The theory behind decibel levels can get pretty involved from a mathematical or engineering standpoint， and I did promise not to bog you down in “techno-hobble”. Here’s the basic idea： the decibel level is derived by multiplying ten times the logarithm of the ratio of the measured power level to a reference power level. Frankly， it’s not something you’ll need to worry too much about unless you’re designing circuits （if you really want to wade through an explanation， see the “suggested Reading” section in the back of the book）， but it does illustrate an interesting point. Much of audio measurement involves logarithms and other exponential functions because：
these numbers make it easier to express the wide range of values that our ears can detect；
exponential numbers more closely reflect the way sound works in the real world.a
For example， every doubling of frequency is a musical octave. Going back to our friend 440 Hz （the note “A” below Middle C on the piano）， we see that 220 Hz is also the note A--but one octave lower-and that 880 Hz is the note A one octave higher. The relative difference between the notes of the lower octave （220 Hz to 440 Hz） and the higher octave （440 Hz to 880 Hz） sounds the same to our ears， but the actual difference in frequency is changing by a ratio of 2：1.
With decibels， we find that doubling the strength of an electrical voltage doesn’t double the sound output level. Instead， every doubling produces an increase of 6 dB.
There’s one final aspect of how the ear works that affects our perception of sound. We’re much less sensitive to low-frequency material at lower volumes. This is why many stereos have a loudness switch to boost the bass when you turn， the volume down. It also makes it more difficult for multimedia producers to create material with decent bass response. Unless the listener has a subwoofer or has cranked up the volume through a regular stereo system， it’s likely that material mixed in a standard studio setting will sound tinnier when played back on small multimedia speakers.
【New Words】
vibration
振动，颤动，摆动
eardrum
鼓膜，中耳
whistle
口哨，汽笛，吹口哨
standpoint 立场，观点
voltage
电压，伏特数
stereo
立体声，立体的
octave
八度音阶
logarithm
对数
8.3 Application of Multimedia
Multimedia is appropriate whenever a human interface connects a human user to electronic information of any kind. Multimedia enhances traditional text only computer interfaces and yields measurable benefit by gaining and holding attention and interest， multimedia improves information retention. When properly woven， multimedia can also be profoundly entertaining.
Multimedia in Business
Business applications for multimedia include presentations， training， marketing， advertising， product demos， databases， catalogues， and networked communications. Voice mail and video conferencing will soon be provided on many local and wide area networks （LANs and WANs）.
After a morning of mind numbing 35 mm slide and overhead presentations delivered from the podium of a national sales conference， a multimedia presentation can make an audience come alive. Most presentation software packages let you add audio and video clips to the usual “slide show” of graphics and text material.
Multimedia is enjoying widespread use in training programs. Flight attendants learn to manage international terrorism and security through simulation. Mechanics learn to repair engines. Salespeople learn about product lines and leave behind software to train their customers. Fighter pilots practice full terrain sorties before spooling up for the real tiling.
As companies and businesses catch on to the power of multimedia and the cost of installing multimedia capability decreases， more applications will be developed both by themselves and by third parties to allow businesses to run more smoothly and efficiently.
Multimedia in Schools
Schools are perhaps the neediest destination for multimedia. Many schools in the United States today are chronically under-funded and occasionally slow to adopt new technologies， but it is here that the power of multimedia can be maximized for the greatest long-term benefit to all.
In March 1995， the White House challenged the telecommunications industry to connect every classroom， library， clinic， and hospital to the information superhighway by the year 2000. The While House has also taken steps to provide governmental support with a program of “Challenge Grants for Technology in Education”， a $ 27-million effort to support state-of-the-art technology in about 20-1ow income rural and urban school districts. Vice President Al Gore is often seen pulling wire in inner-city school labs， promoting installation of computers in schools.
Multimedia will provoke radical changes in the teaching process during the coming decades， particularly as smart students discover they can go beyond the limits of traditional teaching methods. Indeed， in some instances， teachers may become more like guides and mentors along a learning path， not the primary providers of information and understanding the students， not teachers， become the core of the teaching and learning process. This is a sensitive and highly politicized subject among educators， so educational software is often positioned as “enriching”the learning process， not as a potential substitute for traditional teacher based methods.
Multimedia at Home
From gardening to cooking to home design， remodeling， and repair to genealogy software， multimedia has entered the home. Eventually， most multimedia projects will reach the home via television sets or monitors with built in interactive user inputs. The multimedia viewed on these sets will likely arrive on a pay-for-use basis along the data highway.
Multimedia in Public Places