QuestionI'm a mobile DJ. I'm looking to invest in some self powered (Active) speakers. I'm not technically proficient at the details of audio and electrical engineering, but I recognize that, generally speaking, the higher the number of watts an amp and speaker combo are rated for, the more dB's they'll produce. As I tend to find myself in banquet halls and high school gyms, I need a setup that can provide sufficient volume without much distortion. Most of the active speakers I see have power handling ratings in the arena of 100-300 watts continuous, 400-800 watts peak. What is the technical difference between these two ratings, and how can I calculate the dB's that a speaker will put out from a given input level?
AnswerThe statement that you offered "the higher the number of watts an amp and speaker are rated for, the more dbs they'll produce" is only partially true.
The sound pressure output level is a function of the efficiency or sensitivity of the speaker being driven by the amplifier. This measurement or specification is usually given in db ; it is the measure of output sound pressure with 1 watt of power input to the speaker and measured in a standard way.
Now, you must understand the logarithmic characteristics of sound. If you double the audio power input to a speaker it will increase the sound pressure by 3db. In other words, 3db is a 2 to 1 power ratio. If you are driving an amplifier to the 200 watt level and then decide to increase it to 400 watts, that is an increase of 3db in power.
Further, most people notice a difference in sound level of around 2 to 3db. In other words, doubling the power input only increases the audible sound level to the barely noticeable level!
If you want to double the sound level - to the ear - how much power increase does it take? Acoustic testing on many human beings with average hearing shows that it takes about a 9 db increase! However, 10db is 10 times power ratio. In other words, if you want to double the apparent sound pressure level with a 100 watt amplifier you must be able to boost the power level to almost 1,000 watts!
Now, back to the speaker unit. If one speaker has a sensitivity rating of 89 db (typical) and another 91db (still in the typical range) the 89 db rated unit will require twice the power level to achieve the same output as the 91db unit.
Power amp rating are usually given in terms of continuous power delivered into a resistive load with a certain level of distortion. And peak power is usually stated wrongly by most manufacturers. Technically it is merely twice the continuous power since the peak of the sine wave using for a test has a value that is mathematically twice the average. But, what most of them really means is the transient power; or how much can the amplifier deliver for short bursts of music power. Music is very dynamic and the power waveform h as very erratic ups and downs in its power deliver curve. This transient or short burst power rating is a function of the strength of the power supply and the size of the electrolytics in the ripple filter section of the supply. It is a design phenomenon more than anything else.
So, for a big hall with a high ambient background noise level you must have lots of reserve to sound natural and not overly strained. So, look for the highest sensitivity in the drivers and power handling ability for the big amps that are going to drive it. In self-powered systems you are at the mercy of the manufacturer - hoping they have optimized the amplifiers performance against the loudspeaker they have chosen to connect to it.
My personal experience with high school type gymnasiums is that you cannot do it easily with only a stereo pair of speaker systems. It takes multiple drivers and a couple of big subs to get over the acoustic noise generated by the dancers on the floor. A typical system might use a pair of amps to drive two pairs of full range speaker systems and one or two sub amps driving subwoofers.
Let me know if you have further thoughts.
C
PS: There is no way of accurately calculating the sound pressure level you will get in the hall without taking some measurements and establishing a reference. Each hall has its own acoustic absorption and reflection characteristics. Two halls of the same acoustic volume will require different power input due to reflection from the materials used in the construction of the facility. High ceilings, for example, eat up energy that is lost forever; lower ceilings keep it in the room.
That being said, if you have a speaker with a sensitivity of, say, 90db and you drive it with a 100w amplifier it means the output of the speaker would be 20 db (10 db + 10db) above the one watt level, or 110db at 1 meter distance from the driver. A normal fall off rate is about 8db for each doubling of distance away from the speaker unit. But, that is just a wild estimation; it is a function of the room it is loading into. So, in the example above, at 10 feet, or 3 meters distance the sound pressure level would be around 95db or something like that.
