Music Man HD-130 and Ampeg cab review

[Dave W]

Wouldn't it be different for every cabinet tested anyway?  I would think there's no one "correct" graph.  Each graph would be different when you're measuring an actual cabinets.  I think I understand that.

True, but in any given cabinet there would not be that kind of difference between sealed and ported. The principles are known. There's no reason to post a deliberately inaccurate graph.

[Granny Gremlin]

It wasn't deliberately inaccurate (how you can say it's severely inaccurate when it lacks any scale on the axes continues to tickle me); it was half-assed (because the point wasn't to be accurate, so much as show relative shapes, without making any comment about magnitude).   I removed it, so it be done with.

[Psycho Bass Guy]

I never saw the graph, but here are my observations:

Sealed (aka acoustic suspension) has, by defenition, poor transient response

If the cabinet is sealed, below tuning frequency, lower frequencies are attenuated logarithmically, which if plotted relative to amplitude, produces a linear acoustic rolloff, but the sound itself is still being reproduced at the same "speed"- movement relative to time (frequency: measured in Hertz) and "position"- rate of frequency relative to time (phase: measured in degrees). As a matter of fact, sealed cabinets have the BEST innate transient response because there are no phase interactions distorting the output. Ported cabinets have port-phase distortion and open-backed cabinets have secondary coupling (boundary reflections) interacting directly with the driver.

.... then again, so do bass reflex

"Bass reflex" is just a fancy way of saying "ported."

The best transient response comes from open back

Until tuning frequency, ALL cabinets act as open-backed. In an open-backed speaker installation, the low frequency limit is the cone resonance. Below that, the driver literally has nothing to 'grab' and converts all of the power below resonance into heat until either the voicecoil burns up (at the thermal limit- aka "RMS" wattage rating) or the cone suspension tears loose, both resulting in a destroyed driver if enough power below cone resonance is fed to the driver.

(sacrificing bass response unless you  put it in a very large baffle)

...in which case, you've just made it into some sort of enclosed cabinet.

and transmission lines (which require a box that is even bigger than a sealed, assuming the same drive unit).

Transmission lines are tuned subwoofer cabinets (or any portion of the audio spectrum for that matter) that are baffled both above and below a portion of the audio bandwidth.

This is because the speaker is operating in a box that mimics "free air"

No, it doesn't. It is in a box that is acoustically tuned like a crossover to air-load the driver in a way that it is only operating in in small section of the audio bandwidth. It's cheaper and easier to just do it electronically, which is why crossovers are so common and transmission line cabinets are not.

(vs the pressurized nature of a sealed or even ported, aka strategically leaky sealed box).  This minimizes the amount of force required to move the cone so it can react faster whereas sealed cabs hold the speaker back due to internal air pressure (the trade off here is that the speaker can handle a bit more power due to this added resistence to movement; it becomes harder to push the cone/voice coil to the extremes of travel).

You have it backwards. The description of the physical process is mostly correct, but the use of terminology and conclusions are incorrect. A transmission line cabinet only acoustically loads the speaker in the portion of the audio spectrum where it is the most efficient, but it most certainly DOES air-load the driver.

Also, ported cabs (using the same drive unit) do not have an extended range vs sealed

Correct

Ported cabs just have a response that is flatter lower, to the point of the port tuning at which  the response falls off a cliff.

...not so. Ported cabinets have tuning "bumps" at and above tuning frequency (in octave intervals) caused by the phase interaction of the primary baffle, cabinet airspace and the secondary baffle, the port, which produces a VERY uneven frequency response and phase distortion, but with the benefit of better efficiency below natural resonance of a space/driver combination until port frequency, below which point, the driver behaves like it is in open air.

A sealed cab will start to roll off earlier, but much more gradually providing you with lower bass frequencies, but not as much of them.

Correct
 

This tends to sound more natural to the ear vs a ported which can be boomy or hyped in the lower midrange/upper bass (giving the impression of more low bass, without actually having more).

No, there are "more;" they are the tuning "bumps" I was talking about. Additionally, because of the port's secondary baffling effect, some frequencies are cancelled while others are boosted by the tuning resonances, the phase distortion discussed earlier.

Further, some designers attempt to make this psychoacoustic trick even more effective (usually with smaller speakers or ones with relatively high resonant frequencies) set the port tuning so as to produce a bump in the response just before the response rolls off.  I find this sort of cabinet most displeasing soundwise, but I dunno if any instrument cabs are made this way or not.

That is the characteristic inherent in ALL ported cabinets, phase distortion, and it's not a psychoacoutic phenomena. It is measureable in a number of ways. By adding a secondary baffle, a port, connected to the outside air column, a ported cabinet will ALWAYS have phase distortion because of the time/frequency interaction between the two baffles caused by the physical difference in locations of the baffles.

[drbassman]

Wow, my head is spinning.  Interesting stuff for sure.  It could take a lifetime to learn all of the acoustic/physics behind cab design!  I'm short on time these days for picking up new complex info, but I appreciate the comments.  I do know that stuffing the MM cabs with the acoustic stuffin' did help the bottom end of things.  A more pleasing tone out of the cabs now.

That being said, I just go by what my ear hears and transmits to my brain.  If I like the sound, I'm  .  If I don't care for the sound, I'm  .  I want a strong loud output with balance across the range of my bass.  I want some tubey warmth without tons of distortion.  My brain wants to hear something like my old B-15 but a little better and loud enough to compete with my bandmates.  That's pretty much it.

Not to go too far off here, I do have another question about one of my speakers.  When playing through the MM amp into the MM cabs, one speaker emits an audible sound that will be hard for me to describe.  It frankly sounds like the paper cone moving in and out.  If I move away from the cab, I can't hear it, but sitting closer, I can hear that "paper" like sound as the cone vibrates.  It's not distortion or speaker farting, it's a mechanical vibration coming from the speaker itself.  I hope that makes sense!

[Granny Gremlin]

If the cabinet is sealed, below tuning frequency, lower frequencies are attenuated logarithmically, which if plotted relative to amplitude, produces a linear acoustic rolloff, but the sound itself is still being reproduced at the same "speed"- movement relative to time (frequency: measured in Hertz) and "position"- rate of frequency relative to time (phase: measured in degrees). As a matter of fact, sealed cabinets have the BEST innate transient response because there are no phase interactions distorting the output. Ported cabinets have port-phase distortion and open-backed cabinets have secondary coupling (boundary reflections) interacting directly with the driver.

You are wrong, my man.  First, transient response <> phase response though poor phase response can ruin the benefits of good transient response.  Confusion of transient and phase response is additionally not helped by the fact that both can be measured using the same test - square wave response (but different parts of the test speak to transient vs phase response characteristics). Transient response is merely how quickly the transducer will react to program signal input and, accordingly, start moving from it's point of equilibrium.  Phase response is how coherant the aggregate response of the entire enclosure is with respect to the given program element that is being reproduced - e.g. in a multiway system in the crossover region, the tweet and woof can be reproducing the same program element, but they don't (without a lot of design work) do so in the exact same time frame.  Additionally, they are not a point source (unless coaxial) so the distance to your ear creates a further timing differential and therefore phase discrepancy... and yes this happens with ports and those other things you mentioned too).  The fact of the matter is that due to the nature of how acoustic suspension cabinets work, the driver requires more power to move as it is facing greater mechanical resistance due to internal cab air pressure.  That is by definition, poor transient response.

Further, sealed  cabs do have at least one critical phase-related issue: backwave distortion (ported suffer from this as well).

I never made an assertion as to whether the TR of a sealed is better or worse than ported, leaving the matter to something like 'both are not so great in that regard' compared to other types.  

Yes, open back do have a major phase issue, but only (significant) starting at the point where the distance from the cone to the rear opening is 1/4 of the wavelength of the frequency, which is why the bass response disappears there (cancellation due to approximately opposite phase from the front and rear; I say approx because of the time it takes for the sound to travel around the cabinet makes it not exactly opposite even if it is a near-negligible difference - I wouldn't be so particular about that detail if I wasn't talking to you, as you tend to jump on me for such oversimplifications), but since the driver is essentially operating in "free air" it has the lowest mechanical resistance to movement possible and therefore better transient response than a sealed cab.  It is actually the characteristic that (at least somewhat technically inclined) open back lovers (guitards as well as hifi) cite as the reason they love them so much.

"Bass reflex" is just a fancy way of saying "ported."

Yep, I used both terms and thought I was  clear on that (... though to be really accurate, not all cabinets with a port are bass reflex - see Tapered Quarter Wave Tubes, Transmission Lines and Bandpass for example).  I kinda fail to see your point here; maybe I typoed somehow in my original post and you didn't include that in the quote.

Until tuning frequency, ALL cabinets act as open-backed. In an open-backed speaker installation, the low frequency limit is the cone resonance. Below that, the driver literally has nothing to 'grab' and converts all of the power below resonance into heat until either the voicecoil burns up (at the thermal limit- aka "RMS" wattage rating) or the cone suspension tears loose, both resulting in a destroyed driver if enough power below cone resonance is fed to the driver.

With regard to frequency response, yes (if maybe a bit oversimplified, see for example backwave distortion, which affects sealed and ported cabs but not open backs; close enough though).  Transient response I am not 100% sure, but doubt that it has no effect until resonance (but it is probably maximal approaching resonance). Phase response will vary among cab types at least slightly due to size/shape (e.g. baffle diffraction is a function of cab shape/size vs wavelength, rather than a function of speaker resonance).

...in which case, you've just made it into some sort of enclosed cabinet.

No - I think you misunderstood me here.  I did not mean (capital letters) Infinite Baffle, so much as (small letters) a larger baffle (than, say, a guitar combo) - see open back hifi speakers; the baffles can be multiple meters wide and tall and still open back - yes there is still a point of phase cancellation, but the idea is to make the baffle large enough so that this point is moved further downwards, ostensibly below the effective range of the drive unit or below the range of human hearing, but that is not always possible/feasible due to the dimensions/wavelengths involved.  Not very practical but some people swear by them (not me, but never really tried them out due to too damn big).

Transmission lines are tuned subwoofer cabinets (or any portion of the audio spectrum for that matter) that are baffled both above and below a portion of the audio bandwidth.

...

No, it doesn't. It is in a box that is acoustically tuned like a crossover to air-load the driver in a way that it is only operating in in small section of the audio bandwidth. It's cheaper and easier to just do it electronically, which is why crossovers are so common and transmission line cabinets are not.

...

You have it backwards. The description of the physical process is mostly correct, but the use of terminology and conclusions are incorrect. A transmission line cabinet only acoustically loads the speaker in the portion of the audio spectrum where it is the most efficient, but it most certainly DOES air-load the driver.

I'm not sure we are talking about the same thing here (some things you said make me think you're talking about Bandpass cabinets, but not sure).  Transmission line speaker cabinets are nothing like a crossover and not just for subwoofer (edit: oh I see you don't disagree with that, my bad).  They solve the essential problem of what to do with the speaker's backwave by either harnessing it (as useful additional bass response), or nullifying it (depending on whether the line is terminated to a port or a wall, respectively) using what is essentially a delay line (usually with mechanical Low Pass Filter, aka stuffing) which puts the driver's backwave in phase with the front.

This enclosure type is most often used for woofers in a multiway system (or, yes, standalone subwoofers) or full range drivers, but also (at the peak of their popularity in home hifi - the 70s) for midrange drivers in 3-way designs (and since 3 ways as well as TLs are mostly out of style we don't see that anymore).

In the case of a TL that is terminated to a port (99% of commercial and DIY specimens I have ever seen - closed TLs I have mostly seen in use when it is a midrange that is in the TL) the driver does operate in what is very close (if not exactly) free air conditions, as there is no internal cabinet pressure (from cone movement changing internal volume) since there is no bottleneck at the port terminus which is generally at least an order of magnitude, or thereabout , larger than a bass reflex port and presents no mechanical resistance.  A design parameter of TLs  is for the port (line terminus) to be in the area of 0.8 to 1.5 of the driver's Sd or effective piston area (most often 1 or just a hair more) - this means that there is (little or ) no  bottleneck for air moving in and out of the line to maintain equilibrium air pressure and though there may be some effect from having to travel down the line itself (especially if folded to minimise cabinet size) this is negligible (especially compared to sealed/ported).

Anyway, I was talking about these (think of a backloaded horn, with the horn tapered in the opposite direction, or straight in simplified versions.... and linear taper vs log .. and operating on a totally different principle as summarised above):



more info here: http://www.t-linespeakers.org/

and not these (bandpass):

...not so. Ported cabinets have tuning "bumps" at and above tuning frequency (in octave intervals) caused by the phase interaction of the primary baffle, cabinet airspace and the secondary baffle, the port, which produces a VERY uneven frequency response and phase distortion, but with the benefit of better efficiency below natural resonance of a space/driver combination until port frequency, below which point, the driver behaves like it is in open air.

...

No, there are "more;" they are the tuning "bumps" I was talking about. Additionally, because of the port's secondary baffling effect, some frequencies are cancelled while others are boosted by the tuning resonances, the phase distortion discussed earlier.

Yes, I don't disagree with you here.  I was oversimplifying (for your tastes).  By 'flater, longer" I meant that the final bass-end rolloff has a delayed start in ported cabs (relative to sealed), which I am sure you agree with from what you have already said.

That is the characteristic inherent in ALL ported cabinets, phase distortion, and it's not a psychoacoutic phenomena. It is measureable in a number of ways. By adding a secondary baffle, a port, connected to the outside air column, a ported cabinet will ALWAYS have phase distortion because of the time/frequency interaction between the two baffles caused by the physical difference in locations of the baffles.

What do you mean by the first use of the word 'that' here?  

Yes, there are measurable response phenomena here but what I was talking about is the tendency for the auditory system to hear the extended flat bass response (with caveats as explained in the point above - yes a simplification, but you must know what I mean - compared to the early rolloff  start of a sealed cabinet there is an additional period of relatively flat response... and we all know no speaker is litterally flat anywhere in it's response, or not for very long anyway) of a ported cab and feel like there is more and lower bass there which does not actually measurably exist - somewhat as if the rollof slope of the ported cab was more like that of a sealed - like the ear just does not compute a generally broadband signal just stopping right there /expects more.

[amptech]

Wow, my head is spinning. 

No wonder, acoustics is an endless education.

It´s great to see (and read) PBG´s thorough feedback (!) on the subject, but when two ´opposite poles ´have contrasting views on a subject it is sometimes difficult for the average reader to follow the thread and even more difficult to sort out what´s fact and what´s bullshit. Even common sense sometimes falls short when it comes to acoustics..

But the facts are here for those who see (and hear) them

[amptech]

Not to go too far off here, I do have another question about one of my speakers.  When playing through the MM amp into the MM cabs, one speaker emits an audible sound that will be hard for me to describe.  It frankly sounds like the paper cone moving in and out.  If I move away from the cab, I can't hear it, but sitting closer, I can hear that "paper" like sound as the cone vibrates.  It's not distortion or speaker farting, it's a mechanical vibration coming from the speaker itself.  I hope that makes sense!

Yeah, it makes sense - an optimal speaker is a silent one

Without ´researching´anything before answering, I have once in a musicman cab found a loose T washer that made a strange sound that was present at low volumes. A wrong screw had been used, and the nut was not tight against the wood.

Also, check the hardboard gasket around the cone (if you have one), it can make a slight noise if partially loose.

Hope it helps.

[dadagoboi]

...never mind.

[Psycho Bass Guy]

Wow, my head is spinning.  Interesting stuff for sure.  It could take a lifetime to learn all of the acoustic/physics behind cab design!

It's nowhere near as "sciency" as it sounds; I was just being very specific and dry to lessen the chance for misinterpretation. Ironically, what cab tuning has taught us is that simple and direct designs generally are the ones most folks like. Think about how many "hi-tech" bass cabinets have come and gone since the SVT coffin rolled out in 1969. ...and the stage standard is what? BTW, don't confuse phase distortion, sound "smear," with the more familiar amplitude distortion, aka "distortion;" they're different.

Not to go too far off here, I do have another question about one of my speakers.  When playing through the MM amp into the MM cabs, one speaker emits an audible sound that will be hard for me to describe.  It frankly sounds like the paper cone moving in and out.  If I move away from the cab, I can't hear it, but sitting closer, I can hear that "paper" like sound as the cone vibrates.  It's not distortion or speaker farting, it's a mechanical vibration coming from the speaker itself.  I hope that makes sense!

It's something you don't hear much about in the bass world: cone "cry." Basically, it's the mechanical/vibrational resonance where the amount of power being fed into a speaker causes the cone to have pressure feedback and the speaker breaks free of the amplifier momentarily and "farts." It's not the common "bass cab fart"- that's actually a combination of port chuffing and phase problems, but the most analogous way of thinking about it is to envision the process of making artificial fart noises like kids make.  Most modern bass cabinets use high powered drivers that never mechanically get to that point before they toast their voice coils. It literally varies from speaker to speaker. You could try to add doping (mass) to the offending cone and it should move the resonance below the range of the cab. You'll lose some midrange efficiency, but it shouldn't cause any problems and I doubt you'll notice much a change other than the bad noise being gone.

You are wrong, my man.

My comments stand. I have better things to do than argue with someone whose ignorance is only surpassed by his ignorance of that ignorance.

   

[drbassman]

Ah, Confucius said,"To know what you don't know is to possess knowledge."

Anyway, could you give me some clues on the speaker noise I asked about?  "I do have another question about one of my speakers.  When playing through the MM amp into the MM cabs, one speaker emits an audible sound that will be hard for me to describe.  It frankly sounds like the paper cone moving in and out.  If I move away from the cab, I can't hear it, but sitting closer, I can hear that "paper" like sound as the cone vibrates.  It's not distortion or speaker farting, it's a mechanical vibration coming from the speaker itself.  I hope that makes sense!"

[Psycho Bass Guy]

Anyway, could you give me some clues on the speaker noise I asked about?

I answered your question in my last post and amptech had some good suggestions, too. I think you may have glossed over what I said thinking I was replying to something else. Don't let tech jargon make you tune out; like I said, it's not heavy science. Doping is just a cone treatment, a light white glue or varnish coating that stiffens the cone itself and adds mass to it. Sanding sealer works great too. The idea is to keep the finish extremely thin so that it soaks into the paper of the cone or all you'll end up doing is making a hard topcoat that is brittle and will impede cone motion and end up cracking and damaging the cone paper. Speakers become more prone to cone cry as they age because the cone paper becomes more brittle and flexible over time. That's why 'broken in' speakers are louder than new ones.

Only my last sentence was directed towards anything else, and the quote should make it pretty clear what that was.

[drbassman]

Oops, my fault, sorry I missed that!  I got lost in all of the posts.  This is a reconed speaker.  Is that unusual?

[Psycho Bass Guy]

If it's a recone, there's a decent possibility that the cone could be misaligned, not enough to hang in the gap, but enough to wobble in it.

[drbassman]

If it's a recone, there's a decent possibility that the cone could be misaligned, not enough to hang in the gap, but enough to wobble in it.

Thanks.  Is this something I should worry about or have corrected?

[amptech]

Thanks.  Is this something I should worry about or have corrected?

If it is indeed the cone that is misaligned, there is no other way than to re-recone it I´m afraid.
It is possible though, to check  - you can remove the dust cap and measure or look if it´s OK.

Beware, doing so may very well damage the speaker!

If the sound does not bother you at playing volume, just leave it as-is.