Constant directivity loudspeaker designs

Loudspeakers in a domestic setting are used in rooms that are acoustically small in the bass region where the wavelengths are long (e.g. 7.8 m at 50 Hz) and which become increasingly larger as frequency increases and wavelength decreases (e.g. 3.4 cm at 10 kHz). At the low end of the spectrum there can be problems in the response due to room modes and at higher frequencies due to specular reflections. The room can participate in the sound reproduction process at many frequencies depending upon the absorptive and diffusive properties of surfaces and objects in the room and to the degree that these are illuminated by the sound waves emanating from the loudspeakers.

Loudspeakers tend to radiate uniformly into all directions at low frequencies where the radiating surfaces and cabinetry are small compared to the wavelength. They tend to beam the sound at high frequencies because the radiators are typically larger than 1/8th of a wavelength. Thus a loudspeaker, which is flat on-axis, radiates a different spectrum at angles off-axis, where the response is no longer frequency independent. Consequently reflections of the sound from different parts of the room that reach the listener will have different spectral content than the direct sound. The effect of reflections upon imaging from a stereo system has only been studied to a limited degree. It is often assumed that reflections can only have negative effects. This seems reasonable because reflections can only degrade the accuracy of a direct signal. But 2-channel stereo is about creating an illusion in the listener's mind. Neither the microphone signals, nor the ensuing ear drum signals at the listener, are accurate representations of a natural occurrence, of a natural hearing situation. The ear drum signals, though, contain cues from which the listener forms an auditory scene in his mind. If this scene is believable, then great enjoyment can be drawn from it.

The ear drum signals also contain cues about the room, though colored in the case of a loudspeaker that radiates a different spectrum in different directions. It has been my observation that a spatially more open and 3-dimensional auditory scene is created in one's mind when the loudspeakers radiate uniformly in all directions like dipoles or omnis do. The auditory scene has great clarity and distance but not the hard edges and closeness that a highly directional loudspeaker tends to produce

often create a less convincing illusion. The problem must be how a given loudspeaker system blends with the room. Ideally the room and the loudspeakers in it are not part of the auditory scene. That scene should open effortless and untiring in front of the listener, to be enjoyed best with closed eyes.

Other designs

On this web page I want to provide links to engineering information, to websites or forum discussions for the purpose of spreading learning and knowledge about controlled directivity loudspeaker designs. The material should be current and updated to reflect progress that has been made. I will need your help with that and invite you to give me links. The list would serve as a quick entry point to find out where and what the action is. It should therefore also point to relevant activities in languages other than English, since much can be learned from pictures and graphs.

I expect that the list will point to different design approaches. It would be important to know the design objectives and to what degree they were met. To be added to the list please send me an email with the subject DESIGN LIST, with your URL, and with a 50 to 200 word statement of your specific Design Objectives. I will look at the website or forum to determine whether the loudspeaker design or some of its aspects are a contribution to the field of controlled directivity loudspeakers from my point of view and appropriate for the purpose of the list.

Engineering information

Here is a truly ground breaking, sensible and practical application of DSP to the design of crossover filters and the polar response of large multi-way active loudspeakers. Very exciting work! It avoids lobing of the vertical polar pattern by acoustically tight spacing of the driver pairs.

Wide and uniform horizontal dispersion and controlled vertical dispersion up to high frequencies without lobes. The large number of small drivers ensures high output capability, especially in the tweeter range. The floor reflection is part of the design. The sound field is already uniform close to the loudspeaker and suitable for near-field listening. SPL falls off at a low rate with distance (3 dB/oct) and is nearly constant over a distance range. This is ideal for a home theater setup with rows of seats. (SL)
Links

This loudspeaker exemplifies Floyd Toole's loudspeaker directivity requirements. They are the result of extensive listening tests where different box loudspeakers were ranked according to preference. The directivity index increases smoothly from 0 dB to 10 dB, without signs of the two crossovers in its frequency response. Moderately wide dispersion horizontally. (SL)

An active 3-way loudspeaker with a 10" woofer and coaxial 5" mid and 3/4" tweeter. Smooth transition from omni to +/-50 degree, -6 dB, horizontal dispersion. DSP controlled. (SL)

A 2-way loudspeaker with a 15" woofer and compression tweeter. Omni at low frequencies and rapid transition to a beam of +/-40 degree at -6 dB due to a 15" diameter waveguide. The narrow beam widens the sweet spot if the speakers are toed-in to cross in front of the listeners. The contour map would be even more illustrative of reality if it were drawn in polar coordinates with the frequency axis as radius and the angle covering 0 to 360 degrees. (SL)

DIY designs

2p Audio Pages - 4/1/10
Aristoteles and Platon are reference designs for me from which other projects will evolve.

Monte Kay - 3/19/10
- Envision a dipole with an acoustic black hole behind it, completely eliminating the rear wave, leaving only the front, frequency invariant lobe. This best describes my directivity goal. I utilize open baffle dipole and cardioids as tools to eliminate off axis radiation as a partial means of achieving this. This design objective requires significant absorption behind the speaker to approximate the acoustic black hole.
- The “CBT” (Constant Beam width Transducer) as described by D. B. Keele has proven to be an effective means of achieving this goal. My home theatre center speaker combines CBT technology with open baffle cardioids. This focuses the rear wave at the center point of the CBT arc making it very easy to know where to put the absorption. Along with the proper absorption, the open baffle CBT very effectively accomplishes my stated objective.
- Keele and Horbach’s Linear Phase Symmetric Pair approach also accomplishes my objective but with other limitations. The large surface area in the sum of numerous drivers in the CBT solves other problems not related to directivity making it a much higher performance design over the Symmetric Pairs. As this is a discussion of directivity objectives, the other advantages are for another discussion.

keyser ( Martijn Mensink) - 3/17/10
- Flat frequency response, on-axis as well as off-axis by designing for a dipolar radiation pattern from the bass range up to the highest possible frequency.
- Operating drivers largely below the first dipole peak to maintain constant directivity up to about 6.5 kHz. Small U-frame 12" woofers and no baffle at all for the 6" midrange and magnetostatic dipole tweeter.
- Closed box subwoofers will be added at a later date. The stand-alone dipole is currently equalized flat to a little below 40 Hz.
- Sufficient dynamic range. In practice it turns out that at a listening distance of about 3 meters and an average listening level a little over 80 dB(A) and an approximated source material crest-factor of 6 dB and broadband, spectrally dense content, there is no audible compression or distortion. At higher levels the sound becomes a bit congested, but I am not sure if this is caused by the room, the speaker or even my own hearing. This is sufficiently loud for me. I have not yet done any distortion measurements.
- Digital crossovers and equalization. Crossover frequencies are 300 Hz and 2000 Hz, both at 48 dB/oct.

John K - 3/14/10
When designing a speaker system for home use the objectives of any particular design will depend on the specifics of the application and acceptable trade offs. As such it is difficult to state categorically any specific set of design objectives for a CD speaker. With regard to constant directivity, my current interpretation would be that a CD speaker, intended for use in home environment, should have the ultimate goal of maintaining uniform polar response above the Schroeder frequency. As a rule of thumb, this translates to maintaining uniform polar response form about 100 Hz and above. Specific design objectives for my designs may be found at my website.

MOB3W (my open baffle 3-way) - 3/6/10
- Constant radiation up to at least 3kHz
- Symmetrical radiation across the entire spectrum
- Sufficient dynamic range down to at least 40Hz.
- Low distortion
I have made several baffles to test what happens with a midrange woofer on a baffle. I found that for true amplitude and phase symmetry, I had to sink the driver in the baffle. Each midrange has at the backside a construction directly behind the surround that has to be copied at the front. In addition, the slimmer the baffle, the more the radiation remains constant. This way the radiation of the AL130, the midrange in MOB3W, is made constant and symmetrical up to 2kHz. To obtain a radiation that is symmetrical and as much as possible constant for the tweeter, and close to figure-8 at least up to 3kHz, I designed the specific baffle of MOB3W. The AMT2340 tweeter 'sees' as little baffle as possible. It has ridiculously low distortion, not at all like the ESS AMT tweeters and the Eton ER4.

Gainphile - 3/4/10
Affordable lifelike reproduction of music. It is possible to build full-range dipole loudspeakers with lifelike reproduction capability at relatively low cost. The speakers are built under $500 and well under $1000 as complete systems including active 4-way analog crossovers and 8-channel amplifications. There is clear benefit on the accuracy of the drive signals by using active system. Measurable transducer distortions and maximum SPL output are tradeoffs with such budget, yet the loudspeakers are uncolored and loud enough to provide listeners with enjoyable presentation.

StigErik - 3/4/10
True dipole operation over the entire frequency range. Operate all drivers below dipole peak to get better directivity. Decoupled drivers and/or baffles for reduced cabinet and/or baffle vibrations. Active XO and EQ. Choose drivers with good dynamic behavior and low distortion. Use multiple drivers to keep cone excursions far below Xmax. XO the midrange above 300 Hz so its less affected by the typical floor bounce suck-out. I'd like add that my listening room preferences and setup in the room differs somewhat from what is common. I have a LEDE room which kills most of the rear radiation from the dipoles (above 200 Hz). I also like to listen in the near-field - my current listening distance is just 1.8 meters (it should be rather obvious that I dont prefer to have early room reflections....) I also like to position the speakers at 45 degree angle instead of the usual 30 degrees.

Commercial designs

From some time ago - 8/4/10
A variety of significant speaker designs collected by Roger Russel of McIntosh Loudspeaker fame.

Stereolith
A single box stereo loudspeaker with identical drivers on left and right sides and a single, mono tweeter on top. The L and R drivers on the sides are coupled via the internal air volume, which is a spring at low frequencies and becomes a transmission line for distances >l/8 between the drivers. This causes dipole effects with peaks and dips at various angles. It is claimed that the brain handles such radiation favorably.

Grimm AUDIO
A wide baffle 2-way loudspeaker with an IIR crossover, which imposes exact LR4 acoustic slopes crossing at 1550Hz. The phase is subsequently corrected using an idealized inverted all-pass filter, resulting in a maximally linear phase response without any pre-echo’s.

musicelectronic geithain gmbh
RL 901K studio monitor with cardioid response in the bass region. No detail is revealed in a news interview how the two 30 Hz to 300 Hz flow-resistors have been constructed and actively equalized.

Georg Neumann, GmbH
Directivity smoothly increasing from omni to a +/-30 degree beam, forward horizontally

Geddes Loudspeakers
Omni at low frequencies, transitioning to narrow and constant directivity at highs

Controlled directivity loudspeaker designs

Other designs

Engineering information

DIY designs

Commercial designs