The effect distance from a wall has on sound

[ukmusicman]

My Conn 650 has had an issue where some pedal notes play louder than others. At first I put it down to faulty electronics somewhere in the system. But I eventually found that the distance the organ was from my brick chimney stack altered the volume of some notes. When placed very close, 5 inches from the wall, the second octave D and D# were more boomy than all the others, which spoiled the even distribution of the sound. After a little experiment moving the organ back and forth from the wall (I have it on a wheeled platform), I found that the ideal position to achieve fairly good even bass sound was about 23 inches, give or take, from the wall to the back of the organ. It's a bit of a pain, but it's the only way to get a fairly even bass sound. Do any of you sound tech guys know why this is? And what I might be able to do about it. This might be of interest to other members.

[toodles]

The sound waves hit the wall and are reflected, at some frequencies this reflection will be in phase with the direct sound of the speaker making the sound louder, and at other frequencies the reflection will be out of phase with the direct sound, nulling out the sound. Here's a video demonstrating this: https://www.youtube.com/watch?v=b87QZtYKmqo

Note, also, that the reflection from corner walls and ceilings can also support or diminish certain frequencies. The only way to do much about it is to choose the placement in the room where the frequency response is most even. External speakers make this much easier to deal with.

This is a phenomenon that happens with all things that travel in waves--whether it is sound waves or ocean waves. It' can occur with light waves, , often demonstrated with an optical flat and a monochromatic source of light--dust on a flat surface will raise the optical flat slightly in a fashion not parallel to the surface, and you can see dark and light bands on the surface as a result. See: https://en.wikipedia.org/wiki/Optical_flat

[myorgan]

Great video, Toodles! Thank you for sharing it! I may use this with my students.

Michael

[ukmusicman]

Thanks for that. Very interesting, also the links. This begs the question, is there a case for individual volume controls for each note so that an organ could be set up for the room it is situated in? I don't know how that could be done. Just a thought. I've been wondering if the symmetry pots on the bass generator boards could have a bearing on this issue. Could these be to try to even out the relationship between the notes. Does any one know exactly what they are supposed to do? They are mentioned in the maintenance manual, but not what they are for, except that they have something to do with the pedal Bourdons.

[andyg]

There might be a case for individual volume controls, but it's not something that's been done (well, perhaps in some higher end classical or theatre models).

Room acoustics will always have a bearing on things, and I always recommended people not to place the instrument directly against a wall or diagonally across a corner. Standing waves on bass notes can be an annoying problem. My Roland's woofer fires straight across my lounge and down my hallway to the porch door. I have a standing wave on G and G#. Nothing I can do with the organ, it can't go anywhere else. I could perhaps reinstate the lounge door that was once there and shut it, but we like the open plan feel without it!

Sometimes just an inch or two can do the trick, sometimes it needs a foot or two to produce the desired result.

[myorgan]

This begs the question, is there a case for individual volume controls for each note so that an organ could be set up for the room it is situated in?

UKmusicman,

My immediate gut feeling is, "No." If you're basing your question on the video, keep in mind the demonstration was done while moving through the speaker's axis. In order to implement individual volume controls for notes, one would have to make it a moving control, depending on the angle from the speaker. I doubt it would rectify phase cancellation because no matter how loud one makes the controls, they would still be out-of-phase, and would still cancel each other out.

That's just my immediate reaction and thoughts based on your question. I'm not sure if more strategically-spaced speakers would negate the effect, but it may be worth a try.

Michael

[toodles]

The last series of Rodgers analog organs had note by note scaling (volume) adjustments for the bottom octave or two in order to accommodate room acoustics. This does not remove the requirement to place the organ for the most even results, as the boominess and nulls work with harmonics as well as fundamentals, but it can help.

In short, physics is not a science which allows any cheating. You break the rules and you suffer the consequences.

[JamesOrgan]

The last series of Rodgers analog organs had note by note scaling (volume) adjustments for the bottom octave or two in order to accommodate room acoustics. .

That is really quite amazing! Were they very expensive?

[toodles]

In terms of money at the time, yes they were expensive.

[JamesOrgan]

I guess with a modern digital organ you can just edit each note as you wish!