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Old 23rd July 2004, 06:44 AM   #1
JinMTVT is offline JinMTVT  Canada
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Default OK i need a speed crash course!!! have a few questions !! PLEASE HELP! :)

Ok. i've been browsing trhough this site (mainly here in the loudspaker section ) for nearly 2 weeks now...
I do not know how many posts i've read but i know that i've learned alot more here than in 1-2 years of internet looking up stuff randomly

I do have several questions, that i did not find any answers to yet.
And i would appreciate very much, quick information by the "pros"
of this forum and pointers to help me understand what i need to get to the next level of learning.

First off ...

When a driver "plays" a signal, it produces wave in front and back of itself right?
Let's say this speaker is in an infinite baffle enclosure,
that would mean we are only getting HALF of it's power
( front radiated wave? ) since the back wave is losing itself somewhere else ...
Is there any way to use that "back firing" power to get higher efficiency of the drivers?
That is the question that has directed me to 1/4Wave / TL type of designs, because i thought that the point of the design was to USE that back wave to reinforce the sound
( but now i am unsure of that fact, or even if it can be done )
I'd say with myvery limited knowledge of accolyte type
that in a sealed enclosure, near 50% of the "power" radiated from the driver is lost right ? so that makes for always less than 50%
driver/power effieciency for a startup right ?

So any comments on that question of mine will help me understand what we are actually doing here by alot



Then why is it that we try and get the flasttest response curve from all the systems when we could only use an active "equalizer"
with appropriate testing equipment ( fairly cheap right now with all available softwares ) and tune everything for a good response?
I know that for a start we need as flast as possible, but it seems that some are overdoing it, without even testing in room conditions ( wich i know can change everything with modes and reflections .. )
I am not sure also what to think about how to think about our human earing that is surely not lineair from 20 to 20k ,
do we have to take that into account ?


Lastly for today, i do notunderstand the analogy between elentrics and audio, impedance and such ..
i understand the impedance of a speaker varying with frequency
( as with any electrical devices .. ) but other than that .. ???


Ok enough for now,
please take into account that my knowledge is very limited
( limited to MAster Handbook of Accoustics and what i learned on internet and deductions .. ) but i want to learn as much as possible


Thank you very much for your time
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Old 23rd July 2004, 07:05 AM   #2
sqlkev is offline sqlkev  United States
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I'm no expert..but i think i answer some..feel free to correct me please

1. When a driver "plays" a signal, it produces wave in front and back of itself right?

keep in mind different frequencies have different wave length..
the tweeters that play 4khz-20khz will radiate more direct frequencies forward...

2. Let's say this speaker is in an infinite baffle enclosure,
that would mean we are only getting HALF of it's power
( front radiated wave? ) since the back wave is losing itself somewhere else ...
Is there any way to use that "back firing" power to get higher efficiency of the drivers?

The back is essentially an "enclosure" itself...the higher frequencies are more direct and will be push foward...the lower and non-directional waves will make the sound from the enclosure in the back


That is the question that has directed me to 1/4Wave / TL type of designs, because i thought that the point of the design was to USE that back wave to reinforce the sound
( but now i am unsure of that fact, or even if it can be done )
I'd say with myvery limited knowledge of accolyte type
that in a sealed enclosure, near 50% of the "power" radiated from the driver is lost right ? so that makes for always less than 50%
driver/power effieciency for a startup right ?

I'm unsure of this
but when i think of TL designs...i think of horns
i think it serves 2 purposes..
1. to let the speaker breathe as it reaches the lower freq. (port)
2. the horn type outlet will project the lower freq. for better extension?


btw...on the subject of ur ears perceive different freq. at different levels...ure right
try google the term "loudness contour"
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Old 23rd July 2004, 11:28 AM   #3
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Hi, another non expert reply...

When you reinforce the speakers front wave, with the backwave, you do it with a Bass -reflex port, or a TL. The bass-reflex is more common - i.e. easier - most freeware will tell you how.

The idea is to get only the lowest frequencies - where your particular driver is dropping off, i.e. not producing much sound - to come out of the Bass - reflex port... The reason is two-fold a. This sound from the port is displaced physically from the driver - so you have two sound sources. At frequencies in the mid and upper bass - down to about 100Hz IMO - this additional source interferes with the sound from the driver , below 100 Hz, it gradually interferes less and less because of the long wavelengths cause the two sources to act almost as one - so it's not as noticable . b. The sound from backwave is out of phase with the sound from the front of the driver 180 deg.... so they actually cancel each other out... UNLESS the frequency is very low - so that because of the time delay (from when the backwave starts to when it exits through the port) it now almost co-incides with the phase of the next frontwave from the speaker... Thereby increasing output at certain low frequencies This causes some inaccuracy, but in the low frequencies it is not very noticable if you design the port very nicely. Bass-reflex is the most common way to increase lower frequency output anywhere. It is rather flexible ito size and you can change the sound in a lot of ways, depending on what sound you prefer

Transmission Lines use a pipe, sort of like an organ pipe... to increase the low frequencies through resonance and an output at the end of the pipe... a certain frequency will be amplified a LOT if played into a pipe with the same length as the frequency's wavelength... 100 Hz = +/- 3.4m/12 ft. That same freq will be boosted a little less with pipe 1/2 of wavelength and less with a pipe of 1/4 of wavelength. The advantage are a more precise sound, more boost to lower frequencies if you have wavelength size pipes, better delays - so that backwave is more in phase with frontwave BUT TL's are mostly much more difficult to design correctly, damp with wool correctly and build in general NO airleaks whatsoever so if you haven't got the tools, find a sympathetic cabinetmaker... If you find ready made flatpack kit it can be very worthwile.

Sealed enclosures - my fav.s because I don't mind less bass and large boxes - Can be designed properly with good driver selection and good wool stuffing to produce very clean sound They must be designed and built airtight... otherwise you have a not at all designed bass-reflex The other thing with well designed sealed enclosures, are that they drop off fairly gradually (rel. to TL / BR) so placement in a room is more flexible - and your in room response is quite good... definitely not as bad as 50% of BR or TL.

It's your choice... All can be very satisfactory if you design carefully and know what you want and can get...

Active equalisers.... mmmm... It is possible, but mostly make things sound worse unless very well designed digital or super well designed analoque system... Mostly they work better if this filter/equalizer has direct control over each and every driver...

Impedance has two big influences... Say your amp pushes 50watts in 8 Ohm... if it has to produce sound into 4ohm at the same loudness as 50 watts into the 8 Ohm part of the frequency... it has to produce 100 Watts, same 200 Watts into 2 Ohms. 400 Watts into 1 Ohm Most amps don't like this...

The driver impedance also influences the X-over frequency that is designed into your passive X-over - because impedance is one of the determining factors of the X-over Freq... Sometimes not so bad, sometimes quite bad.

It is not a bad idea to have an overall slope of 2dB from bass to tweet, tweet being lower dB than Bass.

Finally my speakers were designed for my room... - a fairly small 14x14x9ft room. So huge bass will always be a problem. I get decent bass down to 50 Hz and usable output down to 40 Hz with two 6.5 inch drivers in a sealed 40 liter enclosure per speaker... Icould get more if I place them closer to walls, but then midrange suffers.
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Old 23rd July 2004, 12:42 PM   #4
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The rear wave of the driver can be utilized via bass reflex or TL or rear-loaded horn enclosures but there is a limitation on which frequencies can be so utilized, basically those from about 100 Hz and lower. Higher frequencies tend to want to go in straight lines and it's a difficult engineering feat to get them to change direction and contribute to useful output. Besides, using both sides of the cone doubles output, which sounds wonderful, but that's only a 3dB increase. Since speakers naturally tend to roll off below 100 Hz anyway the most useful aspect of designs that use the rear wave is to augment response below 100 Hz where the additonal help is required and not higher than that where it isn't.

Active equalizing is the best method, but affordable active equalization is a very recent development. Within 10 years active digital EQ will likely become the norm in consumer-level gear. In the world of pro-touring concert sound it is already the norm. These systems achieve perfectly flat response and do account for room acoustics. See: Behringer DEQ2496 for an entry-level device suitable for home use as well.

www.billfitzmaurice.com
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Old 23rd July 2004, 01:17 PM   #5
smbrown is offline smbrown  United States
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It has been mentioned that the back wave can be used to reinforce the front via a TL or bass reflex and of course that's true, but lets not forget the back-loaded horn, either. This is a very nice solution because you can get very tight, fast bass. The woodworking is a bit more complicted, but of the BLHs I've built, I have not found it too overwhelming of a challenge.

The one problem I see with all active equilazation is that it generally is easiest and least expensively done with solid state components, which are not everyone's cup of tea. I've been using passive line level equilazation on some of my recent stuff and been very happy with it. If you have some gain that you can lose between the preamp and power amp, this might be an ideal solution. No matter what your preference (tubes or SS), it means less components in the signal path, and no components between amp and speaker. I like that.
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Old 23rd July 2004, 03:37 PM   #6
sek is offline sek  Germany
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Well, yet another non-expert point of view
Let me get a little more in theory...

When a speaker moves it's cone forward, it produces a pressure buildup towards it's front side while at the same time producing a pressure drop towards it's back side. Thus everytime the (bass) speaker generates an air wave by moving it's cone, this particular wave (in theory) propagates to both directions with inverted phase (polarity) with respect to each other. The same applies to speakers in boxes, and a tweeter is a speaker with a (very very small) box, just that the box volume is there to "catch" that inverted wave.

The keyword here is acoustical short (circuit). Due to their inverted "polarity" relation, the front and back pressure levels tend to "equalize" and null each other. The idea of using the pressure wave propagating to the back (in the same way one can use the wave to the front) is as old as the electrodynamic speaker itself.

You can actually hear that acoustical short by driving a woofer in free air and moving your head in the area near the basket frame. You will hear an enormous increase in sound pressure level just at the basket border. That's the pressure niveaus of the two waves nulling each other (aiming entropy).

What box types like BR, TL or back loaded horns are doing is:
1. an (acoustical) filtering of the back side's pressure wave (to narrow it's frequency band)
2. an (acoustical) delay by letting it travel through some kind of channel and connecting the opening of that channel to the (acoustical) propagation direction of the front pressure wave.

The result: in a narrow frequency band the sound from inside the enclosure is delayed to the (special) amount of a half duration of a wave period (at the center frequency of the band) and thus adds to the front pressure wave constructively. That leads to a certain gain (amplification) in that band around it's center frequency.

Regarding your second question: if it only were that easy!!!

The main question here is: is the unequalized system capable of providing the (frequency dependent) headroom with still it's original sound quality? And most often this is just not the case.

If you could take any cheap speaker with a given frequency response - terrible but known - and wanted to equalize it flat, then this speaker would have to be capable of delivering a frequency depentent amount of distortion-free sound pressure level headroom at all equalized frequencies. Well, if that was possible with any given speaker, everybody would do it (they keywords here are: overload and distortion).

In order to achive the best results, people tend to start from a flat-enough system, because the influence of increasing the level of some frequencies can be very high on the overall system sound quality. But you're right, some are really overdoing it.

Regarding your third questions: yes, our hearing is non-linear. But as microphones are not, speakers shouldn't be, either. We want to hear what the microphone has detected. It is our ear's non-linearity when listening to the speakers (or headphones, btw.) that replaces it's non-linearity when listening to the sound event that the microphone was recording. This doesn't work sufficiently enough to take a playback as a replacement for a live concert, though. But taking the non-linearity of the human hearing into account when building speakers is extremely difficult. I know of no really good "loudness" device that can really solve this problem. Do a search for Fletcher and Munson (two researchers) if you like.

Regarding your fourth question: the analogy between real-world systems and electrical networks is nothing one can explain in a forum post. But there are plenty of books about it. I know of none in your language that I could recommend you, sorry.

Okay, hope this helps,
Sebastian.
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Old 23rd July 2004, 06:53 PM   #7
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Just wanted to put in a plug for open baffle designs too since they use the back wave to create a null at the baffle boundary which results in more directivity and less room influence. If you don't mind less bass and large systems you may want to add this to your list of considerations.
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Old 23rd July 2004, 07:49 PM   #8
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Not too many years ago, active equalizers and whatnot could muck up the signal. (I once had a Carver "Sonic Hologram" that hummed.) These days, active components are often quite excellent. I have a Richter Scale Series III that I use to straighten out the bass response in a troublesome room. It works real well. The Linkwitz Orion system is very well regarded, and it uses a lot of EQ. However, the cost of active equalization can be significant fraction of the system cost.
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Old 23rd July 2004, 09:49 PM   #9
Solinar is offline Solinar  United States
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The more experienced speaker guys can feel free to correct me if I am wrong, but I believe a sealed box uses the energy from the back stroke of the speaker as well.
A back stroke of the speaker causes a pressure increase inside the box, which is stored as potential energy, which reinforces the next forward stroke when it happens.
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Old 23rd July 2004, 10:20 PM   #10
sek is offline sek  Germany
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The pressure buildup "loads" the air "spring", so to say... That's true of course, at least something has to be done with the energy of the back stroke.

But this actually decelerates the cone, dampens it's excursion, and thus it doesn't add to the output. The best would really be an infinite (open) baffle without enclosed volume and without interaction between the front and the back side...

Sebastian.
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