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-6db below 150 Hz

Cote Dazur

Cote Dazur

pfm Member
When a speaker manufacturer says that a speaker is designed to be placed against a wall, does it implies that this speaker has been designed with a with frequency reponse with a slope of -6db below 150Hz to compensate for the boundary gain?
What is specially done to a speaker to be presented as supposed to play next to a wall?
 
monitorplacement-radiation-space.jpg
James said:
It's called Baffle Step Compensation. It is effected by crossover design. The baffle step frequency depends on the size of the baffle, not fixed at 150Hz.
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Boundary gain is 6db and start to occur at around 150 Hz. That is a fact.
 
It's not so simple. Your assertion is of 6/12/18dB boundary gain is true in an infinitely large room with uninterrupted surfaces. I'm not sure where you got your 150Hz figure from. Of course loudspeakers are designed for where they are intended to be operated. I have designed loudspeakers to take advantage of boundary gain. The magic happens with deliberate driver selection and crossover design. What was the question again?
 
There are some surprises when you calculate the radiation pattern of a circular piston in a baffle.

At low frequencies below 250Hz the sound is radiated in a 360 degree pattern! I know its counter intuitive but its true. As the frequencies increase the sound pattern starts to beam in the forward direction and above 10KHz its is beamed forward.

Thus placing such a speaker close to a wall will cause the rear radiated sound to reflect forward effectively doubling the forward power below 250Hz. So you'll see figures such as +3db (10log2) or 6db (20log2) depending on context at these lower frequencies. I.e bass boost.

A loud speaker designer takes this physics into account.

This is a measured pattern from a real speaker:-

realspeaker.gif[


DV
 
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I have to say I'm slightly confused by this terminology- I thought that baffle step compensation related to the err baffle the edge of whch causes a discontinuity in dispersion.. The room gain at bass frequncies will occur irrespective of the size and shape of the loudspeaker enclosure and arises because the bass frequencies are projected omnidirecitonally and are not affected by the baffle... At higher frequencies the baffle step becomes signifcant. The two things seem to me to be different although the baffle step is affected by proximity to boundaries.
Presumably the point is that adjustign for wall placement might involve more than just adusting bass frequencies. That said would rear wall (as opposed to corner) placement affect higher frequency dispersion? I was under the impresion that they radated forward from a speaker. I pretty sure that rear wall boundary compensaiton swtiches in genelecs work pretty much as the OP suggests with a curve showing up to -6db roll off from either 100Hz or 300Hz.
https://downloads.ctfassets.net/4zj...Monitor_setup_guide_BBAGE125e-lightweight.pdf
 
Whilst obviously related, boundary gain is different from baffle step. The classic example of where two boundaries are used to "gain" bass output is the Allison / AR9 / Naim NBL designs. A loudspeaker baffle mounted flush on an infinitely large wall and well away from the floor and ceiling will radiate into 2pi space whereas a free standing loudspeaker atop a 20ft pylon in open space will radiate into 4pi space below its baffle step frequencies. The latter will lose 6dB of output below its baffle step compared to the wall mounted equivalent.

In my free space designs, I never compensate for the full 6dB. It's closer to 4dB because room gain and boundary reflections will add to that.
 
James said:
Whilst obviously related, boundary gain is different from baffle step. The classic example of where two boundaries are used to "gain" bass output is the Allison / AR9 / Naim NBL designs. A loudspeaker baffle mounted flush on an infinitely large wall and well away from the floor and ceiling will radiate into 2pi space whereas a free standing loudspeaker atop a 20ft pylon in open space will radiate into 4pi space below its baffle step frequencies. The latter will lose 6dB of output below its baffle step compared to the wall mounted equivalent.

In my free space designs, I never compensate for the full 6dB. It's closer to 4dB because room gain and boundary reflections will add to that.
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Yes that makes sense. That said the advantage of wall placement is that the bass reflection is always additive whereas in free space reflections may cancel the direct.
Presumably the degree of compansaion will also vary depending on whether you are sittign close to the speakers (in which case free space boundary efects may be bordering on irrelvant) or far away.
 
James said:
The magic happens with deliberate driver selection and crossover design.
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One would think that a speaker designed to play in free space, away from boundaries, will have a (somewhat) flat frequency response, maybe tilted.
When designing to play near one boundary, as it would be recommend in the installation instruction, using those deliberately selected driver and crossover design, what would the frequency response look like?
 
James said:
It's not so simple. Your assertion is of 6/12/18dB boundary gain is true in an infinitely large room with uninterrupted surfaces.
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Of course not, it is not that simple, not simple period. Once in a room, or should we talk about environment, the sound we hear from the speaker will be affected in many different ways also depending at what frequency (Schroeder). Where we sit will also change the outcome.
When designing for open space, most likely in the assumption that the user will try to minimize the effect from the environment, staying away from boundaries, reducing reflections, etc..., a flat frequency is most likely the goal.
But when designing for a specific location within the room, which is going to be also variable from wall construction to room dimension, etc..., etc...., what does a designer do, beside magic, to get a speaker that plays better near the front wall?
 
you could "design" a speaker with a horrible honk in the midrange like the Mk1 Kan so it has to be used against a wall.
Rgds
Stuart
 
ToTo Man said:
Only if flush-mounted in the wall.
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Well yes strictly but in practice unless your speaker is deep, you are unlikely to have significant nulls from placement against the wall because the wavelength at bass frequencies will be much greater than the distance between the driver and the wall. Genelec recommend front baffle being less than 0.6m from the wall for this reason. (Which will not null until you get to 140 Hz if my arithmetic is working today.)
 
ToTo Man said:
Only if flush-mounted in the wall.
Click to expand...
Well yes strictly but in practice unless your speaker is deep, you are unlikely to have significant nulls from placement against the wall because the wavelength at bass frequencies will be much greater than the distance between the driver and the wall. Genelec recommend front baffle being less than 0.6m from the wall for this reason. (Which will not null until you get to 140 Hz if my arithmetic is working today.)
 
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