JoskaNZ
For what it's worth, this is what I would do
1/ prepare the panel allowing for about a 10mm wide mounting strip around the edges.
2/ make a backing board from a material with poor acoustic properties (IE non resonant --(MDF or similar))
3/. Cut a circle approximately 200 mm dia in the backing board centred on the exciter location
4/. Make an edge strip for the backing board to achieve a final gap between the panel and the backing of no more than 20mm. You can experiment with different edge treatments. I used soft grade EPS strips directly glued to both backing and panel with some success. A more standard approach would be a rigid material (MDF or similar) topped with an absorbent foam seal to which the panel is glued.
5/ I would also line the inside of the backing panel with an absorbent material. I think even 8mm or so closed cell PU foam sheet stapled to the backing (as used for packing protection) would work... This will further reduce the air gap but I'd try it before making the gap bigger.
6/. Corner treatment... Tricky topic but probably try full mounting first before checking if any corner release is necessary. If so, you could release each corner and stuff the gap with fibre fill
7/ Supports to suit
If you do choose to try this I'd be pleased to get some feedback
Cheers
Eucy
For what it's worth, this is what I would do
1/ prepare the panel allowing for about a 10mm wide mounting strip around the edges.
2/ make a backing board from a material with poor acoustic properties (IE non resonant --(MDF or similar))
3/. Cut a circle approximately 200 mm dia in the backing board centred on the exciter location
4/. Make an edge strip for the backing board to achieve a final gap between the panel and the backing of no more than 20mm. You can experiment with different edge treatments. I used soft grade EPS strips directly glued to both backing and panel with some success. A more standard approach would be a rigid material (MDF or similar) topped with an absorbent foam seal to which the panel is glued.
5/ I would also line the inside of the backing panel with an absorbent material. I think even 8mm or so closed cell PU foam sheet stapled to the backing (as used for packing protection) would work... This will further reduce the air gap but I'd try it before making the gap bigger.
6/. Corner treatment... Tricky topic but probably try full mounting first before checking if any corner release is necessary. If so, you could release each corner and stuff the gap with fibre fill
7/ Supports to suit
If you do choose to try this I'd be pleased to get some feedback
Cheers
Eucy
So I have you used cf veil on styrene foam or other materials? if so how thick is the thin version you mentioned? I wonder would non woven polyester sewing interface material work do you think? I would nor expect it to be as strong as glass or cf but in situations where you would not need a lot of stiffness such as thin low density eps rigid foam (the kind used for packing and light insulation) this would be stiffer than a full strength layer of pva on each side and I would guess lighter if using the cf but I am not sure how heavy the polyester non woven interface material is.
If you have built panels out of this material how did they turn out and what materials did you use? Thanks for posting
Hi there folks
I have been reflecting on my experience building my DML panels ( which now sound great ) and how
the initial sound was 'horrible & honky'. It required EQ to get them sounding good.
I also recall a post by another member - calling the sound "honky".
It occurred to me that if both I and another person experienced this same 'Honky' sound, then there would
surely be other people that have also found this shortcoming with their panels. That honky sound.
This got me thinking to present a very simple solution to solve/improve this problem for those experiencing it.
The solution/modification is a simple passive filter requiring only 2 components of standard value. (easy to get)
One coil/inductor and one capacitor.
( this modification will also improve upper region sound by removing a 'masking effect' )
It's a simple series notch filter that places an impedance into the line at about 600hz. The frequency of the notch does not change with load impedance.
The inductor coil in this circuit has a DCR of 2 ohms. The lower the DCR, the deeper the notch and the better the lower frequency response.
Download Xsim. It's very simple. And it's free: https://libinst.com/Xsim/XSimSetup.exe (This link won't take you to a website. It will download the file directly into your default download folder. Look for it there.)
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I do this with my PA panels, but only because I use a sub on stage, and I need a reduction of SPLs behind the speakers.
I'm messing with various positions and shapes and holes on the rear absorber to reduce the amount of attenuation on the bass end. The panels bend in specific positions at specific frequencies, and the rear absorber can be used to flatten the response.
As it is, I use a wide bandwidth absorber (25mm honey-comb-shaped holes with the rear panel lined with carpet felt on the inside) and this does effect the bass end AND the over-all efficiency of the panel.
Thanks Eucy, that sounds like a good method of backing the panels. Have you tried this yourself? I may give it a go if my solution isn't good enough.
For these ones, as I have the frames made out of Oak and I want a slim solution that will look nice in the lounge, I don't have a lot of depth to play with so am thinking of making a back frame with 20mm fibreglass insulation sandwiched between packing blanket, hidden with speaker grill cloth. I did a quick measure and seems to cut above 400hz - 5db to 10,000hz - 10db approx. Below 400hz not affected. I did just have the thought that a layer of mass loaded vinyl could be added, might get some to try.
I wanted to keep away from a solid back as I imagine then I will have the rear sound reflecting back onto the panel which may sound worse?
Andre, I haven't seen that honeycomb absorber I will look it up. Glad to hear it helps you.
I don't want to lower the efficiency too much, obviously it will have less output from the back of the panel but that was causing the issue in the room.
For these ones, as I have the frames made out of Oak and I want a slim solution that will look nice in the lounge, I don't have a lot of depth to play with so am thinking of making a back frame with 20mm fibreglass insulation sandwiched between packing blanket, hidden with speaker grill cloth. I did a quick measure and seems to cut above 400hz - 5db to 10,000hz - 10db approx. Below 400hz not affected. I did just have the thought that a layer of mass loaded vinyl could be added, might get some to try.
I wanted to keep away from a solid back as I imagine then I will have the rear sound reflecting back onto the panel which may sound worse?
Andre, I haven't seen that honeycomb absorber I will look it up. Glad to hear it helps you.
I don't want to lower the efficiency too much, obviously it will have less output from the back of the panel but that was causing the issue in the room.
Carpet felt is great for this...I have used it a lot. I didn't mention it because it's quite hard to get in Aus now .. replaced by crappy faux rubber underlay which fails after a few years.
Andre, on the bass, from what you're saying I gather in your case it's reducing the bass??
The so called air spring is supposed to increase the bass response, but I guess if you open up the rear panel as much as you have that effect may well be lost
Eucy
I was messing around with the panels without the rear absorber in place, and congratulating myself that the bass was coming through nicely. Then, when I put the absorber back in place, all of my congratulations disappeared... I didn't bother measuring.
I think the air spring is supposed to work on the first few modes where large-enough areas of the panel are out of phase with each other. The air spring will possibly reduce the out-of-phase component by a momentary high pressure/low pressure on a specific mode pattern. The air spring is not a totally enclosed back panel. It has large cut-outs in it that appear to correspond to specific modes. I have not spent enough time with it yet to come to any real conclusions.
Very well written post there.
I thought to complete this 'off topic' aspect of sound would be to post something about "the other side of the coin" >
being dullness, and solutions that are ALSO non broadband EQ based.
This is probably the most famous product to correct 'dullness'.
https://www.waves.com/plugins/aphex-vintage-aural-exciter
@Andre Bellwood might even elaborate & expand upon this
Thanks Mister Audio.
Yes, an aural exciter of some description is part of my toolkit when mixing tracks of any kind. It adds second-harmonic harmonic distortion to signals between, say, 3k and 6khz or so (depending how you set it up) which means that sounds are added between 6k and 12khz that were not present in the original signal, and gives the effect of brightness and clarity. It is usually used on any single track that requires an added bit of 'life", but it can also be used in the final mix. Obviously, since this distortion is on the recorded track in the final mix, the end listener does not pick it up as distortion, but rather as a detailed, airy sound.
On the other end of the spectrum, the low end of the mix can also be boosted sans EQ by performing a similar trick with a bass booster (https://www.waves.com/plugins/maxxbass ) which is a different type of add-in, which either halves the lower frequencies (frequencies from 60 to, say 200hz), and adds the resultant signal back into the track, or it adds sub-harmonics to the track to produce a psycho-acoustic illusion of more bass.
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Andrew bellwood.
On the other hand maybe when I listen to the George Michael CD , I can hear the sssss and tttttt on the CD recording, and so can everyone else.
It is on the CD, and I'd say they wanted it there, to enhance the sound, it even has its own echo.
But my point was that these sounds can set off distortions in the panel and exciter, which is very painful if not delt with.
The cavity distortion can be anywhere around the 3k area, and the panel distortion somewhere around the 10k area.
This all depends on the panel and exciter used.
Ignore them at your peril .
Steve.
On the other hand maybe when I listen to the George Michael CD , I can hear the sssss and tttttt on the CD recording, and so can everyone else.
It is on the CD, and I'd say they wanted it there, to enhance the sound, it even has its own echo.
But my point was that these sounds can set off distortions in the panel and exciter, which is very painful if not delt with.
The cavity distortion can be anywhere around the 3k area, and the panel distortion somewhere around the 10k area.
This all depends on the panel and exciter used.
Ignore them at your peril .
Steve.
There is a interesting situation regarding the High Frequency performance of DML panels.
The high frequencies are 'forced' through/out of a medium that is measured in Grams vs Milligrams.
The high frequency performance of a DML can sound 'sibilant', This is part of the physical function = A form of distortion.
This is why I chose to use a Tweeter in my design.
Lets consider music content of 5Khz upwards ...
With a tweeter, the electo-magnetic-motor energy has to only move a diaphragm that has a weight of milligrams.
This allows for fast acceleration and also recovery. It is such characteristics that make tweeters work so well with high frequencies.
( this situation could be considered as > Mechanical Slew Rate )
When a DML panel is used, the high frequency energy of the exciter finds 'opposing mass' regarding very fast acceleration.
So, in practice the high frequency performance of a DML is very much 'a forced thing' > This creates distortion.
Of coarse you don't just add a Tweeter, you also take high frequencies away from the DML panel using a coil/inductor
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I cannot say I recognize that there is an issue with HF distortion with DML. In fact I experience the opposite, especially when playing on high levels, that DML does not distort in the same way as regular speakers. I had several people commenting how clear and non fatiguing the treble is on my system.
Of course like most people I cannot hear things like "singing at 18K", so I might be missing something. I do have a small distortion peak around 4k, but seen no evidence of distortion above that though. Perhaps it depends on material, coating or exciter used?
Do you have any recorded responses in for example REW where it shows?
Also, I would say there is no such thing as "musical content above 5k". While some instruments have overtones stretching in to the last couple of octaves, even the ones that does stretch quite high like piano and violin will produce fundamentals around 4k tops. Put a 48dB HPF at 5k on any track and you will hear some hihats and maybe occasional sibilance, but nothing resembling music. Most fundamentals will be below 1k and you have to go far below 5k to catch enough overtones to actually get the faintest idea about the musical content.
Of course like most people I cannot hear things like "singing at 18K", so I might be missing something. I do have a small distortion peak around 4k, but seen no evidence of distortion above that though. Perhaps it depends on material, coating or exciter used?
Do you have any recorded responses in for example REW where it shows?
Also, I would say there is no such thing as "musical content above 5k". While some instruments have overtones stretching in to the last couple of octaves, even the ones that does stretch quite high like piano and violin will produce fundamentals around 4k tops. Put a 48dB HPF at 5k on any track and you will hear some hihats and maybe occasional sibilance, but nothing resembling music. Most fundamentals will be below 1k and you have to go far below 5k to catch enough overtones to actually get the faintest idea about the musical content.
The DML driver does not move the whole panel at anything above the second fundamental. It moves a small section just in front of the driver, which then creates a transverse wave moving outwards towards the edge of the panel. Same as a pebble dropped into a pond, the pebble does not move the whole of the content of the pond instantaneously. Therefore one only considers the mass of the panel diaphragm at that frequency which is affected by the driver movement. That specific mass at a specific frequency reduces as the frequency increases.
Distortion in DML panels, specifically 3rd harmonic distortion, is generally much lower than in pistonic cones. It's the 3rd harmonic distortion in cones which tells the ear that the speaker is approaching over-drive, and which gives you the impression of how "loud" the speaker is working.
This 3rd harmonic distortion is often extremely low in a correctly edge-terminated DML design. One can play the panel very loudly indeed without getting an impression of loudness as-such until you try to speak to the person standing right next to you.
One can play a DML right up to the driver's mechanical limits without any sense of approaching over-drive, and it might appear that the DML over-drives easily. But that's not the case. It's merely a brick-wall limit that allows the panel to play a perfectly clean and crystal clear signal right up to that point where the driver voice-coil hits its mechanical limits.
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You can pick up ringing in the spectrogram on REW. It's rare to get resonance or ringing at such high frequencies.