Sound And Light Training SALT Manual by Ken Ellis

11. Understanding Sound

The Decibel - dB

• The Decibel (dB) is the unit of measurement used in sound systems



• A Decibel (dB) describes a ration between two quantities expressed as a logarithm. Logarithms are used because our ears hear differences in loudness as a Log function.



• In simple terms, 3 dB represents twice as much power, and 10 dB represents 10 times as much power



• 2 * power = 3 dB



• 4 * power = 6 dB



• 8 * power = 9 dB



• 10 * power = 10 dB



• To make sound twice as LOUD requires 4 times as much power which is 6 dB



• If you have a 100 Watt stereo, and you want one that is twice as loud, you need a 400 Watt stereo



• Loudness of a sound system is measured in dB of Sound Pressure Level (dB SPL)



• Loudness decreases by 6 dB (half as loud) every time the distance from the sound source is doubled



• If you start 3 feet from the speaker and move back to 6 feet, it will sound half as loud (-6 dB)



• If you move from 6 feet to 12 feet, it will reduce in volume by half again



• By the time you are 48 feet from the speaker, the sound will be 1/16th as loud (-24 dB) as it was at 3 feet




---

Frequency Range of the Human Voice

• Voice range covers 300 Hz to 3500 Hz



• Most energy concentrated below 1000 Hz



• Vowels have most of their energy below 1000 Hz



• Vowels contain the "power and impact of the voice"



• Consonants have most of their energy above 1000 Hz



• Consonants are responsible for intelligibility



• Harmonics in voice can go above 3500 Hz



• Poor high frequency response reduces intelligibility




---

Recognizing the Frequency of Sounds

It is important for a sound technician to learn to recognize the frequency of sounds so you are able to quickly and correctly adjust equalization when there is a problem (i.e. feedback, hollowness, nasal sound, boomy, etc.)

When you have time alone with a sound system, put on some good quality music and play with the graphic equalizer to learn what frequency range each control affects. Memorize the tonal quality each control affects.



40-60 Hz	Boomy - a sound over abundant in low lows. These waves move a lot of air, hence Boomy.
60-150 Hz	Fat - the octave above Boominess. Makes things sound big, but not earth- shaking.
125-250 Hz	Woofy - a somewhat nebulous term for sounds that are sort of "covered" - masked by low-end energy.
250-500 Hz	Puffy - is like an octave above Woofy. It's still sort of a cloud, but not as big.
200-400 Hz	Warm - obviously a positive characteristic often found between 200 and 400 Hz. Could easily degenerate into Woofiness or Puffiness if overdone.
500-1kHz	Boxy - seems to remind one of the sound in a small box-like room.
500-1kHz	Low end of Voice
500-5kHz	Power range - mid-range band which contains the 1st and 2nd harmonics of most important sounds.
1.5-2.5 kHz	Telephony - accentuating the limited bandwidth characteristic commonly associated with telephones with a roll-off both above and below.
2.5-4 kHz	Cutting - Here, "cut" means to put an incisive "point" on the sound.
2.5 kHz	Punch - Accentuating this range punches through vocals.
3-6 kHz	Presence - Anywhere from 3-6 kHz can be used to make a sound more present.
7-10 kHz	Sibilance - Dangerous "s" sounds and lots of other trashiness can often be found at 7-10 kHz.
10-12 kHz	Zizz - refers to a pleasantly biting high-end resonance (think of a "harpsichord"-type brightness found around 10-12 kHz.
12-15 kHz	Glass - A very translucent, but palpable brilliance associated with 12-15 kHz.
15-20 kHz	Sparkle - A real smooth stratospheric brilliance almost beyond hearing, but can certainly be sensed.
Above 10 kHz	Brightness - Most generally achieved by a global (shelving) EQ of everything above 10 kHz.
Below 10 kHz	Darkness - The opposite of brightness (a general lack of highs at 10 kHz and beyond).
125-500 Hz	Muddiness - Actually a compound problem: Woofiness plus Puffiness (excess low end and also low mids).
125-500 Hz	Thinness - The opposite of Muddiness (a deficiency of lows and low mid frequencies).

---

Ear Sensitivity

• Sensitivity of most people's ears is relatively smooth between 500 Hz and 5000 Hz



• Ears are most sensitive to sounds between 3000 Hz and 4000 Hz



• Below 500 Hz and above 5000 Hz, hearing sensitivity drops off



• At louder listening levels (rock concerts), the frequency response of the ears becomes more equal over a wider range



• This is why you really can't notice the bass or cymbals in quiet music, but they are quite evident in loud music



• Many stereos have a "loudness" switch to compensate for this effect at low volumes



• A Sound Pressure Level (SPL) of 120 to 130 dB SPL is the threshold of PAIN for most people



• Children and women are more sensitive to loud sounds than men




---

Dynamic Range

• Singers and instruments are capable of performing as quiet as 50 dB SPL and as loud as 110 dB SPL



• This represents a 60 dB dynamic range from the quietest to loudest



• The quietest parts of a performance must still be kept louder than the room noise (called the noise floor)



• The noise floor is typically 50 to 60 dB SPL



• The quietest parts must be amplified enough that they can still be heard above the noise floor in the back of the room



• Assuming 24 dB of loss from the front to the back of a 50 foot long room, the quietest parts would need to be amplified 24 dB to be heard in the back of the room



• Note: It may not be possible to provide 24 dB of gain before feedback (see Feedback Control)



• The speakers must reproduce the quietest parts at 74 dB SPL in order to be heard at the back of a 50 foot room at 50 dB SPL



• The loudest parts of the performance must not be so loud as to be obnoxious or painful



• The loudest should not exceed 110 dB SPL



• The loudest cannot exceed the capabilities of the amplifier and speaker system (also about 110 dB SPL)



• However, since the quietest parts must be amplified 24 dB, the loudest parts (110 dB SPL) will also be amplified the same amount, obviously making them much TOO LOUD



• The useful dynamic range of the speaker system is limited by:



• The quietest part of the program must be amplified to 74 dB SPL to be heard in the back of the room



• The loudest part of the program exceed 110 dB SPL in the front of the room



• This leaves a useful dynamic range of 110 - 74 or 36 dB, much less than the 60 dB dynamic range of a typical music group



• Solutions to the Dynamic Range problem:



• Make the room quieter (requires expensive sound insulation)



• Get a louder sound system (could annoy the audience)



• Use a compressor / limiter circuit (expensive, especially in a system with many microphones and instruments in one system)



• Have the sound engineer turn quiet parts up and turn loud parts down



• ***** Have the performers reduce the demand for dynamic range by getting closer to the mike on quiet parts and backing off from the mike on loud parts, similarly, have instrument players control their own volume according to the dynamic needs of the program




---

Feedback Control

• Feedback occurs when the sound from the microphone is amplified too much.



• Feedback is caused by a repeating circular process of a microphone picking up a sound from a speaker, the sound system amplifying it (too much), the speaker reproducing the sound again, and the microphone picking it up again.



• Feedback usually occurs at one frequency at a time. The frequency of the feedback is affected by:



• Direction the microphone is facing



• Distance between the microphone and speaker



• Frequency response characteristics of the room



• Equalization of the microphone channel



• Equalization of the monitor speakers



• Equalization of the main speakers



• Sound systems should be operated no louder than 6 dB before the beginning of feedback (that is half as loud as when feedback starts)



• Operating closer to the feedback point causes a "hollow" or "ringing" sound



• Adding more microphones increases feedback problems. Every time you double the number of microphones, the maximum gain before feedback is reduced by 3 dB. Apparent loudness of each mike is cut in half when the number of mikes is increased 4 times.



• To go from 1 to 4 mikes halves the maximum volume of each mike before feedback.



• To go from 1 to 16 mikes quarters the maximum volume of each mike before feedback.



• Feedback is controlled by:



• Using directional microphones and carefully aiming them away from monitor and main speakers to reduce feedback



• Performers must be careful not to re-aim mikes towards speakers during performance



• Performers must be careful when hand holding mikes not to point them towards the monitor speakers



• Decreasing the distance between the sound source (performer) and the microphone (so the microphone does not need to be as loud)



• Increasing the distance between the speakers and the microphones



• Using equalizers to reduce the system's gain at the frequencies where the feedback occurs



• Installing acoustic dampening material in the room to reduce sound reflections back to the microphones (expensive solution)




---

Factors Influencing Clarity and Intelligibility

• High monitor levels on stage get into the microphones and muffle the sound because the monitor sound is out of phase with the original sound (don't set monitor louder than necessary)



• High monitor levels can also cause sound to be hollow or ringing



• Instruments playing music louder than necessary on stage causes too much music to be picked up by vocal mikes, making the music muddy (keep music as quite as possible on stage)



• Excessive use of equalization to prevent feedback effects the clarity of the overall sound (again, don't set monitor louder than necessary)



• Acoustical characteristics of the room (reverberation) affects the intelligibility of the sound. Large flat hard surfaces reflect sound which is out of phase with the original sound. Acoustical treatment of walls and ceilings is desirable.



• When performers are too far from the mike, the mike gain must be increased which causes pickup of more background noise and muddies the sound (the optimum distance from the microphones is 6" except for solos where "quiet" singers should be 2" from mike)



• Microphones aimed at monitor speakers, instrument speakers, or drums (tilt mikes up slightly so they point at a "quiet" ceiling rather than behind you at a guitar amp or drummer)



• Breath noises and "popping" on solos (tilt mike up and sing/speak over the top of it, stay 6" from mike)