Bill Fitzmaurice

[quote name='Grangur' timestamp='1386166089' po [quote]st='2296627'] Is this normal for a combo speaker to only take half volume? [/quote]It is. Combos tend not to use the highest quality drivers, and the cabinet size concessions made to keep the rig light and portable are reflected in limited low frequency output capability.

[quote name='flyfisher' timestamp='1386063296' post='2295071'] While not disputing tweeter sensitivity to excess harmonic distortion, is this really a practical issue with modern gear? Is the THD figure for modern amps really so excessive these days that it could damage a properly rated tweeter, even at full volume?[/quote]The THD figure you see listed for amps is what's used as a benchmark when measuring the amp output. That might be anywhere from 0.1 to 1% THD on average. By no means is that the limit to the THD that they can produce. If you crank the amp, or boost the signal level going in to the amp, you can easily reach 20% THD and more. That's how guitar players get their tone, and for that matter why guitar amps never use tweeters, because they'd never last through even one gig. Bass players tend to use amps large enough that they don't have to crank them to get loud enough, but not all do. And bass players who use distortion devices tend to cook their tweeters. [quote]Also, do cab manufacturers not include any simple filter protection for their tweeters? It would be fairly easy to roll off the frequency response above, say, 25khz [/quote]25kHz isn't the problem. Everything a tweeter reproduces is harmonics. A tweeter normally sees only 5% of the broadband power content. That means a tweeter in a 400w rig normally will never receive more than 20w. When you go from 5% THD to 15% THD the level of the harmonics goes up by 10dB. The tweeter that normally is called on to handle 20w now might be gettting 200w. Magic smoke ensues. Woofers aren't phased by the increased harmonics since they would be able to handle 400w anyway.

[quote name='ML94' timestamp='1385849982' post='2292925'] As the title says I'm interested in knowing what could happen if I underpower my cabs. At the moment I have a 600w Genz rig, both at 300W and head being a powerful 600w amp. What if I powered my 600w cabs with a 500w amp. What would the consequences be ? [/quote]For the woofers, nothing. If you have tweeters they can be damaged by too much harmonic content, and excess harmonic content is what's created when an amp (or any device in the signal chain, including fuzz pedals) is pushed to clipping. You're far more likely to push a smaller amp to clipping than a larger one, and the smaller it is the more likely that scenario.

[quote name='bassmachine2112' timestamp='1385898049' post='2293237'] I must echo the comment about how they sound in the mix ,that,s where they shine. [/quote]That has nothing to do with the isobaric design, and everything to do with what's inside, which is this: http://www.bluearan.com/index.php?id=EMIDLIT2510&browsemode=category There are many other sources for Deltalite II 2510 loaded cabs, and if they're not isobaric then a 2x10 will be as loud as a 2x10, not a 1x10.

Related to the OPs original question is if you can get higher sensitivity ported than sealed while maintaining the same response why wouldn't you? This is why: The blue trace is a ported ten with EBP 138, the black a sealed ten with EBP 80, both are 1.5cu ft cabs. At 50Hz, where demands on the amp and driver are the highest, the ported cab has 6dB higher sensitivity. That means it would take two of the sealed to match one of the ported. One reason one might prefer the sealed is if one doesn't care for the stronger low end of the ported. But that's why amps have all those knobs and sliders. Using those devices one can get the same response as sealed, with a major difference, that being headroom in the amp is increased while driver excursion is decreased by using EQ to reduce low frequency output. The other reason why one might prefer sealed is that the reduced amp headroom and increased excursion demands on the driver with sealed will result in a dirtier tone. If that's what you're after all well and good, but if not you should reconsider bias against ported.

[quote name='LawrenceH' timestamp='1385681328' post='2291228'] Your own example is a good one for showing exactly what I'm talking about. Yes, the slope rolls off steeper below 30 just as physics says it must. [/quote] So? That being well below the passband it's moot.[quote] With this particular example the increased sensitivity above that means the group delays between the two cabs will diverge at a much higher frequency ie the transient response of the ported cab will be poorer. [/quote]In this particular case GD of the vented cab is significant only below 40Hz, so it is also moot. GD at 50Hz is 4 and 6ms respectively, putting both well below the threshold of audibility, and GD of the two above 100Hz is identical.

[quote name='LawrenceH' timestamp='1385679616' post='2291203'] Yes, it does mean exactly that. Unless the 'laws of physics' you so humbly equate yourself to are markedly different to everyone else's. You can get it the same in a limited bandwidth, which is what I said at the beginning. You can't get the final slope of a ported cab below resonance to roll off at 12dB/oct no matter how many magical beans you buy from mysterious old men you meet on the way to speaker markets. [/quote]This is a chart of the Ampeg SVT driver, ported in blue, sealed in red, in the same net volume. Of course the gross volume of the ported is larger, to account for the duct, but the increased sensitivity makes that worthwhile. The ported does have a higher roll off slope below 30Hz, but that's hardly of any consequence. This also answers the OPs question regarding what the same driver may do sealed or ported, in this case with a driver that has a moderate EBP that allows it to function in both.

[quote name='LawrenceH' timestamp='1385676173' post='2291136'] The sum of the driver/port below resonance will be a 24dB/oct slope. Always. [/quote]That doesn't mean one cannot realize the same result with vented and sealed, just that one must possess the necessary design skills to do so.

[quote name='LawrenceH' timestamp='1385671086' post='2291055'] I am always puzzled when I see people say that you can design a ported cab to give the LF response of a sealed cabinet, [/quote]You can. I don't know why you'd want to, but it's not all that difficult, if the driver specs are amenable. [quote]In hi-fi, I also think mid-range leakage from ports can be a difficult issue to solve completely satisfactorily as well. [/quote]I've never found it an issue at all. [quote]I don't know why (I'm sure someone on here could explain) but a sealed cab seems to be an especially good match with a valve head. [/quote]Because the impedance peak of a sealed cab with high Q drivers tends to be much lower than the peaks of a vented cab with low Q drivers, and valves don't deal well with high impedance loads. [quote]How does the same driver react in sealed and ported cabs?[/quote]Not all that well, as a driver that works best in a ported cab has a high EBP, that which works best sealed has a low EBP, so putting either into the other is a round hole/square peg situation.

[quote name='Mog' timestamp='1385633808' post='2290385'] These "Smart Power" cabs have intrigued me for a while. The principle is sound but something in the back of my mind keeps saying gimmick.[/quote]The principle has been known for 40 odd years. Take two drivers, configure them so that only one of the cones is exposed to the air and one to the cabinet, and you get the same response curve with half the net box size as is required with one driver. That's a plus. You also get the same net output as with one driver. That's not a plus. This concept was useful said 40 years ago in home stereo applications, because the driver technology of that time often meant it took a box in excess of 12 cubic feet to get strong output at low frequencies. Using a second driver to get the box size down had its advantages. But even in stereo applications advancements in driver technology rendered the isobaric concept obsolete by the 1990s. By the same token isobaric cabs would have made sense in 1965, but not in 1995, let alone today.

[quote name='Iheartreverb' timestamp='1385637197' post='2290443'] The issue is, I can't get the tone I want at a decent volume. What are my options? [/quote]Either they turn down, or you turn up. In your case that means adding another 810. In their case they'll invariably whine "But I can't get my tone if I turn down". That's true, because getting their tone means pushing the drivers to break-up. That brings us right back to the same point: to get the same break-up with four twelves versus one means playing 12dB louder. If you're regularly playing at Wembley or the Royal Albert then 412s are justified. Using a 412 in an average club, let alone a rehearsal space, is like using a sledgehammer to swat at a fly.

[quote name='bassmachine2112' timestamp='1385541554' post='2289374'] Guitarists in the know use 50 watt or less versions so they can get the sound without destroying buildings [/quote]They also use 112s that can be pushed to break up with 1/4 the power that it takes to do so with a 412. 112s also have twice the dispersion angle of a 212 or 412, with no comb filtering of the highs. The trouble is that the vast majority of guitarists aren't in the know, and buy amps based on what they see their heros using, who use what they're paid to use: the biggest, ie., the most expensive, rigs. This guy, however, is in the know:

[quote name='discreet' timestamp='1385502976' post='2289142'] There's your problem right there. [/quote]+1, an 810 can't compete with two 412s. Haul in three more 810s and tell them that's your new rig, unless they get sensible 112 combos.

[quote name='1970' timestamp='1385399670' post='2287766'] Even if this amp is solid state? The wattage doubles @ 4ohms... so I was thinking that headroom/volume would be a distinct bonus of running a 4ohm cab instead of 8? [/quote]Wattage is moot. Volume is limited by voltage swing, and whatever you gain by increasing the voltage sensitivity into a 4 ohm cab is offset by the increased current draw. If you can't get the volume you need from an 8 ohm cab you don't need more power, you need a second identical cab. That will give you a 6dB increase, which equals quadrupling your power/doubling the voltage into one cab, assuming it could handle it, which it probably can not.

[quote name='chris_b' timestamp='1385299906' post='2286515'] The -15db is just a fixed volume control. If your amp has volume controls there is no point in a -nn db pad. [/quote]Amps place the volume control after the first stage of amplification, if not even further back in the signal chain. If the signal chain is overloaded prior to the volume control, causing clipping, the volume control will not eliminate either the overload or the clipping. A pad may reduce the sensitivity of the first input stage, it may attenuate the output of the first stage, it can even be a total bypass of one stage of amplification. But whatever it does it always occurs before the volume control.

The point of the active input is to attenuate the input signal if it's too much for the passive input to handle without clipping. That assumes the output of the active bass in question is some 10 to 15dB hotter than a passive. But not all are, so if it works into the passive input better that's what you should use.

[quote name='bridge' timestamp='1384840206' post='2281290'] has anyone had the experience of using something different with a compact, such as the Shroeder 12L, for sale on here? does it mix ok? [/quote]The two together will probably sound better than either alone, that's usually the case. However, the BF uses a long excursion driver, the Schro does not, so the Schro would be a weak link.

If it was me I'd stop using guitar twelves and get a real bass cabinet. You're giving away an entire octave or more of bass response with the G12-65s, and G12-30s are no better. The only thing 'bass' about the JCM was what they chose to call it.

[quote name='tall_martin' timestamp='1384287893' post='2274937'] Or, is it possible to turn the pair of 8 ohm cabs into 2ohm cabs with the right speakers? Under £200 for new speakers. [/quote]If they're loaded with 4 ohm drivers series wired you just need to wire them parallel to get 2 ohms.

[quote name='subrob' timestamp='1384155395' post='2273210'] Damping factor is also halved. [/quote]Damping factor is moot. http://www.diyspeakers.net/Articles/Richard%20Pierce%20DAMPING%20FACTOR.pdf [quote]What effect does increased current draw have?[/quote]Potential for clipping, and heat, which shortens the life of amp components. Think of an amp as an engine. You wouldn't want to drive a car with the RPMs always red-lined.

[quote name='gsgbass' timestamp='1384132742' post='2273177'] I believe if the 4 ohms is the amps max, it will have an edge over the amp @ 8 ohms. More power normally pushes more sound from it. [/quote]Rule # 1 of loudspeaker design: There's no such thing as a free lunch. What you gain with respect to voltage sensitivity with a lower impedance load comes at the expense of increased current draw. To preserve a balance between voltage and current headroom you should avoid running an amp into its minimum rated impedance load.

[quote name='xgsjx' timestamp='1383745411' post='2268442'] Does running an amp at it's maximum ohms affect the sound that it produces? [/quote]No. Voltage sensitivity is increased by 3dB, but that's at the cost of doubled current draw. Seldom is maximum output affected, as that's determined by the driver displacement limit, which on average is only half the thermal limit.

[quote name='LukeFRC' timestamp='1383350230' post='2263614'] so florescent tube bulbs give a wide frequency light in the greeny bit of the spectrum, and incandescent a narrower frequency of light in the yellowy spectrum.[/quote]Flourescents waste less energy as heat. For proof, take a CFL and incandescent of the same lumens output, put your hands on each. By the same token low sensitivity speakers convert less energy into sound, more into heat than high sensitivity speakers. Equal power applied, totally different results, by as much as a 20:1 ratio best case to worst. So you can have a 5 watt amp and speaker combination that has the same decibel output as another that's a 100 watt amp and speaker combination. Watts? Not totally irrelivent, but almost.

[quote name='RandomBass' timestamp='1383346284' post='2263563'] You are missing the point. Comparing fluorescent to tungsten is irrelevant. As you increase the wattage of either, the brightness increases. Nothing to do with loudspeakers at all.[/quote]Au contraire. A higher sensitivity speaker will be louder than a lower sensitivity speaker with the same power applied. A speaker with lower frequency response will be louder at low frequencies than one with less low frequency response. If one only looks at watts one has no true inkling as to the actual result.

[quote name='RandomBass' timestamp='1383332605' post='2263342'] Even RMS Watts is a marketing ploy. It makes no sense. [/quote]It makes as much sense as using the wattage rating of a lightbulb to gauge its brightness, which is to say, none. But the powers that be decided long ago not to provide the consumer with the specs that really matter.