Raw Speaker Design For Vented vs Sealed Cabinets

[TomCarlos]

Friends....

I've been searching old threads to educated myself on differences in raw speaker design. Specifically, I curious to know if there is a difference in a speaker design where it would work best in a sealed vs vented cabinet.

I looked at the Eminence web site (and others) and noticed that some speakers are recommended for vented cabs, others are recommended for sealed cabs, and others are recommended for either. Some speakers have no recommendation.

I have a Fender GE-112 cab. It is a sealed cab for a single 12". If I am not mistaken, Fender shipped these with either a Celestion or the Fender/Eminence speaker. I have an extra Fender Standard speaker (the 8 ohm, 100 watt that is sold in the Twin Reverb). This is the heavier magnet speaker. I checked the Fender web site but didn't see anything that recommended for sealed or vented cabs.

Maybe this is a big deal over nothing.... I guess you drop the speaker in the cab and do a listen test. U nless you hear something horrifically wrong, you're done?

Thanks for opinions ....

Tom

[tedmich]

short answer: try it a low volume and see if it works, the wrong box can destroy a speaker at high volume.


overly long answer:
almost all guitar speaker cabs are opened back, which is to say NOT sealed. Some are closed back but very few are actually vented or ported (Mesa Thiele is a notable and well loved exception). An excellent white paper is at the Eminence web site:
Sealed vs. Ported Enclosures | Eminence Speaker

Most traditional guitar speakers do not give the Thiele-Small (T/S) parameters so its difficult to design ported enclosures with them, but that is changing. Celestion and Eminence now give all the necessary T/S parameters, but many speakers are unsuitable for different enclosures (or alignments). In general sealed enclosures are smaller with predictable bass roll off of 12db/octave from their tuning frequency, while ported enclosures can be larger, have lower bass response and are often more efficient. Various type of tuned ported cabs are possible including ones which produce a narrow bandwidth very efficiently, the typical car BOOM BOOM bandpass box is one of these. The bass fill may have a couple of notes but you only get to hear one...Open backed cabs are loud sloppy and familiar to guitarists but (I hope) are dying off.

here is a nice clear description of what T/S parameters mean in the real world, avoiding most of the painful math
http://www.speakerplans.com/page89.html

I usually pay the most attention to Qts, descibed well by Eminence as
"As a general guideline, Qts of 0.4 or below indicates a transducer well suited to a vented enclosure. Qts between 0.4 and 0.7 indicates suitability for a sealed enclosure. Qts of 0.7 or above indicates suitability for free-air or infinite baffle applications. "
http://www.eminence.com/support/unde...dspeaker-data/

that translated to ported/sealed/open backed respectively

[The Dude]

Yes, for the most part, there seems to be little rhyme or reason to guitar cabinet design as it relates to TSP. Even the infamous Marshall 4X12 was said to be designed for the least amount of wood waste and had little to do with cabinet volume as it related to speaker parameters (a "happy accident"). Since reproducing lower frequencies is not the main objective (where cabinet design is more critical), box design for guitar is not a big priority or really a big deal. That's not to say that different boxes won't sound differently.

[TomCarlos]

Thanks!!!!

The Eminence papers look interesting and informative. I will dig into those carefully. I think you guys have helped me think about the potential of damaging a speaker in a sealed cab if it wasn't designed for that. While I do not play at high volumes, I am still concerned about "compatibility" between speaker and cab designs.

Tom

[cbarrow7625]

Keep in mind that potential damage to a speaker in any type of cabinet design is cause by one of 2 factors: too much heat (long term) that burns up the voice cool or over excursion from the bass bring too loud or too low in frequency for the design.

For a typical guitar amplifier over excursion, thus cabinet type, doesn't usually come into play very often. The typical response from a guitar amp starts rolling off well above any frequencies that will damage a decent speaker for excursion reasons. That is why just about every guitar speaker ever made has been loaded into sealed 4x12's without any of them having excursion damage problems (that I know of, feel free to correct me).

Bass guitar speakers are a whole different animal due to the potential low frequencies involved. They have to be treated like any other speaker. In other words, closely follow design rules based on T-S parameters.

A 100w guitar speaker is pretty stout; most guitar amps are 100w or less. Of course, be wary of what "watts" means to some manufacturers. I don't think you have any concerns using that speaker in any cabinet configuration for guitar.

Anybody have a respond graph of various guitar amp on axis frequency responses? It's something I have wanted to measure for quite a while and never found the time. I actually wanted to measure response at the speaker terminals (electrical transfer function from the amp into that impedance) vs. the acoustic response of the system to see the real influence of driver impedance on overall electrical response from the amp. Would love to know the -3dB and -10dB LF and HF roll off points of some of our favorite amps and which frequencies end up peaking and getting reduced acoustically due to the drivers and can types. It would be fun and helpful for lots of folks to know. Don't mean to hijack the thread, but it is a tightly related subject.

[J M Fahey]

One real technical reason for some speakers "not being recommended for tuned boxes" is that they wonŽt benefit from them.
Guitar speakers fall squarely in that area.
Their TS speakers *look* so bad that although manufacturers must certainly know them, donŽt publish them so as not to hurt sales.
Although for guitar they sound good.
One *critical* parameter is "Q" , issued in 3 numbers: a Mechanical Q, an Electrical Q and Total Q , which roughly is the first two in parallel, thus being the lowest of them 3.
Q shows (among other things) how well damped the speaker is, and to improve (lower) it, you need f*ck*ng big magnets, very good voice coils, cone weight perfectly matched to suspension and edge elasticity (all of them have cloth edges).
So typical examples of low Q (good) speakers are JBL K/E130 , EVM12/15L, etc.
The cream of the crop, all heavy and expensive, and little used in guitar amps (compared to the million others).
To avoid long Math, for a speaker to benefit from a tuned box which means flat , extended bass, you need Q between 0.35 and 0.45 .
If Q is any higher, youŽll have an impossible to tame peak at the resonant frequency.
Now for closed *flat/HiFi" enclosures, ideal Q (inside the box) is 1 which guarantees flat bass down to the (mounted) resonant frequency and smooth rolloff below, at 12dB/oct .
So designers choose very soft (huge suspensions, foam edge) speakers from 0.65 to , say. 0.9 Q so in the box Q and resonant frequency raise.
All this for Hi Fi speakers.

Now killer Guitar speakers have unmounted Q of 1 and above !!!!
Useless in tuned cabinets and with a very noticeable peak in an enclosed one (think 4x12" chest punch) at a uselessly high frequency for Hi Fi: 90 to 120 Hz .

And in an open back cabinet, their hard suspension and hard thick cardboard edge protect them from overexcursion and. although the open back cabinet may roll down heavily below 150Hz (or worse), the huge free air resonance peak at 80 to 110 Hz provides "fake bass" which we happen to like.

Of course all this was a happy accident, Fender and others used the speakers which were available in the 40's and 50's and open backed cabinets were an unavoidable necessity to vent tube heat.

Just read a few Eminence datasheets which offer a lot of data and youŽll see

[Steve A.]

almost all guitar speaker cabs are opened back, which is to say NOT sealed.

True enough for "real" guitar amps but those darned Marshall cabs are usually closed back.*** But as you say there are very few tuned guitar amp cabs...

While shape and size usually doesn't matter as much as with hifi stereo speakers some cabs are just too darned small- and some are too big. (I remember some big cabs from the 60's- often homemade- which had just a single 12" speaker in them. The idea was to set your Fender combo amp on top of them and then plug it into the ext spkr jack.)

Steve Ahola

*** I prefer open backed cabs myself because it allows the speaker to breathe- with a well-designed cab the sound from the back blends with the sound from the front to create a nice ambience. A closed back cab reminds me of playing through headphones- very tight and restricted!