Friday, September 16, 2005

Understanding Line Array Systems: Part 2


By John Murray

...Understanding Line Arrays> continued...


ISOPHASIC APERTURES

Isophasic aperture is my current favorite high-tech term. It describes the phase characteristic of the slot that loads the horn bell of some line array box HF sections. The perfect line array driver, particularly for very short wavelengths, is a ribbon driver like those used by SLS Loudspeakers. Compression drivers are more rugged and capable of higher output levels than a ribbon driver, but they do not have a linear phase signal at the mouth of a horn.

Ideally, the signal at both the top and bottom of the driver’s horn mouth would arrive in-phase with the signal at the center of the horn mouth to mimic the ribbon driver’s characteristic. Since the center of the horn is closer to the driver’s diaphragm than the top and bottom, the more central paths to the horn from the driver must delay the signal to arrive in phase with the longer paths to the top and bottom of the horn. There are two ways to accomplish this.

The first is to make the path length progressively longer towards the center of the horn via a phase-plug type of device. This technique was employed in the old JBL “slot tweeter” super-tweeter and was adapted by Heil in the V-DOSC system for wavelengths at 1000 Hz and up. Other line array manufacturers have employed similar devices.

The other method is to use variable density foam, which slows the speed of sound through the more dense foam medium towards the center of the horn. Electro-Voice and McCauley use this technique to provide an isophasic horn section in their line array offerings.

Perhaps the most interesting technique for an isophasic device is the patented mid-high frequency aperture by Adamson. It employs the longer path length method, and utilizes directional vanes to prevent excess vertical dispersion as well. This approach is used for both the highand mid-frequency sections of their line array systems. The mid-frequency energy exits via two vertical slots on either side of the high-frequency exit slot. The paths of the mid-frequencies curve around the HF chamber housing. All slots are isophasic.

With the slots of the MF section on each side of the HF slot, diffractional problems of each slot on the other could be very problematic. However, Brock Adamson came up with a unique solution: overlapping the crossover points between the mids and highs. This provides in-phase pressure fronts from the other slots to prevent diffractional interference in the frequency range where it would be a problem.

FREQUENCY TAPERING

The term “tapering” is also commonly called “shading.” They are essentially interchangeable. One of the first tricks used to take advantage of the line array effect was frequency tapering. My earliest exposure to this technique was the Electro-Voice LR-4B column speaker. For low/mids, it used 6-inch by 9-inch cone drivers that had lowpass filters at successively lower frequencies for speakers placed farther out to the ends of the column. This resulted in a longer column at longer wavelengths and a shorter column at shorter wavelengths, producing a similar dispersion pattern and critical distance for all frequencies, which in turns provides a more balanced frequency response at all listening distances.

AMPLITUDE SHAPING

Another tapering/shading technique is amplitude shading. This is used in many current line array products to accomplish front fill coverage where the bottom hook of a J-Array covers the extreme near-field listeners. This technique is simply lowering the volume of the loudspeakers covering the nearfield seating with respect to the longthrow loudspeakers higher in the array.

DIVERGENCE SHADING

Some line array systems offer more than one choice for vertical dispersion of the individual box elements in the array. They do this as a solution to cover the near-field and extreme nearfield seating in most venues. EAW has gone one step further by offering two different models, matching the vertical dispersion and output level so that the drivers produce equal mouth SPL throughout the array. They avoid any amplitude shading for the drivers covering the closer listeners by increasing the coverage angle of those box elements. Why is it important to avoid amplitude shading?

According to David Gunness, EAW director of research and development, whenever two wave fronts with different pressures are combined, there will be a discontinuity at the juncture of the two. This discontinuity will be audible as though it were a separate, non-coherent source (delayed loudspeaker). The result is transient smear and uneven frequency response. Divergence shading provides a wave front whose curvature varies, but whose pressure magnitude does not. Therefore there is no introduced time smear to the signal.

HORIZONTALLY SYMMETRIC ARRAYS

The majority of available line array systems are horizontally symmetric. Ideally, each band pass is a 1/2 wavelength wide strip that runs the entire length of the array. The advantage is that it avoids horizontal lobbing at the crossover-frequency band. It also requires symmetric pairs of inner mid and outer LF drivers flanking the HF sophistic ribbon.

The drawback to this approach is that for the mid-drivers to be within 1/2 wavelength of each other, they must be incorporated into the bell of the HF horn. The normal 90-degree angle causes reflections between the MF drivers and the discontinuous horn walls cause HF problems as well.

HORIZONTALLY ASYMMETRIC ARRAYS

EV, Meyer (on their smaller system), and NEXO have opted for an asymmetric design. This approach avoided the mid-frequencies in the horn bell problem and contends with the horizontal lobbing at crossover problem inherent in asymmetric designs. Choose your poison.

CARDIOID AND HYPERCARDIOID LF SECTIONS

Line arrays have great directional control in the vertical axis. Subwoofer systems, by nature of the very long wavelengths involved, do not have any directional control unless arrayed. Even then, because of the omni-directional nature of each element in the array, there is no front-to-back directionality. This causes muddiness on stage and low-frequency feedback problems. Enter cardioid and hypercardioid low-frequency sections.

Cardioid and hypercardioid loudspeaker systems are similar to microphones, just in reverse. In the case of loudspeakers, two transducers, separated by an exact distance within the enclosure, with delay on the rear driver, create the directional radiation pattern. The cardioids type has maximum level cancellation straight back at 180 degrees behind them and the hypercardioid have maximum level cancellation at about 120 degrees off-axis. As examples, Meyer employs cardioid low-frequency sections while NEXO employs the hypercardioid.

FIR-BASED VS. IIR-BASED DSP FILTERING

IIR (Infinite Impulse Response) filters in a DSP processor act just like analog crossover and equalization filters. Their amplitude and phase characteristics are in a fixed relationship. So much boost or cut produces an exact corresponding change to the phase response.

FIR (Finite Impulse Response) filters are able to manipulate phase independently of amplitude and correct for distance-related cancellations between drivers if each driver is under individual DSP control. Some systems, like Intellivox, employ separate DSP processing and amplification for each driver in the array. These types of systems will define the next big step forward in loudspeaker technology.

GET LUCKY?

So, the next time you want to impress the ladies at the local hall, tell ‘em “We’re gonna hang a logarithmicspaced, articulated spiral array in a horizontally asymmetric configuration employing frequency tapering and divergence shading, which will include isophasic high-frequency and mid-frequency apertures, hyper-cardioid low-frequency transducer sections, is controlled by finite-impulse response filtering digital signal processing, and works well with a psychoacoustic infector.” You might just get lucky…

Live Sound Technical Editor John Murray is a 26-year industry veteran working for EV, Midas, MediaMatrix and TOA. John has presented two AES papers, chaired three Syn-Aud-Con workshops and is a member of the TEF Advisory Committee and ICIA adjunct faculty. If you have a question you’d like to ask John, e-mail him at jmurray@livesoundint.com .

No comments: