The 38% Rule

[Buckle-meister]

A somewhat theoretical question for you:

I have seen it written that setting the listening position 38% from the front wall yields approximately the flattest bass response, but that since this will result in sitting far too close to the display for many, 38% from the rear wall will work equally as well.

My question is: Does the 38% rule assume that the origin of sound coincides with the front wall? If so, wouldn't that imply that for speakers which were drawn away from the front wall, as many commonly are, the 38% would be measured from the front face of the speakers?

Regards

 

[j_garcia]

It entirely depends on the room. No rule will apply to every room and every speaker configuration. Because low bass information has large wavelengths (in the bottom octave, the waves will be longer than the average room), it functions a little differently than midrange and highs, which are fairly short. What happens with these long waves is, they start to intersect and overlap creating standing waves. The waves "excite" the room in a particular way based on where the source is located in the room, which is why location is pretty important for the sub, and also the size and shape of the room will affect where those standing waves collect. Placing the sub in a corner tends to cause the bass to collect more towards the center of the room, while placing the sub in the center of the room would cause the opposite - bass will collect near the center of each wall. Next comes performance of specific frequencies and whether or not collections of standing waves may cancel or exaggerate particular frequencies based on room dimensions (relative to the wavelength of a given frequency) and sub location. A perfectly square or nearly square room can be tough to get good bass in because cancellation is almost a guarantee with the reflected waves occurring at equivalent intervals. This is also the case for a room that is a 2:1 ratio, depending on the dimensions.

Placement creates a basically inverse room response, and what this in turn means is you can place the sub in your listening position and then go take response readings around the room with an SPL meter to determine the best possible locations for the sub. This is referred to as the "crawl" method, as you should be taking readings about where the sub will actually be located.

 

[Audioholics]

You want to find the standing wave modes in your room for first, second, third and fourth harmonics. Here is the formula:

freq = n1130/2D

1130 = ft/second)
where n = harmonic (1st, 2nd, 3rd, 4th, etc)
D = length of the room in a particular dimension (in feet)

So, in a 25' x 15' x 8' room you would find for harmonic frequencies:

<pre>Harmonic | L | W | H
1 22.6 37.7 70.6
2 45.2 75.3 141.2
3 67.8 113.0 211.8
4 90.4 150.6 282.4</pre>
Now check to see where the harmonics are going to collide between the different dimensions to create COMPOUNDED bass modes.

In the above example 70.6Hz for the 1st height mode harmonic and the 75.3Hz of the 2nd width mode harmonic are probabaly going to add to each other a bit, meaning that if you place a chair in those modal standing waves you're going to hear a lot of those frequencies and you are going to get some smearing effects from the lengthy decay times messing with other frequencies in the rooms.

What you read dealt with trying to place the seated position in the best place to avoid being directly in line with the standing wave modes. You're trying not to be seated in any of the modal areas in the room. What you really need to do is plot out your room to see where the real problems are and use subwoofer placement (and eventually EQ once you maximize placement) to lessen the effects of standing waves.

1st harmonic mode is 1/2 way into room
2nd harmonic modes are 1/4 & 3/4 into room
3rd harmonic modes are 1/6 & 1/2 & 5/6 into room
4th harmonic modes are 1/8 & 3/8 & 5/8 & 7/8 into room

Cushy (frictional-only) bass traps don't work as the bass wavelengths are too long to be scrubbed into heat. Flexible walls, Helmoltz resonators (tuned to your problem areas) and diaphragmatic absorption works, however as it transfers the bass energy using resonators.

 

[Ethan Winer]

Buck:

> Does the 38% rule assume that the origin of sound coincides with the front wall? <

Yes, this "rule" is based on the wavelengths as they fit in the room, not from where they emanate (the speaker face).

JG:

> Placing the sub in a corner tends to cause the bass to collect more towards the center of the room <

The center of a room front-to-back is generally where the bass levels are weakest. My sub is in the front left corner, and I've also had it in the front right corner. In both cases the bass level is lowest exactly half way back in the room. Before I added a large number of bass traps the entire center area had a noticeable drop-out. Now with 38 traps (!) the "null zone" is only about an inch long! But it's still there.

Clint:

> You want to find the standing wave modes in your room for first, second, third and fourth harmonics. Here is the formula: <

Of course this is correct, but it's worth mentioning that you really need to measure, not just calculate, because the resonances in a real-world room can vary by quite a bit from what's predicted. In a standard sheet rock room I measured recently, the first axial length mode was 20 percent higher than predicted.

--Ethan

 

[j_garcia]

The center of a room front-to-back is generally where the bass levels are weakest. My sub is in the front left corner, and I've also had it in the front right corner. In both cases the bass level is lowest exactly half way back in the room. Before I added a large number of bass traps the entire center area had a noticeable drop-out. Now with 38 traps (!) the "null zone" is only about an inch long! But it's still there.

Generally speaking, that's where the waves will collect, however, the waves will also intersect at that point and will more likely create cancellation. In my case, I also sit just beyond the center of a 19x20 room and I have a huge null on the left side of the couch. Going to get a BFD soon to see if I can manage that, but since its a null, not sure if I am going to be able to tame my response.

 

[bpape]

The 38% rule is there for one reason - because at that position, it places your ears where there are the fewest nulls and peaks and most of the frequencies somewhat equal in terms of their variation from each other.

Yes - different rooms will place different frequencies at the peak or the null. But, ALL rooms will have a primary mode null at the half way point no matter if the room is 10' long or 100' long - just a matter of what frequency (and harmonics) are 'nulled.'

The same holds true for the other dimensions of the room. Sitting with your ears 38% from the floor will help. Sitting 38% side to side will help (though this is not usually feasible for multiple seats or due to left to right out of symmetry conditions.

 

[Ethan Winer]

JG,

> I have a huge null on the left side of the couch. Going to get a BFD soon to see if I can manage that, but since its a null, not sure if I am going to be able to tame my response. <

Right, EQ can barely make a dent in nulls, mostly because nulls are typically very deep and would require thousands of watts of power to bring back. And then a few inches away, where the null does not exist, you'd be bursting ear drums!

--Ethan

 

[Buckle-meister]

...Going to get a BFD soon.

BFD?

Regards

 

[MDS]

BFD = Behringer Feedback Destroyer, I assume.

 

[Buckle-meister]

Just Googled it. Thanks MDS.

Regards