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.
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).
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.
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.
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.
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!