After attending the AVnu Alliance’s AVB networking technology conference prior to the InfoComm show, I canvassed the exhibit floor looking for other significant technology trends. In the area of loudspeakers for playback and sound reinforcement, I found three companies that have pursued the advancement of linear-phase sound sources. These are speakers that utilize acoustic drivers that reproduce sound in a manner that is both generally flat in their amplitude response and shift the phase (relative time delay) of the signal the minimum amount possible.

Background: A conventional cone type loudspeaker or domed compression driver diaphragm usually has an amplitude response (commonly called a frequency response) that is very irregular across the range of frequencies it can reproduce. A consequence of this is that the phase response (relative time delay) is also extremely irregular. As the amplitude response is modified by filters in attempt to correct it (i.e. flatten it), it introduces even more phase irregularities. The phase irregularities tend to smear the sound and make it less clear. Each of these driver types also tend to have a limited range of frequencies over which they can operate usefully. In order to reproduce a wide range of sound sources they have to be segregated into groups of frequencies where they operate best.

Two companies that have become more visible in the loudspeaker market are SLS Loudspeakers and Stage Accompany. They both manufacture speaker systems that utilize ribbon type high frequency drivers. Ribbon drivers have, in the past, been notoriously weak in their ability to handle high power input levels and reproduce sound at high SPL’s. These companies have succeeded in developing the ribbon driver technologies so that they are not limited to home hi-fi or near-field control booth monitors. They both utilize conventional cone-type drivers for the low frequencies, but in the vocal range, and up into the range where only dogs can hear, they can rock with the best of them without going ‘poof!’ and emanating smoke.

Having a single driver that covers the range of sound from below the vocal range to beyond 20 KHz provides a signal that is uninterrupted by crossover networks and other corrective actions. The transient response of these drivers is amazing. Cymbals and bells sound like, well . . . cymbals and bells!

The other newcomer in the industry is Sievers Audio [update: now Tectonic Audio Labs]. They have developed a flat panel driver technology that roadies are going to love. These speakers have a thin flat enclosure that is phase coherent across the entire surface (a difficult to imagine concept – the sound comes from everywhere on the face of the enclosure – not from a point source). The speaker cabinet is about 23” tall x 60” wide x 3” (yes: 3”) deep, and only weighs about 98 pounds. They can fit flat to a wall or be tilted out slightly as needed to direct the sound field. Although not quite into full manufacturing production [Update: as of 2014 they are now a buyable item], I was given the opportunity to audition the prototypes in a Black Box Theatre near the show. Black Box Theatres are notorious for bad sound due to typically having parallel walls with little or no acoustical diffusion (Who are these people that design theatres with such a bad acoustic performance anyway!? – Ahh, more blog fodder . . .). Two sets of four speaker panels provided all the sound level you could ever ask for in this space. Interestingly, the speakers are almost hemispheric in their sound output, which goes against the grain of common speaker systems design – making a very defined coverage area and combining multiple units to cover the seating area. The surprising result of this device was that an excellent stereo image was maintained throughout a very wide listening area in-spite of the room’s acoustics.

Unlike the prior two speakers that use the ribbon type drivers to provide the sound from the vocal range up to beyond audibility, the Siever devices use the flat panel technology from just above the subwoofers up to around 10 KHz, then transitions into Air Motion Transformers for the high frequencies. The AMT’s are similar to ribbon speakers in their ability to reproduce a flat, linear phase sound field. Coupling the AMTs’ to the linear phase flat panel drivers makes cross-over filter design a simple task and keeps the transition from one driver to the other mostly unnoticeable. [Update: Tectonic Audio Labs has now replaced the AMT's with high-powered Ribbon Drivers, similar to those used by SLS and Stage Accompany.]

So, what’s so special about these three speakers? Why does linear phase response make a difference? The answer is that feed-back (audio regeneration from the speakers to the microphones) is almost a non-issue. You can walk right-up to the speaker with the microphone hot and the system won’t squeal like a stuck pig. Good-bye to notch filters and grabbing a handful of knobs when an actor or presenter goes rogue. And, if you are using better microphones with flat amplitude responses, you can almost shove the mic right into the speaker without fear of sonic Armageddon. How cool is that!?

How does this affect speaker system layout in a theatre? You can place a column of these at each side of the proscenium and not worry about the actors working out in front of them, and you will get a sound field that appears to be coming from the stage instead of from above the stage like you get with flown arrays. The sonic image stays at the same elevation as the performers. Another benefit of well-designed linear phase speaker systems is that you generally don’t need to use delay speakers for balconies and under-balconies – the sound carries clearly without the need for reinforcements. One of the main reasons for using delay speakers is because turning-up the front main speakers will cause feed-back, with that problem significantly reduced, the audience can enjoy a coherent sound that more accurately appears to come from the stage and not the ceiling. The other reason these technologies seem to work well without the need for delay speakers is because the fidelity of the sound coming from the speakers is much better and ‘survives’ the abuse that the room acoustics impinges upon the sound waves (coherent sound waves reflect off of the room surfaces in a coherent manner).

Bottom Line: If you get a chance to beg, borrow, or demo a set of these speakers for a show, give them a shot and take the time to find the best positions for them in your venue. Try to restrain your use of equalization until you have worked through a few rehearsals with them and have gotten to know them in their raw state. Less EQ is generally better for this type of system when it comes to feed-back. Adding EQ will usually make the problem worse, not better.

Note: This review of these three particular loudspeaker technologies does not constitute an endorsement or advertizing plug by the publisher or the blogger, they merely exemplify the development of tools that can potentially make sound system design and operation a more pleasant experience for the audience, staff, and performers. Better sound makes better theatre.