Need help matching monitor to a room

Posted: Sun Sep 26, 2004 5:58 pm

Hi there.

I’m wanted to buy ADAM S3A monitors to my home studio.
But I’m afraid my room is too small for those....

My room is
4 meter long
2.5 meter wide
2.5 high

I have enough acoustic foam wherever I need them.

Any advice?

Thanks.

Posted: Sun Sep 26, 2004 6:51 pm

The cutoff frequency for your room is about 43 Hz, so that might be a bit too much monitor. Let's see what the other say.

Posted: Sun Sep 26, 2004 7:22 pm

David French:

I presume the calculation for the 'cutoff frequency' was something like
4 m = 13.123 ft
frequency = 1130 (ft/s) / 13.123 (ft) / 2 (half wave)
frequency = 43hz.

But what does 'cutoff frequency' have to do with a monitor ?

Posted: Sun Sep 26, 2004 7:44 pm

To my understanding, speakers have a hard time putting out frequencies in the pressure zone of a room (below the lowest resonance) . The result is a reduced ability to produce these frequencies since the speakers must work against the air in the room. Assuming all this is correct, it seems beter to chose a monitor with less low end so you're not paying for size you don't pay for size you don't need and so that you're not driving the monitors hard in an effort to hear the low end. Now, all of this is just what i've read, but I have noticed that when I hear very low bass in a very small room, it sounds very odd.

Posted: Sun Sep 26, 2004 9:37 pm

David French

Quote:

I have noticed that when I hear very low bass in a very small room, it sounds very odd.

I'm wondering if what you noticed might be a modal issue.
I can hear bass in my car, which is 6' wide, which according to the formula should cutoff at around 100hz.

Posted: Sun Sep 26, 2004 10:07 pm

David, below the cutoff frequency there just won't be any modal support. In other words, the room resonances that make some bass notes louder won't exist below that frequency. As you know, the point of preferring some room ratios over others is that with the preferred ratios, the room resonances are spread out fairly evenly.

Below the cutoff, it's not so much a case of the speakers working against the air in the room as it is a case of them just not getting any free reinforcement from room resonance below that point.

Lee

Posted: Sun Sep 26, 2004 11:54 pm

Well, I suppose what i've read is wrong. Luckily, I remember prety much everything I read and where it came from...

David Howard and James Angus in Acoustics and Psychoacoustics 1996 Focal Press wrote:

...in fact, it behaves more like the air in a bicycle pump when the end is blocked. This means that the environment 'loads' any sources of sound differently (such as loudspeakers or musical instruments), and often the effect of this loading is to reduce the ability of the source to radiate sound into the room and so result in reduced sound levels at these frequencies.

What do you guys think of this?

Lee, I thought the Schroeder Frequency was the boundary between the modal region and the diffuse field region. Am I wrong?

Posted: Mon Sep 27, 2004 8:00 am

Quote:

What do you guys think of this?

Why would this apply only to frequencies below the cutoff and not above?

By the way, what's the "it" in "...in fact, it"?

Quote:

Lee, I thought the Schroeder Frequency was the boundary between the modal region and the diffuse field region. Am I wrong?

Mybad. Not getting enough sleep turns the facts in my brain into alphabet soup. You're right. The Schroeder frequency is a crossover point. Below it, you've got discrete modes; above it, the modes are spaced evenly enough so that the room reverberation sounds fairly homogenous.

Lee[/quote]

Posted: Mon Sep 27, 2004 8:27 am

"It" refers to the room.

Quote:

Why would this apply only to frequencies below the cutoff and not above?

That's a good question that I was hoping someone like Eric could answer.

Posted: Mon Sep 27, 2004 9:13 am

David French wrote:

To my understanding, speakers have a hard time putting out frequencies in the pressure zone of a room (below the lowest resonance) . The result is a reduced ability to produce these frequencies since the speakers must work against the air in the room. Assuming all this is correct, it seems beter to chose a monitor with less low end so you're not paying for size you don't pay for size you don't need and so that you're not driving the monitors hard in an effort to hear the low end. Now, all of this is just what i've read, but I have noticed that when I hear very low bass in a very small room, it sounds very odd.

David,

I was still thinking about this hard time thing, in this sense that I wonder if one needs more energy to get the same sound pressure level.

However your comment about the room acting as a pressure room is correct. The sound wave behavior disappears and it starts acting as a pump. At about 1/10 to 1/12 of the wavelength of the resonance frequency it really is as a vessel put under compression/decompression (no directionality whatsoever and a +/- smooth pressure field).
This is why this ratio is also used in panel and Helmholtz resonators as maximum depth to make sure that the resonator really works as a mass-spring system. Once the measures of an Helmholtz or panel resonator become too large versus wavelength, they don't work very well to not at all anymore (is all gradual)

It is comparable with the air as spring in a wall, were below the first cavity resonances (comparable with room modes) the cavity works as a pressure room as well.

But anyhow the other part that it has little sense to have a lower cut-off on the monitors since the sound couldn't be or hardly be generated is not correct. Compare with low frequent sound in a car and with headphones.

One leaf of a wall bring the other leaf in vibration too even when it's a pressure room

SORRY I expressed the wavelength thing wrongly (something wrong with my sentence above so made it easy on myself):
Read as: e.g. for panel resonators:
depth <= Lambda resonance/12
This 12 is often used for Helmholz resonators as 10 = because it's no fixed boundary, see it as an empirical safety 10 to 12. For the frequencies in-between one gets a mixed behavior were nuls, by superposition (not resonant) and/or spatial level differences still occur.

Posted: Mon Sep 27, 2004 10:24 am

David French wrote:

Luckily, I remember prety much everything I read and where it came from...

And here I'm jealous as hell ........................
Wish I was you ..............
How can your head continuing containing all that stuff ??????

I often wondered, if I was capable to remember everything I read and learned how much easier things should be .......
For lots of stuff I was glad I could hold it until the exams were passed

Posted: Mon Sep 27, 2004 11:25 am

Thanks Eric for showing us the truth yet again. I guess Howard and Angus are wrong. As for the memory thing, don't be jealous; it's only been useful for trivia.

So I guess the verdict for the original poster is that he can buy any monitor he wants because there's no such thing as too much monitor for a room, correct?

Posted: Mon Sep 27, 2004 11:55 am

Hi David,

A guy (me in this case) must know his limits. The monitors themselves I leave to the experts/sound engineers.

Posted: Mon Sep 27, 2004 12:39 pm

Thanks a lot for sharing your wisdom with me.

Quote:

So I guess the verdict for the original poster is that he can buy any monitor he wants because there's no such thing as too much monitor for a room, correct?

can someone approve this theory?

Dynaudio Bm6A and 15'a is still an option for me , but i guess this conflict may move to another forum.

thanks again.

Posted: Thu Sep 30, 2004 8:25 am

eric_desart wrote:

However your comment about the room acting as a pressure room is correct. The sound wave behavior disappears and it starts acting as a pump. At about 1/10 to 1/12 of the wavelength of the resonance frequency it really is as a vessel put under compression/decompression (no directionality whatsoever and a +/- smooth pressure field).

Another useful way to think of this is in terms of reflections. Imagine that you bring your speaker close to a wall. At low frequencies the distance from the speaker to the wall is much smaller than the sound wavelengths. So the direct sound from speaker and the reflected sound from the wall are completely in phase, therefore they constructively interfere, raising the sound pressure level.

Now consider even lower frequencies where the wavelengths are much longer that any of the room dimensions. Wherever you place the speaker it is "close" to every wall in the room relative to those very long wavelengths. So the direct sound from the speaker is now completely in phase with the reflections from all the walls. This creates a very significant enhancement of the sound pressure levels at those frequencies. However, like Eric mentioned, this occurs in the frequency range where the wavelengths are several times larger than the room dimensions. For typical size rooms this effect doesn't become significant until you get down around 20Hz.

Thomas

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