Using a Small Sub Woofer in a small room

[MC Bodge]

I was looking for sub-woofer to accompany the Akai AM-25 2-channel amp & Mission 731(?) bookshelf speakers in my small (1.8m x 2.5m) home office.

I picked up one of these locally for a mere £5:




It is an "Optoma Dolcen & Kreiling 50 Watt Active Subwoofer".

I'm not an audiophile, I'm just interested in using it to its potential.

I have connected to the B output of my amp.

I know that they should ideally not be placed close to walls or corners. I'm a bit short on space, and currently have it on large book on the floor behind me, rather than under the desk (the desk is 1.8 x 0.9m takes up the width of the room).

I have set it to give a subtle enhancement to the bass, rather than shaking the books off the shelves.

The port is on the front and the driver grille faces the rear.
-What would be the reason for this? Instinct would tell me to have it facing the opposite way, but it may not matter.

I have had success in taming boomy bass from cheap PC subs in the past by packing or lining them with quilt insulation or stuffing/reducing the port. This sounds quite good anyway, but I may well try it.

Cheers

Edited by MC Bodge on Sunday 19th December 22:01

[Lucid_AV]

The reason the port faces forward is because for this sub that's where most of the sound will come from.

Also, from the look of the picture of the driver relative to the RCA socket, this sub will be playing at high-enough frequencies that you can tell its location anyway. The port frequency will be high, so it's pointless trying to difuse the sound from the port in an attempt to disguise the location; might as well chuck the port sound in to the room.

[MC Bodge]

Thanks

Lucid_AV said:

The reason the port faces forward is because for this sub that's where most of the sound will come from.

I'm interested to know, why would that be the case?

Lucid_AV said:

Also, from the look of the picture of the driver relative to the RCA socket, this sub will be playing at high-enough frequencies that you can tell its location anyway. The port frequency will be high, so it's pointless trying to difuse the sound from the port in an attempt to disguise the location; might as well chuck the port sound in to the room.

Is that due to the small physical size of the driver? It does seem a little directional.

So lining the inside of the port chamber might not have any useful effect? I may try it anyway, of course, as I am intending to open it up for a look inside.

This cheap little sub has been a good addition to my home office. I'm very pleased with it.

Edited by MC Bodge on Monday 20th December 12:37

[Crackie]

MC Bodge said:

Thanks

Lucid_AV said:

The reason the port faces forward is because for this sub that's where most of the sound will come from.

I'm interested to know, why would that be the case?

Lucid_AV said:

Also, from the look of the picture of the driver relative to the RCA socket, this sub will be playing at high-enough frequencies that you can tell its location anyway. The port frequency will be high, so it's pointless trying to difuse the sound from the port in an attempt to disguise the location; might as well chuck the port sound in to the room.

Is that due to the small physical size of the driver? It does seem a little directional.

So lining the inside of the port chamber might not have any useful effect? I may try it anyway, of course, as I am intending to open it up for a look inside.

This cheap little sub has been a good addition to my home office. I'm very pleased with it.

Edited by MC Bodge on Monday 20th December 12:37

A sub's port tuning frequency, and associated output frequency, is generally chosen at or very close to the driver's resonant frequency. In simple terms, the bass comes from the port and not the driver. In the 'nearfield' measurement below the red trace shows the output from the port alone and the blue trace is from the driver alone. The sound heard is a combination of the two. You can see a harmonic spike in the port output at 90Hz.



Your subwoofer is likely to have a built in filter which will only pass the lower frequencies to the driver.............however many subs still allow higher frequency harmonics s out through the port, even the good ones. It is often the case that it is the harmonics that give away the position of a subwoofer in the room even though those higher frequencies have been removed from the signal that the subwoofer amp is feeding to the subwoofer driver itself.



Black is the farfield combined response, blue is the nearfield driver response, red is the nearfield port response including the harmonics being delivered way into the midrange part of the bandwidth. You can see the 'effect' of the harmonic spike at 220Hz impacting the combined output. You can see a harmonic spike in the port output of the top measurement at 90Hz but that is from an extremely big sub.

[MC Bodge]

Thanks for the info.

The sub appears to filter out higher frequencies quite effectively. I'm assuming that it is a simple passive filter.

Switching off output A that drives the bookshelf speakers produce cws only low frequencies from the sub.

[Crackie]

MC Bodge said:

Thanks for the info.

The sub appears to filter out higher frequencies quite effectively. I'm assuming that it is a simple passive filter.

Switching off output A that drives the bookshelf speakers produce cws only low frequencies from the sub.

You're welcome.

Pleased to hear that the sub does a good job of filtering out the higher frequencies and, by the sounds of it, does a pretty good job of limiting the HF harmonics escaping from the port too.

Regarding the filter itself, it is very unlikely to be passive. The passive inductors and capacitors needed to create a filter passively at subwoofer frequencies are physically large and costly high wattage components. The inductors in particular are big coils made using very long lengths of heavy gauge copper wire.

If the crossover filtering is done actively, before the amplification is carried out, the components needed can be low cost, low voltage items; phase adjustments can easily made too.

The bass boost feature, on your sub, is another feature reserved for actively rather than passively filtered units.

[MC Bodge]

Thanks again.

I will open it up for a look inside at some point to see how it is constructed (and if I can improve it) and what components have been used.

The Bass Boost is a little excessive for my small room.

[Howard-]

To be brutally honest, that thing looks like complete and utter junk and is fit for the bin. I'd be very surprised if it could reproduce frequencies lower than your Missions can, and you're likely to be detracting from the overall sound quality by using it, not adding to it.

[MC Bodge]

Howard- said:

To be brutally honest, that thing looks like complete and utter junk and is fit for the bin. I'd be very surprised if it could reproduce frequencies lower than your Missions can, and you're likely to be detracting from the overall sound quality by using it, not adding to it.

I'm surprised that it took so long for somebody to make a comment like that.

You are wrong.

[Howard-]

MC Bodge said:

You are wrong.

I don't think I am, but I'm glad you're enjoying the sound anyway

[MC Bodge]

Howard- said:

MC Bodge said:

You are wrong.

I don't think I am, but I'm glad you're enjoying the sound anyway

For some subtle extra bass, I have added a very cheap, used, sub woofer (that does produce low frequencies, without compromising the bookshelf speakers) to an amp and speakers in a small room, streaming audio from a laptop. It does what I required.

What would throwing the sub-woofer in the bin achieve?

I have not built a special listening room, had my ears ported and polished or had my brain tuned for optimum processing.

There is a thread on here about audiophile bullsh*t, I believe.

[MC Bodge]

Interestingly, I noticed that the Mission speakers in my garage (with an NAD amp) gave a clearer, deeper, bass than the Mission bookshelf speakers in my office.

The speakers in my garage were my wife's before I met her and have grilles fitted -I had assumed that they were the same as the others, but they are actually Mission 700s (compared with 731), slightly larger and with more expensive looking cones.

I'm assuming that the 700s were a better product when new.

The 700s positioning (on shelves, 45 degrees to brick walls) may be better and the garage may have better acoustics too, of course.

700:


731:


(not my photos)

[MC Bodge]

ps. I'm very pleased with the sound from my speakers and sub-woofer in my office. My wife even commented on how good it sounded.

[IanH755]

The Mission 70x range were seriously good for bass response vs the 73x speaker range but were generally considered to be less "defined" overall.

I've had a range of Mission speakers - 703's, 705's (active subs built in) and 753's and the 70x range felt more comfortable as "mid to loudish party/home cinema" type speakers than anything else as at lower volume they were just "OK".

I sold my 705's and bought B&W 605s2's (same layout - 25mm tweeter, 165mm mid-range & 2x 200mm active subs) and the difference in clarity at lower volumes especially was very obvious whilst being fairly equal at higher volumes.

TBH I preferred the B&W's for most stuff but on the rare occasions I could use them for a louder party (amp volume over halfway) the Missions sounded better to my ears.

[MC Bodge]

IanH755 said:

The Mission 70x range were seriously good for bass response vs the 73x speaker range but were generally considered to be less "defined" overall.

I've had a range of Mission speakers - 703's, 705's (active subs built in) and 753's and the 70x range felt more comfortable as "mid to loudish party/home cinema" type speakers than anything else as at lower volume they were just "OK".

I sold my 705's and bought B&W 605s2's (same layout - 25mm tweeter, 165mm mid-range & 2x 200mm active subs) and the difference in clarity at lower volumes especially was very obvious whilst being fairly equal at higher volumes.

TBH I preferred the B&W's for most stuff but on the rare occasions I could use them for a louder party (amp volume over halfway) the Missions sounded better to my ears.

Thanks for the reply.

The superior bass from the 700 is very noticeable. The sound and larger space in the garage probably help the sound too. I could do a side by side test, but I can't bothered too!