Distributed Mode Loudspeakers

My journey into home stereo speakers and high quality audio continues…

This is not a broadcast related post, but I find this type of thing interesting nonetheless. A few years ago, I began fiddling around with flat panel speakers or Distributed Mode Loudspeakers (DML). My first attempt was lackluster, although adequate and not very expensive.

I enjoy listening to all types of music. I notice that some music reproduces better than others. Piano is a particularly difficult instrument to record and faithfully reproduce because of the complexity of the instrument itself. It is a percussive string instrument. When one of the hammers strikes a sting, the string itself does not make a very loud noise. The string is connected to a sound board through a bridge. The much larger soundboard vibrates and amplifies the initial sound. Thus, the wood itself imparts a noticeable quality to the sound. Several other instruments such as acoustic guitar, violin, etc also use tonewood to amplify sound. Thus, when listening to a piano being played back on a set of speakers, it lacks much of the original texture of the actual instrument. It sounds like a recording of a piano, not an actual piano.

This made me think of ways to improve that reproduction.

A DML uses a large flat surface to reproduce sound. The large surface area and the bi-directional nature of the panel means the sound will be defuse and not directional as is the case with conventional speakers at higher frequencies.

The principle characteristics of material used in a DML is stiffness and density. The less dense (light weight) and the stiffer the better. All materials have internal resonances, some decay quickly and others, such as steel will ring for a long time. A ringing reverb sound would be undesirable in most cases. Tonewood has suitable stiffness, a relatively high sound radiation coefficient, varies in density according to species and generally dampens high frequency harmonics. Pianos often use Sitka Spruce for sound boards.

I made two terminated transmission line speakers last summer. Those sound great, but using one driver to reproduce all sound from 20-20,000 Hz has a drawback; the high end lacks detail and is very directional.

A bit of research shows that Poplar has good to very good tonewood qualities while being inexpensive and readily available at the local big box hardware store. Thus, I purchased several 1/4 x 6 x 48 inch (6 x 152 x 1220 mm) planks and a few pieces for bracing to make two 33 x 22 inch (838 x 559 mm) flat panels. The idea here is to gently cross over the transmission line speakers at 4,500 Hz with these flat panels.

The assembly process was pretty easy. It took perhaps two hours to cut all the pieces to the right size. I used a router to chamfer the edges of the back bracing, mostly to reduce the weight of the brace. Everything was glued up and allowed to dry for 24 hours.

Gluing the parts together
Back bracing

The bracing is designed to keep the panel stiff and helps with structural integrity. I sanded the fronts and backs of the panels, then finished them with Tung Oil.

Dayton tactile exciters

Each panel uses two Dayton DAEX32QMB-4 tactile exciter wired in series. There is also a Zobel network which is important for a tube amp. Most driver’s impedance will rise as the frequency increases. This rising impedance is mostly reactance from the driver coil. For solid state amps, this is not much of a problem. However, tube amps, because of the output transformer, need a fairly constant impedance across there output range. This is because the output transformer is a ratio device. The primary impedance is derived from the impedance on the secondary (e.g. what the secondary is terminated with). For example, a certain tube has a 5,000 ohm plate impedance. The output transformer is chosen to match this design specification. The output transformer is designed to drive an 8 ohm loudspeaker. However, the driver impedance rises to 16 ohms at 15 KHz and 24 Ohms at 20 KHz. Under those conditions, the tube would be seeing a 10,000 ohm impedance at 15 KHz and a 15,000 Ohm impedance at 20 KHz. This will lead to added distortion. The Zobel network keeps the impedance fairly uniform across the entire audio range.

Rather than build a cross over into the transmission line speakers, I made an external unit. This allows the speaker system components to operate independently if needed. I can easily change the cross over components for a different crossover point. I can also mix and match things in the future. In other words, it gives me a great deal of flexibility for future projects and experimentation.

Crossover and HF attenuator
Crossover and attenuator components

I made these small boxes to match the speakers. These contain the crossover and HF attenuator. The attenuator is necessary because the difference in efficiency between the transmission line driver and the tactile drivers on the DML.

The schematic looks like this:

Cross over diagram

I kept things as simple as possible. Yes, I could have made a 12 dB per octave crossover with a few additional components, however, I like the gentle blend of both drivers in the mid ranges.

A basic sweep with Room EQ Wizard shows a fairly flat response. There is a room resonance around 90 Hz which I intend to correct with some sound deadening materials in the corners and possibly a bass trap.

Room EQ Wizard waterfall display

This shows that the speaker system is relatively flat from about 200 Hz to 10 KHz. Those bumps and dips from 88 to 250 Hz are all room due to room resonance.

Room EQ Wizard SPL graph

How does it sound? Pretty good! Here is some copyright free music, recorded from youtube, then re-uploaded to youtube! It is not the best video, there is some background noise, etc. However, it give a pretty good idea of how these sound.

Automation Systems

Radio Automation Systems are nothing new under the sun. As Marconi tapped out his famous S, he was likely thinking “We should get a machine to do this…”

Broadcast stations have been installing different types of automation since the mid 1950’s and early 1960’s.  It was touted as a way to free up announcers so they could do more important things.

While cleaning out an old studio/transmitter building and getting things ready for demolition, I found a stash of old product brochures for various automation systems from the 60’s, 70’s and 80’s.  It looks like the previous owner used to go to the NAB every year.  How many radio guys got their start on the overnight shift changing out reels?  I know a few.

Gates automation system brochure, circa 1965
Gates automation system brochure, circa 1965

The Gates Radio Corporation had a fairly standard reel to reel system in three different configurations.  These systems were pretty pricey in their day.  According to the 1966 price list:

  • The Automate 244 cost $7,275.00 ($58,837.00 in 2020)
  • The Automate 484 cost $12,210.00 ($102,946.00 in 2020)
  • The Automate 1007 cost $17,150.00 ($138,799.00 in 2020)

Those are for stereo systems, mono systems cost about $500.00 less.  Each one of those systems ran one station.

Gates Automate 244 events thumb wheel
Gates Automate 244 events thumb wheel

The larger the system, the more events it could trigger.  I have never run into one of these in the wild.

Long form satellite shows began to surface in the early 1980’s. Things like all news formats and overnight talk shows.  I have had nightmares were everyone walks around saying “This is Larry King, you’re on…” while UFO’s fly overhead and next door neighbor, Jim Hightower, buries PCB’s in his back yard not more than 200 feet from his house.

Broadcast Electronics Control 16 radio automation system
Broadcast Electronics Control 16 radio automation system

Broadcast Electronics had the Control 16 system in the 1980’s that ran off of a very basic computer system that could handle 3,000 events with the standard RAM configuration and up to 9,999 events with additional RAM.  This was ideal for multiple program sources; music on reel machines, satellite syndicated talk or music formats, etc.  No longer were these machines simply running the overnights.

Schaffer 901 radio automation system
Schafer 90x radio automation system

Schafer is perhaps the most know analog tape automation, at least to me.  I know of several of these systems that were in operation through the mid 1990’s.  By this time, long form satellite music formats had become the rage.  However, there were still a few stations using reel to reel music services.

I particularly like this flyer from IGM:

IGM (International Good Music) brochure, circa 1971
IGM (International Good Music) brochure, circa 1971

By the mid 1990’s, these tired old dinosaurs where being removed.  Still, the mechanics of the operation were a thing to behold.  It was nice because you could hear the relays snap shut after decoding a 25/35 Hz tone or one of the Mutual Radio be-doops.  The cart-o-matic would chug through the break until the liner fired and then back to the bird.  If there were any issues, one simply needed to stand there and watch which part of the machine malfunctioned.

Schaffer logging system
Schafer logging system

Computer based systems like Computer Concepts DCS, Arrakis Digilink and Audio Vault came along, which got rid of the analog tape.  Instead they stored audio on hard drives.  Those early systems hard drive space was a premium, so usually at least 3:1 compression was needed to fit all the commercials onto the drive space available.

My first brush with Audio Vault was in 1994 at WGY in Schenectady, NY.   It was a pretty good system if you could understand the .ini files.  As the BE tech support guys used to say “You can program it to turn the coffee pot on if you wanted to.”

Nowadays, what used to be a studio location is more akin to a content distribution node.  This rack combines music and commercials stored locally on hard drives with out of town voice tracks and serves as the program source for eleven radio stations.

Radio Automation, 2020 style
Radio Automation, 2020 style

It works remarkably well, as long as the windows operating system stays functional.

Audio Bit Rates and Formats

Occasional reader and blogger Robert has broken down all the information on audio bit rates and audio formats.

His work can be found here: Ultimate Guide to Audio Bit Rates and Formats

It is a good read, especially for those that use audio streaming as their main content distribution method.

Streaming only stations used to be a big thing but have been supplanted by Spotify and Pandora.  I am not a huge fan of either of those services but I do like to listen to podcasts.

Good audio should be near the top of the list for any content provider.  Few things are more annoying than listening to an interesting podcast with low volume, background noise or other technical defects.

The Sonotube subwoofer

Continuing with my speaker projects; I have finished building a Sonotube subwoofer for my Box One speakers. As I discovered, those speakers miss a significant part of the bottom end. I made up for this by using a Polk Audio PSW-10 subwoofer that was part of my home theater system.

Sonotube subwoofer, downward firing configuration
Sonotube subwoofer, downward firing configuration

I found the PSW-10 was certainly loud enough, but lacked definition which became noticeable when the new speakers where installed. Thus, I began thinking about a sealed box subwoofer. When designing the enclosure, there is a good bit of math involved in calculating the box volume.

Sonotube subwoofer, upward firing configuration
Sonotube subwoofer, upward firing configuration

I now have a bit of a dilemma; should I set the subwoofer up downward firing into the floor or should I set it up upward firing into a dispersion cone?  I have tried it both ways and in the downward firing position, it shakes the floor.  That is a great effect for watching movies like “The Hunt for Red October,” where there is a lot of bass rumbling.  It is also great for blasting some Led Zeppelin or Pink Floyd Dark Side of the Moon.   However, the idea of a sealed subwoofer was to add detail to the bass and that it does do.  The Polk Audio PSW-10 certainly had bass, but it was ill defined.  The sealed sono-sub has less low, low bass but the bass instruments now sound as good as the rest of the orchestra.  In the upward position, I get clean omni directional bass which sounds fantastic when listening to P.I. Tchaikovsky Concert #1 in B flat minor, opus 23.  That was the idea when I started making this.  Still, blasting Led Zeppelin is fun and I recommend it for everyone.  Decisions, decisions…

After a couple months of evaluation, I finally decided on the downward firing configuration.  It does add another dimension to watching movies.  Now, I kind of want to get a bass shaker for the floor underneath the sofa.

Piece of left over, 12 inch diameter sonotube
Piece of left over, 12 inch diameter sonotube

In any case, the technical details for this subwoofer are as follows:

  • Driver: Dayton Audio RSS265HF-4, 10″ subwoofer, 4 ohm
  • Plate Amp: Dayton Audio SA70 70 watt
  • 12″ (30.48 cm) diameter sonotube, 19 3/4″ (50.165 cm) long
  • Miscellaneous pieces of wood, paint, screws and foam that were laying around

Sonosub driver, Dayton Audio RSS265HF-4 4 ohm aluminum cone driver
Sonosub driver, Dayton Audio RSS265HF-4 4 ohm aluminum cone driver

Sonotube subwoofer foam lining inside of sonotube
Sonotube subwoofer foam lining inside of sonotube

I found a driver that will work well with a sealed enclosure.  This is important because some drivers work better with ported enclosures and some with sealed enclosure.  Another detail is whether or not the driver can be mounted horizontally.  Some driver cones will deform if placed in the upward or downward firing position.  The Dayton subwoofer selected will not suffer from that.

I used an online calculator called speakerbox lite to calculate the volume of my sealed subwoofer enclosure.  There are several choices for the type response the designer is looking for.  Initially, I thought about critically damped, but the box volume was 125 liters, which is more than a 12 x 48″ (30.48 x 122 cm) sonotube.  I settled on Bessel-2, which has a box volume of 56.68 liters.  That was easily obtainable with the sonotube I had on hand.

The plate amp is a run of the mill 70 watt Dayton unit.  Truth be told, it runs a little bit hot and I’d consider something else if I were making this design again.

For construction, I carefully cut the sonotube to the right length with a jig saw.  The outside of the sonotube was roughed up with some 220 grit sand paper before painting it flat black.

The individual pieces were glued together with gorilla glue.  The wood braces on the outside of the sonotube connect with wood braces in the inside of the sonotube with brass wood screws.  The plywood rounds were cut with a router to a close fit.  Being that this is a sealed design, I took some extra time with a non-silicone based sealant to make sure that the entire enclosure was tight.  All in all, the enclosure certainly feels tight.

It sounds great and I feel like my subwoofer now matches the quality of the other speakers I am using.