in the impedance curve correspond to the natural frequencies (resonance frequencies) of the panel.
The results of another set of tests are shown below. Here I measured the impedance curves for various panel materials. All the panels tested were approximately 400 mm x 580 mm, with the same exciter (DA-25FHE) attached near the 40%/40% position. The panels were hung freely from two wires (or tape strips). I would not normally make a "speaker" this way (hanging from strings, without any frame), but the purpose of the test was to examine the materials themselves, without any other influence.
I stacked them in order based on the height and sharpness of the resonance peaks. I think it's pretty reasonable to assume that the sharper and taller the peaks are, the lower the panel material's internal damping is. On that basis, these results seem to show that the acrylic panel has pretty strong damping, followed by the plywood. Interestingly, my CF balsa composite appears to have quite low damping, similar to XPS and EPS. Aluminum, unsurprisingly, appears to have the least damping.
Eric
The results of another set of tests are shown below. Here I measured the impedance curves for various panel materials. All the panels tested were approximately 400 mm x 580 mm, with the same exciter (DA-25FHE) attached near the 40%/40% position. The panels were hung freely from two wires (or tape strips). I would not normally make a "speaker" this way (hanging from strings, without any frame), but the purpose of the test was to examine the materials themselves, without any other influence.
I stacked them in order based on the height and sharpness of the resonance peaks. I think it's pretty reasonable to assume that the sharper and taller the peaks are, the lower the panel material's internal damping is. On that basis, these results seem to show that the acrylic panel has pretty strong damping, followed by the plywood. Interestingly, my CF balsa composite appears to have quite low damping, similar to XPS and EPS. Aluminum, unsurprisingly, appears to have the least damping.
Eric
Thank you Eric for sharing you results. First time we have a material comparison by the impedance measurement.
For XPS, EPS : are thy row or coated as usually recommended for DML use?
For the aluminum, the peaks seem smaller then the EPS, XPS or CF/balsa one. Is it a reading effect?
Christian
Hi Christian.
This was the part of this patent that I was most interested in, at the time.
He found that the mass loading the panel roughly half way between the exciter point and the edge of the panel, filled in the bad dip in the response at 200hz .
This is basically the same method i use to find these points on my small panels, but i use my fingers while watching the response change ,in real time.
it is very simple.
The first part also describes the bowing in the centre of the panel , which i regard as the direct radiating part of the DML panel ( pistonic in nature 😀).
This is where most of the hf above 10k comes from on an EPS panel.
steve.
quote.
When the vibratile system 22 is thus relatively stifiiy clamped, and energized at its central point by voice coil form 18, the vibratile system bows in and out with maximum excursion at the central driven point, and the degree of excursion gradually diminishes towards the peripheral edges. It may be thus generally classified as a clamped piston bowed at the center. It is well known in the art of physical vibrations of membranes that when a clamped membrane is thus energized there are set up various nodes of vibrations with trough and peaks determined by the energizing frequency, by the shape of the diaphragm, and by the physical constants of the material of the membrane. Whereas such nodes of vibrations are relatively well known for standard circular; and homogenous membranes, the determination of such nodes and anti-nodal points can be determined only by approximate empirical methods for other than symmetrically contoured areas. For the type of fiat vibratile systems which has been reduced to practice in this invention where one side of the vibratile system is approximately one and one half times the length of the other, and where the piston overall dimension is approximately 15" X 22", the node of vibration is such that at approximately 200 cycles per second there is a pronounced anti-resonance in the acoustic response of the system as indicated by the dotted curve of FIG. 9
By exploring the vibrations of the panel with a sound probe, it was found that there were anti-nodal points of vibration very nearly equidistant between the furthermost clamped extremities of the panel and the voice coil driving point. To eliminate the anti-nodal vibration areas, it is feasible to load these points with masses of proper weight that will cause the overall modal vibration display to change. The effect of adding these masses 38 (shown in FIG. 8) is to eliminate the pronounced acoustic drop at 200 c.p.s. and to smooth out the overall response curve as a whole as indicated by the solid curve of FIG. 9. This invention thus teaches a method whereby it is possible to improve and make uniform the vibration of irregular piston shapes by probing them acoustically for anti-nodal points, and then mass loading these points to upset their anti-resonance effect.
This was the part of this patent that I was most interested in, at the time.
He found that the mass loading the panel roughly half way between the exciter point and the edge of the panel, filled in the bad dip in the response at 200hz .
This is basically the same method i use to find these points on my small panels, but i use my fingers while watching the response change ,in real time.
it is very simple.
The first part also describes the bowing in the centre of the panel , which i regard as the direct radiating part of the DML panel ( pistonic in nature 😀).
This is where most of the hf above 10k comes from on an EPS panel.
steve.
quote.
When the vibratile system 22 is thus relatively stifiiy clamped, and energized at its central point by voice coil form 18, the vibratile system bows in and out with maximum excursion at the central driven point, and the degree of excursion gradually diminishes towards the peripheral edges. It may be thus generally classified as a clamped piston bowed at the center. It is well known in the art of physical vibrations of membranes that when a clamped membrane is thus energized there are set up various nodes of vibrations with trough and peaks determined by the energizing frequency, by the shape of the diaphragm, and by the physical constants of the material of the membrane. Whereas such nodes of vibrations are relatively well known for standard circular; and homogenous membranes, the determination of such nodes and anti-nodal points can be determined only by approximate empirical methods for other than symmetrically contoured areas. For the type of fiat vibratile systems which has been reduced to practice in this invention where one side of the vibratile system is approximately one and one half times the length of the other, and where the piston overall dimension is approximately 15" X 22", the node of vibration is such that at approximately 200 cycles per second there is a pronounced anti-resonance in the acoustic response of the system as indicated by the dotted curve of FIG. 9
By exploring the vibrations of the panel with a sound probe, it was found that there were anti-nodal points of vibration very nearly equidistant between the furthermost clamped extremities of the panel and the voice coil driving point. To eliminate the anti-nodal vibration areas, it is feasible to load these points with masses of proper weight that will cause the overall modal vibration display to change. The effect of adding these masses 38 (shown in FIG. 8) is to eliminate the pronounced acoustic drop at 200 c.p.s. and to smooth out the overall response curve as a whole as indicated by the solid curve of FIG. 9. This invention thus teaches a method whereby it is possible to improve and make uniform the vibration of irregular piston shapes by probing them acoustically for anti-nodal points, and then mass loading these points to upset their anti-resonance effect.
Christian,
True, the three you mentioned all have one or two peaks that are higher than any of the peaks in the aluminum. I'm not sure I'd put much weight on individual peaks, but still, it is indeed surprising to me that the they are not more differentiated from the aluminum.
The PS foam results I included where uncoated. But I did also test another XPS panel that was coated with several coats (four as I recall) of PVA/water. Unfortunately, it is not a perfect comparison with any other panels as the coated panel was much thicker (25 mm vs 14 mm) and the corners were rounded. Results are shown below with the 25 mm coated/rounded XPS panel in red, and the 14 mm uncoated panel in green.
For the thicker panel, the peaks are fewer within any given frequency band, but that would be expected as the thicker panel is much stiffer, and hence all the natural frequencies are more spread out. But as far as the sharpness of the peaks, the effect of the coating (if any) is not dramatic.
Eric
Hello Steve.
This patent is a treasure. Thank you for the guidance (reading a patent in English is not my favorite task!).
This extract matches fully with what is explained by Tectonic in its paper introducing the BMR principle. The weights are added at antinodes.
What is funny putting the 2 papers together is the patent propose a practical method :
- Find in the FR the dip
- Play this frequency and find the antinode (place with minimum of emission). Tool : a mic close ot the membrane... or a pipe as just read yesterday as you suggested in a post to detect the source of parasitic noise in an axciter
- Add a mass there (value to be determined by several test)
All of that fill the hole in my understanding : "commercial DML have additional mass, not mine... why?"
Christian
Christian,
I'm not so sure that is correct. From the Graham Bank BMR patent (https://patentimages.storage.googleapis.com/1d/e6/29/d4bdc6e58aff65/US7916878.pdf):
Here, I understand the "locations" mentioned above to be referring to the locations at which to place weights (or drive means). But here in the patent Bank appears to be saying they should be added at (or near) the nodal lines of the highest mode to be balanced, rather than at antinodes. Or am I missing something?
Eric
Hello Christian.
Can you explain this quote for me please, I'm not sure what you meant?
A BMR is basically a bending wave panel that goes down to pistonic motion to produce a full range driver , if mounted in a box.
Using your finger's to find the best position for the mass weights is easy , if you have a small panel, not so easy on a large panel.
You can vary your grip from a light gentle touch to a hard pinch.
I try to stay near the edges as moving closer to the exciter can tend to alter the sound too much in the higher regions, in a bad way .
It is difficult to generalise about all panels as they tend react differently.
Steve.
Hmm... Possible I made a link too quickly... but I would surprise that the same problem (dip in the FR) has 2 opposite solutions.
For now, this evening, it is cake cooking for my colleagues tomorrow. I will try to go further in the comparison by the end of the week.
Christian
I have in mind different pictures of commercial DML, starting by Tectonic that have local weights added. Sorry I can't collect the different brand names or pictures now. It is just in my last design that I started to experiment to "finger method" but in this case it is by changing the exciter location (same by looking at the RTA while the panel is playing a pink noise) I found the smoother FR.
So thinking to this old patent (Cohen), the pictures of commercial DML like Tectonic, the BMR where several rings of mass are added to restore the initial modes of a free disc. This solution of added inertia seems quite frequent.
I haven't, but you remind me to pull my finger out and install my rear speakers - after moving house.
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I think a point worth making re. above measurements & results is that exciters do have
their own natural resonance (Fs) , so results at or below Fs are really quite academic.
The Dayton EX32's I am soon to use have a Fs of 395Hz. This makes it obvious to use a woofer.
( The panel mass will reduce Fs , but how much = variable )
Maybe panel weight is a parameter people should start using ?
I would love to see other peoples information re. other exciters Fs figures.
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I think a point worth making re. above measurements & results is that exciters do have
their own natural resonance (Fs) , so results at or below Fs are really quite academic.
The Dayton EX32's I am soon to use have a Fs of 395Hz. This makes it obvious to use a woofer.
( The panel mass will reduce Fs , but how much = variable )
Maybe panel weight is a parameter people should start using ?
I would love to see other peoples information re. other exciters Fs figures.
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Not really, but I have noticed that spatial imaging is fantastic, and use them in a quad setup but with a stereo signal.
Sorting out DSP unit and soundcard now so I can do actual surround playback on them since I think they will really shine at that.
Nicolas ojala
This recording on page 180 was made on my surround sound system with the front speakers replaced with small card panels.
The sound was amazing, sounding so realistic.
Steve.
Hello Christian.
I treat every type of panel differently, I cannot generalise as to the sound or response of varying dml panels.
The same is for bending wave, bmr, and dml ,with all the problems of bleed through from one type to another .
It all starts from pistonic action in the coil area, I use all of these three modes to produce the best possible panels.
Different methods are used to tame different panels and types of panels.
It all depends on what you are trying to achieve.
A used to use pink noise and music to find the best places for exciter mounting.
But now I either place the exciter in the NXT position or the centre, depending on the panel.
Supposedly the NXT position gives a good spread of modes with a good central position, which I prefer mostly, depending on panel size.
Steve.
I treat every type of panel differently, I cannot generalise as to the sound or response of varying dml panels.
The same is for bending wave, bmr, and dml ,with all the problems of bleed through from one type to another .
It all starts from pistonic action in the coil area, I use all of these three modes to produce the best possible panels.
Different methods are used to tame different panels and types of panels.
It all depends on what you are trying to achieve.
A used to use pink noise and music to find the best places for exciter mounting.
But now I either place the exciter in the NXT position or the centre, depending on the panel.
Supposedly the NXT position gives a good spread of modes with a good central position, which I prefer mostly, depending on panel size.
Steve.
Hello Eric,
Good reading! The methods described by both papers (BMR vs Biphonic) are different... In BMR, the mass are at nodal lines starting by the highest mode as you pointed it while in the Biphonic patent, it is at anti-nodes of the frequency (which is low) that creates the dip in the FR (anti-resonance in the text).
Well.. the link between both is not created...
From M Cohen's patent :
Mister Audio,I think a point worth making re. above measurements & results is that exciters do have
their own natural resonance (Fs) , so results at or below Fs are really quite academic.
The Dayton EX32's I am soon to use have a Fs of 395Hz. This makes it obvious to use a woofer.
( The panel mass will reduce Fs , but how much = variable )
Maybe panel weight is a parameter people should start using ?
I would love to see other peoples information re. other exciters Fs figures.
Where does your 395 Hz Fs come from? The spec sheet for that exciter reports 160 Hz for Fs.
I agree that a subwoofer is warranted in combination with most DML's. But that said, I don't think you can interpret the stated Fs figure for an exciter the same way as you would interpret it for a conventional cone speaker. As you correctly pointed out, the panel mass (and stiffness and mounting) will influence Fs, but perhaps a lot more than you might expect! In fact, I've found that all the 20-ish watt DA exciters behave similarly, that is, exhibiting an "Fs" of 20-40 Hz when mounted to an actual plate, despite a stated Fs (spec sheet) in the range of 200 Hz.
The 32u and similar Dayton exciters can easily produce strong and undistorted bass down to 50 Hz and perhaps lower. In my experience with DML, the resonance that determines the bottom end of the frequency range is generally not that of the exciter, but rather that of the fundamental frequency of the panel, as mounted. The real (and fun) challenge in designing a good DML (in my mind, anyway), is finding a way to get smooth bass response starting from the panel's fundamental though the next four octaves or so.
Eric
How exactly does one find this frequency? I've seen some complicated suggestions throughout this epic thread, but is there a concise method?
I've built DMLs with 3/32" aircraft basswood ply, 24" x 24" & 12x12" ... they sound good! (still need sub). But I'm struggling to find the right support structure -- I'd like to keep them open like Spedge's but this means a hanging installation, which isn't ideal for me.
As my parts & equipment (some on back order) slowly come together for my DML's
I will post description and weight of the panel, both without and with the twin exciters.
I will also post impedance/resonance information.
Re. the support of my panels >
The top is going to be HUNG - basically using ladies flexible hair-ties. (x2)
However, the bottom is to be restrained & controlled (anti-flap) by 2 long rubber strips.
(note: there will be an open baffle 12" woofer below my panels)
The 2 long rubber strips, roughly 550mm long, will go down to the 'base plate'.
I will post description and weight of the panel, both without and with the twin exciters.
I will also post impedance/resonance information.
Re. the support of my panels >
The top is going to be HUNG - basically using ladies flexible hair-ties. (x2)
However, the bottom is to be restrained & controlled (anti-flap) by 2 long rubber strips.
(note: there will be an open baffle 12" woofer below my panels)
The 2 long rubber strips, roughly 550mm long, will go down to the 'base plate'.
You could hang them from a clothes rack like these. Or make your own frames.
https://www.walmart.com/ip/Oukaning...1?wmlspartner=wlpa&selectedSellerId=101001636