What is a DML speaker?
Distributed Mode Loudspeaker (known as DML) technology is the use of one or more electro-acoustic exciters to induce a uniformly distributed vibration into a flat panel made of a resonating material.
These speakers are usually in the form of large flat panels that can be decorated or integrated into a wall or ceiling. They can be invisible or they can be the center piece of the room. In almost all cases, if you have them, you are going to want them big and plenty of them. Arrays of DML speakers can be used to fill large spaces with balanced and well supported sound.
Tech Ingredients "World Best Speakers" Video
If you've never seen nor heard DML Speakers before, the YouTube channel Tech Ingredients has a great video explaining DML speaker technology with a demonstration of their performance.
Choosing the Right Material
Choosing the right material will greatly affect how a DML speaker sounds. Different materials will produce different ranges of frequency response. Ideal materials are things that resist compression while also allowing the panel to flex. Flexibility and compression resistance are key when creating an efficient DML speaker.
Materials that have these characteristics are polystyrene (often used as insulation panels in buildings), pallet board (a heavy duty cardboard used to make pallet boxes), balsa wood and resonant spruce (used to make guitars and other musical instruments).
Other resonant woods used in instrument making can also be used to create DML speakers. Spruce is just the most common example.
Tech Ingredients has another great video going into detail demonstrating DML speakers created out of different materials.
The types of panels we have been making use polystyrene and we have found the frequency response of the un-equalized DML speaker can be over responsive on the high end, while providing almost no response below 120 Hz.
In almost every case, you will want to pair your DML speakers with a subwoofer and a good equalizer or digital signal processor. When done correctly this setup can create a wonderful listening experience in almost any sized listening space.
What makes DML speakers different?
Instead of using a piston-like motion moving a diaphragm like conventional speakers use, DML speakers produce sound across the entire surface of the panel. This creates a number of unique features of DML speakers over normal pistonic speakers.
DML Speakers are bi-directional (sound emits from both the front and the back), while also emitting almost no sound from the side. Because the back of the panel will emit the sound 180 degrees out of phase from the front of the panel, the area around the side of the speaker where these two waveforms meet, cancel out and produce almost no interaction on the side walls of a room.
By emitting sound over a large area (DML speakers are usually 2 or more feet wide) directional sound and speaker separation are maintained, while also mitigating room modes that may cancel out certain frequencies within a confined space.
DML speakers are also highly resistant to microphone feedback. It is possible in many cases to even put the DML speakers behind a live band playing on a stage without getting any feedback into the system.
In large spaces like auditoriums and cathedrals, DML speakers can fill a room with crystal clear sound, while also managing the echoing and interference that can occur in very large spaces.
The Pros and the Cons
Pros:
- High power efficiency (when built with optimal material)
- Unique flat panel shape can be integrated into walls or other architectural or design elements within a room (often DML speakers are hidden by making them look like the other sound treatment panels in the room)
- Broad frequency range (180 – 18000 Hz)
- Air radiation resistance is small and constant with frequency providing directivity that is not affected by the size of the speaker.
- Bipolar (sound radiates in two directions, front and back) which also limits sound interactions with side walls.
- Slow fading of sound away from the speaker allows it to fill a room with a very even sound.
- The distributed sound waves do not penetrate walls as much as normal speakers. It's possible to list to them quite loudly before the neighbors will complain.
Cons:
- They're big, and you need many of them.
- Limited low end frequency response (although large panels can be made to produce a low frequency response, this is usually a balance between speaker size and the desired frequency wavelength).
- Very bright peaks on the high end require equalization
- The large size of these speakers can be a negative in small spaces, although the ability to hide them inside walls or as artwork can easily overcome this limitation.
- Slow fading of sound away from the speaker can cause off-center imaging that reduces stereo separation.
Cost
The cost of materials to make DML speakers can be quite low. Although I don't list this as a Pro necessarily as the cost of the resonating material alone may not be very much. Building a cabinet to house the speakers and the investment into a DSP which is almost certainly going to be a necessity can drive the cost of DML speakers back into the same price range as other types of speakers.
If I was a college student that didn't care what it looked like, I would rig up a pair of polystyrene panels somewhere they wouldn't rattle, throw a couple fluffy towels over them them and turn the treble all the way down. The quality of the sound will surpass what you will get with some midrange bookshelf speakers. But your ears will ring everywhere you go.
The Extreme Peaks
The performance of DML speakers can be outstanding. When making DML speakers from polystyrene (which is an optimum material in terms of price and performance), DML speakers can have an extremely strong frequency response on the higher end. This spectrograph shows the frequency response of a set of 4' x 3' speakers made from 2 inch polystyrene. Notice the "white hot" area between 3k and 4k.
This huge response spike between 2,000 - 5,000 Hz is very noticeable when listening to large DML speakers for the first time. I even covered the panels with soft but heavy towels to dampen down this spike. Just turning the treble down is usually not enough. It can be quite painful to listen to these types of speakers very loudly without a Digital Signal Processor (DSP).
Digital Signal Processing (DSP)
Digital Signal Processing can be a little involved. Certainly more than I can cover in this introduction. But in a nutshell, it is quite easy to manage this frequency peak in DML speakers with a couple of wonderful tools. You will need a piece of hardware called a DSP. A company called MiniDSP creates a wonderful little DSP for under $100.
You will also want to use a free piece of software called Room Eq Wizard. In addition to the Room Eq Wizard, you'll need a calibrated test microphone and an SPL meter. MiniDSP makes a wonderful USB test microphone that is also very affordable. You can get an app on your phone to work as an SPL meter, or you can also get a dedicated SPL meter on Amazon for $20-$50.
The image below shows the current DML sound system we have in our workshop after it has been corrected with a DSP. The frequency response now is almost perfectly flat across the main range of audible frequencies.
Conclusion
DML speakers can be a very attractive option when considering a sound system. If you're willing to couple the DML speakers with a DSP and a subwoofer, then you can create a system with really amazing sound.
If you have a large space, like a church, auditorium, theater, nightclub, etc. the special characteristics of DML speakers in terms of filling a room with clear sound, while mitigating interference with room modes and system feedback, can make DML speakers a very compelling solution.
The learning curve of Room EQ Wizard can be a bit steep. Also, at first look, the robust controls available to you with a DSP system can be a bit overwhelming. To help knock the edge off of that learning curve, we created this article: