Originally Posted by raynist
Thanks. I haven't listened to my new 228's yet to determine if I need more mid bass. If I do, and decide to add an MBM, could it by run as a supplement to the 228's, where the 228's are crossed at 100 and the MBM crossed at the same? Essentially the MBM would be replicating the 300-100 range that the 228's are.
There is a reduction of SPL caused by deconstruction interference between the direct sound of a speaker and the reflected sound. This cancellation is called Speaker Boundary Interference Response (SBIR) or Allison Effect. It is discussed in the Acoustical Treatments Master Thread
, at GIK Acoustics
, and elsewhere. Since the effect is based on the speaker's position, the cancellation occurs regardless of where you sit.
SBIR's effect can be calculated by the following formula: Speed of sound (1130 ft/s)/distance in ft X .25
The formula can be simplified to 282.5/distance in ft
This is 1/4 wavelength and can also be calculated using the calculator at Real Traps
and viewing the 1/4 wave frequency.
For a speaker 2 ft from the front wall, the cancellation occurs at 141.3 Hz (285.2/2).
For most people, SBIR causes issues with their midbass since the cancellation ranges from 80-200Hz.
There are several ways to reduce/eliminate SBIR:
1. Use a baffle wall. This eliminates reflected sound.
2. Use absorption on the front wall. It takes a lot of absorption to reduce reflections at the midbass frequencies.
3. Position sub(s) and speakers so they each have interference at different frequencies. Set the crossover around the 1/4 wavelength frequency. This will help get a little overlap. The problem is that many use subs with lots of excursion for low frequencies. Some of these rolloff early above 80 Hz or have lots of distortion (IMD). If playing below 40 Hz, you don't want the sub playing above 150 Hz.
4. Use an MBM in a different location and overlap the midbass frequency range. For example, you can use a 200 Hz low pass on the MBM and a 80 Hz high pass on the speaker. They will then overlap in the problem frequency range. To correct the SPL, you use a high or low shelf filter.
#4 is basically the same as using flanking subs
as advocated by Wayne Parham. In order to get the maximum midbass coverage, the driver of the MBM should be a different distance from both the front wall and side wall than the speaker. I think Wayne recommended no more than 4 ft away, though, so their aren't audible timing issues in the frequencies. The MBM under the speaker changes the speaker to floor distance, but the other two are the same unless you rotate the MBM so it faces a different direction.
Below 1/4 wavelength the direct/reflected frequencies couple and you get bass reinforcement.
This is a long way of saying that I agree with you that using an MBM can be of benefit if implemented properly.
I have a pair of dlbeck's 212HT-LP speakers and last weekend built some boxes for MBMs that will use Acoustic Elegance PB18H+ drivers. I still need to round over the edges, coat with Duratex, and load everything. I hope to take some measurements in my room demonstrating the benefit of using the MBMs. In his theater, we plan to use a 150 Hz high pass and 40 Hz low pass on the MBM's with an 80 Hz high pass on the 212HT-LP's. Below 40 Hz will be routed to two S2's.