9 replies to this topic

[_Anthony_]

After asking a question about arrays, I want to know how this is helpful in a car. I understand that the SPL levels will be greater and that there is less chance of distortion in higher volume levels, however that's not the part I'm interested in at the moment.

How can beaming be used to good effect in system design? My understanding of the effect is that frequency signals travel in a straight line rather than in a wave form from the cone itself. Is my understanding correct/ incorrect? How is this used to good effect in presenting a wide and open sound stage with excellent imaging? I'm gradually going to redesign my system, however I'm looking at getting the design correct so to give me the best sound possible.

[~thematt~]

In an environment as bad as a car, Arrays and waveguides are the Golden goose. Unfortunately, not enough people really understand them to be able to enjoy the benefits.

When a speaker plays a particular lower frequency, the speaker will act like a pure point source. As the frequency increases, the ratio between wavelength and cone diameter will change and multiple point sources will begin to appear on the cone.

What happens then is, multiple point sources causes multiple 'mini' waves, and therefore interaction between these waves produces peaks and nulls. The further you move off-axis, the greater the distance between the closer and farther point sources (on the same cone) and therefore the greater the amount of nulls. When on-axis, all the point sources are approximately the same distance to the listener, and therefore peaks occur. Overall, the effect is akin to a torch-light, where right in front is the 'brightest' points, and to the sides its 'darker'. This effect is called 'Beaming', and as you can gather, is relative to the cone diameter and the wavelengths (frequencies) of sound being reproduced (hence why cone diameter has the biggest effect on off-axis response).

The main purpose of an Array, is to connect multiple drivers so closely together, they produce a single wavefront across their frequency band. Cleverly enough, this wavefront actually starts to beam at two different frequencies. First is due to the distance across the singular unit (width) and the second is the distance across the combined units (length). The overall affect is 'sharpening' your focused wave towards a particular direction.

In a car, this has incredibly wonderful benefits. Honestly, how many things in the car cause troubles with your response? Heaps yeh?! Well, arrays allow you to 'direct' the outgoing wave away from these troublesome response areas. Examples include focusing your tweeters away from the windscreen and glass, focusing your midrange directly towards the listeners etc. etc. Less early reflections in the final sound gives you sharper images, and more ambience and depth to the recording (because its these reflections that cause all your problems).

Added to that you have the benefit of multiple drivers for greater SPL (and therefore smaller sizes can be used), lower distortion and better power matches with amps (greater flexibility with impedance). If you choose something like the HAT 3" for an MTM array (for example), the results (when done properly) will be unbeatable by anyone using conventional setups.

For purely SPL reasons, multiple smaller 6.5" mids can be arrayed together to give you a massive thump in the pants. Wont go lower, but will definitely hit harder. Definitely a recommended option for those who like kickdrums! Find a medium to high Fs and high Qts driver (like the Rainbow kicks), array them and prepare to defibrilate your heart!!

Edited by ~thematt~, 26 February 2008 - 10:37 AM.

[Matt VIP]

and the drivers must be as close together as possible for the greatest benefit?

so, for example, on a car a-piller, we could use an MTM array (eg 3" midrange driver, 1" tweeter, 3" midrange) extending up the a-piller in a F/G baffle?

or would we be better off doing this on the door itself so that the door trim could be used as a baffle?


and why MTM?

[~thematt~]

The closer the drivers are to one another, the more effective the coupling.

Yes, you can most certainly use an MTM in the pillars, if you have enough room. However, we fall back to the old 'equal pathlengths' adage, where the most benefit is gained by having equal pathlengths. Something not obtainable with A-pillar MTM's, unless you are sitting in the back seat!! Also, seeing as you asked specifically about moving UP the A-pillar, naturally you are also moving TOWARDS the driver. Unless you have individual T/A per driver (which could be available on a carputer) then its not the best option.

An MTM array is simply the most common, but not the only choice. So long as the baffle is appropriate, it will work. MMTMM, MTM, TMT etc. etc. Each has its benefits/fallbacks. The best place to put an array like this though, is the good old kicks. Horizontally mount an MTM, and you could (with a bit of work) avoid the underdash problems.

[austin-towers]

The reason for use of vertically symmetrical driver layout (most commonly mtm), is because it results in zero vertical lobing error at the crossover frequency.

In regards to driver spacing, generally it is recommended ctc should be less than half the wavelength of the highest frequency expected to be reproduced, in the nearfield. In the farfield, less than the wavelength.

www.audiodiycentral.com/resource/pdf/nflawp.pdf

I can see nearfield listening as having obvious advantages for left/right amplitude differences for two seat listening(direct sound anyway), however the lines start getting unpractically long lower into the midrange. IID begins ~ 1kHz ??

[_Anthony_]

But aren't equal pathlengths impossible as well in the kicks? I will have TA shortly however.

Drivers I'm looking at have an FS of 62Hz and and total q is 0.55. Perhaps these may not be suitable as the q is lower.

[_Anthony_]

So basically the wave goes through a focused beam to the listener and the other reflected waves do not make such an impact yes?

At what frequencies is beaming of benefit on an array such as an MTM?

[~thematt~]

_Anthony_, on Feb 27 2008, 04:47 PM, said:

But aren't equal pathlengths impossible as well in the kicks? I will have TA shortly however.

Drivers I'm looking at have an FS of 62Hz and and total q is 0.55. Perhaps these may not be suitable as the q is lower.

Yes they are still impossible. You dont need EQUAL pathlengths though, simply pathlengths closer then 12 inches in difference. The closer to zero though, the more effective and realistic the soundstage.

The drivers are mids yeah? In which case, they should work fine. In fact, with a decent enclosure size, they'll work really nicely.

_Anthony_, on Feb 27 2008, 04:51 PM, said:

So basically the wave goes through a focused beam to the listener and the other reflected waves do not make such an impact yes?

At what frequencies is beaming of benefit on an array such as an MTM?

100%, yes. Beaming is a benefit at any frequencies where the wave has passed into the far-field before it contacts a reflective surface. You can estimate transition using this http://upload.wikimedia.org/wikipedia/en/3...USP-4998112.PNG

[_Anthony_]

Yep, they're 6.5 inch mids (Morel supremo's). I'm thinking of getting a second pair and mounting them in my doors. I had it rumbling around in the back of my head that there was a benefit of using arrays but I had no idea what the benefits were. Thanks for putting it in simple terms for easy understanding.

How would you define near field or far field?

[~thematt~]

In the near field, most of the energy detected is direct as opposed to reverberant. Also, the sound doesn't dissipate with distance, so it acts like a pressure chamber with all points in the field being approximately of the same energy. If a reflective surface is in this zone, the reflections combine with the direct energy fairly quickly, and are difficult to audibly seperate. This is demonstrated by having tweeters VERY close to the windscreen, and firing directly into it. The reflection becomes the 'direct' sound.

In the far field, a greater portion of the overall energy is reverberant and energy decreases with the square of distance. Involve reflective surfaces here, and you'll start to notice severe combing effects, lobing and poor frequency response due to the environment.