Hoparlörlerin dinleme odalarındaki ses kalitesinin araştırılması
Investigation of sound quality of loudspeakers in rooms
Yazar: KEREM BAŞARAN
Danışman: PROF. DR. HALİT TEMEL BELEK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Makine Dinamiği, Titreşimi ve Akustiği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Bu tez çalışmasında, oturma odaları ve stüdyolar gibi küçük boyutlara sahip dinleme odalarında sıkça kullanılan direkt yayınımlı elektrodinamik hoparlörlerin ses kalitesi araştırılmıştır. Bu tip hoparlörlerin ses kalitesinde, dinleme odasının akustik özelliklerinin, hoparlörün kendisi kadar yüksek bir öneme sahip olmasından ötürü araştırma hem hoparlör hem de odanın akustik özelliklerini içerecek şekilde yapılmıştır.
Hoparlörler, bir elektromanyetik motor aracılığı ile hareket ettirilen diyaframın titreşerek havada ses basıncı oluşturması yoluyla çalışır. Bu işlemi verimli bir şekilde ve frekansa göre değişmeyen, nötr bir karakterde gerçekleştirebilmek için hoparlör diyaframlarının hafif ve esnemez olması istenir. Bu iki özelliğin aynı anda sağlanması ve hoparlörün ilgili frekans bandında yüksek kalitede ses yayını yapabilmesi için, diyaframın yapısal özellikleri ve modal davranışının incelenmesi gerekir. Bunun yanında hoparlör diyaframını dengeleyen süspansiyon sistemi ve kuvveti oluşturan motor sisteminin de tıpkı diyaframda olduğu gibi, yüksek doğrusallıkta ve frekansa bağlı değişimleri minimum olacak şekilde çalışması gerekir. Tüm bunlar hoparlör ünitesinin mekanik ve elektriksel tasarımının temelini oluşturur.
Hoparlörün mekanik ve elektriksel öğeleri dışında, hoparlörün sesi ne yönde yaydığı ve bu sesin oda içerisinde hangi noktalara ne şiddette ulaştığı da ses kalitesinde belirleyici birer etmendir. Bu noktada da hoparlörün kabini, diyafram geometrisi ve modal davranışları büyük öneme sahiptir.
Hoparlörden yayılan ses, bir telefon ya da kulaklıktakinden farklı olarak yalnızca dinleyicinin kulağına ulaşmaz. Bu sebeple oda da dinlemenin bir parçası olur ve oda yansımaları ve mod şekilleri dinlemenin kalitesinde belirleyici rol oynar. Yapılan araştırmalar odadaki ses sönüm miktarının, odanın doğal frekanslarının ve dinleyiciye gelen erken ses yansımalarının hoparlörün sağlıklı bir şekilde dinlenebilmesi için çok yüksek öneme sahip olduğunu göstermiştir. Bu sebeple yüksek ses kalitesinin elde edilmesi için odanın da tıpkı hoparlör gibi belirli tasarım kriterlerini sağlar olması gerektiği anlaşılmaktadır.
Hoparlörlerin ses kalitelerinin bunca farklı etmene bağlı olması, hoparlör ses kalitesi ile ilgili genel kabul görmüş bir ölçü ya da tanımın halen ortaya çıkmamış olmasına sebep olmuştur. Bunun önemli bir sebebi de ses kalitesinin kişisel bir görüş olması ve dinleyici beğenilerinin, dinleyicilerin o güne dek dinlemiş oldukları hoparlörler tarafından şekillenmesidir. Tüm bunlar, zayıf temeller üzerine kurulu bir ses kalitesi arayışına sebep olmuştur.
Ses kalitesine dair bilginin kaynağının insan olmasından yola çıkarak, tez çalışması kapsamında bu konuda yapılacak bir jüri testinin esaslarına dair de araştırma yapılmıştır. Bu noktada kazanılan bilgiler ışığında bir jüri oluşturulmuş ve dinleme odalarında hoparlörlerin oluşturdukları ilk yansımaların dinleyicide bıraktığı izlenim üzerine bir jüri testi tasarlanmıştır. Test için özel bir dinleme odası hazırlanmış ve tavan, yan duvar ve arka duvar yansımalarını yutmak için üç farklı düzenleme oluşturulmuş, dinleyicilerden bu farklı düzenlemelerin her biri ile stereo dinleme yapmaları istenmiştir. Bu dinlemeler sonrasında dinleyiciler bazı sorulara cevap vermiş ve sesteki farklılıklar üzerine fikirlerini sunmuştur.
Yapılan çalışma tek bir kısım duvardan (tavan, yan duvar gibi) gelen yansımaların yutumunun ses imgesi üzerinde çok büyük bir etkisinin olmadığını göstermiştir. Jüri üyelerinin cevapları örnekler arasında düşük varyans göstermiştir. Detaylı varyans analizi sonrasında elde edilen bulgular ise literatürde yer alan çalışmaları destekler niteliktedir. Kuvvetli yansımalar, özellikle yan duvardan ve yeterli gecikme ile geldiklerinde sahne genişliği ve doğallık gibi algıları arttırmıştır. Hoparlörün arkasındaki duvardan ve tavandan gelen yansımalar ise tercih edilmemiştir. 
Çalışmanın son aşamasında jüri testinde yer alan örnekler bir baş ve göğüs simülatörü kullanılarak dinleyicilerin bulunduğu konumda kaydedilmiş, dijital sinyale dönüştürülmüştür. Bu sinyal üzerinde kabalık, tonalite, keskinlik gibi endüstride sıkça kullanılan on farklı ses kalitesi parametresinin hesaplaması yapılmıştır. Daha sonra bu veriler jürilerin verdiği cevaplar ile karşılaştırılmış ve ses kalitesi parametrelerinin dinleyici algılarından herhangi birini ölçer nitelikte olup olmadığı araştırılmıştır. Bu çalışma sonucunda, keskinlik değerinin dinleyicilerin yüksek hacimli olarak nitelendirdiği örneklerde tutarlı olarak daha yüksek olduğu görülmüştür. Bunun haricinde ses kalitesi parametrelerinin dinleyici yanıtları ile herhangi bir ilişki göstermediğine kanaat getirilmiştir.

Abstract:
This thesis aims to investigate the factors that affect the quality of sound portrayed by high fidelity speakers in a listening room. The present study includes analysis of factors both related to the loudspeakers and the room acoustics as it well known that the rooms can be as important as the loudspeakers. Within the scope of this thesis the sound quality performance of the two-way high fidelity speakers of a certain size are studied in living rooms with an ordinary size and acoustic properties. The thesis starts with the basic introduction to loudspeakers using examples from cone type dynamic speakers, which are easy to describe and are the most common type of speakers. These are the speakers commonly used as bass drivers in aforementioned two-way speakers and share many basics with the high frequency drivers.
Loudspeakers are devices that turn electrical energy into sound. This conversion starts with an electromagnetic motor, creating force proportional to the electric current it receives from the amplifier. This part of the speaker is recognized as the motor and includes a magnet system and a coil. The coil is placed in a high density magnetic flux created by the magnet system and the current on the coil creates forces in both directions.
The force created by the motor system is transferred to the diaphragm of the loudspeaker. The diaphragm is usually a cone with a low mass and a large surface. Its role is to create sound pressure through displacements. For the highest efficiency, they are expected to be as large as possible because this reduces the impedance with air and allows movement of a larger volume of air. Yet making the cone larger and keeping it stiff enough causes the cone to have a higher mass and partly decreases the efficiency. If the stiffness of the cone is compromised then at higher frequencies the cone will cease to act like an ideal piston. Modal behavior will start and the phase of the movements in different points on the cone will be different. This will cause irregularities in both the sound pressure level and the directivity thus the sound will be perceived by the listeners as artificial and harsh.
The diaphragm and coil inside the magnet system is suspended by what is called a suspension system. Usually consisting of two parts, the suspension system makes sure the displacements of the diaphragm take place to and from a described resting position which is optimal for the magnet system. The suspension also limits movement in extreme displacements and dampens the cone movement at the resonant frequency.
Most of the deficiencies of this kind of loudspeakers are caused by changes in mechanical and electrical characteristics when the diaphragm goes through displacement. These type of deficiencies are recognized as nonlinearities of the motor and suspension system and have a direct connection with the amount of distortion a loudspeaker introduces to the sound. The other important aspect is the diaphragm itself, which is a solid body with finite stiffness. Diaphragms introduce damping and can have relative displacements within their surface. In addition, they become more and more directional with increasing frequency. This affects the total energy distribution against frequency and directivity. These particular properties of the diaphragm usually dictate that each speaker driver is used for a limited bandwidth, usually about four octaves.
Loudspeaker measurements shared with customers usually consist of a steady state frequency response measurement. This measurement is usually taken from a 1 meter distance directly in front of the loudspeaker in an anechoic room. While giving some idea about the speakers' capabilities, these kinds of measurements do not take into account any effect related with time, directivity and distortions. Although their use is common in consumer market, audio professionals know very well that these kinds of measurements do not even come close to identifying a speaker. Plenty of measurements are required to characterize a speaker's acoustical behavior.
When it comes to assessing a loudspeaker's sound quality in a room, things get even more complicated. The direct sound projected to the listener will be identical to the one in an anechoic room or free field, but all the radiation in other directions is combined to create a complicated sound field. The listener will be subjected to countless number of reflections, each having their own frequency and phase characteristics. These reflections, along with the material in the music will define the spatial characteristics of the listening experience.
Acoustical treatments for rooms can be divided in two main categories. Treatments for room modes and treatments for reflections. Treatments for room modes are low frequency absorbers located in high pressure zone of the acoustical modes in the room. These help dampen the modal resonances in the room and correct the time and frequency response. Usually, resonant type absorbers are utilized for these kinds of applications.
For the treatment of room reflections, panels made from porous materials like fiberglass are the most common choice. These types of absorbers absorb energy proportional to particle velocity and to absorb lower frequencies, they need to be located further away from the wall they are attached to. Using these absorbers, certain wall reflections that are detrimental to sound quality can be absorbed. A similar effect can also be attained using diffusers, which distribute sound in other directions rather than absorbing. Which wall reflections should be absorbed and which ones should not is a complicated topic rooted mostly in psychoacoustics. To have a better understanding of this topic and hopefully reach some resolutions, this thesis includes an experiment with real subjects.
The experiment that has been carried out aims to understand which of the early reflections from the walls in a listening room have a positive effect on sound quality and imaging. For this experiment, a listening environment representative of a standard listening room was created and a stereo hi-fi system was installed. All the acoustical properties of the room and speakers were selected so as to fit common applications as described in the British Standard 'BS 6840-13 - Listening Tests on Loudspeakers'.
After creating the listening environment, special hangers were installed to allow mounting of wall absorber panels in locations where early reflections occur. The locations were on ceiling, side walls and the rear wall of the speaker (speaker wall). Predetermined acoustic setups, where reflections from only one direction were absorbed were created and questions were asked about the quality of the sound. The listening room was modified so that the subjects did not see the actual installations. A computer interface was introduced so that they would pick the acoustic setup they wish in a mixed order and vote easily. During the test an operator was employed to mount the absorbers according to the subject's choice. The subjects were allowed to freely rate different acoustic setups between 0 and 10. This led every subject to have different means and variances.
When the tests were finished, the first step was to analyze the results of the jury for reliability. Then the same setups introduced to the subjects were recorded with a head & torso simulator for further objective evaluation.
The statistical methods used for reliability and data extraction were item-total correlation, Friedman's test and Chi-Squared test. With item-total correlation, the answers of each listener was compared to the rest of the group on an overall basis through calculation of  variances. With Friedman test and Chi-squared, the answer to each separate item and consistency of the answers were compared and acoustic setups that were consistently ranked higher were revealed. With the help of these tools some of the subjects were identified as providing unreliable data and were left out of analyses. Data extracted from the others were investigated further to gather information about their preferences.
The analyses of the listening tests have shown that removing early reflections from a single wall (two walls for side reflections) had a small but noticeable effect. These effects were mostly about the size of the sound stage portrayed by the loudspeaker. Side-wall reflections were preferred and rear wall reflections were not preferred by the subjects. This was in line with previous research on the subject. Questions about intimacy and naturalness gave ambiguous results, which was interpreted as them not having any obvious connection with early reflections. The listeners do not prefer absorbing ceiling reflections in the listening room. 
A separate investigation was carried out using the recordings from the head & torso simulator. This involved running sound quality metrics commonly used in the industry on the recordings. The music piece as recorded by the head and torso simulator was divided into three pieces and the sound quality metrics were calculated for each of these pieces. This way it was possible to observe if the sound quality metrics would change within the song and if these variations in the metrics based on the acoustic setup were consistent between these recorded samples.
Most of the sound quality metrics did not provide consistent data within the music piece and even less had a linear relationship with the preferences of the listeners. The only correlation was between the sharpness measure and the spatial characteristics of the sound stage. Setups with higher sharpness measurements also received higher rankings from listeners. Whether this was a causal relationship or not has to be further investigated.
The fact that some of the acoustic setups have consistently received different ratings is a proof for the significance of even a single wall's reflections. In this case, the perception of the sound stage is affected when the reflections from a single wall is removed, even though the listener is relatively close and directly in front of the speakers. Keeping in mind that these reflections are  most likely resulted due to the off-axis radiation of the speakers, the significance of this off-axis radiation is once again emerged as an important parameter. 
Along with the vast amount of electrical and mechanical variables within the speaker itself, the coupling with the acoustic domain and the path to the listener presents a complex problem to optimize the sound quality. For this reason, sound quality of a loudspeaker in a listening room will stay as a subject with plenty of room for further research.