optimized using advanced simulation and measurement tools
Ole Wolff Elektronik A/S, Roedengvej 14, 4180 Soroe
sales@owolff.com, T: +45 5783 3830
sales@owolff.com, T: +45 5783 3830
20-03-2023
Audio in smart glasses
Glasses with extra features
Glasses with built-in screens, cameras and - most importantly - loudspeakers are appearing on the market. These types of devices allow the user to perform many of same tasks as they would on a portable device (phone/tablet) while having hands free for other tasks. These pages specifically deal with the audio part of the smart glasses, where loudspeakers are placed in the temples as close to the ear entrance as possible in order to deliver the best sound. The speakers' job is to direct sound waves towards the user's ear while the ears remains unoccluded, allowing the user to hear the surroundings while listening to audio at the same time. This is where Ole Wolff's expertise in maximizing sound performance from tiny enclosures can make a notable difference.
Sound in the near field
Compared to earphones and headsets, which are positioned in the ear canal or directly onto the ear, the loudspeakers in smart glasses are situated in a so called near field related to the ear entrance. Therefore the tuning of the acoustic parameters are quite different from ear- and headphones, both in repects of desired frequency response and loudness. The acoustic impedance "seen" by a near field driver is very different from the in-ear impedance, which is why it takes much larger driver to produce a satisfying bass response compared to sealed earphones.
Stuffing the temple
Along with all of the electronics, battery etc. neccesary to make the glasses "smart", the temple also has to contain the driver with the tuned front and rear cavities. Every mm^3 counts and even what may seem as minor adjustments can have a huge impact on the resulting sound performance.
Read more about speaker system design
The tuning has to account for reflections coming from the head as well as low frequency cancellation caused by sound waves from the rear side of the loudspeaker's diaphragm that can reach the ear through a rear vent. Such rear vent can be necessary if there is not enough space in the temples for a large enough air volume behind the speaker to allow for a decent bass content, but the placement of the vent is critical to avoid cancellations. Using simulations various port placements can be tested without the need for physical prototypes.
Read more about Transducer design
Simulations
Ole Wolff uses a model of a dummy head and a detailed human ear similar to Head & Torso simulators found in our labs.
Typically the driver is modelled using Thiele-Small parameters to keep computation time down but full mechanical models can also be used.
The air volumes surrounding the driver and the holes, leakages and meshes to the outside world are modelled and so is skin- and eardrum impedance of the head.
Read more about simulation in Comsol Multiphysics®
Changing speaker properties
Example of Thiele-Small data of a 8x15mm speaker used in a pair of glasses with a built-in speaker.
Measuring the real deal
A satisfying acoustic performance is always the ultimate goal. Objective performance is tested using a head and torso simulator (HATS) and subjective performance is evaluated by both the end customer and the trained listeners at Ole Wolff.
A head and torso simulator (HATS) is being used to measure the acoustic performance of the speaker glasses.
Choosing an appropriate driver
When designing a pair of smart glasses it is tempting to choose as small a driver as possible to keep the overall thickness of the temples down. However, the laws of physics still apply no matter how smart the smart glasses are. In order to get as much "bass" frequencies as possible, a certain volume of air must be moved which, in general, correlates with both the diameter and the thickness of the chosen driver. Other important parameters to consider are
- High linearity (low THD)
- Low resonance frequency
- High VAS
- High sensitivity (to keep battery drain low)
The OWR-1653T-20 driver is an excellent driver with good compromise between size and performance that will work well in applications such as smart glasses.
Do you want to know more about audio in smart glasses?
Involve Ole Wolff early in your design process and you will get the best result.
Send an e-mail or contact your local sales contact to learn what Ole Wolff can do to help developing your products.