Speakers and their interaction with the room

[NestorM]

I would like someone to shed some light on the subject for me.I've heard that each type of speaker interacts differently with room modes. Is this true, is there evidence or are they just assumptions?

 

[MarkS]

I would think it would depend to some extent on the locations of the bass driver(s) (including ports and/or passive radiators) on the speaker body, and of course the frequency ranges each covers.

 

[DonH56]

Where room modes matter is deep bass, at those frequencies where wavelengths are so long that the speaker looks like a point source so they all behave more or less the same. A sound wave at 100 Hz is more than 11 feet long, larger than most speakers. At higher frequencies, reflections from other surfaces matter, and different speakers have different dispersion (radiation) patterns so react with the room differently. Planar dipole speakers like ESLs and Magnepans send sound forward and back with little energy to the sides; conventional speakers send sound from the front and off to the sides (and top/bottom) to nearly 180 degrees so the room can significantly influence their sound.

There are numerous online calculators and papers showing modes for rectangular rooms. Modes in asymmetric rooms are more difficult to calculate.

 

[tmuikku]

Totally, sound is affected by the room. Its more useful to think other way around, you listen sound in room that speakers put in there. You are listening the room not speakers, unless you get the speakers close enough so that direct sound is much louder than room sound. Room modes are subsection of all the effects room has to sound, first reflections, flutter echo, spectral balance of early reflections and how long reverberance there is and so on. Then there is psychoacoustics, how sound is perceived, and all of it changes with frequency.

As sound is invisible its helpful to try and connect it to something more tangible which helps to imagine and understand it better. For example blink of an eye lasts about tenth of a second, 100ms. If two walls in your room are 5 meters apart sound travels between the two walls roughly 7 times during eye blink! As there is typically six boundaries on a room and sound reflects spectacularly number of reflections that hit ear within the eye blink are calculated in thousands. Its amazing how hearing system can weed out the message from the bombardment. There is difference how early reflections within few milliseconds affect perceived sound compared to late reverberance, direction and content of early reflections matter and so on.

Another, 5 meters is wavelength of 68Hz. There would be modes between these two boundaries 5 meters apart from half wavelength and multiples of (in frequency). Pressure minima center of the distance at 34Hz, maxima 68Hz and so on. Height of a room could be 2.5 meters making modes between floor and ceiling from 68Hz and multiples of. Basically sound of bass is dominated by room and how speakers and observation point are positioned in there, and it is just not deep bass but all the way up to kick bass >100Hz and beyond the smaller the room is. Size of the room defines density of the modes, the bigger the room the denser they become (closer in frequency). At some particular frequency tied to room size modes get so dense they are now called reverberation, this is called Schroeder frequency. Modes are easiest to visualize on computer, this example was to illustrate wavelength is the glue between sound to physical world, it ties how speakers and room interact and how you perceive it.

I hope this gets your imagination rolling. Its best to read on acoustics, plenty of books and web resources to read. Have fun!

 

[abdo123]

that each type of speaker interacts differently with room modes.

Do you mean dipoles vs monopoles or something else?

 

[NestorM]

Do you mean dipoles vs monopoles or something else?

Exact. There is a lot of text out there that says that an open cabinet influences the room less than a closed box and this in turn less than a reflex. IS THERE PROOF OF THIS, "SCIENTIFIC" INFORMATION?

 

[DVDdoug]

Exact. There is a lot of text out there that says that an open cabinet influences the room less than a closed box and this in turn less than a reflex. IS THERE PROOF OF THIS, "SCIENTIFIC" INFORMATION?

I would think the opposite since the sound coming out of the back reflects off the back wall and around the room in different ways, depending on the room.

Some people like open-back speakers but they are obviously a minority. Most good speakers on the market have an optimized sealed or ported cabinet. (Optimized for the particular driver and for what the designer wants and usually taking multiple compromises into account.)

Acoustics is a "big subject"! As tmuikku, says there are books on the subject. You can get a degree in acoustics!

Real Traps* has some good information. I think REW has some good information.

Or maybe start with understanding Standing Waves. Standing waves are most-problematic at low frequencies, especially where the direct and reflected waves cancel-out creating a null. Since the sound is (mostly) the nulls can't be corrected with EQ. (Bass traps can kill the reflected bass, smoothing-out the frequency response). Standing waves happen at other frequencies too but our brain is not "bothered" as much.



* Real Traps has a lot of good information but the are trying to sell bass traps and other acoustic treatment and they seem to advocate for a "dead room" (lots of absorption and minimal reflected sound) but not everybody wants that. Of course, a dead room does mean that the room has less effect on the sound.

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[sarumbear]

Exact. There is a lot of text out there that says that an open cabinet influences the room less than a closed box and this in turn less than a reflex. IS THERE PROOF OF THIS, "SCIENTIFIC" INFORMATION?

Not less but different.

 

[tuga]

Exact. There is a lot of text out there that says that an open cabinet influences the room less than a closed box and this in turn less than a reflex. IS THERE PROOF OF THIS, "SCIENTIFIC" INFORMATION?

Think about how the sound is propagated by different topologies and interacts with the room boundaries.

 

[DonH56]

Exact. There is a lot of text out there that says that an open cabinet influences the room less than a closed box and this in turn less than a reflex. IS THERE PROOF OF THIS, "SCIENTIFIC" INFORMATION?

Seems backwards. Planar dipoles have a figure-8 pattern for frequencies with wavelengths shorter than the panel dimensions, so do not radiate as much to the sides or top and bottom, but the back wave is as large as the front. And deep bass transitions to a point source pattern. Sealed speakers radiate from the drivers, typically the front of the box, in a pattern roughly 180 degrees across horizontally and vertically (subject to box dimensions) and narrowing as frequency goes up. They have little response behind the speaker unless there is an additional driver on the sides or back. Ported designs add radiation from the port, but that is usually tuned low for bass reinforcement. As frequencies drop below driver dimensions the radiation pattern moves more to a point source that radiates equally in all directions. Radiation patterns for speakers (among other things) are typically represented in "Pi space", the amount of a sphere the speaker covers, or angles from a centerline.

Any acoustic text will walk through all of this. I have probably listed my graduate text in a post someplace on ASR but that is not what I would recommend as a starting point...

In any reasonable listening position you should be getting mostly direct sound with reflections delayed and lower in amplitude as they lose energy interacting with surfaces (unless infinitely stiff) and due to normal wave dispersion (spreading) on their way to the listener.

I have an article about room modes and comb filter effects on ASR, see the list linked in my signature. It is not a topic I spent much time on so the articles are pretty brief; there is far more info on the Web and through various other resources.