Having owned Magnepan 3.6/R for 20 years and set them up in 3 very different listening rooms, I’ve learned a few things. I want to capture the important things here.
- Front wall: in front of the listener, behind the speakers.
- Rear wall: behind the listener, in front of the speakers.
- SBIR: speaker boundary interference response
- The total response at the listener position includes sound reflected from the front and side walls near the speaker.
- This response depends on the distance and angle of the speaker to these walls, and the treatment of those walls.
- LBIR: listener boundary interference response
- The total response at the listener position includes sound reflected from the rear and side walls near the listener.
- This response depends on the distance and angle of the listener to these walls, and the treatment of those walls.
- Speed of sound: 1130 f/s at sea level and 70*. Slower when cold, faster when warm.
All speakers are sensitive to room setup, but planars are dipoles which are more sensitive than conventional speakers. This is both a blessing and a curse. The blessing: iIf something isn’t right you can often fix it with simple rearrangement. The curse: for ideal sound, the speakers are going to be further into the room away from the walls, than conventional speakers.
All speakers (even forward-firing cones) propagate both forward and back. But a dipole’s back wave has inverted amplitude. This is often called inverted phase or 180* out of phase, which is technically inaccurate; it would have that effect for a steady-state signal like a sin wave. Yet for a constantly changing musical signal, inverted amplitude is different from being 180* out of phase.
Example 1: consider a speaker parallel to the front wall, 3′ away, which is 1/4 wavelength of 94 Hz. The back wave hits the front wall, reflects and as it passes the speaker it has traveled 1/2 wavelength, so it is 180* out of phase with the direct (non-reflected) wave from the speaker. This attenuates 94 Hz. But if the speaker is a dipole, it does the opposite (boosts) because the back wave started out with inverted amplitude, so shifting it 180* out of phase brings it back in-phase.
Conclusion: due to SBIR, dipoles boost the 1/4 wavelength frequency.
Example 2: consider what that same speaker does at 188 Hz (twice the frequency, half the wavelength). Now the 3′ distance is 1/2 wavelength, so the distance traveled is a full wavelength. A conventional speaker will boost this frequency because it’s in phase. A dipole will cut this frequency.
Conclusion: due to SBIR, dipoles cut the 1/2 wavelength frequency.
Direct vs. Reflected
Dipoles have a flatter impedance vs. frequency curve, without the strong Q resonances that conventional speakers have. This makes them a near-resistive load which is easy for amps to drive and gives them flatter phase response and group delay with a big, open, transparent sound. Conventional speakers sound thick and muddy in comparison.
With all speakers, the sound you hear is a mix of direct and reflected. With dipoles this mix has relatively more reflected, less direct. This can make them sound big and phasey in underdamped rooms. With dipoles your room typically needs more damping than it does with conventional speakers.
One way to tackle this is to damp the walls behind the speakers to reduce reflection. How much damping you need depends on the room size, shape, materials, and your personal preference. Too much damping and the dipole will sound thick & muddy like a conventional speaker.
Conclusion: in small to medium sized rooms, you will need to damp the wall behind dipoles to some extent, but not entirely. This damping must be effective down into bass frequencies, so it can’t just be acoustic foam; it must be tube traps, bass traps, etc.
This topic doesn’t at first appear to be unique to dipoles, but it turns out to have an important difference. Consider a listener 3′ in front of the rear wall. Sound from the speakers reflects from the rear wall and comes forward, having traveled 6′ when it reaches the listener again. At 94 Hz, this is half a wavelength, so it attenuates that frequency. At 188 Hz this is a full wavelength, so it boosts that frequency.
What’s different about dipoles: the LBIR and SBIR distances, when equal, negate each other’s effects. With conventional speakers, they exaggerate each other. That is: if the speakers are 3′ from the front wall and the listener is 3′ from the back wall, the dipoles give flat frequency response: SBIR cuts the same frequencies that LBIR boost. Conventional speakers give a double-sized cuts and boosts at the same frequencies.
Conclusion: when setting up dipoles in a small to medium sized rooms, try to make the LBIR and SBIR distances roughly equal.