Scooby Doc

Thanks to Niall from Eire (Venting Bass)

Anybody got any experience of building subwoofer enclosures. Seems like for an afternoons DIY I can save myself enough to have 2x10" subs instead of 1.

From what I've seen on the internet there appears to be a bit of science behind it - does this matter that much for in car use say compared with the home?

If the dimensions of the box are specific to a particular subwoofer than I can't believe the stack of boxes in my local ICE shop are that special.

Thoughts please. Chiark?

Martin

mattski

the science does matter :O) I used to be a sound engineer before getting a 'real' job and it must be right for the driver to operate effectively.

There are probably some pretty good freeware programs around for enclosure design that take into account driver characteristics, try doing a search on

Mr.Cookie

Martin

All the subs i've bought have given enclosure size as either a volume needed (example say 5 litres) that they work with ported and non ported just need to design box to theese requirements or actual box design like the MFI type instruction. Sorry cant remember equation would just say make sure you use 18mm MDF and to include thickness of MDF in calculations.

Simon
Ps non ported are a damn site easier to get right and save having to work out port length

Scooby Doc

Thanks guys.

There is also a website at

chiark

Sorry, been out of communication for a couple of days...

In short, "size matters".

Designing the box can be done using something like "Perfect Box", which is an ageing DOS program, or check freshmeat as Mattski says.

You need to have an idea of what sort of driver (speaker) you want, and if possible find its Thiele-Small parameters. Feeding this into your program should tell you an ideal volume to aim for. No doubt it will recommend dimensions, but you'll probably want to ignore these. Basic rules for building are:

Your sub will probably come with a recommended volume - this normally works well in most cars.

Whatever you get from your program, your response will be little like the graph plot that the program produces because of the car's varying response to various frequencies. I seem to recall something called the bass transfer function of an envionment, but could be spouting again... I don't know of one for the Impreza.

If you want two speakers, you may want to build two separate chambers as I have found in my experience that this gives a cleaner sound than using two drivers in one chamber. Others advocate using just one chamber , so there's no "right" answer.

In car against home use: the calculations are the same, but the driver needs to be built for the car environment (more extreme temperature and vibration, ...).

As for the stack of boxes in your ICE shop: they are trying to be all things to all speakers, and they simply can't be.

Regarding ported (or vented, or bass reflex, or passive radiator) vs sealed box design, everyone has an opinion on this. A port reinforces the output from the driver at certain frequencies, but has the unfortunate property of failing to provide suspension to the driver below a certain frequency, meaning you can knacker a driver with a bit of effort. Also, to some people, ported boxes can sound a bit "one-note"ish.

Again, try to listen to a couple of designs and subs before forking out.

DIY sub box building is good fun, and I enjoy it. Although I've made one for the scooby and can't ever see me changing it as it was a bit of a labour of love involving mdf, fibreglass and much swearing

If you're around the North, you're more than welcome to listen to my stuff and I'll give you a hand, for all the good that will do you

harryh

Anyone have any tips on how to attach the sub-enclosure to the car?

chiark

Yes: securely. Mine is attached by bolting through the back seat supports, but that's due to its design.

Check three times before tapping into the floor. One unfortunate chap on here had an ICE installer mount their gear to the fuel tank in rather an indirect manner.

Look extremely carefully before screwing into anything!

Scooby Doc

Chiark

Thanks for the advice. The problem I have is that I have 2 8" Denon subs from a previous install in an acoustic shelf. The problem would be solved by buying a new pair of subs with the necesary information for designing an enclosure - precisely the info I don't have for the Denons. Whilst they are not the last word in sub bass the thought of life with no subwoofer in the system is just not the same. I guess I could contact Denon but the subs are about 6 years old - probably due for retirement.

Martin

Scooby Doc

Chiark

Thanks again. I'll give it some thought this weekend and let you know about the end result. I am going for Infinity Kappas up front with my Nakamichi CD45Z hu.

I had a look at your FAQ website. Nice install in your boot. Is that one big bit of MDF with separate subenclosures behind? Its kind of neat.

Cheers

Martin

chiark

You could always chuck the Denons under the parcel ledge and do things that way... Remove the rear speakers and cunningly mount your denons below the hole.

As for boxing them, don't give them too much power and I should think that a box less than 1 cu ft for both combined should be more than enough.

We've gone from advising all sorts of scientific things to a bodge it approach . This type of install would get you some bass with little to no expenditure and a bit of effort.

chiark

The box is a fairly large thing that is the width of the gap behind the back seats, full height in that area. Front is flush to the back seats.

It's made from MDF and lined with fibreglass to save weight (supposedly). You can probably work out that there's three chambers in there - one in the middle for the amps/bass cube and power gubbins, and one at each end for each sub, giving it about .8 cu.ft. to play with.

Only problem is how to stop the bloody car rattling

Scooby Doc

Harryh

Apparently velcro stops the box sliding. It remains to be seen whether this works in practice.

Martin

ScoobyDoo555

I too, like Mattski, work in the audio industry as an engineer. You've probably guessed it anyway, but size is really important, as the speaker enclosure is very important for bass reproduction.
One thing nobody's mentioned yet, is the port hole. This HAS to be a set size - depending on the speaker you're using.....

Normally, it has to be air-tight to generate an 'air' cushion for the speaker to rest on.
If you combine this with a port AND baffles inside the box (known as Transmission Line - normally VERY expensive in HiFi!!!) you should get clearer, more defined bass......

Scooby Doc

ScoobyDoo555!

Thanks for your reply.

I opted for the off the shelf box and an Infinity reference 10". Interesting. I could not hear the woofer except at idle initially but a quick fiddle with the set up of the DCA 800 seemed to get the old spoiler vibrating nicely to the amusement of my neighbour. Base is good, perhaps not quite as well defined as would be perfect. That may well be the choice of sub - recommended by my local shop because it would be easier to drive than say a Perfect 10. The off the shelf enclosure is close to but slightly largere than what would be ideal for this sub. I might try swapping the sub or trying 2x 10".

I was tempted as was NAF to remove the rear speakers to vent the bass but could not get the grills off ( it was going dark and was in a hurry). The speakers are faded out completely in the rear.

I'll keep you posted. Any suggestions gratefully received.

Its good to have decent sounds once again

Martin

Scooby Doc

A question for our sound engineers ScoobyDoo555 and mattski.

If you have 2 say 10" subs in a sealed enclosure is it best to split the box into two or are the volume requirement for the subs additive.

Martin

ScoobyDoo555

Scooby Doc,
If you've got 2 drivers, definitely split the two - having the two drivers together can cause phasing problems in the case.
Just in case you don't know what phasing is, it's when the 2 outputs (waves) cancel each other out, resulting in minimal sound output (in full bandwidth music, this results in a 'wispy/swishing sound')

Hope this helps.....


Dan

Scooby Doc

Dan

Thanks

Sensitivity of speakers is measured in dB. The higher the figure - the easier to drive. I ask because the dealer told me that the reference infinity subs would be easier to drive with my (old) amp than say perfect 10's.

Any thoughts. Also they didn't use my 4 guage cale from battery to amp because it was too short, but replaced it with 8 guage. Not a problem with the Denon but I feel might just be a little short sighted should I upgrade the amp in the future.

What are you guys using?

Cheers

Martin

Richard Gledhill

Just a quickie - have a look on

ScoobyDoo555

My system isn't too serious at the moment -
Sony Minidisc Head Unit & 10 disc CD
JBL replacement speakers (5dr turbo)

In fact, I'm off tomorrow to buy a 10" sub and amp (can't have anything too big coz of the baby's stuff and the dogs....)

Speakers - dB (or Decibels) are a measure of Sound Pressure Level (SPL) - 140dB is the threshold of pain (blood coming out of your ears!!!!), not necessarily just a measurement of sensitivity.

You need to have an amp that's MORE powerful than the speakers, otherwise you're causing the amp to 'work too hard', thus inducing distortion - not what you want. I think what your dealer was getting at was that your sub(s) could be handled by the amp, as stated above......

Regarding the cable, it sounds OK - normally need serious guage cable if you're getting into 1000-1200 watt country, and definitely if you're using a cap.....

It sounds like you're going for bass that can detatch your retinas.. ;-)

Hope this helps

Dan

Scooby Doc

Dan

Thanks for your advice. What did you buy?

What I was wondering when I as ked about decibesl was whether a speaker with a sensitivity of say 97dB would be harder to drive than say one with a sensitivity of 90dB?

Thanks to Richard - the RF site is pretty good.

Did my first commute with the new system today - big smiles!

Keep you posted

Martin

ScoobyDoo555

Once again - I totally missed the point!!! Bugger.
In answer to the question, yes, the higher the 'sensitivity' the harder it is to drive...

I've been looking at the JBL 'liquid cooled' subs, just need to re-mortgage the house to pay for 'em :-(
I've found a discount store that does everything - I'm gonna have a serious look at the weekend - I'll let u know.

Dan

Richard Gledhill

Hang on... the higher sensitivity, the easier it is to drive for a given SPL (i.e. volume level). Sensitivity is defined as being the loudness (in decibels, dB) when you run 1 watt through at 1kHz, standing 1m away from the speaker. Therefore a 97dB/W/m (the proper definition) will give you more volume than a 94dB/W/m - in fact twice as much, as the decibel scale is logarithmic, not linear. Going up by 3dB = twice as loud.

This is why Cerwin Vega and Wharfedale floorstanders (in house "hifi" now!) give you so much volume from weedy amps.

About the "who needs to be bigger" debate... Well, in theory, you're right - the amp needs to be bigger if you're looking for hitting the maximum values the speaker can hold. The danger, of course, is you don't know if you're going to far, and then... fffzzzttt.. pop.... If you have a less powerful amp, then when you get near its limits, you will hear the distortion setting in. Finally (particularly on cheaper amps with no protection), the amp will saturate it's outputs at dc 14.4V (batt level) and will burn the coils out. I think most people use speakers that can take it, and make sure the sound isn't distorted. Basically distorted = trouble...

Cheers
Richard

Scooby Doc

Richard

I was sold on the infinity reference 10" sub because the llikes of perfect 10's would be too much for my elderly amp. The latter have a sensitivity of 97dB as opposed to 90dB for the cheaper references. Therefore the perfect 10s should actually be easier to drive and they are simply trying to sell me a knew amp.

However the system sounds good so far although it was my ears that were hurting the other day - subsequently I realised it was just bloody loud although I'll check the speakers are inphase when I stick in the dynamat.

thanks for your thoughts


martin

ScoobyDoo555

You're dead right Richard - my mistake, got it the wrong way around!!!

Went back through my textbooks (bit old now though!) Incidentally, anybody who wants to get into audio& hifi theory, get Newnes Audio & HiFi Engineers Pocketbook (about £10)VERY VERY good......
And found the stuff.....

My interpretation was that the dB sensitivity was the amount of dBs required to run the speaker optimally...... Oops ;-)

Dan

john banks

Richard wrote, "Going up by 3dB = twice as loud." Maybe you can correct me, but I thought that 10dB was twice as loud therefore requiring approx. 10X the amplifier power to double the volume. So a 50W amp running 93dB/W speakers will give the same volume as 100W amp running 90dB/W speakers and to make double the volume would require approx. 500W with the first and 1000W with the second set of speakers. That's why I haven't changed my front Kenwood coax to Infinity with sensitivity -4dB/W. (cos it would sound murine).

Do you mean "Going up by 3dB = half as much amp power for same volume"? I am not trying to be picky, but just understand exactly what is going on.

I am still trying to find a set of components to power off my 50W amp that will give better power handling without a drop in sensitivity of more than 1dB/W (ie 92dB/W/m or better) but all the ones I see the specs of are nowhere near for a decent price or they are cheating by using 2.83V (=2W @4ohm) or measuring sens. at 0.5m etc etc).

Yours etc (stuck with some sensitive & loud Kenwood coaxs. which will hopefully improve with awaited XTC speaker baffles kindly being shipped by Rockin'Ru).

ScoobyDoo555

Technically, the measure of 3dB is the LEAST amount the human ear can detect........

john banks

The threshold for human hearing is 0dB by definition (=0.021dynes/square mm). [reference: ENT textbook]

The "average" human ear can just detect approx. 10% increase in amplifier power [reference: Car stereo Cookbook] This is a 10*log(1.1) = 0.4dB increase.

So I don't understand where 3dB comes from.

Back to the (sub)topic though; there is a big difference between a doubling in amp power and a doubling in volume. 500W is only twice as loud as 50W if sensitivity is equal. So you have to put a lot more power in to get a worthwhile increase in volume. So sensitive speakers are very important.

Still looking for some sensitive components with good power handling

And we all need subs with good sensitivity unless we have money to burn on amplifiers that run cool at ridiculous power outputs!

Scooby Doc

John

Glad to see you embracing the more scientific aspects - did you crack on and dynamat your doors?

I suppose one of the advantages of paying a premium to have somebody do the work for you is that they are willing to swap the kit about. Going to try perfect 10's towards the end of the week as a swap. Theoretically a more sensitive speaker, so hopefully an easier drive.

As an added thought, as my DCA 800 is 6 channel amp with 2 channels bridged into mono to run the sub and 2 to run the front components ( my rear speakers are faded out completely) - could I bridge the unused 2 channels into my two channels running the components? Just a thought.

Martin

john banks

Doors - yeah 9 sq metres! And some sticky fingers! Speaker baffles on their way from Rockin'Ru to kill the last vibes.

Are the Kappas in yet? What do you think of them? I am considering them, but with claimed 89db at 2.83V I feared they would be quiet? My amp is probably weedier than yours - only 50WRMSx4.

If you have 2 channels bridged to run the subs, and are using two channels for your components and 2 for the rears which are faded out, then why not consider using active crossovers to biamp the components. If I add another amp, which I am thinking of depending on what components I get, I will certainly be thinking of this. I suppose if you bridged 2 channels for each side of the components you would get ?c.50% extra power (compared with biamping) to your components but with less adjustability and more distortion? If you biamped the fronts then you could run the rears off the HU esp if they are faded out? So a choice - you could double your power to the fronts by biamping or possibly triple it by bridging.

[This message has been edited by john banks (edited 19 November 2000).]

Scooby Doc

John

9 square metres! I am impressed. Was that all dynamat or what. Did you try any of that spray stuff?

My rear speakers are connected to the head unit but are faded out. I don't seem to miss them and was planning to take them oout if I have a dry weekend off.

The possibility of putting in an active crossover has crossed my mind. I have posted elsewhere that as most of my driving at present is at motorway speed then road noise drowns out the mid frequencies a little although it does appear to be dependent to a degree on the choice of music. Whats the chat on active crossovers.

The Denon is quoted at 6x50W although this is not RMS as is typical for japanese amps.


I must say that I have been quite pleased with the Kappas. The sound good with an nice tonally ballanced sound and good imagery. Maybe as they are not quite as sensitive as some then maybe this explains why I am having difficulties at high speed.

Cheers

Martin