HiWave HIBM65C20-8 Line source

[Twinmeister]

I want to build a line source array using the 3-1/2" HiWave BMR drivers, HIBM65C20-8. I have two questions. First, to obtain optimal constructive interference across the frequency spectrum in the most desired-to-be-affected frequencies, how many drivers should I use per channel? (And how far apart should they be spaced? And what IS the most desired range to have radiate as line source? I'm aware it may be impossible to achieve line source across the entire spectrum with a ""one-way"" or "full range" driver. I know the ratio between width and height affects which frequencies radiate as line source vs point source, but I lack the application know-how. I hail mostly from the pro field, does it not matter as much in home audio? I've never seen anyone concerned about doing the math in home audio applications. They just stack a bunch of drivers (or worse, a few.) and call it a "line array.")

Second, I would like to discreetly amplify each driver-- unless there will be around "hundreds" of them. I know it will be costly, but if I can manage a reasonable amount of drivers (see above question) I would like to try. Should I use a pre-amp before splitting the signal running to each amp, or will simply splitting the line have minimal effects on signal quality? Does splitting the signal divide the line impedance? I know single ended home audio amps are considered impedance balanced. Not sure if splitting effects this "balance."

Thanks everyone! This is my first post here. I hope to stick around.

[johnnyrichards]

Re: HiWave HIBM65C20-8 Line source

Welcome!

This driver will perform well in an open back line array, as well. I am no good on advice on line arrays, but we have some of the best in the biz that pop in from time to time.

[Jim Griffin]

Re: HiWave HIBM65C20-8 Line source

These balanced mode radiators from HiWave aren't known for their high sensitivity but if you array enough of them, that will mitigate some of that concern. There is a builder from the British Isles (Derek from Overkill Audio) who has built an array with a neo magnet (higher sensitivity) version of this driver. But those were special order and a more expensive array than you likely wish to consider. It is interesting to read his experiences and observations. Notice that he is dabbles into commercial ventures so his comments may have a more optimistic slant than an unbiased person. He uses a DEQX DSP unit to optimize the frequency response.

My suggestion is to build a 16 driver per side array wired in 4 series and then 4 parallel groups for an 8 ohms impedance. Space the drivers as close as possible so that the center-to-center distance is minimized. You'll have dynamic range to die for and improve the sensitivity into the 90's dB SPL. The downside is that without tweeters, the high frequencies will roll off so you may wish to add a line of small tweeters if you wish to have optimal performance.

Google 'bmr line array' for some leads on what others have done.

Read my near field line array line paper for design details on straight line arrays at:

http://audioroundtable.com/misc/nflawp.pdf


Good luck,

Jim

[Twinmeister]

Re: HiWave HIBM65C20-8 Line source

Thank you for the information Jim! So far I have read your PDF once. It was extremely enlightening. I don't think I got everything on the first read-through however, I'll give it another go. You have a superlative vocabulary. My main question wasn't directly answered, however, I believe. I understand infinite height is the "best," and that as you go lower in frequency height matters more, conversely as you increase in frequency height matters less. I also picked up that a second-best to infinite height is an array that reaches from floor to ceiling, thus obtaining the best coupling with said surfaces.

But this is my first line array project, I would like to start a little smaller than 6 feet tall. My unspoken vision for this project was something that could sit on my desk at my computer. Now, certainly that would be extremely near field. I supposed it would have been beneficial to have previously mentioned that. I have the impression the answer to my main question is linked with the distance at one will be listening to the array. So if my understanding is correct, (please correct me if I'm wrong) if I'll be listening at my desktop less than 3 feet away, I can get away with a much shorter (and thus cheaper) array. Is this correct?

You mentioned the highs might roll off... In your PDF it sounded more like they'll cause utter chaos and wreak havoc. I was previously aware this may be an issue. However, I was somewhat hopeful for the BMRs, as they have superior off axis radiation and their over all geometry is flat. Am I mistaken to think the planer-like radiator is of any benefit? Possibly that it might "fake" being a driver of "smaller" diameter, thus allowing the higher frequencies to couple more appropriately. But perhaps that thought is in vain.

I have yet to Google around or look up the guy with the neodymium version. All of my (little) spare time went to that PDF. I will follow through on that soon.

Thanks!
-Strunk

[duanebro]

Re: HiWave HIBM65C20-8 Line source

I have 4 3" drivers on an open baffle in my bedroom. While I'm not sure that makes a line array, once eq'ed flat they sound very nice. Fr is 100 - 10khz. The nice thing is that this can be done very cheaply. I don't see an advantage of amping each driver unless you can setup a delay to make them act as a curved array. You could apply shading this way but i don't think that that is a big deal on a very short array - and it can be done much cheaper passively.

Have fun and let us know how you like them.

[Jim Griffin]

Re: HiWave HIBM65C20-8 Line source

Strunk,

You can start smaller--perhaps 4 or 9 drivers in each line. Comb lining can be mitigated somewhat by spacing the array (too close may be a issue if you move around a lot). I haven't heard the balanced mode radiators but they do feature more even dispersion across their aperture. Hence, they will would mitigate comb lines but some cross interference will still be possible.

A open baffle as suggested by Duane would be easy to build. But you would need a sub to supply the lows and you would have to place the arrays away from the wall behind them to minimize back wall reflections. You can find a set-up that will sound good by experimenting a bit. A treble control can help with high frequencies and such.

Jim

[Twinmeister]

Re: HiWave HIBM65C20-8 Line source

Thanks for the feedback guys. I'm not concerned about build difficulty. I am sufficiently skilled to build as complex of a design we can dream up. Your informative paper was on monopole line arrays Jim, so I am unaware of the ramifications of an open baffle design. I am willing to consider it by all means, but...

I am secretly in love with push-pull isobaric configuration. I built a quad 18" driver dual isobaric push-pull pair sub, and I fell in love with it. I recently experimented on push-pull isobaric with a pair of small full range drivers, and found the pair sounded better than ever vs the single driver. Here's the trick I found though, in both cases of the 18" sub and 3" full range, the front and back drivers must be discreetly amplified to gain the most immediately noticeable benefit. Both clarity and bass extension I found to increase with discreet amplification.

So ultimately, I am looking to crate a push-pull isobaric line source array with these HiWaves. One could argue, buy drivers that cost twice as much and forgo push-pull isobaric. But wheres the sense of fun and awesome in that? (Also, not sure if moving from a $7 driver to a $14 driver is going to be that much more impressive anyway, where as I know first-hand the potential of push-pull isobaric.)

-Strunk
(My first name is David, but its such a common name I figured signing it on a forum would be practically meaningless.)

[duanebro]

Re: HiWave HIBM65C20-8 Line source

Im not up on push pull. But to me i would have to compare a push pull pair to a pair of drivers... also i am using ob because i had some mdf, a hole saw and a case of drivers staring at me. I will be using 18-20 per side in a sealed box soonish. I just posted that to say that it can sound good with out being huge. One thing to note is you need to be at the same height as the array. Highs drop off big time above and below them.

[Twinmeister]

Re: HiWave HIBM65C20-8 Line source

also i am using ob because i had some mdf, a hole saw and a case of drivers staring at me.

I would do the same! Experiment a lot as soon as possible. Which drivers are you using? I don't think you mentioned that.

Technically, all frequencies should drop off above and below a "perfect" line array. In a "realistic" one, the lower frequencies drop off less and less above and below the array as you move up in the frequency spectrum. This is a function of array height, but I'm still trying to figure out if it's also a function of listening distance. It seems so, but no one has answered my question yet.

I would be really interested to know, duanebro, when you build your taller array with the same drivers, will the perceived frequency point that drops off above and below the array change? In other words, will now the midrange also drop off above and below ? Or even better, will the midbass drop off? Perhaps the highs will still noticeably drop off sharper due to comb filtering, and this is what you're hearing at present vs the effect of the line array.

If I may reiterate my desire for clarity, I want to create a line source array where I can listen to as close to the full range frequency spectrum in nearfield (Fresnel, 3db decrease per doubling of distance.) as possible. I now understand this only concerns lower frequencies, high end frequencies can be effected by lobing. I lack clarity in knowing if minimum listening distance can minimise required design height. Or, if there is some universal minimum which exactly corelates to a spesific frequency, aka, x-height will get you y-minimum frequency at nearfield radiation reguardless of distance from array-- even at zero distance.

D-Strunk

[Twinmeister]

Re: HiWave HIBM65C20-8 Line source

I've just about convinced myself to use tweeters. I've found these which seem ideal to me. I like what I read about them. (Well, what I read about their faceplated brothers.)

I also love planer and ribbon tweeters. I was doing some reading around forums, and I came across a thread which mentioned the original Heil AMT tweeters. People have said they're hard to cross over to, but in fact it's the fault of the piston driver they're trying to cross to, not the AMT. It seems conventional piston drivers are too slow in comparison to the AMT-- Have too much moving mass. I read that a team developed a piston type speaker to overcome this. The driver was light and fast enough to keep up with the AMT, and it has a flat frequency response past 20kHz to testify to this.

I rather like that concept. If a driver is light and fast enough to play through 20Khz and beyond, it'll be all the more articulate in the designated passband. In other words, use a full range driver as a midbass. That being said, I think I'll still use the HiWave BMR drivers in my linesource array, but cross them over to a tweeter. That ring radiator tweeter, if I can do it.

The BMRs have an impedance peek and frequency dip in the 1.6k area. I would like to avoid this by crossing over at or below this point. The ring radiator tweeters have an Fs of 1030Hz, and a recommended crossover of 2k. However, this is a line array design, and I have the impression because of the lower power seen by each driver, I'll be able to cross the tweeters over at a lower point. Is this accurate? Does anyone speculate I'll be able to cross them over in such a way to avoid the impedance peek of the BMRs?

D-Strunk