General Sound Information
How this was done, and how to find your pitch:
The following speaking fundamental frequency norms have been provided as a result of Dr. Daniel R. Boone's work published in 1991:
|Sex||Age||Normal Speaking Pitch||Frequency|
|Women||21||G below middle C||196 Hz|
|Men||21||C below middle C||130 Hz|
|Women||51||F below middle C||174 Hz|
|Men||51||A2 (8 notes below middle C)||110 Hz|
Middle C = 262 Hz. For more info, see: Frequencies of Musical Notes
For men, the main Formant range is around 2.5 kHz to 3 kHz, while in women, the range is roughly 3 kHz to 3.5 kHz. There are also low Formant ranges: 500 Hz for men and 1 kHz for women. There are harmonics to deal with as well, but they will not be as powerful as the Formants - assume that the will be at least one octave above (2 times the frequency indicates a 1 octave increase).
As a result, the Midrange frequencies from 1 kHz to 4 kHz are critical for Vocal intelligibility in a PA system, however in order to maintain richness in the sound, you need to also provide for the Fundamental (which should be at least as low as the persons normal speaking voice). You should also be able to provide up to 18 kHz to keep the voice crisp.
PA systems were created primarily for voices, however, they often include other instruments run thru the PA system. In order to determine what ranges you need to cover, you need to know more about the frequency ranges they produce.
Realistic Usable Frequency Ranges
20 to 200 Hz - Sub Woofer
100 to 5000 Hz - Woofer
500 to 5000 Hz - Midrange Speaker
800 to 7000 Hz - Midrange Horn/Compression Driver
1800 to 20000 Hz - Midrange Piezo
2000 to 8000 Hz - Tweeter
4000 to 20000 Hz - Piezo Tweeter
While it looks like the Woofer should be able to handle most everything by itself, what you see are the ranges that it makes sound at, what these specifications don't tell you is the level (in dB) that is output for each of these frequency ranges. A goal of high end Home Audio speakers is to attempt a flat a response as possible across the whole range - this is not often the case with Musical Instrument speakers - they are optimized to give you more output volume (SPL) at the expense of flatness, and its assumed that you will have to adjust the EQ to compensate.
To get a good sound thru the PA system, you really need to have at least 2 speakers (a Woofer for the lows and something to cover the mids and highs), or a 3 Speaker system that has optimal coverage of the Bass, Mids and Highs. For a vocals only PA system, there is not much need for a Sub Woofer. If you need support of the extreme low end, then you add Sub Woofers as separate boxes.
Please see Speaker SPL Information before running out and buying some thing that you see on sale. You get what you pay for and for PA and Musical Instrument systems, you do not want to use Low SPL Home or Car Audio speakers.
An example of that is the outdoor public address systems that are basically a bunch of waterproof horn enclosures hung high above a crowd. These Horns are very specific for Midrange and above. They have very small diaphragms and cannot move enough air to reproduce much signal below 1000 Hz. When you hear people talking thru these, you hear the Formants in action, but you never hear the fundamental frequencies. To understand a spoken message, you don't need to hear the fundamental. When music is played thru these, all of the bass portion is lost.
To regain the missing Bass, you could add a Woofer, however you really don't want to load of 2 speakers on the power amplifier when all you want is the Woofer to cover for the Horn when it can't get down to specific notes.
The solution to this is a Cross-Over network. A Cross-Over network splits out frequencies, separating highs and lows from each other. Complex waveforms will have high and low frequency components; all the Cross-Over network cares about is separating them into 2 or more parts based on frequency range. There are 2 types of Cross-Over networks, Active and Passive.
Active Cross-Over networks are used between the Audio Mixer and the Power Amplifiers. I say 'Amplifiers' because you will need a separate power amplifier for each frequency range that you have broken the signal out to. This tends to be expensive, but is by far the most efficient and controllable use of Power Amplifiers and speaker systems. All high end stadium system split at least the Sub Woofers from the PA speakers using this method (and usually more parts).
Passive Cross-Overs attempt to split the raw power amp signal into different frequency ranges that you assign a specific speaker to - normally, you choose a speaker/horn/tweeter (all with the same impedance) that best matches the range desired. Since the individual speaker/horn/tweeter is not conducting the same frequencies, your Power Amplifier thinks that it is seeing a single speaker out there instead of multiple speakers. Of course, you have to make sure that you choose the Cross-Over points so that there is no overlap. Most Multi Speaker Pro-Audio cabinets use Passive Cross-Over networks.
For more information on Passive Cross-Overs see:
You have a 200 Watt power Amplifier. You want to build a 2 way speaker system for it that has a 12 inch woofer and a Midrange Piezo horn (No Cross-Over is required for this configuration). The Piezo naturally starts responding at 1800 Hz. What should the speaker power ratings minimally be for safe operation?
You have a 300 watt power Amplifier. You want to build a 3 way speaker system for it that has a 15 inch woofer, a midrange compression horn and a tweeter. The Cross-Over points are 1000 Hz and 4000 Hz. What should the speaker power ratings minimally be for safe operation?
Your music may be quite different than the 'average' cited; many people push more bass than average recorded music, so you might want to make the Woofer closer to the power amplifiers rating. Keep in mind that the peak power rating of speakers for Musical Instrument use is normally 2 times the RMS rating. So, in the above 'Example 2', the 210 watt Woofer can handle peaks of 420 watts.
Adding a Sub Woofer
Normally, the lower the frequency, the more power is needed to get it heard. Sub Woofers need a lot of power when used in a Pro-Audio situation. Often these will have their own power amplifier (many have built in Active Cross-Overs). If using a Passive Cross-Over, you typically split into a Low Pass out for the Sub Woofer and a High Pass out for the rest of your speaker system. In many cases, effectively driving a Sub Woofer will require that you increase the power Amplifiers output by 50% or more. Plan for this if you expect to add Sub Woofers later.
Feedback is a bad thing for your Tweeters
As you will note. The Cross-Over network can minimize the power going to different speakers. One exception to this rule is Feedback. A blast of sustained feedback can provide close to the full power of your power amplifier into your tweeters - their voice coils will not last long if this happens. Be aware of things that cause feedback and do what you can to avoid it.
Guitar/Bass Guitar use
In most cases, these instruments are best served by speakers alone, and you can assume 100% of the sound should go to speakers. A lot depends on your choice of speakers for the sound that you want. Guitar and Bass Guitar have interesting Formants and your style of playing can really affect the need for additional high frequency support. When I add high frequency support to a guitar or bass guitar speaker cabinet, I'm trying to boost the overtones and harmonics. Distortion boxes create odd harmonics, which results in high frequency that cannot be represented in a Woofer alone (sometimes you want the speaker to roll this off naturally for you). In the case where you do decide to add a high frequency boost in this environment, assume that you will need to be able to handle approximately 25% to 50% of the normal amplifiers power.
Questions? Comments? .
© 2004, Shavano Music Online