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Speaker off axis response: forward firing cone / dome speakers
by Nyal MellorJanuary 14, 2014
This blog article is the third in a series on speaker directivity and off axis response. Previous articles in the series established the psychoacoustic as well as subjective importance of speaker off axis response and looked at different ways to measure speaker directivity. Today we will look at the directivity characteristics of forward firing cone / dome speakers. This category of speaker design represents the vast majority of speakers manufactured today and what most people own. These speakers are comprised of two or more forward facing drivers with cones for the reproduction of low to mid frequencies and domes for the high frequencies. Variations on this approach such as dome midranges, ring radiator tweeters and the like all have similar issues. Future articles will examine other speaker designs including co-axial, dipole, dipolar hybrids and horns.
The basic cone / dome speaker: nearly always flawed lateral off axis responses
This type of speaker typically has an off axis response that deviates from the on axis response. The classic issue is mismatch of drive unit directivity at the crossover point between the midrange and tweeter. Typically what you will see is that the midrange driver starts to beam (i.e. the directivity narrows) as the frequency is increased. Yet when the tweeter is brought in its response is very wide. On axis things look good but off axis analysis reveals a lack of energy due to midrange beaming. These issues are most pronounced in speakers using 5"+ midrange drivers with 1" tweeters. A couple of examples are shown below. The left hand graph, from a StereoPhile review of the Vienna Acoustics Mahler is particularly bad. Even at relatively shallow off axis angles such as 30 degrees there is an obvious ridge at 3kHz. The right hand graph, from a StereoPhile review of the B&W Nautilus 805, is well behaved with a quite consistent off axis response to angles of around 30 degrees, after which the lack of off axis energy from the midrange becomes visible in the overall shape of the graph. From a room acoustic treatment perspective speakers with off axis performance like this force the designer's hand. The speaker on the left, the Vienna Acoustics, will sound better with the lateral reflections fully absorbed in all but the largest rooms where the speakers and listener are far from boundaries and hence reflections are low in level relative to the direct sound. In most rooms leaving the lateral early reflection points untreated would create an obviously colored sound, particularly in the region from 1-4kHz due to the significant variation in the off axis frequency response. We have more flexibility with the speaker on the right (the B&W) but leaving the lateral reflection points untreated at angles over 30 degrees would still lead to timbral coloration in the 1-4kHz range. this StereoPhile review. Your guess is as good as mine as to where the crossover point is between the midrange and the tweeter! This is a very well designed speaker with no obvious off axis issues due to mismatched midrange / tweeter directivities. The waveguide has clearly done it's job! This type of speaker gives a skilled acoustic designer huge flexibility in how they treat the room. The room's early and late reflected contributions to the measured and perceived sound can be shaped to the client's subjective preference. Early lateral reflection points can be left reflective for maximum soundstage width and envelopment or can be absorbed for more precise imaging. In the right room, where the listener is relatively far from boundaries, diffusion can be used to break up and redirect reflections.