Agedhorse's Speaker Failure Explanation & Kindness on amp/speaker pairing

[agedhorse]

As an E.E. in this industry, I have designed speaker products for "several" companies over the past 25+ years, including Genz Benz. I have worked very closely with transducer (driver) engineers and I also designed your amplifier. I have also designed and analyzed pro audio products for some of the largest companies in the country.

I received an e-mail from somebody on the forum asking if your "advice" was indeed true since he owns a rig very similar to yours, and he had been using it for a while and was suddenly concerned that he might be "blowing" his speakers. He forwarded this thread and I understand why he was concerned.

I read your post and it's full of mis-information concerning how speakers work and how power ratings relate to the real world. I am not bashing with any attempt to sell anything... in fact you have already bought something! I am also not bashing the speaker manufacturer since they are free to do whatever they feel they need to do to sell their product. What I am saying is that your comment about damaging any cabinet by underpowering it is false and bad information to others here on the forum.

Speakers (low frequency drivers for this discussion) fail for one reason and only one reason... excessive power. This damage can be either thermal or mechanical in nature.

Thermal failure is the result of too much power causing excessive temperature rise in the voice coil and bobbin assembly, causing a softening of the adhesives, blistering of the former and a breakdown of the wire insulation. Thermal failure can occur when the speaker is used with an amplifier that is rated to large for the duty cycle the speaker sees. If the signal is heavily compressed, the duty cycle increases and the speaker power rating (thus amp size) must be decreased. Another cause ofthermal failure is using an amp that is too small for the job at hand, that amp being driven heavily into clipping. When an amp is driven heavily into clipping, the power delivered to the speaker can be as much as twice the RMS rating of the amp. This is, IMO, the primary reason behind the myth that underpowering a speaker can blow it. The failure mode is still too much power.

The second failure mechanism of a speaker is due to mechanical damage. This is always caused by too much power. It's pretty simple for a manufacturer to determine how much power causes mechanical damage. The problem is putting it into a specification that is useful to the average musician. The mechanical power handling of a speaker is not a fixed number, but varies greatly by decreasing as the frequency decreases. This is due to the mechanical loading that the air mass creates to the drivers and below some point the driver is no longer adequately loaded and flops around like an undamped spring. It's not uncommon for a speaker to show a power rating of 1/4 the original mid-band rating when you approach 25-40Hz. Combine this with a grossly large power amp and you have the single most common cause for bass driver failure.

Also note that an amplifier rated at say 1200 watts "RMS" by definition produces a peak 2400 watt rating because of the conversion factor for a sine wave between RMS values and peak values.

I have attached a good overview of loudspeaker power ratings from my good friends over at Harman. Note their discussion relating to derating for musical applications and their references to such in examples 2 & 3.

http://www.jblpro.com/pub/technote/spkpwfaq.pdf

Hope this helps.

[Kindness]

I receive a lot of private messages asking me about pairing amps and cabinets, some regarding specific pairings and some just looking for general guidance. I also have responded haphazardly to threads and posts where a fellow forum member may mention they are "pushing 3,000 watts into a "Brand X" 4x10. I figured I'd use this thread to consolidate some of what I've written.

First, no thread about amp and cabinet pairings would be complete without a discussion of "underpowering" a cabinet. The short story, there is no such thing as underpowering a cabinet. When someone uses that phrase, you need to question whether the speaker/writer actually understands the subject matter.

Agedhorse's thread in the FAQ regarding speaker failure explains the two mechanisms for speaker failure, both caused by sending too much power to a cabinet. If you haven't already, read and re-read this thread until you understand it. You don't need any technical background to understand the concepts as they are presented, so if you don't understand the entirety of that post, you may want to ask questions about where you are falling off track. Once you understand everything written in that thread, you'll be well equipped in understanding your gear.

Agedhorse's Speaker Failure Thread (http://acapella.harmony-central.com/forums/showthread.php?t=1796660)

Got it? Good.

One of the questions I am asked the most is some variation of this:
"I have "Brand/Model A" amp. What cabinet will handle the "X" watts from my amp?" The real question isn't what cabinet will take the wattage from your head, but which cabinet will produce volume/tone sufficient to meet your needs with your amp. You can have an enormously powerful amp and a cabinet that can't handle much power, but if you are achieving your desired tone and volume everything is just fine, you just might be carrying around more power than you need.

Speaking of more power than you need, what about the guys playing with huge power amps into standard bass cabinet? Does anyone actually use 2,000 watts into a typical 4x10? Nope. No one. There just aren't bass cabinets that can actually handle that much power. Remember from the FAQ post linked above, there are two limits to cabinets, thermal limits and mechanical limits. Nearly every commercial bass cabinet is actually displacement limited (mechanically limited) - not thermally limited. What the heck does this mean? Go back to Agedhorse's post and reread the mechanical limit section. In short, due to the mechanical limits of a cabinet, it is not uncommon for a bass cabinet to start farting out in the bass region at about 1/4 the published thermal rating of the cabinet. Your 1,000 watt 4x10 probably can't handle much more than 200-300 watts below 100 Hz. Crank the volume, boost the bass and bask in the flubbiness. :)

How am I going to know if I am exceeding the cabinet's mechanical limits? If you are listening to your cabinet you should be able to tell when it is farting out and you should dial the amp back. If it sounds good, it is. If you are interested in learning more about these limits, download WinISD and start modeling cabinets. If you have questions about that, post them and I'll help.

Then why do I need a 2,000 watt power amp to get sufficient volume out of my cabinet? Many bass specific preamps are incapable of driving the power amp to full power. Power amps have an "input sensitivity" which is the voltage required to drive the amp to full power. If the preamp cannot output sufficient voltage, you will not be making use of the full power of the amp regardless of volume/gain settings. Thus, if your components aren't well matched (and it is often difficult to even find published specifications to make the determination) you might think you have your 2,000 watt amp on full go, but you might only be driving 200 watts. If you put a mixer or preamp in front of the amp to boost the signal, you might find you've been carrying a lot more reserve power than you expected.

Lots of people want to know what equipment they need to be as loud as they want to be. Many know that doubling the power or doubling the displacement will give you a 3 dB increase in volume. +10 dB is generally accepted as "sounding twice as loud." In order to achieve a +10 dB increase, you need ten times the power or just over eight times the displacement (eight times the displacement is +9 dB). In other words, a 1x12 at 50 watts may be twice as loud as the same 1 x 12 at 5 watts. Similarly, eight of the same 1x12s may be twice as loud at 5 watts as the 1 x 12 at 5 watts. One of the reasons adding speaker displacement is generally a great way to boost volume is that most solid state amps will deliver more power to a lower impedance. Therefore, if you take a typical 1x12 on a solid state amp and add another 1x12, you have both doubled the displacement and (approximately) the power for a +6dB gain. +6 dB isn't double the volume, but it is a noticeable change. The other reason is power compression. Many speakers do not operate linearly within the range of power we use to drive them. In many cases the first 50 watts to the cabinet have a much greater effect than the next 50 watts. When you split the amps power into a greater number of drivers, you reduce the effects of power compression.

Lastly, a little something about sensitivity ratings. I sometimes get asked something like: Cabinet A is 103 dB/w/m and Cabinet B is 99 dB/w/m - this means Cabinet A is louder, right? Well... it means that even if the specs can be trusted that Cabinet A is louder with an input of 1 watt. It doesn't tell you anything about what happens when you turn up. Maybe cabinet A will start farting out at a much lower power level than cabinet B. So even though it takes more power for Cabinet B to get loud, it might be that Cabinet B has a higher maximum output. Then again, maybe not. The point is, those specs are more related to how power hungry a cabinet may be, not how loud the cabinet can get. (In practice they tend to connote how mid oriented a cabinet is. Generally speaking, the higher the sensitivity cabinet will tend to be more mid oriented.)

Hopefully these few short paragraphs are helpful in some way. They are just thoughts off the top of my head based on common questions I get and I would be happy to expand on any of them if anyone has questions.