Speaker off axis: omnidirectional speakers

This blog article is the sixth in a series on speaker directivity and off axis response. This article will consider the off axis response of omidirectional speakers ("omnis") and consequences for acoustic design.

Previous articles in the series established the theoretical foundation in terms of the psychoacoustic as well as subjective importance of speaker off axis response and different ways to measure speaker directivity. We followed this up with articles that examined the specifics of:

 

What is an omnidirectional speaker?

The theoretical model for an omnidirectional speaker is a simple pulsating sphere that radiates every frequency from a point in space. Suffice to say there are no perfect omnidirectional speakers in existence, as it's impossible at this point to generate a 20Hz sine wave from a small pulsating sphere! In reality even speakers marketed as omnidrectional only exhibit that characteristic over a portion of their passband.

MBLs Radialstrahlers are perhaps the most well known and highest end omidirectional speakers in existence today. Their lower end (still expensive) speakers incorporate only two omnidirectional drivers that cover the upper midrange and treble, whilst their higher end models include multiple units covering everything upwards of 100Hz. The MBL omni drivers are technologically quite interesting, being comprised of vertically oriented petals made from carbon fiber or aluminum affixed at the top and driven from the bottom with a typical moving coil assembly. More details can be found in this video.

MBL 101 Xtreme and its triple omindirectional drivers

MBL 101 Xtreme and its triple omindirectional drivers

Mirage make loudspeakers with drivers that fire onto lenses which disperse the sound very widely in the lateral plane. These speakers, like the MBLs, aren't really true omnidirectionals in terms of their radiation pattern since they still have directivity changes in the vertical plane. B&O make some speakers that work along similar principles, using the lenses developed by Sausalito Audio Works. The characteristic of all these speakers is that sound pressure levels do not change laterally off axis, except for a drop off towards the rear due to enclosure shading.

Mirage "Omnidirectional" bookshelf speaker

Mirage "Omnidirectional" bookshelf speaker

Omnidirectional lateral off axis performance

Pretty amazing, huh? Whilst the off axis is not as smooth as that from say a YG speaker due to the small ripples that can be seen, there is no evidence of any narrowing in directivity as the frequency increases. Sound pressure levels off axis are the same as on axis.

SPL difference between 0 and 60 degrees laterally, from SoundStage review of Mirage OMNI60

SPL difference between 0 and 60 degrees laterally, from SoundStage review of Mirage OMNI60

The chart above shows the difference between the on axis response at 0 degrees and the response off axis at 60 degrees. You can see that the SPL levels are very consistent, save for the minor bumps and wiggles in this unsmoothed measurement. This is another good example of the off axis performance of an omnidirectional speaker.

It's worth remembering that the performance of a loudspeaker is not just about the off axis. It's a combination of a number of things. Many of the Mirage speakers have pretty poor on axis response with variations of 3-6dB within their omnidirectional working range!

 

Room acoustic considerations of omnidirectional speakers

Omnidirectional speakers put a lot more energy into the early and late reflected fields than other speaker types. This energy is spectrally consistent with the direct sound, especially in the lateral plane, and maybe also in the vertical depending on the speaker design. Spectral consistency is a good thing, since what you hear is the fusion of the direct and reflected sounds. See my article listening room reflections and the ETC for more about spectral consistency.

A frequent criticism of omnidirectional (and dipole) speakers is that imaging is vague compared to a traditional (box) speaker. The key early reflection point that's responsible for this difference is the front wall behind the speaker. Comparing the amplitude and spectrum of early reflected sounds for a front firing box speaker and an omni will reveal the biggest differences from the front wall reflection. For a box speaker this reflection is mostly comprised of bass frequencies that wrap around the speaker baffle and bounce off the front wall (read about "baffle step" to understand this effect). It contains little mid and high frequency sound except that caused by diffraction off the baffle edge. In contrast the front wall reflection for an omni contains a lot of mid and high frequencies. What's more this reflection is normally high in level relative to the direct sound, since the front wall is often the closest wall to the speakers. This front wall reflection arrives at listener's ears from a direction which does not add much beneficially and should therefore be absorbed, redirected or diffused. If the energy return from the front wall is properly managed then the other early reflections can be considered and treated similarly to the acoustic design for a wide dispersion conventional forward firing cone/dome or coaxial speaker.

The late arriving field (otherwise known as the reverberant field) will be higher in amplitude for an omni than for all other speaker types. We perceive high levels of energy in the late arriving field as spaciousness, which is generally a good thing. Care should be taken to preserve the spectral balance of the late arriving field with all speaker types, but this is not a design consideration specific to omnis.

Thanks for reading this article, and next time we'll look at dipoles! Be sure to leave your thoughts and questions below.

Speaker off axis: coaxial speakers

This blog article is the fifth in a series on speaker directivity and off axis response. This article will consider the off axis response of coaxial speakers and consequences for acoustic design. Previous articles in the series established the psychoacoustic as well as subjective importance of speaker off axis response, different ways to measure speaker directivity, the off axis characteristics of waveguided and non-waveguided forward firing cone / dome speakers and the off axis characteristics of constant directivity horn waveguide speakers.

 

A coaxial speaker is one where two or more drive units are physically co-located such that the sound they radiate comes from the same point in space. The most typical arrangement is a coincident midrange and treble, as seen in speakers from KEF, TAD, Tannoy and others.

 

KEF LS50

KEF LS50

 

Consistency of both vertical and lateral off axis: a primary characteristic of coaxials

The major advantage of a coaxial is off axis response consistency in both lateral and vertical planes. Speakers using a vertically stacked driver array can be engineered to measure extremely well laterally off axis but as soon as one moves into the any other plane there will be some lobing resulting from phase cancellation due to path length differences between the two drivers. This can be seen by comparing the vertical off axis measurements of a coaxial and conventional speaker as shown in the KEF LS50 and YG Anat III measurements below.

 

KEF LS50 vertical off axis

KEF LS50 vertical off axis

YG Anat III vertical off axis

YG Anat III vertical off axis

Acoustically the vertical lobing seen in any conventional vertically stacked speaker means early reflections from the ceiling will timbrally distort the sound perceived at the listening position. What's more the ceiling reflection in most rooms is often high in amplitude since it is often closer than the side walls. Aggressive treatment, typically using absorbers, of the vertical reflection is therefore required to prevent timbral distortion in rooms with lowish ceilings. In rooms with higher ceilings the reflection can be redirected such that it becomes part of the late arriving field which contributes less to perceived sound. Coaxial speakers allow more freedom in terms of dealing with the ceiling reflection, and could be left untreated.

 

Midrange as waveguide: a secondary characteristic of coaxial speakers

A second characteristic of some coaxial speakers is that the midrange cone acts as a waveguide for the tweeter, which results in matched directivity. I say some because not all coaxial speakers use the mouth of the midrange cone in the same way.

Coaxial drivers using tweeters with small voice coils can have the tweeter physically co-located with the midrange voice coil. KEF uses this approach in their Uni-Q drivers. The Uni-Q also shows evidence of great care taken to reduce discontinuities at the midrange surround which can cause issues.

KEF Uni-Q

KEF Uni-Q

Drivers using compression drivers in coaxial arrangements typically have them behind the voicecoil of the midrange. This can be clearly seen in the B&C driver below [as used in Seaton's Catalyst speakers].

B&C 8" coaxial driver.

B&C 8" coaxial driver.

Some manufacturers such as Fulcrum Acoustics have used novel approaches in order to improve their performance and remove perceived issues with the design. Fulcrum list distortion of the tweeter's output due to modulation by the movement of the woofer at higher volume levels as one of the principal issues with 'normal' coaxials - see this page for more details on issues with 'normal' professional coaxial drivers.

Fulcrum Acoustics Coaxial

Fulcrum Acoustics Coaxial

That wraps up our look at coaxial speakers. Well engineered coaxials are probably the easiest speaker type to design for acoustically because of their superb off axis performance. They don't offer as much pattern control as some waveguides but their even, relatively wide dispersion makes acoustic treatment design straightforward.

In future articles we'll look at other speaker designs such as dipoles and CBTs. In the meantime please leave any comments you have below!

Speaker off axis: constant directivity horn waveguide speakers

This blog article is the fourth in a series on speaker directivity and off axis response. This article will consider the off axis response of constant directivity speakers and consequences for acoustic design. Previous articles in the series established the psychoacoustic as well as subjective importance of speaker off axis response, different ways to measure speaker directivity and the off axis characteristics of waveguided and non-waveguided forward firing cone / dome speakers.

Many speakers are labeled "constant directivity" because the term seems have become cool in recent years. True constant directivity (I'll use the abbreviation CD from now on), however, would mean that the directivity of the speaker does not change from the lowest frequency all the way to the highest frequency. It would also mean that the directivity is constant in the horizontal and vertical planes. The only true constant directivity speaker would be a full range, point source hemispherically radiating speaker, something that does not exist in reality.

So what we have left are quasi-CD speakers. We'll look at the two way designs that have become popular in home theater recently in this article and look at other design approaches such as Constant Bandwidth Transducers (CBTs) and dipoles in future articles.

 

Constant directivity speakers using waveguides

The design of these speakers takes the form of a woofer with a horn loaded tweeter. The tweeter is generally a compression driver, but does not have to be. There are a few different types of horns, but the one we are interested in is the constant directivity horn. This type of horn is actually often called a waveguide and is just a larger version of those found on forward firing cone / dome speakers. Geddes has written a nice paper that explains what a waveguide is and how it differs in theory from a horn.

The larger the waveguide the lower in frequency the directional control extends to. These speakers are often two ways with a relatively large woofer. The woofer is run up much higher than it would be in a cone/dome speaker and is purposely used to the point at which its response beams. The narrowed response is then matched to the controlled directivity provided by the waveguided compression driver. The theoretical directivity design model for this kind of speaker is wide directivity at low frequencies that narrows as the woofer starts to beam and then stays constant throughout the rest of the frequency range.

Procella P8 - a good example of the CD breed

Procella P8 - a good example of the CD breed

These types of speakers have found their niche in high performance home theater. The compression driver used in the waveguide provides very high sensitivity and therefore does not need to dissipate a lot of power to hit cinema SPLs. For Acoustic Frontiers high performance home theater at this point equals constant directivity compression driver based speakers. That really narrows down the list of manufacturers! Companies that design speakers like this are Pro Audio Technology (formerly Professional Home Cinema), Procella (the brand we love and recommend) and the JTR Noesis line. If you want to DIY check out DIYSoundGroup, who provide flat packs and SEOS CD horns. Seaton speakers do use compression drivers but they don't use true waveguides (they fit into the coaxial driver category).

SPL vs frequency vs lateral off axis angle plot for the Geddes Abbey 12. The black line is the -6dB line. You can see the basic characteristics I referred to earlier - directivity narrows as the woofer starts to beam then stays constant from 1.5kHz through 10kHz.

SPL vs frequency vs lateral off axis angle plot for the Geddes Abbey 12. The black line is the -6dB line. You can see the basic characteristics I referred to earlier - directivity narrows as the woofer starts to beam then stays constant from 1.5kHz through 10kHz.

With these types of speakers the larger the woofer, the larger the waveguide and the lower down in frequency directional control extends. Another example of a well executed CD speaker is JBL's M2 speaker featuring their strangely shaped Image Control Waveguide. Mix Magazine published an article about the story behind the M2 that is worth reading.

JBL's image control waveguide, as featured in the M2 and Studio line.

JBL's image control waveguide, as featured in the M2 and Studio line.

We published this graph earlier, but here it is again: the JBL M2 off axis measurements. You can see by the shape of the off axis curves that the crossover to the waveguide is around 800Hz. Above this frequency the directivity, and therefore off axis spectral balance, is constant through 8kHz or so.

Off axis measurements for JBL's M2

Off axis measurements for JBL's M2

 

Room acoustic implications of constant directivity waveguided speakers

CD waveguided speakers, like the JBL M2 and Procella P8, have some important implications for room acoustics. The off axis sound, at least in the range the waveguide is working, has similar spectral content but it lower in level than the direct sound. Reflections from major boundaries in the room are therefore lower in level than they would be with a wide directivity speaker such as a cone/dome speaker and have fewer perceptual effects. Below the waveguide transition frequency the spectral content of the reflected sound will have a different composition to the direct sound, which may cause unwanted effects. However Floyd Toole's research shows that image shift, source broadening and timbral coloration from reflections are primarily related to frequencies above 1kHz so maybe the CD design is solid if large enough to provide control down to a low enough frequency.

We would expect the frequency response at the listening position to exhibit a slope from bass frequencies to the point at which the waveguide takes over at which point it should flatten out. This kind of frequency response will look very different to that from a cone / dome speaker. Automated room correction routines that attempt to correct to an arbitrary target curve do not take differing speaker directivities into account, and this is a good reason not to use them unless you can manually draw the target curve. If you find one that lets you set the target curve, such as Dirac Live, a good approach is to measure the uncorrected speakers, use a high level of smoothing such as one octave to show the shape of the frequency response at the listening position, and then fit the room correction target curve to this.

60x40 coverage CD horn (18Sound XR1464).

60x40 coverage CD horn (18Sound XR1464).

One negative of CD speakers is coverage. Some speakers, such as the JTR Noesis are using very narrow coverage waveguides, with only 60 degrees coverage to the -6dB point (i.e. 30 degrees laterally off axis). When used as screen channel speakers in a home theater this means that at minimum the speakers need to be toed in to provide consistent mid / high frequencies across a row of seats in a home theater.  In some wider home theaters the speakers may not have wide enough coverage. When used as surround speakers each row generally needs its own speaker due to narrow coverage. This limitation can be worked around, but needs proper coverage analysis during theater layout. Other speakers use much wider dispersion waveguides, such as the apparent 120 degrees in the M2, or the 90 degrees in the Procella P6.

In future articles we'll look at other speaker designs such as coaxials, dipoles and CBTs!