The Aurium Waveguide

Open Baffle Speaker System

(c) copyright, Robert B. Richards, 2014

 

The Aurium Waveguide speaker system is a tri-amp'd open-baffle (mostly dipole) speaker system, with an active 4 pole Linkwitz-Riley crossover, which has active woofer EQ and open-bafle EQ. It's a satalite/subwoofer arrangement, so the woofers can be positioned independently from the satalite speakers for better interaction with room acoustics (less boominess). I've also included my variation of the Bob Carver "Holographic Generator" in the crossover chassis (bypassable).

The two sealed woofer cabinets are "monopole". The tweeter is monopole from 1.4kHZ up to about 7kHZ (The 7kHZ can be easily modified). It's my opinion that between roughly 1kHZ and 6kHZ monopolar is going to give you better imaging clarity than dipole, since fewer room acoustics interactions will cause less balance deviations over frequency. From 100HZ to 1.4kHZ this speaker system is dipole, and from about 7kHZ to 25kHZ it is dipole.

Inter-aural crosstalk, which exists in virtually all playback systems that use speakers, significantly damages imaging cues below about 1kHZ. This is because inter-aural crosstalk needs to happen only once, or our ear-brain mechanism won't know how to interpret it. In a true stereo recording, this will happen at the recording end. If it happens again at the playback end, our ear-brain mechanism gets confused, and imaging isn't at all what it could be.

There are two ways to minimize this issue (that I know of) in a typical playback system. One is to use an inter-aural cancellation function (Carver or Polk methods), which require the listener to sit exactly centered between the L and R speakers and back a certain distance, forming some variation of a triangle with the speakers. (and have little or no side wall reflection issues). Another approach is to have speakers that are "open-baffle" below about 1kHZ, to reintroduce an arguably false sense of depth in the frequency range below 1kHZ, where we sense image location by timing or phase differentials, rather than amplitude differentials. This "false" sense of depth synthesis works very well in most rooms, and is not particularly sensitive to where the listener sits, as opposed to the Carver or Polk interaural cancellation methods. For the person who's willing to sit in just the right place, there is an inter-aural cancellation circuit similar to the Carver one, that can be engaged, for that extra sense of being there with the musicians. These two methods of bringing back a sense of being there in the lower midrange frequencies compliment each other, rather than getting in each others way.

Above about 6kHZ we sense image location differently due to the size of the wagelenths. The shape of our outer ear comes into play and helps us sense height as well. It's my opinion that high treble, above 7 - 12 kHZ sounds significantly more "real" when it's emitted out both the front and rear of a speaker, especially if the rear tweeter can be angled upward, so the highest frequencies come from above as well. Comb filter effects from having two tweeters is a non-issue because of the path length differential being so much larger than the wavelengths involved. The combfilter cancellations will be too close together for us to perceive them. This adds "air" to the cymbals, and doesn't really damage anything.

The rear tweeter has it's own Banana jacks for the highpass capacitor and/or resistor, so it's fully adjustable in both cut-off frequency and amplitude, depending on what works best in a given room.

Front View

 

Rear Side View

The Tweeter Tower shelf never quite touches the front baffle board.

The tweeter tower has its own mechanical "return to ground".

This way, the tweeter output will avoid the vibrational modulation, caused by the 5 inch drivers.

Probably a minor issue, but I wanted to have the waveguide fin on the back side to help diffuse the rear waves, so hot-spots would be minimized.

Rear Tweeter

Snell had a similar rear firing tweeter on their top of the line system in the mid 1980's.

They were in one of the listening rooms at Dolby when I worked there back then.

 

Rear Tweeter Angled Upward

The rear tweeter is a 3/4 inch Dayton dome with a neodymium magnet.

Tweeters Being Wired

I'm not convinced that it makes any significant difference, but I wired the rear tweeter to be "di-pole" (out of phase with the front tweeter) rather than "bi-pole"

Side View

You can't see it, but there's a gap of about 1/8 inch between the tweeter shelf and the front baffle board.

The front of the baffle board has two layers of 1/4 inch thick acoustic padding (like felt), to minimize any emission from panel vibration.

One layer is glued to the baffle, and the other with grill cloth wrapped around it and glued to it, is held on with velcro.

The 10 degree tilt reduces floor bounce comb filter effects, and reduces time alignment issues with the four 5 inch drivers.

Any vertical line array of any length will have less of a floor and ceiling bounce comb filter issue. The longer the better.

The narrower the acoustic output on the horizontal axis (by using smaller drivers), the cleaner the imaging when used with an electronic inter-aural cancellation circuit.

 

Rear View

I put acoustic dampening material in the lower rear area to minimize any cavity effect on the rear emission.

All bracing is Oak. The vertical triangular oak braces at the sides of the back of the baffle minimize acoustic impedance and cavity effect issues, and further brace the baffle board.

Looking South

 

Banana Jacks Epoxied into the underside of the Oak Shelf for the Rear Tweeter allow fine tuning for a given room situation.

That line cord wire that goes horizontal in the above picture, is the stabilizer mechanism for the tweeter tower.

It attaches to the baffle board at the sides, and to the tower-stand/waveguide-fin in the middle.

Keeps it in place over time horizontally, but dampens out virtually all vibration on the Z axis.

Electrical Interface

 

 

The Building Process

Most of the big pieces

The reddish wood is called Bloodwood or Satine. It's harder and heavier than oak. Apparently a bit rare.

 

Making the Baffle Board

I routered the back side of the driver holes (1/2 inch radius) to minimize acoustic impedance effects. The drivers were mounted with wood screws and silicone rubber glue.

After the laminating glue dried (1.5 inch total thickness), I ran it through the table saw again so the sides of the front would be absolutely flat, and bond fully to the side panels below.

I consider this to be particularly important.

 

Attaching the Sides

There's probably an easier way to make these...

Making the Bases

Six big bolts attach the bases to the baffle.

 

Checking the waveguide/tweeter stand for any issues before bolting it in.

 

Creating the waveguide finn.

The triangle brace is there to minimize acoustic impedance issues.

 

 

The Woofers (2)

Peerless XXLS 12 inch driver in a sealed box, driven with active EQ making them acoustically flat down to 20HZ (with a fast drop off below that).

 

The Mid and Tweeter Drivers

I researched a lot of drivers and ultimately went with the same drivers that were good enough for Sigfried Linkwitz, who's technical expertise I have a lot of respect for.

The Midrange Drivers

Same as in the Original Linkwitz Pluto system.

The special 3 layer cone (whatever that's supposed to mean) is hard enough to have a small break-up peak of a few dB in the frequency response at around 5kHZ,

but the 4 pole crossover cuts them off at 1.4kHZ, so it's a non-issue.

 

The Front Tweeters are Seas Millenium 1 inch Domes.

Same as is used in the Linkwitz Orion system.

There were not many 1 inch dome tweeters that could be used all the way down to 1.4kHZ

and not have I.M. distortion or frequency response issues, even with the 4th order rolloff.

Frequency Response from Zaphaudio website

 

Distortion graph from Zaph website (absolute scale may be off some according to his website)

 

CSD graph of Millenium dome tweeter from Zaph website

 

The Electronics

The big loaded board above is my variation of the Carver hologram circuit.

3 way 4 pole active crossover network and active woofer EQ, Open-Baffle EQ and Carver style Holographic Generator (can be bypassed)

Since I'm not a software engineer, and I want to know exactly what the processing is doing, all my circuits are analog.

Below is that actual measured frequency response of the crossover circuit.

The midrange part is lower because of an efficiency difference. Four 5 inch drivers have about 6dB more efficiency than one.

The Y axis is dB. The red vertical lines are the actual crossover frequencies.

The bottom of one of the boards I built into the crossover chassis.

 

 

Acoustics 101

The "comb Filter" effect

In the case where there is one reflected acoustic signal mixing at your ear with a direct acoustic signal, you get a frequency response issue as in the graph below.

At the frequency where the delay of the reflected signal is a half wavelength, you'll get a substantial cancellation when the two signals add at your ear.

Due to the laws of physics, you'll also get cancellations at all the integral multiples of that "fundamental" frequency.

At higher frequencies where the energy gets more easily absorbed by the room surfaces and furnishings, the cancellations are typically less drastic.

12dB cancellations right in the midrange frequencies are not rare, based on my 1st hand experience.

This drawing only shows a 5dB cancellation depth. I beilieve it's a theoretical drawing, not an actual measurement.

This will be worse in smaller rooms.

 

Here's one example of how a room destroyed an otherwise very flat speaker system

A high end speaker system

 

It's frequency response measured up close

 

Same speaker measured further back, in a relatively typical listening room,

with the calibrated mic about where a typical listener would be located.

 

Most listening rooms will butcher the frequency response of any speaker. Especially smaller rooms.

There's both the combfilter effects, and room resonances, only the latter rings.

I found that putting acoustically absorbtive material (2 inch cotton rope or foam rubber) in the corners of the room reduced room ringing substantially.

Comb filter cancellations average out better (get filled in) in "live" rooms (rooms with lots of relfections). This is typically true for frequencies above about 300HZ.

From about 80HZ - 300HZ, depending on the room, there will usually be so few effective reflections, due to the size of the wavelengths,

that there is likely to be some bad frequency response issues in the lower midrange/upper bass frequencies that typically cause "boominess".

"Boominess" is usually more because of the room acoustics, than the speaker itself. Below 80HZ, my room had no problems of any significance.

 

If you decide to use a graphic or parametric active EQ to even out the response (only recommended for below about 300HZ),

you can attenuate peaks, but trying to pull up cancellations doesn't really work and causes problems. You would create peaks elsewhere in the room.

Moving the woofers to different locations can be the best way to even out the lower mid/upper bass issues, which is why I prefer the Satalite/Subwoofer arrangement.

I don't recommend puting woofers in a 3 surface corner.

Old photo of Setup in my living room, with previous tweeter towers.