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Old 1st July 2006, 07:51 AM   #1
garymck is offline garymck  Australia
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Default room gain?

Hi,
as a beginner to designing subwoofers, I was wondering what level of room gain and at what frequency it starts at. I realise that this will depend on the room,, but are there any rough rules of thumb? I've read that floor adds 3db, against a wall another 3db, and corner positioning yet another 3db. Is this reasonable, and if one is modelling an enclosure what frequency should one start at allowing for room gain? - 50hz?

I previously posted about a woofer that seemed to model with extended low frequency, but with room gain at the low end, I was concerned that that it would become overpowering so I modelled it in a closed box.

a 120l enclosure shows -12db at 20hz and an f3 of 47.5, with a system q of .57

sorry for all the newbe questions.

cheers
gary
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Old 1st July 2006, 08:42 AM   #2
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You are getting room gain mixed up with boundary reinforcement.

If you imagine a speaker is radiating high up a pole in the middle of a big field there are no boundaries to reinforce it. When you place it on the floor you are radiating into half space and get +3dB gain. Add a wall perpendicular to the floor and it's quarter space and another +3dB. Finally add another wall so the speaker is now in a corner and you are in eighth space with another +3dB.

Now room gain is totally different and occurs when the wavelength being produced by the speaker is the same as a half-wavelength of the room, and the driving mode changes from wave mode to pressure mode. This can only occur if walls are solid enough and there are no leaks or openings. The curve will depend on how lossy the room is and the individual dimensions, but a well sealed room can yield 12dB/oct slope from the half-wavelength frequency.
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Old 1st July 2006, 09:08 AM   #3
Svante is offline Svante  Sweden
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Richie00boy is right in that reflections from single walls is different from the gain that occurs inside a closed cavity (=the room). Due to multiple reflectoins, specific frequencies gets amplified and also, if the cavity is airtight, there will be a boost of 12 dB/octave towards really low frequencies.

If you want to understand the gain that occurs from a corner (which is a bit different from the room gain) have a look at this thread about corner reflections
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Old 1st July 2006, 09:22 AM   #4
garymck is offline garymck  Australia
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Thanks for clearing up the difference between room gain and boundary reinforcement.

I guess the question then is, is there a frequency that boundary reinforcement starts to become significant, and should therefore be included in the modelling of a subwoofer?

Assuming I intend to place it close to the junction of a wall and the floor, would it be reasonable to use as a rough extimate 6db of reinforcement starting from 50hz on down?

cheers
gary
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Old 1st July 2006, 10:53 AM   #5
Svante is offline Svante  Sweden
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Well there are three "frequency ranges" that are important for the acoustics of a listening room.

At very low frequencies, the room serves as a big box, and yields a boost of 12 dB/octave towards low frequencies (compared to having the speaker in free space). This range at the frequency where the largest room dimension equals half a wavelength. For a room with the largest size=6 metres, this frequency becomes f=c/(2*x)=345/(2*6)=29 Hz. So this range is rarely fully developed in the audible range except for very small rooms.

The lowest room resonance occurs at this frequency, in this case 29 Hz. Above that frequency there are several resonances, and these become more and more closely spaced as the frequency increases. The peakedness of the resonances depend largely on how much damping material there is in the room. This "middle" frequency range is often problematic in normal rooms, due to the large variation in the response with frequency.

At higher frequencies the resonances appear so close together that there are several of them within the width of each individual peak. There is even a frequency defined, above which one considers the individual peaks as indistinguishable, and this frequency is called the Schröder frequency. It can be calculated from the room volume V i m³ and reverberation time T in seconds: fs=2000*sqrt(T/V), which typically ends up at a few hundred Hz in normal rooms.

Of these three ranges, the middle one typically ranging from 30-200 Hz is the most problematic. Here, individual frequencies become amplified, whereas others don't. Seen as an average, though, there is a gain in this region, and this is called room gain. The individual differences between rooms are large, often more than 10-20 dB. So, compensating for room gain is not easy, contrary to the impression that some simulation softwares seem to imply.
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Old 18th November 2009, 01:46 AM   #6
form109 is offline form109  United States
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the Room Gain in my Room is Annoying...Bass around 50-70 Hz is Greatly Accentuated and made Boomy.
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Old 18th November 2009, 03:21 AM   #7
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Oh not true, please. The problem you're probably referring to are room modes. Those are spots in the room that due to the dimensions of the space have either high or low pressure points at some frequencies. There's that old adage... great speakers in a bad room make lousy sound. I was very aware of this when I used to do tech work for recording studios. You wouldn't believe how awful most professional recording studio control rooms are. This one room, that shall remain nameless, had multiple peaks and valleys right in the mix position due to the room being a seven sided circle almost. 17' front to back and 19' to the sides. mix position was barely a foot south of the center point. Even on paper an acoustician could have seen the problem before the place was built.

F. Alton Everest wrote a great book on acoustics years back. Master Handbook of Acoustics, I think it was. I say great because at the time it was one of the first to go into detail on the LEDE principle.

I was even lucky enough to have designed a control room and went nutts on the detail to make it sound good.

Ok, trivia... What is the worst aspect of a listening environment?

For me, it's stereo image mush caused modes and/or non-equal walls that don't reinforce bass response equally (the opening to the dining room on the right vs. the closed wall on the left). as well as an rt60 that's too high or unequal versus frequency and seen by a TEF waterfall graph.

Bass reinforcement caused by the room is good thing when it is controlled. It's bad when the room is full of axial, tangential, oblique and much more complex pressure bubbles.

This one room I designed that had weird dimensions 11'x30'. Console was centered on the long side against the wall which gave an 11' depth to the listening spot. 1/2 wave build-up was around 110 hertz. So opposite each of the EV Sentry 500s mounted on the front walls, I built into the rear wall a couple of slat bass trap (Helmholtz resonators) tuned for 110 as per the fudge factors shown in that book (or was it another studio acoustics book?). Well, anyways, the sound of the NS-10s were amazing. So many people complimented me on it. There were no smeary room modes messing it all up compared to before the bass traps (and the custom made QRD).

Google for L.E.D.E. and bow in its presence and give offerings of NOS RCA tubes.
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Last edited by davygrvy; 18th November 2009 at 03:40 AM.
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Old 18th November 2009, 01:47 PM   #8
PASC is offline PASC  Brazil
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Hi!

Also value to mention other Alton Everest book, Acoustic techniques for home and studio, which I followed before of the construction of mine main room.
The golden ratio technique helps a lot.
So he also teaches us about room fundamental ressonances of length, w and height and their following harmonics in order to avoid any coincidences...

Regards,
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Old 19th November 2009, 02:14 AM   #9
form109 is offline form109  United States
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Quote:
Originally Posted by davygrvy View Post
Oh not true, please. The problem you're probably referring to are room modes. Those are spots in the room that due to the dimensions of the space have either high or low pressure points at some frequencies. There's that old adage... great speakers in a bad room make lousy sound. I was very aware of this when I used to do tech work for recording studios. You wouldn't believe how awful most professional recording studio control rooms are. This one room, that shall remain nameless, had multiple peaks and valleys right in the mix position due to the room being a seven sided circle almost. 17' front to back and 19' to the sides. mix position was barely a foot south of the center point. Even on paper an acoustician could have seen the problem before the place was built.

F. Alton Everest wrote a great book on acoustics years back. Master Handbook of Acoustics, I think it was. I say great because at the time it was one of the first to go into detail on the LEDE principle.

I was even lucky enough to have designed a control room and went nutts on the detail to make it sound good.

Ok, trivia... What is the worst aspect of a listening environment?

For me, it's stereo image mush caused modes and/or non-equal walls that don't reinforce bass response equally (the opening to the dining room on the right vs. the closed wall on the left). as well as an rt60 that's too high or unequal versus frequency and seen by a TEF waterfall graph.

Bass reinforcement caused by the room is good thing when it is controlled. It's bad when the room is full of axial, tangential, oblique and much more complex pressure bubbles.

This one room I designed that had weird dimensions 11'x30'. Console was centered on the long side against the wall which gave an 11' depth to the listening spot. 1/2 wave build-up was around 110 hertz. So opposite each of the EV Sentry 500s mounted on the front walls, I built into the rear wall a couple of slat bass trap (Helmholtz resonators) tuned for 110 as per the fudge factors shown in that book (or was it another studio acoustics book?). Well, anyways, the sound of the NS-10s were amazing. So many people complimented me on it. There were no smeary room modes messing it all up compared to before the bass traps (and the custom made QRD).

Google for L.E.D.E. and bow in its presence and give offerings of NOS RCA tubes.

my room...as mentioned before has a strong "Mode" in the 50 Hz Region a slightly larger room in the house has a Lower "Mode" around 25 Hz....larger rooms seem to have lower room modes.
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Old 19th November 2009, 02:27 AM   #10
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No, you didn't mention it before. You were calling your situation room gain. Room gain is a condition were the pressure in the room is contained below a certain point rising on a slope of 12dB/oct. That is, not peaking or nulling at anything.

Let's try again... bass traps are what you are looking for to remove the modes, because moving walls is usually not the better choice.
Click the image to open in full size.
Even after treatment, he still has problems at 60, 80 and 140 with anti-modes at 70, 95 and 180. Properly tuned and placed Helmholtz resonators can surgically remove them.

But that's just the absorption aspect. There's diffusion techniques as well.

Square room just stink. Form, read some articles or books on acoustics for listening environments.
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Last edited by davygrvy; 19th November 2009 at 02:53 AM.
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