Rear Ported Speakers, Backs to Wall?

BEWARE of guys that spell "bass" base!!

The Linn Kan's (LS 3/5 size) were design to work on the back wall. Cain & Cain is making a speaker called "The Wall Of Sound" that was made to be close to the back wall. It comes in three sizes. The one in the photo is the middle size. Around $4000. It's on the front page of their web.

Port location doesn't matter as long as you have a few inches of clearance to avoid slot loading and aerodynamic noise.

Speaker location matters a lot, arguably as much as the equipment. You get a substantial increase in output (perhaps 5dB out of a theoretical 6dB with infinitely rigid walls) as you halve the space a speaker operates into. This occurs as you get within 1/8th wavelength of the front wall.

With a foot from the wall to the front of the speaker, you're going to have 3-4 times the energy at 140Hz on down that you'd have with a free-standing placement. You'll have some boost at the bottom end on male vocals that makes guys sound like they have chest colds. If the speakers weren't specifically designed to be placed near a wall they will be noticeably bass heavy and boomy.

This can be fixed with a shelving high-pass filter on a parametric equalizer.

The other problem you're going to have is with constructive and destructive interferance like the quarter wave cancellation noted by Rich. It's not just at one frequency - you'll get suckout at odd multiples of the first frequency that decrease in magnitude as the speaker becomes more directional with increasing frequency. You can't fix this with equalization.

While conventional speakers only have drivers on one side, the wavelengths involved are large compared to any domestically acceptable speaker (a 100Hz bass wave is over 11 feet long, and a 1KHz wave is still a foot long) so they wrap arround the speaker and interact with anything to the sides and rear.

IMNSHO, this means that there are exactly two reasonable ways to use conventional speakers:

1. Well away from any walls. The greater of 4' off the front-wall and half the distance to the listener is a nice starting point although more is better. This means the reflections have 1/4 the power of the direct sound, can't tweak it more than 1dB, and are delayed 7ms so your brain interprets them as ambiance instead of confusing it with the direct sound.

2. In the walls. There is no separate front-wall reflection to cause problems.

Controlled directivity speakers are another solution - waveguides and cardioids lack rearward radiation. With their off-axis null dipoles are more tolerant of being close to the side walls.

with that kind of electronic, I would suggest the psb m2 monitors on stands. They're front-ported and fantastic sounding. Astounding value used.

Totem Acoustics Arros are rear ported and can be placed within 6 inches from the rear wall. Check out Totem's website under FAQ.

It may not be optimum but you can still get good results will rear ported speakers.

As stated in previous posts, rear wall placement almost always requires sealed box/acoustic suspension speakers or front ported speakers.

Are you looking at both monitors and floorstanders? NHT speakers match up very nicely with NAD amplifiers. Try something like the SB3 (discontinued) or the new Classic 3.

Regards, Rich

Speaker placement will be all the more important with tuned port speakers, which have their own non-linear resonances. You would probably be better off with a sealed box: a design which will be more forgiving on placement.

Placing speakers anywhere close to a rear wall will boost base response by a few db, but will also cause a frequency dip from rear reflected base frequencies that cancel the forward radiating energy. This can be as much as a 20db hole in your frequency response in the base. As you place the speakers closer to the wall you will move this interference upwards in frequency and make it more evident. (This problem does not affect mid and higher frequencies as they radiate almost entirely forward from the speaker and not backwards)

If you can get your speakers 1.8 Meters from the rear wall (measured from front baffle) then you will have a dip at 50Hz ....this is a good distance to use for linear sound quality but obviously impractical in most rooms! It also means you lose a few db in base response as the reflected energy does not add to the forward radiating energy.

If you place your speakers at 50 cm from the rear wall (measured from front baffle) then the problem will occur at 170 Hz: a problem!

At a mere 25 cm from the wall then the hole in LF frequency occurs at 340 Hz: a definite problem!

Generally this means that almost no practical home speaker setup will be immune from this problem; a fact which many audiophiles are unaware of, but one of the principle reasons people keep struggling with speaker placement. Of course, some source material brings out the problem more than other material, but it is hardly practical to keep moving speakers according to what you are listening to!!

The formula for where your frequency dip/cancellation will occur is 340 / (4 * distance from front baffle to rear wall in meters).

A base trap on the wall behind the speakers can help but the only way to completely eliminate this problem is to soffit mount the speakers - just as they do in studios. This is what I have done at home.

I'll back Narrod. Also, from what I've always been taught, you should spend equal amounts on each piece in a system. More than a slight imbalance with Revel/NAD?

I suggest looking for a front ported speaker and dumping the equalizer.