Sub Size vs Directivity

[vrowish]

Hi all,

Bigger is better, but are there any directivity issues when putting a large sub with a small main/satellite? My understanding is that per given frequency, a large driver will beam more than a smaller one. If a large sub has to cross over high to a small main, will it draw attention to itself? Shouldn't a small speaker have a small sub? Is there a recommended sub size per lower frequency extension of a satellite?

Thanks.

[Zen Traveler]

Welcome to the forum vrowish.

I believe there are too many variables in your question to give you a concrete answer. The exact speakers in question could help with the responses. In general if you have to cross your sub higher than 80 Hz and it's considerably larger than your satellites I believe you will have the problem that you stated.

[sfogg]

" My understanding is that per given frequency, a large driver will beam more than a smaller one. "

Yes...but... beaming gets to be a problem when the wavelength of the sound the driver is reproducing is getting close to the size of the driver.

That won't happen with a sub and any normal sized woofers.

Shawn

[dragonfyr]

I think we may be confusing intensity with directivity.

Generally speaking, a LF device such as a SW exhibits a low Q polar pattern, where MF and HF exhibit higher Q polar lobing that exhibits both more lobes for the same coverage area and narrower (higher Q) lobes.

This relationship will impose a greater coverage issue on the MF/HF devices and the SW should be essentially unaffected - in other words, if 'looking' solely at the sub...it doesn't matter.

Matching intensities (SPLs) between the various components can be a more problematic issue as the localization of sound is greatly effected by relative SPL levels. And, without dealing with the extensive area of pyschoacoustics, and variables such as the potential for having multiple units addresing the same bandpass, etc., I would simply address this by matching the relative SPL levels of the various components. Observing this simple guideline, you can then focus on issues such as LF extension, etc., of the SW.

[DrWho]

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On 7/21/2005 12:21:19 PM vrowish wrote:

Hi all,

Bigger is better but are there any directivity issues when putting a large sub with a small main/satellite? My understanding is that per given frequency, a large driver will beam more than a smaller one. If a large sub has to cross over high to a small main, will it draw attention to itself? Shouldn't a small speaker have a small sub? Is there a recommended sub size per lower frequency extension of a satellite?

Thanks.

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What kind of speakers and subs are you looking at and how high will the crossver be set to? Generally, any sub crossed over above 80Hz is going to sound localized simply because your ears get more directional as frequency increases. So if you absolutely must use a high crossover, then by all means get two subwoofers and place them in the same vertical plane as your mains. Basically, the localizeability of the sub is not related to the size of its driver.

Another thing to keep in mind is that a lot of subwoofers have a high Le value, which just means they were built using a trick to extend the LF response of the system. The sacrifice being that the HF performance of the driver suffers, which isn't a concern in a typical subwoofer system crossed over at 80Hz. So when looking for a sub, pay attention to the HF response of the driver as well. You won't start noticing directivity problems until you get into the 500Hz range, which should be way higher than you plan on crossing over anyway (otherwise I would just invest in new mains).

[PrestonTom]

Although I agree with the above comments, please remember that with wavelengths this large (regardless of the cutoff) the SPLs that you are going to hear (at your chair) are highly dependent on room acoustics. Even at 100Hz the wavelength is about 10 ft and these low-frequencies are (relatively) unaffected by furniture, carpet and drapes. So the (appearent) directivity and level will be more affected by the geometry of your room & placement of the listening chair. At 100Hz and lower the directionality is not strong and typically there will be little stereo separation (i.e., the signal wil be similar in level at each speaker but will probably interact differently with the room geomerty).

Good luck,

-Tom

[vrowish]

One more question:

Main/Satellite speaker in question spec'd at: +/-2db, 70-20,000hz with a 55hz lower extension (although I don't know how far down it is at that point) and a sensitivity of 89db. Recommendations???

Thanks.

[dragonfyr]

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On 7/21/2005 4:35:30 PM PrestonTom wrote:

Although I agree with the above comments, please remember that with wavelengths this large (regardless of the cutoff) the SPLs that you are going to hear (at your chair) are highly dependent on room acoustics. Even at 100Hz the wavelength is about 10 ft and these low-frequencies are (relatively) unaffected by furniture, carpet and drapes. So the (appearent) directivity and level will be more affected by the geometry of your room & placement of the listening chair. At 100Hz and lower the directionality is not strong and typically there will be little stereo separation (i.e., the signal wil be similar in level at each speaker but will probably interact differently with the room geomerty).

Good luck,

-Tom----------------

Tom, this is intended simply as an aside comment...

You are right about the long wavelength low frequencies - which become even greater when we near 30Hz and below! But I cannot help but observe that for every claim of the inability to localize low frequency sound, that in an acoustically small room (by definition - one that lacks a reverberant field) I can without exception identify the location of the subwoofer. Now some rooms have been better then others, but the placement of subwoofers, particularly for music IS important to me. ( ...as I don't really care about the realism of an exploding building or crashing plane or exploding planet - after all, when was the last time you heard an exploding planet!?) And placing them in alignment with the mains is important to me. (And in this application separate channel information rather then an LFE is definately preferable as well. After all, all LF information is NOT mixed mono nor summed between the various channels.)

But I realize that many seem to like this with HT applications and when using the LFE out (rather then separate channel feeds with an active crossover), that if you don't mind the planet's explosion coming from behind the couch, then I can suspend my disbelief long enough to enjoy it too!

[vrowish]

Actually, I guess at this point I should move over to the sub forum for final suggestions, thanks everyone for your input.

[PrestonTom]

I think DRAGONFYR and I are in agreement. The reason I invariably bring up the issue of standing waves in low-frequency sound reproduction is because of a demo that I always did for the new lab assisstants and visitors to the lab.

I would set up a speaker in a long, narrow room that was carpeted and furnished. I would play a low-frequency sound around 100 or 200 Hz (wavelength of about 5-10 ft). The signal would either be a sustained pure tone or a very narrow band of noise. Yes, this is not the same as music but it is revealing of some of the difficulties when you put a two cabinets and a listening chair in a room.

The listener was not confused by the location of the cabinet, so there was an accurate perception of overall direction. However, they would remark the image was not punctate but rather "large" and "diffuse" when compared to a higher frequency signal. This is not unfamiliar and is in part due to the fact the sound was continuous.

The main point of the demo was to have them walk around the room (sometimes even holding a SPL meter & mic). The variation in SPL (and loudness) was huge. Even with relatively small changes in location (less than a foot) the differences could be as much as 20 dB (4 fold change in loudness).

Now to the living room.... Now if the oputput from the left speaker, due to standing waves & comb filtering, differs from that of the right speaker than the two channels can produce very different SPLs (even the they may be similiar when measured immediately at the cabinet). This, of course, is a prime cue for directionality. In the real world of listening to music in the living room (is that the real world, hmm... yeah)I don't think we are bothered all that much because the low frequecy components are directionalized but they are "diffuse", however the rest of the signal (at higher frequencies) is both "punctate" and well-directionalized. These other regions of the spectrum are what is being relied on for the perceived imaging. I guess the conclusion is: thank goodness that music is broadband.

-Tom

[Colin]

what the Dr said, except it might be as low as 40Hz, try the sub in the middle first, between the sats....