tet@uvaee.UUCP (Thomas E. Tkacik) (10/28/85)
Does anyone know how the speaker configuration (vented, sealed, etc.)
affects a woofers sensitivity? Acoustic suspension speakers are known
for being less efficient than bass reflex speakers :-)
Is this because IT IS an acoustic suspension speaker, or is it because
the drivers used are less efficient than drivers used in vented speakers?
Why would this be the case?
I have also seen a statement which helps a builder decide wether a particular
woffer will work better in a sealed or vented cabinet.
If fs/Qe (resonence frequency divided by electrical Q) > 100 use a vented
speaker. If it is less than 100 use a sealed speaker.
Does anyone know how to decide if a driver will work in a TL or horn?
Tom Tkacik
...!decvax!mcnc!ncsu!uvacs!uvaee!tetsaf@bonnie.UUCP (Steve Falco) (10/29/85)
> Does anyone know how the speaker configuration (vented, sealed, etc.) > affects a woofers sensitivity? Acoustic suspension speakers are known > for being less efficient than bass reflex speakers :-) > Is this because IT IS an acoustic suspension speaker, or is it because > the drivers used are less efficient than drivers used in vented speakers? > > Does anyone know how to decide if a driver will work in a TL or horn? Acoustic suspension is less efficient than bass reflex for a couple of reasons. First, in bass reflex, the sound from the port adds to the volume while in acoustic suspension, that sound never leaves the box. Second, acousic suspension is very springy because the air is trapped. Consequently, it retards cone motion. For horns, you want a driver with a stiff suspension. Not rolled polyfoam! Klipsch uses an old style cloth accordion pleat which is quite stiff (made by Eminence (sp?)). Now here's my question: If you seal the box for acoustic suspension, how do you handle changing barometric pressure - it tends to displace the cone from center... Maybe there should be a valve on the side of the box to allow mountain dwellers to bleed off some of that extra air the factory put in the box. (Guess I'd better put a :-) in to avoid the flamers...) Steve Falco
rdp@teddy.UUCP (10/29/85)
In article <480@uvaee.UUCP> tet@uvaee.UUCP (Thomas E. Tkacik) writes: >Does anyone know how the speaker configuration (vented, sealed, etc.) >affects a woofers sensitivity? Acoustic suspension speakers are known >for being less efficient than bass reflex speakers :-) >Is this because IT IS an acoustic suspension speaker, or is it because >the drivers used are less efficient than drivers used in vented speakers? >Why would this be the case? > There are a variety of factors affecting sensitivity. In a simple-minded case, take the example of trying to attain reasonably low cutoff frequency in a sealed box enclosure (acoustic-suspension). One is working against a combination of suspension compliance and the compliance of the trapped air. This results in fairly stiff restoring force. Inorder to get a reasonably low cutoff, it becomes necessary to have a correspondingly high moving mass. This combined with the motor assembly, results in a low efficiency. Well, one might say, why not increase the "efficiency" of the magnet-voice coil to increase overall efficiency. Well, that is certainly possible for the mid-band regions, but the reult is a system that would be over-damped in the bass (presuming, of course, it was correctly damped to begin with). Interestingly enough, if one plots amplitude versus frequency for a given system, changing just the BL factor (the combined electro-magnetic "efficiency"), one end up with a family of curves showing expected changes in mid-band efficiency, but the output at and below resonance remains relatively unchanged. This is because below resonance, the voice coil is no longer in charge of the cone. There will be, granted, slight changes, but these will be negligable by comparison to the changes in midband efficiency. Now, in a bass-reflex (or passive radiator) system, the box no longer presents a simple compliance to the driver. It is now a complex reactive load. The designer is now faced with many more degrees of freedom in trying to optimize the system. Such optimization schemes might include maximizing efficiency and bandwidth in exchange for maximum output level (as was done in the BBC monitor series LS1, etc), or trading reasonable bandwidth, moderate efficiency and very low low-frequency distortion for high power handling (as in the KEF RS104aB), or simply sacrificing bandwidth and distortion to get the absolute maximum efficiency and power handling (as JBL had tended to do). THe answer is, unfortunatly, not a simple one. I had spent some time consulting to several companies, attempting to design systems for them. My approach was to specify the system as completely as possible, then use these figures to derive the specifications for the needed bass driver. Looking in driver catalogs from a variety of manufacturers, it was nearly impossible to find anything that would match. Not that I was designing anything wierd, but it seemd that the fascination with magnet size resulted in drivers that were markedly unsuited for closed-box systems. Bass reflex designs were out, primarily because of the stigma associated with such systems ("no, I do not want my speaker to sound like a JBL!") Richard H. Small, in the JAES (see references below) has thoroughly dealt with the efficiency issues of closed-box, bass-reflex, and passive radiator system in his series of articles. I would recommmend the technically minded among you to look these articles up, as they present the entire physical basis of direct-radiator loudspeaker design in a coherent, scientific and unprejudiced manner. Small, Richard H., "Direct-Radiator Loudspeaker System Analysis," Journal of the Audio Engineering Society, vol. 20, no. 5, pp. 383-395, Jun 1972. Small, Richard H., "Closed-Box Loudspeaker Systems," Journal of the Audio Engineering Society, "Part I: Analysis," vol. 20, no. 10, pp. 798-808, Dec 1972. "Part II: Synthesis," vol. 21, no. 1, pp. 11-17, Jan 1973. Small, Richard H., "Vented-Box Loudspeaker Systems Part I: Small Signal Analysis," Journal of the Audio Engineering Society, vol. 21, no. 5, pp. 363-372, Jun 1973, "Part II: Large-Signal Analysis," vol. 21, no. 6, pp. 438-444, Jul/Aug 1973, "Part III: Synthesis," vol. 21, no. 7, pp. 549-554, Sep 1973, "Part IV: Appendices," vol. 21, no. 8, pp. 635-639, Oct 1973. Small, Richard H., "Passive-Radiator Loudspeaker Systems," Journal of the Audio Engineering Society, "Part I: Analysis," vol. 22, no. 8, pp. 592-601, Oct 1974, "Part II: Synthesis," vol. 22, no. 9, pp. 683-689, Nov 1974. Dick Pierce
seifert@hammer.UUCP (Snoopy) (10/30/85)
In article <601@bonnie.UUCP> saf@bonnie.UUCP (Steve Falco) writes: >Now here's my question: If you seal the box for acoustic suspension, >how do you handle changing barometric pressure - it tends to displace >the cone from center... Maybe there should be a valve on the side of >the box to allow mountain dwellers to bleed off some of that extra air >the factory put in the box. (Guess I'd better put a :-) in to avoid the >flamers...) > > Steve Falco The box isn't sealed *that* tightly. Snoopy tektronix!tekecs!doghouse.TEK!snoopy
rfg@hound.UUCP (R.GRANTGES) (10/31/85)
[] Well, one more bit of information. Its not the type of box, so much, folks (horns aside) but the sound pressure level the designer is aiming for. If you're not using a horn and you want loud sound at low frequencies, then you have to move a lot of air. For a given diameter cone, louder means more cone excursion (in and out). Speakers designed for big excursions (so called "long throw") have the problem, "How do you keep a long travel linear?" 1) You have to keep the suspension linear - solutions discussed earlier. 2) you have to keep the voice coil seeing a constant magnetic field throughout its travel. For long travels, the easiest most economical way to do this is to make the voice coil much longer than the magnetic field. That way however it moves the coil still sees the same field. However, it means that only a small part of the field of the voice coil is being used at any time. Hence, a loss of efficiency. If you decide to live with not so big excursions then you can still get your frequency response and efficiency in the small box, only don't play loud. Horns act like transformers and couple the speaker to the room. You can get loud sounds, large air motion, with small speaker excursions, hence no need for long throw design (but watch out for the speaker suspension when you drive the speaker below its cutoff frequency. -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg
rdp@teddy.UUCP (10/31/85)
In article <601@bonnie.UUCP> saf@bonnie.UUCP (Steve Falco) writes: > >Now here's my question: If you seal the box for acoustic suspension, >how do you handle changing barometric pressure - it tends to displace >the cone from center... Maybe there should be a valve on the side of >the box to allow mountain dwellers to bleed off some of that extra air >the factory put in the box. (Guess I'd better put a :-) in to avoid the >flamers...) > Two answers: first, have YOU ever tried to build a wooden box that was absolutely airtight? It's real tough, especially when you start cutting holes in it to put drivers, terminals, etc. Secondly, I have, myself, never seen an absolutely airtight enclosure that would suffer from the problems you describe. In fact, many manufacturers seem to be acutely aware of this problem and build a deliberate leak into the box. Probably not worth the engineering and manufacturing time to worry about. These leaks have a long time constant (2-4 seconds) and, because of this, are of no consequence in effecting the performance of the enclosure as a sealed box. The time constant should be, of course, much longer than that of the woofer/enclosure system. This means that for a 30 Hz system, whose time constant is on the order of 10's of milliseconds, anything substantially greater than that is suitable to make the driver think that the box is truly sealed. The danger arises from having the leak "whistle" under certain conditions. Of course, if the :-) you have is global to the paragraph, then you shouldn't have read my reply :-). Dick Pierce