Frequency Response

[guildbass]

May I add my bit?

I used to hang out at a PA hire company and am bessy mates with a rather talented engineer (currently the sound guy for Porcupine Tree).

Basically, very few electric basses reproduce much in the way of fundamental notes. My Guild 302 has a truly monstrous bottom end, so much so that a lot of smaller PA rigs get into trouble with it. We measured the output and about 50% or the Guild's output on the 'E' was 41 hz fundamental. The rest was 1st harmonic. Now, the Guild is a big heavy set neck instrument, most bolt neck basses have a lot more 1st harmonic than fundamental. In other words, we hear a lot of 82 Hz when we pluck our 'E' string. If you want to hear a genuine fundamental 41Hz note, listen to a double bass or one of those strange Guild/Ashbory things with the rubber strings.

That's one reason why most bass cabs are have a bump at 80-110 hz. The other is....

Just take a look at the frequency response of a 10" driver...It's running out of steam well above that fundamental and putting it in a big box with similar units won't make the slightest difference to its electro/mechanical performance of the driver

You CAN make a cab work at 41Hz or whatever, First you have to use a really big driver that can move enough air at those lowrer frequencies, a 15" or an 18" unit. Then you port the cab with the correct lengths of tube to extend the speaker's range below it's natural 3db point. You have to use at least two and the ports work like an organ pipe..or like blowing over a bottle....they are the right length to resonate at the chosen frequency. You use one as a transition pipe to get from say 70hz to 50 hz, then another to get down to 40 odd Hz..

However...

....the other thing is that a lot of amps, and particularly valve amps, don't have nearly enough current and a high enough damping factor to control a speaker moving that slowly. |You need literally HUNDREDS of watts and some serious current to do that and basically your best bet is a really big bipolar transistor power amp or an even bigger modded Mosfet amp biased on to class AB (Mosfets can be great sounding as they are voltage amps, like valve amps, and can be very sweet when biassed up properly)...But to control really big speakers running at low frequencies, you need good old fashioned transistors cuz transistors are current devices.


Couple that with a valve pre-amp to deal with the tone and you'll get there.

I have a Peavey Maxx preamp with a crossover output going into a Crown MA2400 (1200 watts rms into 4 ohms per channel) with one side running my 15" in it's ported cab, and the other side running a pair of 7" speakers... But then I don't want distortion at the loud end. It's a really smooth fat sound with lovely crisp highs and will make your flares flap about if you stand in the way!

[stevie]

[quote name='Bill Fitzmaurice' post='1305759' date='Jul 16 2011, 01:42 PM']It's a bass reflex cab, nothing special or unusual except for the external driver mounting, and that serves no purpose, it could be inside with the same result.[/quote]
It's not a reflex cab. It's a Helmholz resonator - it more or less says so on the drawing. It has more in common with a classical guitar than a ported cabinet.

[alexclaber]

Stevei, tsn't a reflex cab a Helmholtz resonator? Though I see your point, you're suggesting that the cab works by solely exciting the resonant system of the two masses of air in the ports vibrating against the spring that is the volume of air in the enclosure divided by the area of the port:enclosure opening. But what's happening with the woofer backwave, why wouldn't that act like the frontwave from a woofer in a normal ported design?

Guildbass, interesting stuff about the bass! Did you compare how the output varied depending upon pickup choice and plucking position? That has a significant effect based on my studies. I'm going to be doing some research into this and writing some articles for BGM in the near future. Your comments on getting a speaker to reproduce those low fundamentals are about halfway there! The reason most bass cabs have that ~100Hz bump is due to the cabs being too small for the driver's Vas. You don't need a very big driver to move air at those low fundamentals - you need a driver with very high volume displacement, which is a function of cone area and clean cone excursion. Obviously increasing the driver size increases one of those parameters - but in the process it also increases Vas which makes it harder to get deep response from a cab without it ending up impractically large.

A speaker port doesn't work like an organ pipe (a transmission line speaker does) but it does indeed work like blowing over a bottle. However if you put two speaker ports into one enclosure you don't get two independent tuning frequencies, you get one whose frequency is the function of the total port area, enclosure volume, and total port air mass. I haven't come across a problem with most modern amps driving speakers that can go low - apart from valve amps which really do struggle - but most modern amps are seriously powerful!

Regarding the output from a double bass, I believe it essentially works like a dipole speaker (thus there's a lowpass filter effect which depends upon the frontal area of the body) plus a Helmholtz resonator whose tuning is set by the F-hole areas and body's internal volume. Although I've yet to measure one, from what I've heard in person and based on other stuff I've read I doubt there is very little low fundamental content in the radiated acoustic sound. An electric upright would be another animal entirely.

Edited July 17, 2011 by alexclaber

[ShergoldSnickers]

[quote name='guildbass' post='1306372' date='Jul 17 2011, 02:22 AM']You CAN make a cab work at 41Hz or whatever, First you have to use a really big driver that can move enough air at those lowrer frequencies, a 15" or an 18" unit.[/quote]
The diameter of a speaker won't tell you much about the lowest frequencies it can produce. It may tell you something about how high the SPL will go when reproducing those frequencies though, but even this is not certain. The eight inch drivers in my hi-fi speakers cut off rapidly only below 36Hz, so have no problem reproducing the 42Hz E fundamental. Getting them loud enough to be useful as a bass rig however... well that's simply not on as they don't have the efficiency required. If I could get enough cabinets together though, then I'd be able to shift enough air to get loud enough, though it would probably sound awful higher up the range with multiple cabinets causing all sorts of phase cancellation mayhem off axis.

The 'larger speakers go lower' myth may be in part due to smaller diameter speakers having a higher useful frequency range generally, and thereby fooling the brain into hearing less bottom end by comparison. Rather than larger diameters, one or two manufacturers have gone the route of increasing excursion limits and increased power handling for their designs, both in terms of watts outright, and also in terms of thermal capacity to handle those watts. In essence they have increased the stroke of the piston driving the air, negating a larger diameter. It's the volume of air encompassed by the 'piston stroke' that matters. More SPL from the same diameter speaker.

There comes a point when decreasing the diameter of a bass speaker is going to make it non-viable for gigging, so we commonly see 10" 12" and 15" designs used for bass cabinets. The notable exception is the 5" unit used in the Phil Jones range of cabinets. These units are usually ganged up with 24 being used in one design. I'm sure the bottom end is equally as deep as other designs using larger speakers. They claim a 'flat' response down to 25Hz. Whilst I'm slightly skeptical, 42Hz would seem well within that range.

[url="http://www.philjonespuresound.com/about/reviews/394.pdf"]The Phil Jones 24B - review as pdf link[/url]

To get the required gig level SPL you may need to multiply drivers when using smaller diameters, but the lowest frequency produced within a defined dB drop limit is not a direct function of their diameter.

[ShergoldSnickers]

[quote name='alexclaber' post='1306489' date='Jul 17 2011, 11:02 AM']Stevei, tsn't a reflex cab a Helmholtz resonator?[/quote]
Isn't a Helmholz resonator just a name that identifies a high Q? Tentative question as I'm still learning about all this stuff. Fascinating though isn't it? Hooked.

[stevie]

[quote name='alexclaber' post='1306489' date='Jul 17 2011, 11:02 AM']Stevei, tsn't a reflex cab a Helmholtz resonator? Though I see your point, you're suggesting that the cab works by solely exciting the resonant system of the two masses of air in the ports vibrating against the spring that is the volume of air in the enclosure divided by the area of the port:enclosure opening. But what's happening with the woofer backwave, why wouldn't that act like the frontwave from a woofer in a normal ported design?[/quote]
A reflex cab contains a Helmholtz resonator, which is the vent, but a reflex system is more than just the vent. This design seems to rely solely on the vent to produce sound - just like blowing over a bottle. The contribution from the rear of the driver would be what you would expect from a driver in free air (or from the air blowing over a bottle)- practically nothing. I think the only reason the driver is not boxed in is because it would raise its resonant frequency. However, if you imagine this system with the driver boxed in, it starts to look very much like a bandpass system.

Edited July 17, 2011 by stevie

[alexclaber]

I'm not convinced by that. What happens if you turn the driver around? Then it looks just like a bass reflex cab! The only difference I can see between cone-inwards and cone-outwards mounting is that the latter gains you fractional extra internal volume and the woodwork required for rear mass-loading (via the hole it fires through adding air mass to the cone as on the Omni15) is a bit easier. Now if the cab was designed to be mounted in a wall so that the back of the driver is an a different room to the listening room then that would make it a different sort of cab but that doesn't appear to be the case.

[Bill Fitzmaurice]

[quote name='alexclaber' post='1306489' date='Jul 17 2011, 06:02 AM']Stevei, tsn't a reflex cab a Helmholtz resonator? Though I see your point, you're suggesting that the cab works by solely exciting the resonant system of the two masses of air in the ports vibrating against the spring that is the volume of air in the enclosure divided by the area of the port:enclosure opening. But what's happening with the woofer backwave, why wouldn't that act like the frontwave from a woofer in a normal ported design?[/quote]
Exactly. All the parts are the same as a standard bass reflex, they're just rearranged. It would be easier to visualize as a bass reflex with the driver mounted in the usual fashion, but that's what one's imagination is for. Now if one has no imagination...

[ShergoldSnickers]

Patent awarded to Graham Holliman for the 'Velocity Coupled' infra bass cabinet design is [url="http://www.fear-of-bicycles.co.uk/BC/pdfs/GB2037534A.pdf"]available here[/url], as a 650k pdf.

I'll let the experts draw any conclusions from the numerous diagrams and technical description.

[alexclaber]

Aha!

[endorka]

[quote name='Bill Fitzmaurice' post='1303817' date='Jul 14 2011, 05:41 PM']If you play a pristine 40 Hz or lower sine wave through a capable speaker with very low THD you can't even discern pitch.[/quote]

Agree with the difficulty discerning pitch on the pure low notes. On occasion I've had to transcribe synth bass lines where all the upper partials have been rolled off, and I sometimes find it quite tricky. On a causal listen you think you can discern pitch, but on a close listen you realise it just isn't there.

To transcribe the line accurately you have to listen to it while not listening, and try to catch the pitch by surprise. Very strange.

Jennifer

[dc2009]

[quote name='alexclaber' post='1306715' date='Jul 17 2011, 02:44 PM']Aha![/quote]

Hi Alex,
Finally found the time to read through all the technical gubbins you linked me to on your site! Very impressed - clearly explained and not too waffley, and cleared up a lot of stuff in my head.
Out of interest, what would you say (based on your opinion and experience) is the head that works best with your gear?

[stevie]

[quote name='Bill Fitzmaurice' post='1306659' date='Jul 17 2011, 01:45 PM']Exactly. All the parts are the same as a standard bass reflex, they're just rearranged. It would be easier to visualize as a bass reflex with the driver mounted in the usual fashion, but that's what one's imagination is for. Now if one has no imagination... [/quote]
It doesn't look like a reflex cab no matter where you stick the driver. There is no Vb. The resonator chamber isn't loading the driver as it would in a normal reflex cab, because it's separated from the driver by a hole. Take away the resonator chamber and what you've got an arrangement that is closer to a transmission line than anything else. Not only that, but according to the patent, the design is flat to 7Hz using a 15" driver without any special characteristics. I'd like to see that in WinISD.

[alexclaber]

Stevie, if you enlarge the hole between the long ports and the main enclosure it would become a conventional ported box. If you blocked that hole it would indeed become a TL. But the fact that that hole acts as a second port changes everything! It's quite a fine detail though, had to read the patent to see it...

[alexclaber]

[quote name='dc2009' post='1306765' date='Jul 17 2011, 03:54 PM']Out of interest, what would you say (based on your opinion and experience) is the head that works best with your gear?[/quote]

Hard question to answer! Depends very much on the tone you're after, the bass/strings you're using, the loudness of your band and the specific cab. They're aren't many amps that won't work well for someone with one of our cabs. I was going to write a shortlist of amps that are popular with our customers but I keep thinking of more and more models!

[Prime_BASS]

[quote name='alexclaber' post='1306872' date='Jul 17 2011, 06:06 PM']Hard question to answer! Depends very much on the tone you're after, the bass/strings you're using, the loudness of your band and the specific cab. They're aren't many amps that won't work well for someone with one of our cabs. I was going to write a shortlist of amps that are popular with our customers but I keep thinking of more and more models![/quote]

The list will be incredibly long. As long as the head is pumping out real watts in a region of around 300 up you'll be fine with the midget or pushing it to the compact.
Had a SVT-3 pro putting 350watts into a compact and it felt rather under powered, yet I had a OTB running in 4 ohm mode into it Whig should be about 300-350watts an it was ridiculously loud still.

Markbass tend to be good companions, as are the TC heads, I've used both with a variety of Alexs cabs and they both sound wonderful at serious volume. A lot of praise comes from the harkte LH500 aswell.
I always tell people you get what you out what you put in sound wise, as long as the watts are there to drive it.

[Mr. Foxen]

[quote name='alexclaber' post='1306859' date='Jul 17 2011, 05:49 PM']Stevie, if you enlarge the hole between the long ports and the main enclosure it would become a conventional ported box. If you blocked that hole it would indeed become a TL. But the fact that that hole acts as a second port changes everything! It's quite a fine detail though, had to read the patent to see it...[/quote]

Where does the hole stop being a port and just being a hole? Like with half open back cabs, is that a giant port tuned really high to the point where it makes no odds, or is there a point where it ceases to be?

[stevie]

Yes, there is a point where it ceases to be a port and it just becomes a gap. The cabinet then changes from a reflex to a dipole. If you blow across the neck of a bottle you'll get a sound, but if you try it with a jam jar you get nothing.

[dc2009]

[quote name='Prime_BASS' post='1306940' date='Jul 17 2011, 07:06 PM']The list will be incredibly long. As long as the head is pumping out real watts in a region of around 300 up you'll be fine with the midget or pushing it to the compact.
Had a SVT-3 pro putting 350watts into a compact and it felt rather under powered, yet I had a OTB running in 4 ohm mode into it Whig should be about 300-350watts an it was ridiculously loud still.

Markbass tend to be good companions, as are the TC heads, I've used both with a variety of Alexs cabs and they both sound wonderful at serious volume. A lot of praise comes from the harkte LH500 aswell.
I always tell people you get what you out what you put in sound wise, as long as the watts are there to drive it.[/quote]

Not a Markbass fan but if I got sick of my Hartke speaker bit I could always try the head bit through one of these cabs.
Of course I'm sure many amps will work well, but was just wondering if Alex perhaps had a personal preference (or what does he/do most of his employees use)?

[deepbass5]

Love you guys for your passion, really like these techno threads.

Harmonics. It’s worth remembering it is our ear and brain's ability to sort harmonics that we can tell if a note e.g. (C concert) is played on a Clarinet a Piano or a Trumpet. Mens voices have more harmonics than a female voice, this makes a male voice clearer and easier to understand than a female voice, which is why women should not be used to make anouncemense at train stations.

So from all this you may sort of understand why a good sounding cab has more to do with selection and matching of components and construction and much testing than just going for the 0 - 60mph figure and the go fast stripe most manufacturers want us to go for.

I also understood that a Bass Viol sounds low mostly due to its unique harmonics due to its scale length

Edited July 18, 2011 by deepbass5

[Bill Fitzmaurice]

[quote name='Mr. Foxen' post='1306946' date='Jul 17 2011, 02:09 PM']Like with half open back cabs, is that a giant port tuned really high to the point where it makes no odds, or is there a point where it ceases to be?[/quote]
It is technically a ported enclosure, but the resonant frequency is so high that its function is moot and the enclosure functions as an open baffle. As to how the enclosure in question really works that becomes definitive only when both frequency response and impedance sweeps are examined; the one or the other in and of itself isn't definitive. The lack of impedance sweeps in the patent abstract is most curious, and is one missing detail that should have resulted in outright rejection of the application.

[guildbass]

[You don't need a very big driver to move air at those low fundamentals - you need a driver with very high volume displacement, which is a function of cone area and clean cone excursion. Obviously increasing the driver size increases one of those parameters - but in the process it also increases Vas which makes it harder to get deep response from a cab without it ending up impractically large.

And...

The 'larger speakers go lower' myth may be in part due to smaller diameter speakers having a higher useful frequency range generally, and thereby fooling the brain into hearing less bottom end by comparison. Rather than larger diameters, one or two manufacturers have gone the route of increasing excursion limits and increased power handling for their designs, both in terms of watts outright, and also in terms of thermal capacity to handle those watts. In essence they have increased the stroke of the piston driving the air, negating a larger diameter. It's the volume of air encompassed by the 'piston stroke' that matters. More SPL from the same diameter speaker.

Yes, agreed... It's a while since I thought about that... And it was late!

As regards the bass note and how much was fundamental, which pick -up etc... We just plucked the 'E' and looked at it on a scope. Almost certainly both pick-ups running together at full volume so a 50/50 mix. Tones back and front full up, on my fully passive Guild 302.

we were testing my HiFi amp at the time which was doing service as the power amp in my rig. it was a 60w into 8 ohm per channel Mosfet Ambit which had been hot-rodded by the Engineering half of IAS Loudspeakers (who remembers THEM!). in those pre-CD days they were using Max Townsend's original Elite Rock turntable prototype with it's silicon fluid damped arm and some nice reel to reel kit to get the low notes.Tthe other half of the company, Alan Willis, had perfect pitch and was a trumpet player so the speakers always sounded incredibly musical but they needed the Amp to go down to properly low frequencies! I saw that amp with a set of folded horn Beaulieus push a Linn Sondec into oscillation, Something that Ivor Tiff said was impossible... Sadly, Willis boasted about it and Tiffenbraun rather sternly told his distributors to dump IAS or lose the Linn concession...Exit IAS!

[Phil Starr]

[quote name='fatback' post='1306026' date='Jul 16 2011, 05:35 PM']Presumably it's the longer scale length that's allowing for the different harmonics? How does that work?[/quote]

A double bass is pretty complex acoustically. The string sits on a bridge which excites a resonant chamber. Movement of the bridge will damp the movement of the strings killing some harmonics quicker than others. The front face of the bass is under a bit of tension and will have its own series of resonances and the body of the bass forms a helmholtz resonator which will again have a resonant frequency. there will also be resonances and standing waves inside the cavity which is highly reflective and probably considerable sound contributions through the back and sides.

Add in the possibility that the strings could be made from nylon, gut, polyester steel or some sort of hybrid and would in any case be different in construction from most electric bass strings and you can see there are plenty of reasons for the different sounds.


[quote name='Mr. Foxen' post='1306369' date='Jul 17 2011, 02:07 AM']The ratio of harmonics depends pretty heavily on the sampling point I think. As in the pickup position.[/quote]

Absolutely and also the point where the string is excited/picked.


[quote name='stevie' post='1306453' date='Jul 17 2011, 10:14 AM']It's not a reflex cab. It's a Helmholz resonator - it more or less says so on the drawing. It has more in common with a classical guitar than a ported cabinet.[/quote]


[quote name='stevie' post='1306601' date='Jul 17 2011, 12:55 PM']A reflex cab contains a Helmholtz resonator, which is the vent, but a reflex system is more than just the vent. This design seems to rely solely on the vent to produce sound - just like blowing over a bottle. The contribution from the rear of the driver would be what you would expect from a driver in free air (or from the air blowing over a bottle)- practically nothing. I think the only reason the driver is not boxed in is because it would raise its resonant frequency. However, if you imagine this system with the driver boxed in, it starts to look very much like a bandpass system.[/quote]

I love these old cab designs, I don't think the designers knew themselves how they worked half the time. Reading the article in the link about adding weighs to the tone arm to extract more subsonics and needing to protect the room from the speaker reminds me of some of the loony articles more recently written about cables in hi-fi systems.

For what it is worth, to me this is an attempt to get 'free' bass from the resonace of the cavity and to couple them along the pipe. The air space between the speaker and the cab would decouple the resonator from the speaker to a fair degree so it wouldn't act as a reflex cab in the conventional sense. I wouldn't want to have to do the maths to see how much of the pressure from the speaker went into the cavity and how much straight down the 'horn'. You could equally argue that the 'horn' would act as a port for the air space and you might with the right bit of luck get the resonances to work as a coupled design. Alex is right that the back of the speaker would radiate sound in the same way as the front but it's dispersion would be interesting with the basket and magnet in the way. The 'horn' would have its own resonances and also act as an acoustic filter losing almost all the high frequencies and selectively some of the lows. The design would be very sensitive to changes in the driver and to the way it would couple with the floors and walls of the room all things the designer alludes to.

I suspect this speaker would have had a large resonant hump which would have been very satisfying at a time when very few speakers had any real bass, a bit like the in car systems the kids install, but not very accurate. I picture the owner only listening to 'Zarathustra' and the intro to the Toccata and Fugue in Dm so they can hear that lovely resonance.

I love Basschat.

[stevie]

[quote name='Phil Starr' post='1307456' date='Jul 18 2011, 09:20 AM']I love these old cab designs, I don't think the designers knew themselves how they worked half the time. Reading the article in the link about adding weighs to the tone arm to extract more subsonics and needing to protect the room from the speaker reminds me of some of the loony articles more recently written about cables in hi-fi systems.[/quote]
The fact that the design was published in Hi-Fi Answers speaks volumes.

[Bill Fitzmaurice]

[quote name='Phil Starr' post='1307456' date='Jul 18 2011, 04:20 AM']I love these old cab designs, I don't think the designers knew themselves how they worked half the time.[/quote]
Quite right. The best example of that is the Karlson. Karlson himself called it a horn, but it had not a single characteristic of a horn. It was actually a series tuned dual chamber bandpass reflex, but as it was introduced some 20 odd years before T/S theory became well known he could be excused for not knowing that. OTOH he made response claims for it that were nonsense, and produced SPL charts that could only have been concocted.