Investigating Dipole Bass in a Small Room

[Eric Weitzman]

I recently became aware of a theory that says dipole speakers are incapable of producing deep bass in small rooms. Essentially, the theory asserts that below the resonant frequency defined by a room's lowest modal frequency corresponding to its longest dimension, the dipole cannot pressurize the room and so it cannot produce any bass below this frequency.

I decided to perform an experiment to test the theory. I placed a dipole speaker in a small room and measured its frequency response. The results of my experiment are at http://www.landtime.com/perm/smallroomdipolebass.htm.

- Eric

[richie00boy]

Originally posted by Eric Weitzman
http://www.landtime.com/perm/smallroomdipolebass.htm

- Eric

Link corrected

[richie00boy]

Interesting report. I would like to see details of the speaker setup used and also expansion on your 'correction for the SPL meter inaccuracies'

[Eric Weitzman]

Thanks for correcting the URL. Those darn periods. . . .......

I used a Linkwitz Orion speaker. The Orions are a three-way actively crossed-over system. The midrange and bass drivers are configured as dipoles, while the tweeter is not. The crossover frequency between mid and woofers is about 140Hz IIRC. There are two woofers mounted in an H-frame baffle. Since they are sitting on the floor, they have a 2pi radiation pattern, while the midrange is 4pi. Dipole eq, 2pi/4pi equalization, etc, is performed by the standard electronics in the Orion's crossover/EQ. In other words, the speaker was set up to play as usual for full-range music reproduction.

The speaker was placed 1.5' from the side walls, 2' from the wall in front of it, and about 3' from the wall behind. The front/back asymmetry was due to the step in the floor. I would have tried it in different positions but the options were pretty limited in this small room.

The corrections applied to the RS meter measurements are 100Hz/0db, 60Hz/+1db, 40Hz/+2db, 30Hz/+4db, and 20Hz/+9db. There are different sets of corrections floating around the net. I got this particular set from a friend who did his own analysis of the meter. In any case, the raw data shows no significant drop off in the bass below resonance.

- Eric

[richie00boy]

OK thanks for the new info. It seems you did not take into account the frequency response of the Orions themselves in the results?

[Eric Weitzman]

It seems you did not take into account the frequency response of the Orions themselves in the results?

You are correct. I saw no point in doing this for several reasons.

1. The room is swamped by modal resonances.

2. There must be significant room gain too. This is contrary to what John Kreskovsky wrote. If you read the first part of his article, he says that room gain has no effect on dipoles because it's a phenomenon caused by room pressurization -- and since dipoles cannot pressurize the room (in theory), there's no room gain to be had. That doesn't seem to be the case though.

3. The Orions roll off fairly low (-3db/30Hz and -6db/20Hz). This is well below the frequency of interest (56Hz) where theory predicts that there should be precipitous drop in bass levels. If I did adjust the figures for the Orion's rolloff, they would show even more bass production below 30Hz. What is shown, unadjusted, is sufficient to test the hypothesis.

- Eric

[nunayafb]

Couple of things you might want to consider:
1. Room gain, a room that small it is going to kick in early and give a large boost to low frequencies.
2. Distortion from the woofer, It may be very small if they are good woofers but if they produce alot of harmonic distortion when you put the 20-30hz and maybe a little higher like 40hz then you will get some higher harmonics being produced which will interact with the room the same as when you play the actual higher frequency. The spl meter does not know what frequency it's hearing, only spl. This will throw off your measurements, obviously.
3. Why did you choose such a small room,(bathroom?) why not run this experiment in the living room? The statements from the website are for realistic size rooms and Im sure he didnt feel it necessary to discuss such small areas.
4. Also keep in mind that the dipole cancelation effect is not equal in every square inch of the room. It is constantly changing from perfectly in phase(constructive interference) to perfectly out of phase(destructive interference) and everywhere in between. The importance of this is the low decay time/arrival time of the small room. A larger room will have longer arrival times(reflected and direct) and you will notice more profound cancellation. This
Java applet will help you visualise the effect, set it to room mode, 1source/1freq, with boundary, set freq so it appear to be about 10- 20 pulse emissions per second(youll see what I mean),3d mode helps but not necessary.

[richie00boy]

Originally posted by Eric Weitzman
If I did adjust the figures for the Orion's rolloff, they would show even more bass production below 30Hz. What is shown, unadjusted, is sufficient to test the hypothesis.

- Eric

I do tend to agree with that statement.

[Eric Weitzman]

Originally posted by nunayafb 1. Room gain, a room that small it is going to kick in early and give a large boost to low frequencies.

Unless I misunderstood John's articles on his web site, the underlying rationale for the theory is that a dipole cannot pressurize the room and that room gain is a due to room pressurization. So if room gain is present and accounts for the measured response, the theory is based on an incorrect premise.

Originally posted by nunayafb 2. Distortion from the woofer ...

I believe the XLS drivers were chosen for the Orions in part because of their very low distortion.

Originally posted by nunayafb 3. Why did you choose such a small room,(bathroom?) why not run this experiment in the living room? The statements from the website are for realistic size rooms and Im sure he didnt feel it necessary to discuss such small areas.

Small is small, isn't it? Shouldn't the physics be the same in a small room or smaller room?

Actually, it would take some extremely large dipoles to test the hypothesis in a "normal" living room. I just did the calculations for a 20x16x8 room. The transverse distance is about 26' corresponding to a resonant frequency of 22Hz. Is it of any practical significance to anyone if a dipole can or cannot produce high SPL bass below 22Hz? Virtually no speakers can, other than subwoofers of course.

By choosing the smallest room in my house, the resonant frequency could be pushed up well above the point where the speakers roll off. My living room is much too large for the experiment because the lowest resonant frequency there is about 12Hz.

Originally posted by nunayafb 4. Also keep in mind that the dipole cancelation effect is not equal in every square inch of the room.

The room clearly has lots of modes and irregularities in its response, perhaps some of it due to unequal dipole cancellation. I wouldn't want to listen in there, that's for sure. Still, I don't see how this changes anything about the hypothesis.

- Eric

[Rudolf]

Eric,

are you going to measure the sound profile of your bathroom or the validity of John K´s theory?

I believe that this missing (or not missing) room gain of the dipole speaker should not be measured against the anechoic frequency response of that speaker but relative to the room gain of a conventional loudspeaker, preferably a closed box, in the same room.

You surely have noticed that John K. did this in all his diagrams. Would be nice to see that comparison in your experiment too.

I have heard a few comments of dipole owners who found their speakers working "according to John" in their rooms. So I am not yet a believer in your results.

But thanks of course for looking deeper into this problem.

Rudolf

BTW: Are your room dimensions really translating to 1.12x1.65x2.25 m? If that is possibly true you could not honestly expect a speaker with the size of the Orion to work as a dipole in that space. It could behave more like an infinite baffle with some big leaks inside.