Buying a sub what do the numbers mean?

Cluelessjock

Standard Member
In building a system I am aware the difference the sub is amkes however what do the numbers actually mean in sound terms and how much do I need to spend to get it right?

When checking out subs I note some are rated not a lot different to my fronts (40HZ), so would these be a waste of time? Many also go to 20HZ but what does 20HZ difference mean, is it a lot, or not? Do I need to go below 20HZ or is 1HZ a significant difference?

Thanks
 

Badger0-0

Distinguished Member
Simply put, the lower the number in Hz, the more rumble you hear.

Your speakers may be rated at 40 hz, but how much is that figure down on the baseline (ie average)?

The speaker spec figs should be -3dB to have any respect :smashin:
-6dB is worse, but still gives you an idea.
If the spec doesn't mention either, it's garbage and meaningless.
And probably crap.
 

Badger0-0

Distinguished Member
Sorry mate, I re-read my post and thought it would probably be confusing :suicide:

Point is, if your speakers are genuinely capable of a decent 40 hz (you should say what make/model they are) a 40 hz sub isn't going to help much, because a 40 hz sub is going to be cheap and poor, on the whole. Having said that, a sub is designed to put out sub frequencies, so you may see a bit of an advantage.

As with anything, price generally has the biggest bearing.
Unless you know what you're looking for ;)

Very simply put, have a look at the SVS PB10 and BK Monolith (forum search).

Both £400 and guaranteed to give you a lot more bass than your speakers.
Are they worth the money?
Absolutely, if you can go that high :thumbsup:

There are slightly cheaper versions not quite so good eg BK XLS200 @ £289, I believe, springs to mind.

Any of those will stomp your speakers for decent low bass.
 

Mr Incredible

Distinguished Member
Just a few sweeping generalisations:

(1) Unless you are paying oodles of money, front speakers will not be designed to reproduce frequencies at lower than 40Hz which have the same loudness as those frequencies above 40Hz.

(2) A speaker may be able to produce the same level of loudness (decibels) over a wide frequency range, say 40Hz to 20,000Hz, but at either end of this spectrum the loudness tails off. In order to give the manufacturers some consistency in how they define their frequency response, they are allowed to report what frequencies they can produce at up to 3 decibels below the average decibels over the main range. The rate at which the frequency will "roll off" below these lower and upper points, will depend on the speaker design. Some speakers will be designed to roll off their output at the rate of 12 decibels for each octave either below the lowest range point, or an octave higher then the highest range point. Some speakers do 6db, others higher like 24db. An "octave" is the halving or doubling of frequencies from any particular point. So an octave below 40Hz is 20Hz. an octave above 40Hz is 80Hz.

(3) As a result of (2), normal speakers are not adept at reproducing those frequencies in a film which go below say 40Hz. This is where the "rumble" and feel of a sound becomes more apparent than actually hearing the sound. Hearing changes from person to person, changes with age and with lower frequencies the perceived loudness changes with the frequency itself. So in order to get a true home cinema experience you need to supplement your front speakers with sub. Some people can "hear" as low as 25Hz, other can "feel" much lower even down to sub 20Hz. Depends on many factors.

(4) The power to reproduce low frequencies can be immense, disproportionately higher per decibel than at higher frequencies. Consequently it is advantageous to have a subwoofer take over some of the lower frequency reproduction that the speakers produce. Subs are designed to deliver power at the low end. The THX standard for multi channel sound reproduction is that an amplifier should be set to "split" the frequencies above and below 80Hz to the speakers and subwoofer.

(5) Now the 80Hz "crossover" setting in an amplifier is not a brick wall. The amplifier will "roll off" the frequencies, either side of the crossover, it sends to the sub and speakers. In this way there should be a smooth and level transition of sound that is produced simultaneously by both speakers and sub. It is fortunate that when frequencies get to about 80Hz (depends on people) they become difficult to locate. What i mean by this is that you could play a 80Hz tone from each of the main speakers and you not be able to tell from which speaker it was produced. So in theory and ideally, a sub's output should be heard and not "seen" as it were. It is there to supplement the main speakers sound-field for that all important "thump" in the chest moment!

(6) Because the THX standard is 80Hz (some prefer other settings depending on their front speaker capability), getting front speakers to go lower than 40 Hz (an Octave below the crossover) is somewhat fruitless as those frequencies will be routed to the sub by the amplifier. However, a rule of thumb and the fact of the mechanics of how an amp will "manage" the roll off of frequencies around the set cutover point, it is suggested the amps cut-over be set at twice the frequency of the lower frequency response of the main speakers. However, if you have the "new" satellite speaker type arrangement, these typically will not go much lower than about 60-80Hz. In this instance the crossover could be so high as to send frequencies to the sub which can be "located" by ear: not a good thing. So then you have a compromise between smooth and level transition between speakers and sub, and the ability to make the sub's output "disappear".

(7) Low frequencies that a sub produces say at 30Hz and below, have no regard for most barriers in a home, including brick walls. It is these lower frequencies that will permeate you entire house, and your neighbours if you have them. So you need to be careful when you choose to ratchet it up!

(8) The reason why you may have lower frequency producing fronts is if you are a bit of an audiophile and want to play stereo. Many purists will not touch a sub to supplement the speakers in this scenario. However, even if you get speakers which are "rated" at 40-20,000 Hz at -3 decibels, for home cinema the amp will manage these and the sub well for film reproduction. You can then use them on their own for "stereo"

Hope that helps a bit more without confusing you further.

This is a graph of the frequency responses of my speakers when the amplifier iss et with different crossovers: you can see how the 80Hz crossover reduces the frequency output below the 80Hz mark.

purexo.jpg


Conversely, her eis a frequency plot of my subwoofer showing the opposite effect on the sub output:

80xo.jpg
 

Member 96948

Distinguished Member
Nice graphs Bob.:smashin:

The speaker graphs make it perfectly clear why you should run your main speakers full range rather than let an Eq'd sub take the load.:rolleyes:

Russell
 

Mr Incredible

Distinguished Member
Apart from the fact that I can't EQ the mains, otherwise there may be a case for running my mains full range! There's too much room interaction to let the mains have free reign!

Unless there's a touch of irony/sarcasm in your comment? :D
 

Cluelessjock

Standard Member
Thanks guys, I have just learnt more about subwoofers in the last 15mins than in my life time, superb!

I have been finding that irrespective of where I go they try to sell me the brand they stock without as much as discussing the suitablity. This has left me a tad confused and wary of what I am being told. It seems to be a bit of an afterthought?

The important thing is to get it right, there is no point in spending £200 if it will only give minimal improvement on the fronts. I would rather not spend anything and live with the fronts only, or spend £400 and have a decent improvement. Last suggestions from a dealer was a Sunfire D 8 at £650. It seemed a lot of dosh for not a lot of box so he said I could have the D 10 at £800??

The fronts I currently have are Ruark Talisman II (floor standing speakers) rated at 40HZ, not extremely low but certain base can be felt.

Rel, the monolith, Velodyne are the ones that seem to be mentioned most!
 

Cluelessjock

Standard Member
Not well educated enough it seems!

Toddled off to look at some of the suggestions and now becoming confused about whether I should have a downward firing or forward firing, room is 4.5m x 5.3m? Also excuse the ignorance but could someone please educate me on the difference between high and low level connections?

Thanks
 

badbob

Banned
High is speaker outputs from your amplifier.
Low is RCA from subwoofer pre-outs.

I wouldn't put too much into down firing versus forward, although down firing offers better driver protection from damage.

Those Sunfire's offer good bass for the size of the sub, but they're better subs for the money.
 

Cluelessjock

Standard Member
High is speaker outputs from your amplifier.
Low is RCA from subwoofer pre-outs.

I wouldn't put too much into down firing versus forward, although down firing offers better driver protection from damage.

Those Sunfire's offer good bass for the size of the sub, but they're better subs for the money.

Thanks badbob, it seems the Monolith is a better option at a couple of hundred less, any views on Rel?
 

badbob

Banned
Buy the very best subwoofer you can afford. £400 is a the minimum I would spend on a subwoofer. Things like BK Minotaur are a waste of money. Rel's are a bit overpriced.
 

Mr Incredible

Distinguished Member
The fronts I currently have are Ruark Talisman II (floor standing speakers) rated at 40HZ, not extremely low but certain base can be felt.

Looking at this spec page for your speakers, they are rated at 42-22,000 @ +/-3db.

Note that my speakers rated at 39-22,000 @ +/- 3db and yet if you look how they perform in my room you'll see that with the interaction of "room modes" (i.e. room dimensions which will create standing or null sound waves for any given set of frequencies), that my speakers actually output higher levels of sound below their rated output than they do in their rated range. The trouble with these frequencies is that left uncontrolled they can become overbearing and boomy. With a separate sub driven from the dedicated subwoofer line out on the amp, there is an opportunity to make an attempt to calm these peaks using a bit of kit to massage the frequencies before they hit the sub. This is often referred to as the BFD, or Behringer Feedback Destroyer. A bit of kit originally designed to cut frequencies and avoid feedback on PA equipment. It just so happens that it is a wonderful bit of kit for using with a sub.

I bought my SVS sub secondhand based on the fervent (some have said biased) support of this make in this forum. It was a decision I have never regretted. I had auditioned a couple of set ups in demo rooms, but looking back they were sooo poor in their set up as to be laughable.

There is no doubt that both individual's impressions and formal testing show that the Monolith has some serious output with low distortion. Whatever you get, and provided you can get it tweaked satisfactorily in situ, then just be prepared for that first moment you find yourself watching a film for a "sub moment" and then become aware you'll be sat there with a big inane grin on your face!

TBH I don't think it is worth spending less than around £400-£500 on a sub, and equally I'd consider secondhand. The SVS people have introduced a new upgraded range of drivers and many owners are upgrading. You may get a good bargain in the classifieds.
 

Cluelessjock

Standard Member
Thanks Mr Incredible, am I right in the impression I am forming that the sub can be viewed more on it's own specifications and, within reason, it's less critical to have it match the other components?
 

Mr Incredible

Distinguished Member
That is a fair observation. I think you'll find that the "matching" to other components is achieved through correct integration around the crossover area rather than matching the top end output of a sub with the bottom end output of speakers. As the crossover is set at 80Hz (typically, and appropriate for your set up IMO) you don't need to worry about "matching" per say. The AV amp will do the integration for you. What you then pay for is an ability for a subwoofer to produce high output with low distortion and to go "low" (typically sub 30Hz and even lower). The more ability a sub has to do all of these then the more wonga you'll need!
 

Cluelessjock

Standard Member
Guys,

I have spent some time looking at the options for subs recommended and see there are various options for connecting them. I have an Onkyo 875 and have the front's bi amped.

What are my options?

Is it still possible to have a high level connection with bi amping ( I assume it is and either speaker output can be used)?

I see some subs have a dedicated input for .1 theatre use, is this necessary and will the other options; high level, one or two phono outputs or pre outs work just as well with surround?

If it is necessary to connect low level for surround and I also connect high level for stereo music will receiving signals from both high level and low level simultaneously during movies cause a problem for either the receiver or the sub?

Thanks
 

badbob

Banned
In a standard system you only need the single cable RCA-RCA from the subwoofer/LFE pre-out.
You normally would use high level in a hi-fi system if your amp doesn't have pre-outs.
 

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