Detectability and audibility

Attention signals should on the one hand be loud enough to draw the attention of the listener. On the other hand, they should not be so loud as to startle the listener, certainly not if the signal can be heard by other listeners.  It has been shown that, when a signal is about 15 dB above the background noise level, it is well audible and not too loud. Furthermore, ring tones composed of many different frequency components are less readily masked by other sounds such as music than sounds composed of only one or two frequency components.

 

Identifiability

When someone hears a warning sound, it should immediately be known what the sound means. For ring tones this is only rarely a problem.  Only when operating in a control room in which other equipment can produce attention or warning sounds, one runs the risk of confusing ring tones with one of those other signals. 

 

Discriminability

This is a major problems for ring tones. Many people have more or less similar ring tones. Only after hearing a few tones, listener can distinguish the ring tones of his/her own telephone from that of someone else. As a consequence, when one is reading a book or just falls asleep, the attention is diverted from the book or from the day dreams into which one was just musing away.  

 

Localizability

It is a well known situation. In a train a ring tones sounds and, besides the owner of the ringing phone, various other people start looking for their mobile telephones.   The problem is that most ring tones consist of tones with gentle onsets and frequencies in the range between 1500 and 3000 Hz. These are the sounds which are more difficult to localize than any other sound. For sounds to be well localizable they should be wide band and have short attack times. As already mentioned, this has the additional advantage that they are less easy masked by other sounds. This can be realized by including higher harmonics and by replacing the trapezoid temporal envelopes by, for instance, envelopes which start very rapidly and then decay exponentially giving the tones a plugged timbre.  

 

Perceived urgency

The perceived urgency of an attention signal should carefully reflect the urgency of the actual situation. Most ring tones start calling and only stop when the listener answers the call. This is a property that can be very annoying even for real alarm calls. There is no reason not to interrupt the sequences of tones by relatively long pauses. These give the listeners the opportunity to communicate undisturbed. Silence is really an important component of well designed warning signals. Perceived urgency can then be increased by  increasing the tempo of the next tone sequence, increase the bandwidth of the tones, increase the interval between the tones, make them inharmonic, louder, or apply frequency modulation. Note however, that this may increase the annoyance of the bystanders. 

 

Perceived annoyance

Some aspects have already been dealt with. Moreover, the tones of some ring tones consist of pulse-like sounds. Such sounds have the property that the higher harmonics are so close in frequency that they cannot be resolved by our hearing system. As a consequence these high-frequency harmonics interfere, which induces a buzzy, nasal timbre, generally felt annoying. Too abrupt attacks also contribute to annoyance, certainly if not followed by rapid decays as in plugged sounds. Very loud sounds with abrupt attacks are excellent for creating panic. Ironically most fire alarms are of this kind. The risk of panic, also for very loud sounds, is much less when starting softly, giving them gentle onsets, and including long silent pauses in which people can deliberate what to do. 

 

Conclusions

The function of ring tones is to signal to the owner of the mobile phone that someone is calling. Hence, ring tones have the function of drawing someone's attention. For all listeners except the owner of the phone these calls are false alarms and, hence, disturbing. But the annoyance of ring tones can be diminished by taking a few simple measures. Do not make them too loud!  If you do not want to miss it, let the ring tones start softly only to increase in intensity after a few tone sequences.  Include long pauses between successive tone sequences, especially when the ring tones must be loud. Make the tones wideband, but do not pack the high frequency partials too close in frequency as this will make the timbre unpleasantly nasal and buzzy.  Wideband sounds are easy to localize, and are not so easily masked by other sounds.  Consequently, wideband sounds do not need to be as loud in order to be well audible.   

Remarks

Now try to realize how few ring tones fulfill the abovementioned requirements!  You see? That is why they are so annoying.   

Designers will classify much of what is said above as "purist". In my opinion the challenge is to realize ring tones which obey the abovementioned design principles and meet the demands of the creative designer.

 

Dik Hermes

Subdepartment of Human-Technology Interaction

TU/e, Eindhoven, The Netherlands

 

Literature

Edworthy J.  & Adams A.  Warning design: a research prospective.  London, UK: Taylor & Francis, 1996. 

Stanton N. E.  & Edworthy J.  (Eds. ) Human factors in auditory warnings.  Aldershot, UK: Ashgate, 1999.