Driver offset and its effect on frequency response

Aim

To decide whether to place the full range driver on the centre line of the baffle, or to offset it to the side by some amount.

Method

I decided to build two baffles, and use the measured frequency response with first the driver located on the baffle centre line, then with it offset 10cm (4 inches) to the left to decide which is the preferred solution for this particular open baffle speaker.

Using my universal baffle for locating the full range driver, I made up two baffles that were as similar as I could make them, but with the driver hole being located in the centre on one, but offset on the other.

I proceeded to measure the frequency response at 1m.

Results

On axis

Above 3kHz performance is pretty much identical. Below 3kHz the blue line (driver offset) is slightly smoother than the read line (driver centered) which has a dip at 850Hz and about 1800Hz.

15 degrees off axis

For the offset driver, whether you are listening off to the left or to the right makes a difference. The green line (listening 15 degrees to right) is generally the smoothest. The blue line (listening 15 degrees to left) has the largest peaks and dips. The red line (driver on baffle centre axis) has more peaks and dips than the green line, but they are otherwise of similar magnitude.

30 degrees off axis

The green line (30 degrees to right) is better than either the blue or red lines.

Driver on centreline of baffle

Pretty good grouping up to about 3kHz. Driver breakup visible at 10kHz and above.

Driver offset to left of baffle

Compared to the driver on the centreline of the baffle, this situation shows better grouping of on axis and off axis response up to 3kHz. Above 3kHz, no real difference.

Conclusion

The location of the driver on the baffle does indeed affect the frequency response, and quite substantially at that. In both cases however the frequency response had issues, and as is often the case chosing one over the other will have benefits in some areas, but drawbacks in another. The offset driver location showed an improved frequency response when listening off to the right side of the speaker, but a degraded response when listening off to the left side.

In the end I decided to go for the offset driver location. These speakers will be listened to from a couch positioned between the speakers, and so it will be possible to locate the couch at the sweet spot. If the speakers were required to sound good over a wider range of seating positions, including off to the outside of the sweet spot, I would have used the on axis driver as this would not suffer from the poorer left side response the off axis driver exhibited.