QUESTIONS & ANSWERS.

This page is all about those questions that we get asked at AMG, with the answers that we give, which to the best of our knowledge are true and based on facts.

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1. Help ref. Dual Bandpass Box?
2. WHY DOES MY MTX BOX SOUND SO BAD?
3. I Need help with my bass and choosing some component speakers?
4. How do I tune my bass box?
5. What is peak power/rms power?
6. I need help, re: speakers for my MERC 230?
7. How do I wire up my Isobaric box?
8. I need basic help with SPL?
9. How do I get some bass into a small boot space?
10. Advice about dual voice coils?
11. Next set of Q & A's.

Subject:
CONGRATULATIONS
Date:
Tue, 3 Jun 1997 16:13:42 -0400
From:
Barry cairns <106343.3313@compuserve.com>
To:
"amg.loudspeakers@virgin.net" <amg.loudspeakers@virgin.net>

HELLO
WHAT AN EXCELLENT PAGE YOU HAVE PUT TOGETHER I JUST HAD TO MAIL YOU TO LET
YOU KNOW THIS IS ONE OF THE BEST PAGES I HAVE SEEN (NO BULLS*** THAT IS )
.I AM CURRENTLY DESIGNING THE BOX TO INSTALL INTO THE BOOT OF MY RENAULT 21
(SALOON). IHAVE 2 HIFONICS ZEUS 12 INCH DRIVERS AND I AM INTENDING TO BUILD
A DUAL BANDPASS BOX AND FIRE IT THRU THE SKI HATCH .I HAVE LOTS OF SOFTWARE
TO HELP WITH THE BOX CALCULATIONS OF MY PREVIOUS BOXES SEALED AND PORTED
AND HAVE HAD SOME EXCELLENT RESULTS AND SO AM QUITE CONFIDENT ABOUT BUILDING
A BOX MYSELF(ESPECIALLY AFTER I FORKED OUT £120.00 TO HAVE MY FIRST BOX
CUSTOM BUILT BY A WELL KNOWN MANUFACTURER ONLY TO DISCOVER IT WAS CRAP
AFTER I DROVE 150 MILES TO COLLECT IT .MY PROBLEM IS WITH THE CALCULATION
FOR THE PORT SIZE FOR THE SKI HATCH (55 SQ ").
I WILL HAVE AS MUCH SPACE AS NECESSARY FOR THE CENTER CHAMBER AS THE BOOT
IS HUGE,MY PROBLEM IS THAT THE LENGTH OF THE PORT FOR THAT AREA AND TO TUNE
THE BOX TO A SENSIBLE LEVEL I.E. ANY WHERE BELOW 100HZ IS VERY LONG ,WOULD
YOU SUGGEST USING SEVERAL SMALLER PORTS OR GO WITH A VERY LONG PORT ,THE
DESIGNS I HAVE SEEN AND HEARD AT SHOWS SEEM TO HAVE NO PORT THEY USE THE
THICKNESS OF THE EDGE OF THE BOX PRESUMABLY TO PORT THE BOX .I AM AS YOU
CAN TELL NOW VERY CONFUSED I WOULD BE VERY GRATEFUL IF YOU COULD HELP ME
OUT WITH MY PROBLEM
WITH THANKS
IN NEED OF BASS BARRY

Hi Barry, thanks for your kind words.

I am assuming that your present computer software will allow you to work out both Isobaric and Tri-chamber boxes. If not let me know. I am also assuming that your 12” speakers like bandpass boxes. How you tune a bandpass box depends on what you want out of it. Basically there are two ways to approach tuning for bandpass boxes. Either sound quality or pure raw SPL. I usually go for sound quality which means using either a 4th order bandpass or a 6th order bandpass. 6th order’s have a port from the rear to centre chamber though you can also port it to the outside of the enclosure but that way lends itself more towards SPL. if you opt for a port from each rear chamber into the centre then it is possible to get, depending on speakers a figure of about 40Hz or less, you need to use a port of 75mm x 110mm for the best results as this makes power handling easier and reduces internal port noise. The centre chamber is tuned for the upper bass cut off. For sound quality I have found that this should be tuned to 3-4x the frequency of the rear ports, about 120-160Hz. This means that the box will sound nice, though not as efficient,but you will not need excessive port size, this is why so many boxes that you have seen have used just the thickness of the wood as the port length. I have found though that the centre chamber also controls power handling and the transient response times, this often needs to be tuned to a length greater than the thickness of the wood. Using multiply small ports is not the same as using one long port, e.g. 2 x 2”D x 4”L doesn’t equal 1 x 2”D x 8”L, the maths just don’t work in such a linear fashion. 2 x 2” x 4” is almost equal to 1 x 3” x 4”. At a guess, and without running up your speakers on my program, I would say the following, and this is based on my own experience using Eminence 15” drivers. The centre chamber should be about equal to half of one of the rear boxes. For sound quality, or equal to about one of the rear chamber for SPL. And the centre chamber should be tuned to about 3.5x the F3 value of the rear chambers. For SPL then the centre chamber should be tuned to about 2x + 10Hz the value of F3 of the rear chamber. The rear port should be on the same panel as the speaker but on opposite sides to their respective speaker panel, otherwise they blow air from one port straight into the other port and cause cancellation to each other. This port should be at it’s smallest 1 x 50mm x 110mm but more respectively and for better power handling then 1 x 75mm x 110m. The front port if you use the thickness of the wood will probably need to be at least 150mm but much more likely to be between 200-300mm. I personally prefer to use at least 4-10 x 75mm x 110mm depending on what effect I want, but getting the front port wrong causes more problems than getting the rear port wrong. Also if you are walking the tri-chamber path, both speakers MUST be wired to be in phase together, i.e., push together and pull together, the easiest way to do this is to wire them +ve to +ve and -ve to -ve, for a 2ohm load or +ve in and then -ve to +ve and -ve out for a 8ohm load.

I have tried to offer as much info as possible but you must appreciate that bandpass boxes especially tri-chamber and isobarics take some getting used to and even with a computer it has taken me some 200 boxes to learn the little tricks that have learnt. And I’m still learning.

Good luck and stay in touch and watch out for MAX POWER June 17th 1997 and read their review of AMG Bass boxes.
ERSKINE.

Subject:
Re: BANDPASS BOX
Date:
Wed, 4 Jun 1997 18:41:54 -0400
From:
Barry cairns <106343.3313@compuserve.com>
To:
ERSKINE FENTY <amg.loudspeakers@virgin.net>

THANK YOU VERY MUCH FOR THE ADVICE YOU HAVE GIVEN ME ON THE DESIGN FOR MY
BOX ,THIS IS THE SORT OF DETAIL THAT THE LOCAL ""SPECIALISTS"" ARE NOT ABLE
TO SUPPLY AND I GREATLY APPRECIATE THE TIME YOU HAVE TAKEN TO REPLY TO MY
MAIL . I AM GOING TO AIM TO BUILD THE 6TH ORDER BANDPASS SUGGESTED ALTHOUGH
THE SOFTWARE I HAVE ONLY ALLOWS ME TO WORK WITH SINGLE CHAMBER BANDPASS
BOXES AND NOT TRICHAMBER THIS WHERE MY PROBLEMS LIE AS I CANT AFFORD TO PAY
OUT FOR THE LIKES OF LEAP OR TERM PRO FOR THE LIMITED NO OF ENCLOSURES I
BUILD .I WILL BE TRYING TO MAXIMIZE THE BOX TOWARDS SPL , SO WOULD I BE
BEST TO PORT THE REAR CHAMBERS OUTSIDE THE BOX AND HAVE ONLY THE CENTER
CHAMBER PORTED THRU THE SKI HATCH , OR HAVE BOTH REAR PORTS FIRING INTO
THE CENTER CHAMBER AT OPP SIDES OF THE BOX .I HAVE INCLUDED THE THEILE
SMALLS FOR THE DRIVERS WITH THIS MAIL AND WOULD BE VERY GRATEFULL IF YOU
YOU COULD GIVE ME YOUR SUGGESTIONS FOR THE CENTER CHAMBER AS THIS IS THE
ONLY PART OF THE BOX I AM GOING TO STRUGGLE WITH AND I AM AWARE IT IS
PROBABLY THE MOST IMPORTANT PART TO GET RIGHT ,I WILL HAVE NO PROBLEMS WITH
THE PORTS FOR THE REAR CHAMBERS AND CHAMBER SIZES ,I KNOW THIS IS A BIT OF
A CHALLENGE FOR ME BUT I BELIVE WITH A BIT OF HARD WORK I SHOULD BE ABLE TO
COME UP WITH SOMETHING VERY PLEASING .
I WOULD ALSO BE VERY INTERESTED IN SEEING SOME OF YOUR ALREADY COMPLETED
PROJECTS IF YOU ARE GOING TO BE DEMONSTRATING ANY VEHICLES OR HAVE INSTALLS
ALREADY COMPLETED THAT MAY BE AT ANY SHOWS,CRUISES ETC WOULD YOU LET ME
KNOW WHERE I COULD SEE THESE .I AM EAGERLEY AWAITING THE NEXT ISSUE OF MAXY
P TO READ YOUR REVIEW ,
THANKS AGAIN ERSKINE
BARRY

Hi Barry

Glad to have been of some help, I do understand what you mean about local dealers not being able to do much more than get you to part with your money.
At present I am awaiting for my new software to be delivered as my current one has got corrupt and is being replaced, so hopefully that will be in a couple of days. I shall keep your speaker details on file and get back in touch. Normally we offer a speaker design service to people for a nominal fee and use that money within local charities in our area like the RSPCA, if you wish to make a donation that would be nice, as this is a service that I am happy to do for other car audio nuts like myself but it does take a few hours to come up with a spot on design. With respect to cars with our bass boxes, well that depends on where you live, we are in Berkshire and there are loads of our cars around, this makes it quite easy for me to call someone up and get them around to give a demo. My present car is off the road, it is being prepared for Sound Off 98, and this is one seriously serious piece of audio sex. Our intentions are to aim for the European finals and I know we are going to not just make it but also do very well. To date I have not heard a car that really takes my breath away, other than Martin Ellis’s BMW from 94/95 sound off, often because the gear has been brought to look flash especially from an installation point of view. My Mazda MX 6 will look the part in a qute sort of way but from a sound quality point of view is going to sound like you are wearing a pair of head phones in the car. Anyway we have a year to get it right. With reference to cruises, well Chelsea seems to be the one that some of our customers go to, lookout for a Black 205 - 2 x15” in a 4th order bandpass, Red Astra GTE - 2 x15” in a 6th order tri-chamber, Blue Clio - 2 x 15” in a 6th order tri-chamber, Blue Orion - 2 x 15” in a Isobaric 6th order bandpass and a Gold 309 - 1 x 18” in a Quasi 6th order bandpass box. Of these I know two of the guys and one of the girls has already been fined in court under the new criminal justice bill...what a life ahh!
I’ll be in touch as soon as I get my software, be patient, it will be worth it. Sends us your fax number or address as it is not possible to e-mail the graphs and copy that is associated with the work that is produced that is not ASCII text.
ERSKINE.

WHY DOES MY MTX BOX SOUND SO BAD?

Hi ROB

29th, May, 1997.

I have only ever seen and heard one MTX Road Thunder box and that was from another dissatisfied customer. I said to him that on paper the box should perform well. He had a good amp and everything seem sweet. About 12 months ago I went through the MTX book and checked each of their box calculations and suggestions. I found their sealed and vented boxes to be spot on. Their bandpass boxes weren't my personal choice of settings as I felt that they could be improved upon but they were still good.

In my personal opinion I believe that the problem is two fold. First, like many other bass box manufacturers, the desire to make the front chamber all nice and pretty often means that their are no port holes in that chamber. In most system the front movement of the cone ought to be played into the smallest chamber, which is usually the front, if this is reversed as in your box, then you could try wiring your box out of phase, i.e., turn the -ve and +ve back to front, at least in theory this should correct that problem. The second problem is also directly related. In most bandpass boxes the front chamber is usually used to tune the upper cut off frequencies. Carefully tuning of this chamber will control how loud it is. The rear chamber is tuned to control how much bass it will produce. When I looked at the MTX box I found that the front chamber was small, well if there is no port holes in this chamber and you do as I suggest and reverse the phase, it is almost like pretending the magnet is on the other side of the speaker, then the cone side becomes the rear chamber and it is just too small. The now front chamber is just too big, what I am saying is that like many bass box builders they have got it wrong. Even if you do not reverse the phase it is still wrong. At present I am awaiting for a new software program as my old one has gone on the blink but I believe that with the information about the exact size of your box that it could be retuned. When I get my new software and if you wish then I will happily run the information up on the computer and tell you what if anything can be done to retune your box, it will mean drilling new port holes into the box but if it is retunable it will be well worth it and cheaper than buying a new box. I often get asked to retune Branded Bass Boxes, with some makes it isn’t easy or isn’t practical or worth while. Although the sketches below use two bass speakers the principle is the same, the front chamber should be at least 1/3rd the size of the rear chamber and it should be tuned to one octvave plus 5-15Hz above the rear box. Your box is effectively a 4th order bandpass box, changing it to a well tuned 6th order might just solve your problem, by improving its efficiency and might release an extra 2-6dB of SPL.

Bass
Date:
Mon, 16 Jun 1997 15:00:24 +0100
From:
Andrew Richards <ARICHA11@ford.com>
Organization:
Ford Motor Company
To:
amg.loudspeakers@virgin.net

Hello..
I have just had a look through your web-pages and I thought that I would
take you up on your offer of advise to anyone.
Firstly I would like to congratulate you on your most informative +
enjoyable web site. I am fed up with the amount of rubish + bullshit in
the car stereo industry and it is most refreshing to come across a
company such as yourselves who obviously have propective customers' best
intent at heart.

I have a couple of questions + views that I would really appreciate your
comments on - so here goes:
Firstly some background: I am currently doing up a mk1 golf Gti and part
of this includes trying to build up some really respectable sounds for
it. I have changed some of my equipment from my last car, and at the
moment my equipment list is as follows:
Kenwood 667RL Head Unit + CD604 Changer
Kenwood KAC 626 Amp (claimed 2 * 50 wrms)
Alpine MRV1000
Rockford Fosgate RFA 614 Audiophile Component set (6 1/2 mid + tweeter +
Xovers)
Two Rockford Fosgate RFA Audiophile 12" Subs

I have 2 questions - one related to the front components + one related
to the subs..

The Front components: I quite like the overall sound at the moment - The
6 1/2" mids produce a very punchy mid bass sound - but I feel there is a
fair bit lacking in the upper midrange. Therefore I am thinking about
getting a pair of 4" drivers to do say everything from about 500Hz -
approx 3500Hz and to use the Rockford 6 1/2 " mids to do the frequencies
from 80Hz - 500Hz. I would get another amp of about 2 * 90 wrms to do
the 6 1/2" mids and use the kenwood to do the tweeters + 4" mids split
by the x-overs. Later on I could get rid of the kenwood amp and get a
better 4 channel amp say 4 * 50wrms to power the mids + tweeters
actively.
For a 4" driver I am thinking of either the Eminence Pure range - or of
Oz Audio - these seem to have had the best reviews - for the money. I
was wondering which you would recommend - I want them to be nice + sweet
- not at all harsh.. and to be able to go quite loud.
Do you think this would be a good way to go? Also - is it worth thinking
about powering the mids + tweeters actively - or would the difference
not be worth the cost ? I here a lot of people saying that running
speakers actively is the best way to go - but I see a lot of competition
cars using passive x-overs + so I wonder if it's just all hype.

Bass: I really love these Rockford Audiophile subs.. They are the
sweetest 12's I've heard and produce lots of bass without even more
boominess, which is good. They also take all the power that my Alpine
MRV1000 can throw at them and look like they could handle twice as much
- despite being rated at "only" 250 watts RMS (they are 8 ohlm subs
wired in parallel + bridged acrross the amp). However I do not feel that
I am getting the best out of them at the moment. I have been running
them in individual sealed enclosures of 1.2 Cubic foot as recommended by
my local Rockford Dealer. They are real punchy, but don't drop as low as
I would like and don't go as loud as I would like either - in these
sealed enclosures. I have to admit to liking a lot of bass - but I like
it tight + with the ability to drop (a lot of people have told me that
it is not possible to have both - but I don't believe that is true.. but
I know it depends a lot on enclosure design).
Anyway - I am trying to work out an alternative enclosure design. I am
thinking about a very large ported box - tuned quite low say about 33Hz
or so - in order to give a bit more volume over a bigger range than the
sealed box + to enable them to go a lot lower.
The problem is that I need my boot space. I regularly go whitewater
canoeing + so I need to have the ability to carry a lot of stuff. I have
thought about this long and hard + have come up with an idea, but I
don't know if it will work. I would be interested to hear what you
think..
This is my idea: Basically - to use the whole boot area as a big box -
BUT .. to have the top of the box acting like the parcel shelf with the
subs upside down firing into the box from the top. This top side would
be hinged somehow - so that it could be opened. i.e. so that the whole
inside of the box could be stuffed full when I need the bootspace (with
the subs turned off obviously) - so that it would be a very large box -
about 8- 10 cubic foot for both subs to play in, and yet when I need the
space - the inside of the box would effectively be my boot as normal -
but with MDF sides. I would just need to figure out a way to Seal the
top down effectively - but still allow it to be opened easily.
What do you think ?? Probably that I"m mad and that it won't work ?
I have got some basic box design software and the response of the subs
in such a box looks awsome on paper. I am worried that it may spoil
their "tight" sound though - and that It may be very difficult to seal
the box sufficiently.

Well I know this was a long message so I thankyou for your time - and I
would be real grateful for your views and/or any other ideas..
Many thanks,
Andy..

Hi Andy

Thanks for your kind words.
Quick plug, have you read this months MAX POWER, p192. Check it out.

Re: your questions. I have never listen to the Oz Audio so I can’t comment. As for the Eminence Pure, well, if you like good sex then you’ll love these, they are very sweet. I have used their 6”x9” and 5.25” and they are the biz. Also if you have the room, then there is a company called BANDOR, they make full range speakers out of aluminium, they are a fair price and look even better than the EMINENCE, but their magnets are extra large, e.g., their 4” speaker has a 4” magnet, that is bigger than some subs.

Active or not? Well that depends on your budget, I have always gone non active but next year for the first time I will be fully active, personally I would first try non active, try a 47uf Bi-Polar Capacitor on your mids, this will give you a cut a -6dB at about 800Hz but will still play down to about 200Hz. Try either 3.3uF or 2.2uF Bi-Polar Capacitor for your tops. Let your mids play to about 5KHz or even 7.5KHz, this way your tops will be a gentle fill rather than the main stage. Also try running them off your head unit, there is enough power there to do them justice. These suggestions will cost you about £5, if you don’t like the results then consider an active set-up. Remember the only reason to have an active set up other than it looks good is to get total control over a difficult speaker/environment set-up and to control power handling..

Have you consider a Isobaric Bandpass box, it helps keep space down to a minimum and sounds great. Under normal conditions most boxes have a hard time getting quality sound and low bass, but I know how to do it, though it only works with some drivers and nearly always in a “Quasi-Horn Loaded 6th Order Bandpass Box”.

With most box size suggestions, dealers aim to give your ears a bit of a bashing. I have taken many a speaker recommendation and improved upon it. Also a small box will sell itself better than a big boot hungry one. If you want load you go via a vented box, if you want punch you go via a sealed box, careful design of each can give you a reasonable amount of low. But if you want low, low bass, without distortion, then you have to consider a bandpass. I believe that I have cracked the secret of how to get a bass box to produce quality sound and deep bass, please read the MAX POWER review, of 8 box submitted, AMG got the best mark for sound quality, Bass, and power handling, check it out and if I can be of any further help let me know.

Martyn Iles wrote:
>
> I'm currently designing a new box for my JBL T150 sub and am using
> blaubox as a guide (as well as suggestions from JBL's on-line
> representitive who has been very helpful) but I'm not sure of the best
> way to go about tuning the box when its in the car.
>
> I can try different port lengths when I've got the box in the car and
> then listen to each for a while (this will probably take several days
> to get a realistic opinion), but what's the most effective (ie
> quickest) way of determining the REAL tuned response.
>
> I have an SPL meter (as well as a Laptop with home written RTA
> software) and a test disc - will the greatest output (SPL) be at the
> tuning frequency?? Can I just run a 100Hz to 20Hz sine sweep and see
> at what point the reading is highest?
>
> I'd be grateful for any suggestions as I'm sure they'll both get me a
> better sounding box and probably mean it takes less time for me to get
> it sounding right.
>
> Martyn Iles (kryten@dial.pipex.com)

There are two easy ways of checking your box tuning. Both require a
signal generator and one a multimeter and resistor. The quickest ways is
to connect a sine wave generator to the input of an amplifier and the
amps output to your speaker. Set the volume to about 90dB using your SPL
meter and adjust your signal generator to about 30Hz. Then just slowly
begin a frequency sweep. If you closely watch the cone of your speaker
there will come a point at which your speaker appears to stop moving.
This is easier if your speaker has a logo on it. At the point where the
movement appears to have stopped, the port hole will then be moving air
at its maximum speed. This is then your box’s tuned frequency, as read
from your signal generator.

Or you could hook up a 5ohm/10watt resistor in series with the live fed
to your speaker. You need to measure the AC voltage across this resistor
and adjust your amp output until the voltage is about 2-3 volts. Slowly
begin a frequency sweep and note the highest voltage on your meter. At
the point of that highest reading note your signal settings from the
sine wave generator, this is the tuned frequency of that box.

Also note that when tuning a box, in my experience the lowest tuned port
setting is nearly always made better by the addition of an extra port.
I’ll explain, lets say your box is tuned with 1 x 3” x 4” port and this
gives you an F3 of 35Hz. By the addition of another port say 2 x 3” x
4”, the tuned frequency might rise to say 45Hz, but the power handling
is seriously increased and so to is the transient response. This is so
because it is a good idea in practise to keep the Fb figure to within
5-10Hz of the F3 figure, even if that F3 figure seems a little high or
even if the Fb figure is higher than the F3. Although 45Hz might not
seem as exciting as 35Hz, your speaker will appreciate the extra
breathing room, this example is just an off the cuff example but when
calculated properly makes a world of difference to both sound quality
and overall performance. And remember a speaker that is tuned to about
50Hz will sound really good in a car, as you go down the hertz scale to
achieve lower bass tuning figures you give up some of its SPL qualities.
==================
Yes, this is good advice.
You should really divide the voltage measured across the resistor by the
voltage measured on the amp output. This will compensate for frequency
response variations in the sine wave generator/amp AND in the
multimeter. Both can be substantial at these frequencies.
1/ Port tuning does not affect broadband sensitivity.
2/ Lowering tuning frequency usually improves transient response.
3/ Raising tuning frequency can cause unacceptable driver excursion
below the tuning frequency.

Cheers, John

--
John Graley Home: 01784 469404
7, Buckingham Court, Mob: 0976 892045
Kingston Road, Work: 0181 5603171
Staines, Middx, England TW18 4JG
======================
In a car a 45Hz F-3dB isn't at all wrong when designing a bass reflex
enclosure.
So 35Hz isn't usually anything to strive for since it will in most cases
sound muddy (even if the transient response is improved)

And when lowering the tuning freq, the length of the ports increases so
there might be practical limitations involved.

IMO tuning isn't for finding a point where the cone excursion is
virtually
zero, it's for getting the accurate sound and that does not necessarily
mean the proper tuning frequency described previously in a post.

AND of course sealed enclosures is a whole different thing when we are
taking
F-3dB points.

Please note that I'm not criticising John or his opinions since they
aren't
faulty, just sharing my opinions and adding some experiences,


> Cheers, John
>

Jonas
--
----------------------------------------------------------
Note:Written above are my own opinions, not my employer's!
----------------------------------------------------------
Jonas Holmgren
Product Engineer - Ericsson RF IC's & Modules
e-mail: ekajonh@eka.ericsson.se, voice: + 46 8 4046136
http://hem.passagen.se/ekajonh

On Sat, 31 May 1997 18:05:33 GMT, ar826@lafn.org (Irving Shiro) wrote:
>In a previous article, stroker@mv.itline.it (Alberto Treves) says:
>
>>What is exactly peak power handling?
>>Is it just the duoble of rms power handling? (or it changes between differents
>>amps)
>
> RMS is what the amp can put out all day long...
> Peak power is not just double the rms. It usually is that much,
>I dont think... What this refers to is how much it can put out for a
>very short time, such as a hard bass hit, or something similar.
>
>>Is a 800watt rms amp capable of pushing a short burst of 1600watt (peak)?
>>How much a peak is going to last ? (40-50-60 milliseconds?)
>>I know RMS power handling (in a driver) refers to thermal resistance of driver
>>(how much a driver can handles without over-heating for continued program), but
>>peak power handling (in a driver) is maybe much more a matter of mechanical
>>strengh of driver.(how much a driver can go forward and inward without going out
>>of cone allignment or on the opposite without distorting(with too much rigid
>>suspension)), isn't it?
>>Thank you in advance for your infos
>> Alb
>
> There is a spec on all drivers called "X-max". This refers to
>how far the cone can move back and forth. This tells you how much air is
>being displaced, and pretty much how loud the thing is... :).
> If you are trying to match up a driver with an amp, it is always
>better to OVERpower it than UNDERpower it. Underpowering could result in
>clipping your speaker and it being broken! However, you dont want to
>give a 30W speaker 600W, either. Hope I helped...
>later
>-sb
>
It's called marketing.......

RMS stands for "Root Mean Square" and it refers to the math involved
to get to its value. An RMS value aka "effective value" is actually a
measure of the "heating effect" of a sine wave.
The power ( P ) of a current ( I ) flowing through a resistance ( R )

is I x I x R, therefore Power ( Watts ) = Current squared x
Resistance. The RMS value of an AC current is the value that DC
current uses to produce the same power or heating effect.
When buying an amp or speakers always refer to the *RMS* value, the
higher the number the more effective power it produces or handles.

Peak, Peak to Peak and RMS

This is where the marketing comes in .....


/ \ + Peak 10volts
/ \
0 volts__/_____\_________ sine wave
\ /
\ /
\ / - Peak - 10volts


Peak = 10 volts
Peak to Peak = 20 volts
RMS Value = 7.07 volts

convert Peak to RMS = Peak x .707 = 10 x .707 = 7.07
convert Peak to Peak to RMS = Pk to Pk x .3535 = 20 x .3535 = 7.07
====================
You're right up to here. But the ratio to WATTS RMS vs WATTS peak is
NOT .707, it's .5.
.707 is the ratio for volts, and for amps, and power is volts x amps, or
as you stated above, current squared times resistance, go figure. This
is where the idea that peak value is twice the rms value.
======================

These values are all true for the above sine wave ( a real sine wave
is actually more rounded at the top and bottom but this is the best I
can do with ASCII). Notice that the RMS value is the lowest. The
measurements will work the same way for watts. Now if I was going to
advertise an amp or a speaker, and I didn't care about my reputation
or my customers, I would advertise it ( in HUGE letters ) as 200
Watts, and somewhere on the box (in tiny little letters ) I would put,
if I bothered to put it at all, that this was the Peak to Peak value.
What this 200 watt amp/speaker is effectively capable of
producing/handling is 70.7 watts RMS, quite a difference wouldn't you
say? Also with amps you have to consider at what % of distortion that
it is being measured at, in other words if you can't turn it up over
half way ( because it really sounds like crap if you do ) you bought
an amp that puts out 35W RMS, a really big difference, huh?
Now if you didn't know much, which would you buy a 50W RMS @ .01
distortion for $199 or a 200W with a nice box that says in huge
letters 2 0 0 W A T T S for $199?

Peak Power handling in a driver............

Thermal-limited acoustic power level is what this spec is usually
referring to, in other words, it is the point when the voice coil
burns because it can not handle any more electrical power. Depending
on what the voice coil is made of, the Peak power may be higher than
the acoustic power, but the acoustic power can never be higher than
Peak as the voice coil would simply burn up.

Xmax.........

The maximum linear travel / excursion of the speakers piston. If this
value is exceeded the voice coil leaves the magnetic gap (in a forward
phase) or bottoms out (in a negative phase). In either case the voice
coil becomes deformed causing friction and eventually burns up. I have
seen pistons/cones that where blown right out of the basket, spider
and all, nasty!

Sensitivity (sens) aka SPL...........

Measured using 1watt or 2.83volts as reference, it is how loud the
speaker is at 1meter (just over 3 feet) the unit of measure is dB or
decibel. The higher the number the louder.
In order to get an increase of 3dB you need to double the power, so if
you get a sub with a high sens to start off with you can have a louder
sounding sub with the same amp.

This is getting long so I'm going to stop,


Hope this helps,
Alex
ramjam@globalserve.net

Subject:
Re: HELP ME... Speakers
Date:
Tue, 27 May 1997 21:20:19 +0100
From:
Roger Peter Jones <roger@gogi.demon.co.uk>
To:
ERSKINE FENTY <amg.loudspeakers@virgin.net>
>> I have a merc 230e and i want to buy some new speakers for the front of
>> the car. I am thinking of buying custom fit Pioneer's but i believe that
>> i may be better with 10cm mid-ranges and spearate tweeters.
>>
>> The custom fit speakers i where looking at are the pioneer TS-H125 and
>> are 50 Watt max. I would like to achieve the best sound for money and
>> just need some advise. So please suggest!!
>>
>> They will be running of a Denon DCA-550 amp which i think is 30 x 4 RMS.
>>
>> So Thats that!!!
>>
>> RoGER
>The Merc 230, like most Merc in that range tend to have their speakers mounted
>on the
>dash.
>This is an ideal place for any speaker in the front of any car. Many people
>believe that
>the main sound of music should appear to be coming from one place, i.e, a single
>point
>source of sound. In this respect 2-way co-axial speakers serve this purpose
>quite well.
>The advantage of a separate mids and tweeters is that each speaker can work well
>within
>its effective frequency range, and combined handle more power. You then have the
>problem
>of deciding where to hide the tweeters.MAC AUDIO make a range of speakers which
>are
>custom fit for Mercedes, I had a set in my AMG 190E a few years back. I choose
>them over
>other brands not just because they looked better or because they cost more, but
>simply
>because they sounded better. They had a power handling of about 35wrms and they
>were the
>business. An amp on your front speakers always makes them sound so much better
>than
>running them from the headunit so your Denon should do a great job. You might
>also like
>to consider putting a 47uf Bi-Polar capacitor in line with each front speaker.
>This will
>reduce the bass that these speakers can't really deal with properly, it will
>make them
>come alive and produce crystal clear music that will just take your breath away.
>47uf =
>about 800Hz/-6dB. this might seem drastic but it still lets through frequencies
>at
>100Hz, it is just they are less damaging to your speakers and allow the speakers
>to
>almost breath again. If 47uF seems too much then you can try 100uF which will
>effectively let through twice as much bass.
>
>Good Luck. ERSKINE.
Thanks for the excellent advise, have you had any experience with subs
for Mercs. The w124 230e's have the fuel tank behind the rear seats
which makes it quite differcult to choose a good sub. I was thinking of
a Pioneer 12inch after reading a review in Revs. What do you think. Also
what type of power would be needed to run that in a sealed box.
--
Roger Peter Jones

Hi Roger.
Too many people, that petrol tank is a real pain. There are a few ways of over coming your problem. On the rear shelf of your car is a first aid kit. This can be easily removed to allow the bass from the boot to breath into the car. I believe that AUTO ACOUSTICS now make a custom built chamber to take advantage of this fact and offer a choice of speakers to do so. Ref. page 10 of June’s Car Hi Fi - + 44 (0)1932 849211. They quote a price of £75.00 for the shelf and £200 for the box + the cost of a 10” speaker that works in a 25litre box. In my experience, although I love Auto Acoustic to death, I brought 3 shelves for my 190 and they were not a perfect fit, they did need a fair bit of delicate machining to get a near perfect fit.
You could use a decent bass tube like a Bazooka or Eminence bass tube and just prop the flap on the first aid kit open to let the bass in. I choose to use a A/A Shelf and build a 4th order Bandpass box using a 15” sub. When all was finished the install looked the biz, you couldn't really see the box, but then the boot build reduce my boot space by about 2/5th. It did sound good though. With respect to power, well that depends on your box or tube. What ever the speaker, check its WRMS rating and divide this by two. An amp should never be played at more than 75% of its WRMS output. To put this into context if your speaker is rated at 200wrms, then its real handling power is probably about 100wrms in a well made box. If you want to drive this at full power then you need an amp that can produce at least 135wrms, this will allow for the amp to be run at it’s optimum, it will also preserve the life of both the amp and speaker, more power is fine but it must be controlled via the gain settings. This is also dependant on the voltage output of your head unit and any other devices in the chain like graphic eq’s etc. If you have a high voltage head unit i.e.,. 4v+ then you need to watch your gain settings and keep them lowish. Again in my opinion in cars like BMW’s and Merc’s, bandpass boxes tend to produce the sort of efficiency that is need to over come all that metal and good insulation from these cars. If you use the first aid kit hatch then your choice of box is not so restricted, but I just love bandpass boxes.

stay in touch.

ref: to Isorbaric wiring.

Hello Denis.

In order to answer your question I have to make a few assumtions. But it will all make sense in the end.

Isobaric. It is not easy to see from your drawing but I assume that you have a box with a center panel and have a 8” speaker on either side that is bolted together to form an air tight seal between themselves.

The speakers need to move in a push pull configuration in order for them to work correctly. Assuming that your speakers are 4ohms.

If you want a 2ohm load then this is how you do it.
* from your amp connect the +ve output to spk A +ve terminal.
* from your amp connect -ve output to spk A -ve terminal
* from spk A +ve terminal, connect to spk B -ve terminal.
* from spk A -ve terminal, connect to spk B +ve terminal.

If you want a 8ohm load then this is how you do it.
* from your amp connect +ve output to spk A -ve terminal.
* from your amp connect -ve output to spk B -ve terminal
* from spk A +ve terminal, connect to spk B +ve terminal.

good luck
ERSKINE.

ref: Need some help on the subject of SPL.

Hi Chris, thanks for your e-mail. I am sorry to say that all the pages in my designs are with reference to Bandpass Boxes, though because of the nature of some of my designs they might look a bit like Transmission Line Enclosures. SPL is a funny thing in cars. If your are serious about it all, then these basic tips will help you on your way. You will need serious battery and alternator power to start with. Amps have to be chosen carefully, i.e., high power output/low current needs. With respect to your bass box, well simple physics will help, the more speakers you use in stacks of two’s and four’s and multiples thereon in the better. With respect to an actual box, well a vented box gives you loudness at high volumes but is not very good at power handling, a sealed box is not as efficient but power handling is good. If you put these two boxes together you come up with a 4th order bandpass box. In tri-chamber mode this will use 4 identical speakers of your choice, if they are 12” or 15” then there is scope to build a very serious box. It should be possible depending on size and choice of speakers to produce a box that is capable of at least 105-110dB from a 2.83v input which is equal to about 1watt. Again depending on your chosen box and speakers it should be possible from an amp which is bridged into 1ohm and producing about 200wrms to be able, taking the transfer function of a car into consideration to get at least 145dB. I say at least because you might have more power and different cars affect acoustics differently, but tri-chamber isobaric in a 4th or 6th order configuration are the way to go. You could go the 6th order route, this is where the rear chambers are vented to either the outside world or to the inner chamber. Both will give you about 2-6dB more than its equal 4th order brother but you run the risk of excessive cone movement which could lead to speaker damage if you opt for the vents to be vented out of the box rather than into to the middle chamber, even though “out” they produce the most SPL. If money was no object, then I would say buy the best set of 4 x 15”’s you get, like the Cerwin Vega Strokers and get a 6th Order, Isobaric Tri-Chamber box built. With an amp capable of say 500wrms+ you would be moving into the 150-155dB arena, and that is serious. There is another design you might like to think about, and this one really depends on the shape of your car and how much space you have to play around with. The box I have in my Mazda uses 4 x12” in a very serious vented configuration. It is design for pure SPL and makes no allowance for sub bass, i.e. it comes in at about 90Hz, but then in the same box is an 18” upside down in a Quasi-Bandpass box. This requires you to choose speakers very carefully, and I must admit it took be hours of going through my database of speaker parameters to get it right, but the 18” takes care of bass below 90Hz and box works in a compound loaded push/pull arrangement. If you were to use say 4 x 12” stokers and either their 15” or 18” you could do some serious damage on the SPL scene. Anyway if I can be of any further help you know where to find me. ERSKINE.

ref: need bass in a small and tight boot space.

Hi.

The simple answer to your question is what is good bass? Different boxes/speakers produce different types of bass, and the person looking for SPL bass might not necessarily want Point of Reference bass. To produce large amounts of distortion free bass, speakers of larger sizes tend to accomplish this task easier than those of its smaller cousins, so 2 x 8’s will not sound as good as 1 x 15”, all other things being equal. Don’t listen too much to those people who like to play silly mind games with certain numbers within speaker parameters. An 8” speaker isn’t designed to compete with a 15”, even if you take 4 x 8” and put them in a fancy box, they will still have a hard time up against a single 15” in aan equally fancy box.

With respect to the porting of your box, this will depend on the present size of the box and whether the speakers you have actually like ported boxes, without the speaker details and your current box size this question is hard to answer, but I suspect that if your after quality bass, then you might already have a good box. If you wish to improve on your box then there is always room for improvement with out having to waste lots of money over beyond the cost of your current box.

Two chambers or one? Is there any air coupling between the two speakers, i.e., although they have separate compartments is there any porting between these two compartments. If there is, then differences can be found if these speakers are wired in series or wired as 2 x 4ohm assuming a left and right channel input. In my experience, and this backed up page 91 of The Loudspeaker Cookbook, that speakers with separate compartments but receiving a single signal from a single amp often, upto about 3dB, perform better than two separate boxes powered from two separate amps. It is all about the mutual coupling of acoustic wave forms.

I hope this helps you along your way, if you would like a more serious look at all the different types of enclosure and their pros and cons explained in detail but in plain simple English, have a look at our web site, look in the section CAR AUDIO and lets us know what you think.

Good Luck. ERSKINE.
http://freespace.virgin.net/amg.loudspeakers/

ref: dual voice coil speakers.

The vast majority of speakers available on the market
today are referred to as electrodynamic. All
electrodynamic speakers share a fundamental aspect
of operation: the reaction of a fixed magnetic field
against a changing one. In most electrodynamic
speakers, a voice coil, which is a single coiled length of wire
wrapped around a cylinder called a former, produces the
changing magnetic field when alternating current from the
amplifier flows through it.

This current is an electrical representation of the sound that
was produced by the musicians in the recording studio and
causes the voice coil (and therefore the cone or dome
attached to it) to react against the fixed magnetic field
produced by the speakers fixed magnet. A positive pulse
should cause the cone to move outward and a negative
pulse should cause the cone to move inward. When the
cone moves as a result of being propelled by the voice coil,
it produces the changes in the air pressure of the listening
environment that we perceive as sound.

What is a dual voice coil speaker?

A dual voice coil speaker is simply one in which
two separate lengths of wire are wound together
around the same former and terminated
independently. Except for some exotic exceptions,
both voice coils have the same number of turns
and length of wire, resulting in identical electrical
characteristics.

In most cases, one coil is wound onto the former first, and
the second one is wound over the first one. Naturally, it is
more expensive to wind and terminate dual voice coils and
you will typically pay a small premium compared to a similar
single voice coil speaker. So what do you get for the extra
few bucks? Do dual voice coils offer a performance
advantage? Not really. Do they offer any benefit over a
conventional single voice coil design? Definitely.

What is the advantage of dual voice
coils?

The primary advantage of the dual voice coil
speaker is wiring flexibility. A single dual voice coil
driver offers the user three hookup
choices...parallel, series and independent. In a parallel
hook-up the drivers impedance will be half that of each
individual coil (a dual 4 ohm speaker would be a 2 ohm
speaker in parallel.) A series hook-up results in twice the
impedance of each single coil (a dual 4 ohm speaker results
in 8 ohms if its coils are wired in series.) Finally, you can
wire each voice coil to a separate channel of your amplifier,
which can be useful if your amplifier is not mono-bridgeable
or if you are bridging a four channel amplifier down to two
channels to run your sub.

The independent wiring application is the one that brought
about the need for dual voice coil speakers in home audio.
Unlike most good car amplifiers, home amplifiers and
receivers are typically not mono-bridgeable. For this reason,
dual voice coil woofers were developed so that a subwoofer
or center speaker could be driven from the left and right
channels of the average stereo home amp/receiver. Since
sub-bass frequencies are hard to localize, the dual voice
coil subwoofer allowed sub-bass reinforcement within one
cabinet and one speaker. This cabinet could be placed
inconspicuously in a corner or along a wall of the listening
room, with the obvious benefits being space-efficiency and
lower cost than two independent bass cabinets or a larger
cabinet with two subs in it. Many popular home subwoofer /
satellite speaker systems still use this basic configuration.

What happens when you run different
signals into each voice coil of a
dual voice coil speaker?

Essentially, if there is any difference between the
signals driving each coil at any given point in time
at a given frequency, the voice coils will either
fight each other or help each other, depending on
the phase relationship of the two signals at that
frequency. This is not the same thing as bridging an
amplifier and can create undesirable non-linearities and
distortion because different input signals at each voice coil
create shifts in the speakers electrical parameters.

For this reason, it is advisable to mono-bridge the amplifier
whenever possible and connect the voice coils of the dual
voice coil speaker together in parallel or series. If a dual
voice coil subwoofer must be wired to two independent
channels, the inputs to both channels should ideally be the
same (summed mono) and every effort should be made to
match the gains of both channels as closely as possible.

Whats the point of a dual voice coil
speaker
if I have a mono-bridgeable amp?

Because most decent car audio amplifiers are
mono-bridgeable, it is just as easy to run a single voice coil
sub in mono as it is to run a dual voice coil sub in mono.
Where a dual voice coil subwoofer has an advantage is in
giving the user greater wiring flexibility while avoiding
speaker-to-speaker series connections.

As an example, a dual 4 ohm subwoofer with its coils in
series behaves as an 8 ohm speaker. The same speaker,
with its voice coils wired in parallel, behaves as a 2 ohm
speaker. If your amplifier is designed to run at higher
impedances, you would wire the coils in series. If your
amplifier is a high-current design which produces optimum
performance at lower impedances, the parallel connection
makes more sense.

The benefits of the dual voice coil design are even more
apparent with multiple subwoofer installations. Lets say you
are using a five-channel amplifier with a single mono
subwoofer channel that produces optimum power at 4 Ohms
mono. You want to run four subwoofers in your enclosure.
You have a choice between using four 4 ohm single voice
coil subs or four dual 8 ohm voice coil subs. With the single
voice coil speakers, the only way to achieve a 4 ohm mono
load is to wire them in a series-parallel arrangement. The
problem with this, is that it is less desirable to wire
subwoofers in series to each other (as opposed to parallel).
Because of slight and unavoidable differences between
speakers and because of the high likelihood of uneven
loading between different speakers in a car, there will be
slight differences in the mechanical behavior of the two
speakers in series. These differences in movement result in
induced voltage (called back EMF) being created by the
speakers across the series connection. This effect causes a
problem when two speakers which behave differently are
connected in series because the speakers can modulate
each other

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Last revised: November 14, 2002.

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