Archive for January 2010

Cardioid Subwoofers

January 27, 2010

A hot topic of discussion is cardioid subwoofers, so this will be the place to get that topic going. At the moment I will use this as a test to see if I can upload a few graphics from the book here.  The first will be some pics that describe the behaviour of end-fire arrays.

The above figure shows the timing chain for a set of four speakers in the end-fire configuration. The basic principle is a game of acoustic “leap frog” where the rearmost speakers jump sequentially over the front unit. The timing is set up so that all four speakers are synchronised at the front of the 1st (the rightmost in the figure) speaker. This “in phase” situation causes the signal to sum additively in this forward direction. The phase angles shown at the right for 3 different freq ranges are color coded to reflect their position on the phase cycle (green = +/- 90 deg, yellow = 90-120 deg and red = >120 degrees. The key item here is NOT the exact phase angle, but rather the amount of AGREEMENT in phase between the 4 speakers. In this case 31 Hz shows perfect agreement at 98 degrees so the addition wiull be strong. 63 Hz shows 4 speakers all synch’d at 198 deg and will achieve the same effect.  

Meanwhile in the rearward direction (shown on the left ) the timing chain reveals four speakers out of time as they move over the rearmost speaker. The 4 elements are all at different times and range over 17.4 ms apart. The phase responses also fall apart – ranging from a 1/6 cycle (65 deg at 31 Hz) to 2.16 cycles (125 Hz) and all sorts of values in between. These disparities in phase values cause the amplitude response to sum very poorly in the rearward direction, the intended result of the design.  In sync at the front – scrambled at the rear. The side directions fall somewhere between and the end result is shown in the 3 polar plots at the bottom of the chart. 

This figure shows an alternate spacing/timing configuration. Instead of a constant spacing (as the upper version shows), with a consistent delay timing, this config has a log spacing – and log adjusted timing. The leap-frog game is still played the same at the front – everybody in sync at the front cabinet, but thing play out somewhat differently at the rear. The difference is small but illustrates the options we have available to us.

This 3rd figure has a different twist to it. In this case the intent is NOT to have perfect synchronicity at the front of the array. Instead the timing sequence is set up so that they are slightly off – such that there is about a 90 degree spread at 125 Hz, 45 degree spread at 63 Hz etc.  This creates a less than perfect addition at front/center – but causes a better addition at the front corners. The result  is a flattened front and an overall triangular shape.  This configuration was shown to me by Mitchell Hart way down in Australia.

Just added first war story

January 26, 2010

I turned on the way-back machine to tell a tale from a LONG time ago. You can find it in the War Stories tab on the right

6o6

Away on vacation – will return the 24th of Jan

January 13, 2010

Heading south to get some heat. Stay – or get warm folks. See you soon

6o6

Breaking the line

January 9, 2010

A friend of mine who is not an audio engineer went to a show and later told me it sounded too loud and was unintelligible. Sound plausible? Sure. Now try this: a friend of mine went to a show and said it sounded like the line of the speakers had been broken. Still sound plausible? No?  I figure everybody knows what this sounds like since it is such a big issue for discussion. Every time I tune a system or hold a seminar someone tells me all the things I can’t do because I will break the line. Whatever you do don’t break the line because it will sound like…………………. Help me out here… sound like what?

The deaf and blind test

I invite you to blindfold me and roll me around the venue. We can listen to pink noise or any music besides Steely Dan. As we move I will use my trained ear to tell you when it gets louder/softer, more reverberant or less, brighter or duller. I can tell you specific peaks and comb filter areas, and even identify transitions between elements of the sound system or the timing and strength of echoes. This is not because I am special – any audio engineer with a trained pair of ears can do this.

Similarly you can stuff my ears and put me in a remote room in front of my FFT analyzer and I will be able to identify all of these same features as you move a measurement mic around the room. Admittedly this is not something everyone can do, but with sufficient training and experience you can. The reason is these are objective, verifiable, audible characteristics of the sound system in the space. A 6 dB level difference between two locations is not a theory – it is true or not true. It can be directly experienced and measured.

The sound of breaking line

Here is what I cannot tell you by either of the above methods: whether or not the line array theory has been violated and the line is broken. When the line breaks do we hear a snap? Does the frequency response show tear marks? These are absurd questions but please tell me: what are the tell-tale signs? Why is there such widespread fear of breaking the line? The best I can figure is that it is the fear of breaking the party line, as much or more than the acoustical line.

There are two principal manifestations of the “Don’t’ break the line” strategy. First is the prohibition of level tapering within a multi-element array. So if it’s 6 dB too loud in the front area (a verifiable fact I can hear and measure) we should not solve this with level tapering because we will break the line (a theoretical construct that I cannot characterize sonically or measure).

The second is the prohibition of spatial separation between sections of an array. So when a balcony depth is more than half the hall depth we should not split the array into upper and lower sections (which would have measurably superior uniformity) but rather keep it together to preserve the line.

Level Tapering

Let’s start with level tapering. Let’s consider a basic arena shape in the vertical plane: Longest throw to the top – shortest to the bottom.  Is the relationship of level taper to line breaking a digital phenomenon? i.e. one or zero? Broken or unbroken? If we have 16 identical boxes in a line and they are run at the same level it is assured that our line is unbroken. If one box is down 0.1 dB is the line broken or just slightly bent? If it is broken already I have bad news: You have never heard an unbroken line, because manufacturing tolerances aren’t that good. How about variations of 1 dB? Again, nobody can deliver you 16 boxes that are within a single dB.  

 Let’s step it up: Turn the bottom box down 6 dB. This makes it effectively half a box. The loss in the lowest frequency range (where directionality is so low and wavelengths so large that they sum well) that the combined response is reduced by less than 0.25 dB. This does not seem so scary for the big picture does it? Let’s go further and reduce 3 boxes by 6 dB. The lowest frequencies now lose 1 dB. The high frequencies meanwhile are substantially reduced in the area where the bottom three boxes are pointed – such as the early rows of seats. This is a tangible benefit (up to 6 dB of HF control) for a minimal cost (1 dB loss in overall LF power).  Is the line broken now? If so, how can I tell? Is it broken everywhere or just at one place. Is it broken at all frequencies?  It should be easy to find the break point at 10 kHz, since there is enough directional control to hear the isolated areas on either side of our fault line. You could find it blind-folded or measure it with an analyzer. Do you think you can identify a break point at 100 Hz? Good luck. These large wavelengths do not turn on a dime. When people talk fretfully of breaking the line is it the VHF range they are worried about? Not from what I hear. The concerns I hear are much more about the lows than the highs.

A practical taper is usually more gradual with 1-3 dB steps being more typical and which makes it difficult to locate the transitions in the space. If we tapered the bottom 5 boxes at 3-3-3-6-6 dB we would get an overall LF loss of 1 dB, and gain substantial HF steering. The same price would be paid for a taper of 1-1-1-1-2-2-4-4 dB on the bottom 8 boxes of our 16 element array. These are just two of a myriad of options that can be employed to help tailor the response of our bent/broken system to the shape of the audience.

To offset the un-characterizable, un-measurable theoretical problems this tapering will cause there are additional benefits: besides increasing front-back level uniformity in the HF range it also improves uniformity (to a lesser extent) as frequency falls – all the way down. The dB that we lose in the low end is offset by the fact that the beam center is steered upward off the floor and therefore spread more evenly front to back. (This is covered extensively in the book Sound Systems: Design & Optimization.)

Many experienced engineers have espoused to me the perils of breaking the line. Yet all strive for front/back level uniformity. There is a limit to how much we can squeeze these systems by splay angle asymmetry alone. Once that option has been exhausted and the cluster ain’t coming back down we need to look at how much level disparity is still left. If it’s too loud in front, I vote we break the line and turn down the lower boxes. If you have a better measurable, verifiable method, I would love to hear it.

Theory world

One funny historical aspect related to this subject: 25 years ago when we started bringing FFT analyzers in to tune sound systems, many engineers would deride us. They resisted our use of an analyzer to help make decisions, saying we lived in “theory-world” and that they lived in the “real-world”. Nowadays some engineers still want to limit the actions we perform based on the analyzer but now the problem is that I live in the “real world” and they are in the “theory world.”  Funny that one eh?

We will save the balcony battles for another day……………

Line Array Tuning

January 6, 2010

I’ve opened a new page on line array tuning entitled

“The ABC’s of Line Array Tuning”   you can find it in the tab at the upper right corner.

Comments welcome

6o6