Center channel(same as fronts) sounding MORE boomy now it 80cm off wall.

[keenly]

I have 3 PMC DB1ispeakers as LCR. https://www.stereophile.com/standloudspeakers/pmc_db1iii_loudspeaker/index.html

I have 4 RPG modex 32-500hz plates on front wall. I am experiementing with speaker position. For a while they were closer to wall, but I could not hear bass notes. Now I have center isolated 80cm off wall(modexon wall), and I am playing MC albums I know has bass guitar only in center. I can finally hear notes, but some linger and boom more than they did before. I am sitting 3.9ft from back wall in a 15x 15x8ftroom.

why would there be more boom off wall as opposed to towards it? Cheers Paul

 

[TLS Guy]

I have 3 PMC DB1ispeakers as LCR. https://www.stereophile.com/standloudspeakers/pmc_db1iii_loudspeaker/index.html

I have 4 RPG modex 32-500hz plates on front wall. I am experiementing with speaker position. For a while they were closer to wall, but I could not hear bass notes. Now I have center isolated 80cm off wall(modexon wall), and I am playing MC albums I know has bass guitar only in center. I can finally hear notes, but some linger and boom more than they did before. I am sitting 3.9ft from back wall in a 15x 15x8ftroom.

why would there be more boomhh off wall as opposed to towards it? Cheers Paul

Probably because there is a reflective wave in phase with the speaker output.

 

[William Lemmerhirt]

This image may or may not be useful but it’s the shortest way I can think of to illustrate what may be happening. While bass does collect at boundaries, and boundaries will reenforce bass notes, what I think you’re experiencing simply that after moving the speaker, you’ve changed where some of the peaks are in the room. They just happen to be where your head is at. Try moving half the distance back.

 

[TLS Guy]

Probably because there is a reflective wave in phase with the speaker output.

That speaker is unusual. It is a small labyrinth enclosure, with a line length of roughly 4.5 ft. However there is not much if any taper. PMC also tend to under damp their lines. This is very evident to me, when I have listened to their speakers.

This is the FR of your speakers as measured by Stereophile.

So they have gone for the lowest F3, which is -3db around 50 Hz. However the line is rolling off fast at that point and you can see the third harmonic at around 95 Hz is actually higher in amplitude than the fundamental. The port output with a line is quite potent and that is on the rear firing towards the wall.

Now when I go the calculations you happen to have placed your speakers just in the right spot to reinforce a reflection from that second harmonic.

I would say that speaker is prone to a bit of boomer even with optimal placement.

Those designers have not really made the choices I would have made in designing that labyrinth. The problem is they want to boast about how low that small speaker can go, and pay other penalties.

 

[keenly]

That speaker is unusual. It is a small labyrinth enclosure, with a line length of roughly 4.5 ft. However there is not much if any taper. PMC also tend to under damp their lines. This is very evident to me, when I have listened to their speakers.

This is the FR of your speakers as measured by Stereophile.

So they have gone for the lowest F3, which is -3db around 50 Hz. However the line is rolling off fast at that point and you can see the third harmonic at around 95 Hz is actually higher in amplitude than the fundamental. The port output with a line is quite potent and that is on the rear firing towards the wall.

Now when I go the calculations you happen to have placed your speakers just in the right spot to reinforce a reflection from that second harmonic.

I would say that speaker is prone to a bit of boomer even with optimal placement.

Those designers have not really made the choices I would have made in designing that labyrinth. The problem is they want to boast about how low that small speaker can go, and pay other penalties.

Thanks for input. I went through 6 lots of speakers before I settled on these(i mean within 10monthsof trying). All the others were metal tweeters and had some sibilance, these did not. I would like to change them at some point as they do not have the flattest response, however I have to say the vocals sound smooth, no sibilance or harshness(unless a crappy mix). Getting decent bass in my new room, despite spending thousands on treatment is a nightmare. It is actually depressing. Would you put deep bass traps behind speakers? The modex plates do not seem to do much.

 

[TLS Guy]

Thanks for input. I went through 6 lots of speakers before I settled on these(i mean within 10monthsof trying). All the others were metal tweeters and had some sibilance, these did not. I would like to change them at some point as they do not have the flattest response, however I have to say the vocals sound smooth, no sibilance or harshness(unless a crappy mix). Getting decent bass in my new room, despite spending thousands on treatment is a nightmare. It is actually depressing. Would you put deep bass traps behind speakers? The modex plates do not seem to do much.

Well, you can see that those speakers have a good on axis mid and high end FR response.

Now, I suspect you may have overtreated your room. I do not use any special treatments just the architecture of the room.

Actually, room treatments, I pretty much regard as futile speaker treatments.

The reason is we need an ambient field and not a dead room. The problem comes with speakers is that so many either have a very narrow response and or, and usually and the off axis response does not match the axis response. This means the ambient and direct fields do not match. The ear immediately cries foul. This is why there are so many products to make a futile attempt to try and correct rooms, for what is largely a speaker problem.

Now your speakers were designed by ex BBC engineers, and those speakers are principally for close monitoring of BBC outside broadcasts, for instance for their location broadcast trucks, like they would use to Broadcast from Kings College Chapel Cambridge for instance.

Now, this engineers are used to TLs. The BBC had legendary triamped TLs in there main studio. The design was by the consortium that worked with the Radford group so many years ago, to improve the TL. These speakers were dismantled some years ago. Electronics was by Peter Walker of Quad. The speakers have been replaced by TLs from PMC.

I have designed and been fascinated by TLs all my life. In 2000 George Augspurger published what I consider the definitive model for TLs in the AES journal.

George is now very old. However he bequeathed to me his software, and you can download it for free on my website.

TLs are tricky and difficult, they also need to be big.